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Versions: 00                                                            
INTERNET-DRAFT                                          Adam M. Costello
draft-ietf-idn-amc-ace-m-00.txt                              2001-Feb-12
Expires 2001-Aug-14

                         AMC-ACE-M version 0.1.0

Status of this Memo

    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC2026.

    Internet-Drafts are working documents of the Internet Engineering
    Task Force (IETF), its areas, and its working groups.  Note
    that other groups may also distribute working documents as
    Internet-Drafts.

    Internet-Drafts are draft documents valid for a maximum of six
    months and may be updated, replaced, or obsoleted by other documents
    at any time.  It is inappropriate to use Internet-Drafts as
    reference material or to cite them other than as "work in progress."

    The list of current Internet-Drafts can be accessed at
    http://www.ietf.org/ietf/1id-abstracts.txt

    The list of Internet-Draft Shadow Directories can be accessed at
    http://www.ietf.org/shadow.html

    Distribution of this document is unlimited.  Please send comments
    to the author at amc@cs.berkeley.edu, or to the idn working
    group at idn@ops.ietf.org.  A non-paginated (and possibly
    newer) version of this specification may be available at
    http://www.cs.berkeley.edu/~amc/charset/amc-ace-m

Abstract

    AMC-ACE-M is a reversible map from a sequence of Unicode [UNICODE]
    characters to a sequence of letters (A-Z, a-z), digits (0-9), and
    hyphen-minus (-), henceforth called LDH characters.  Such a map
    (called an "ASCII-Compatible Encoding", or ACE) might be useful for
    internationalized domain names [IDN], because host name labels are
    currently restricted to LDH characters by [RFC952] and [RFC1123].

    AMC-ACE-M is a cross between BRACE [BRACE00] (which is efficient
    but complex) and DUDE [DUDE00] (which is simple and provides case
    preservation).  AMC-ACE-M is much simpler than BRACE but similarly
    efficient, and provides case preservation like DUDE.

    Besides domain names, there might also be other contexts where it is
    useful to transform Unicode characters into "safe" (delimiter-free)
    ASCII characters.  (If other contexts consider hyphen-minus to be
    unsafe, a different character could be used to play its role, like
    underscore.)

Contents

    Features
    Name
    Overview
    Base-32 characters
    Encoding procedure
    Decoding procedure
    Signature
    Case sensitivity models
    Comparison with RACE, BRACE, LACE, and DUDE
    Example strings
    Security considerations
    References
    Author
    Example implementation

Features

    Uniqueness:  Every Unicode string maps to at most one LDH string.

    Completeness:  Every Unicode string maps to an LDH string.
    Restrictions on which Unicode strings are allowed, and on length,
    may be imposed by higher layers.

    Efficient encoding:  The ratio of encoded size to original size is
    small for all Unicode strings.  This is important in the context
    of domain names because [RFC1034] restricts the length of a domain
    label to 63 characters.

    Simplicity:  The encoding and decoding algorithms are reasonably
    simple to implement.  The goals of efficiency and simplicity are at
    odds; AMC-ACE-M aims at a good balance between them.

    Case-preservation:  If the Unicode string has been case-folded prior
    to encoding, it is possible to record the case information in the
    case of the letters in the encoding, allowing a mixed-case Unicode
    string to be recovered if desired, but a case-insensitive comparison
    of two encoded strings is equivalent to a case-insensitive
    comparison of the Unicode strings.  This feature is optional; see
    section "Case sensitivity models".

    Readability:  The letters A-Z and a-z and the digits 0-9 appearing
    in the Unicode string are represented as themselves in the label.
    This comes for free because it usually the most efficient encoding
    anyway.

Name

    AMC-ACE-M is a working name that should be changed if it is adopted.
    (The M merely indicates that it is the thirteenth ACE devised by
    this author.  BRACE was the third.  D through L did not deliver
    enough efficiency to justify their complexity.)  Rather than waste
    good names on experimental proposals, let's wait until one proposal
    is chosen, then assign it a good name.  Suggestions (assuming the
    primary use is in domain names):

        UniHost
        UTF-A ("A" for "ASCII" or "alphanumeric",
               but unfortunately UTF-A sounds like UTF-8)
        UTF-H ("H" for "host names",
               but unfortunately UTF-H sounds like UTF-8)
        UTF-D ("D" for "domain names")
        NUDE (Normal Unicode Domain Encoding)

Overview

    AMC-ACE-M maps characters to characters--it does not consume or
    produce code points, code units, or bytes, although the algorithm
    makes use of code points, and implementations will of course need to
    represent the input and output characters somehow, usually as bytes
    or other code units.

    Each character in the Unicode string is represented by an
    integral number of characters in the encoded string.  There is no
    intermediate bit string or octet string.

    The encoded string alternates between two modes: literal mode and
    base-32 mode.  LDH characters in the Unicode string are encoded
    literally, except that hyphen-minus is doubled.  Non-LDH characters
    in the Unicode string are encoded using base-32, in which each
    character of the encoded string represents five bits (a "quintet").
    A non-paired hyphen-minus in the encoded string indicates a mode
    change.

    In base-32 mode a group of one to five quintets are used to
    represent a number, which is added to an offset to yield a
    Unicode code point, which in turn represents a Unicode character.
    (Surrogates, which are code units used by UTF-16 in pairs to
    refer to code points, are not used and not allowed in AMC-ACE-M.)
    Similarities between the code points are exploited to make the
    encoding more compact.

Base-32 characters

        "a" =  0 = 0x00 = 00000         "s" = 16 = 0x10 = 10000
        "b" =  1 = 0x01 = 00001         "t" = 17 = 0x11 = 10001
        "c" =  2 = 0x02 = 00010         "u" = 18 = 0x12 = 10010
        "d" =  3 = 0x03 = 00011         "v" = 19 = 0x13 = 10011
        "e" =  4 = 0x04 = 00100         "w" = 20 = 0x14 = 10100
        "f" =  5 = 0x05 = 00101         "x" = 21 = 0x15 = 10101
        "g" =  6 = 0x06 = 00110         "y" = 22 = 0x16 = 10110
        "h" =  7 = 0x07 = 00111         "z" = 23 = 0x17 = 10111
        "i" =  8 = 0x08 = 01000         "2" = 24 = 0x18 = 11000
        "j" =  9 = 0x09 = 01001         "3" = 25 = 0x19 = 11001
        "k" = 10 = 0x0A = 01010         "4" = 26 = 0x1A = 11010
        "m" = 11 = 0x0B = 01011         "5" = 27 = 0x1B = 11011
        "n" = 12 = 0x0C = 01100         "6" = 28 = 0x1C = 11100
        "p" = 13 = 0x0D = 01101         "7" = 29 = 0x1D = 11101
        "q" = 14 = 0x0E = 01110         "8" = 30 = 0x1E = 11110
        "r" = 15 = 0x0F = 01111         "9" = 31 = 0x1F = 11111

    The digits "0" and "1" and the letters "o" and "l" are not used, to
    avoid transcription errors.

    All decoders must recognize both the uppercase and lowercase
    forms of the base-32 characters.  The case may or may not convey
    information, as described in section "Case sensitivity models".

Encoding procedure

    The encoder first examines the Unicode string and chooses some
    parameters.  It writes these parameters into the output string, then
    proceeds to encode each Unicode character, one at a time.  The exact
    sequence of steps is given below.  All ordering of bits and quintets
    is big-endian (most significant first).  The >> and << operators
    used below mean bit shift, as in C.  For >> there is no question of
    logical versus arithmetic shift because AMC-ACE-M makes no use of
    negative numbers.

     0) Determine the Unicode code point for each non-LDH character in
        the Unicode string.  Since LDH characters are encoded literally,
        their code points are not needed.  Depending on how the Unicode
        string is presented to the encoder, this step may be a no-op.

     1) Verify that there are are no invalid code points in the input;
        that is, none exceed 0x10FFFF (the highest code point in the
        Unicode code space) and none are in the range D800..DFFF
        (surrogates).

     2) Determine the most populous row:  Row n is defined as the 256
        code points starting with n << 8, except that this definition
        would makes rows D8..DF useless, because they would contain only
        surrogates.  Therefore AMC-ACE-M defines rows D8..DF to be the
        following non-aligned blocks of 256 code points:

            row D8 = 0020..001F
            row D9 = 005B..015A
            row DA = 007B..017A
            row DB = 00A0..019F
            row DC = 00C0..01BF
            row DD = 00DF..01DE
            row DE = 0134..0233
            row DF = 0270..036F

        (Rationale:  Whereas almost every small script is confined to
        a single row, the Latin script is split across a few rows,
        and the row boundaries are not especially convenient for many
        languages.)

        Determine the row containing the most non-LDH input code points,
        breaking ties in favor of smaller-numbered rows.  (If a code
        point appears multiple times in the input, it counts multiple
        times.  This applies to steps 3 and 4 also.)  Call it row B.
        Let offsetB be the first code point of row B.

     3) Determine the most populous 16-window:  For each n in 0..31 let
        offset = ((offsetB >> 3) + n) << 3 and count the number of code
        points in the range offset through offset + 0xF.  Let A be the
        value of n that maximizes this count, breaking ties in favor
        of smaller values of n, and let offsetA be the corresponding
        offset.

     4) Determine the most populous 20k-window:  If the input is empty,
        then let C = 0.  Otherwise, for each input code point, let n =
        code_point >> 11, and count the number of non-LDH input code
        points that are not in row B and are in the range (n << 11)
        through (n << 11) + 0x4FFF.  Determine the value of n that
        maximizes the count, breaking ties in favor of smaller values of
        n, and let C be that value.

     5) Choose a style:  One of the base-32 codes used in step 7.3 has
        two variants, and so base-32 mode is subdivided into two styles,
        narrow and wide, depending on which variant is used.  Compute
        the total number of base-32 characters that would be produced
        if narrow style were used, and the number if wide style were
        used.  The easiest way to do this is to mimic the logic of steps
        6 and 7.3.  Use whichever style would produce fewer base-32
        characters.  In case of a tie, use narrow style.

     6) Encode the parameters.  If narrow style is used, then let
        offsetC = (offsetB >> 12) << 12, and encode B and A as three or
        four base-32 characters:

            00bbb bbbbb aaaaa        if B <= 0xFF
            01bbb bbbbb bbbbb aaaaa  otherwise

        If wide style is used, then let offsetC = C << 11, and encode B
        and C as three or five base-32 characters:

            10bbb bbbbb ccccc              if B <= 0xFF and C <= 0x1F
            11bbb bbbbb bbbbb ccccc ccccc  otherwise

     7) Encode each input character in turn, using the first of the
        following cases that applies.  The mode is initially base-32.

         7.1) The character is a hyphen-minus (U+002D).  Encode it as
              two hyphen-minuses.

         7.2) The character is an LDH character.  If in base-32 mode
              then output a hyphen-minus and switch to literal mode.
              Copy the character to the output.

         7.3) The character is a non-LDH character.  If in literal
              mode then output a hyphen-minus and switch to base-32
              mode.  Encode the character's code point using the
              first of the following cases that applies.  Square
              brackets enclose quintets that can be used to record
              the upper/lowercase attribute of the Unicode character
              (because the corresponding base-32 characters are
              guaranteed to be letters rather than digits) (see section
              "Case sensitivity models").

               7.3.1) Narrow style was chosen and the code point is in
                      the range offsetA through offsetA + 0xF.  Subtract
                      offsetA and encode the difference as a single
                      base-32 character:

                          [0xxxx]

               7.3.2) The code point is in the range offsetB through
                      offsetB + 0xFF.  Subtract offsetB and encode the
                      difference as two base-32 characters:

                          1xxxx [0xxxx]

               7.3.3) The code point is in the range offsetC through
                      offsetC + 0xFFF.  Subtract offsetC and encode the
                      difference as three base-32 characters:

                          1xxxx 1xxxx [0xxxx]

               7.3.4) Wide style was chosen and the code point is in
                      the range offsetC + 0x1000 through offsetC +
                      0x4FFF.  Subtract offsetC + 0x1000 and encode the
                      difference as three base-32 characters:

                          [0xxxx] xxxxx xxxxx

               7.3.5) The code point is in the range 0 through 0xFFFF.
                      Encode it as four base-32 characters:

                          1xxxx 1xxxx 1xxxx [0xxxx]

               7.3.6) If we've come this far, the code point must be
                      in the range 0x10000 through 0x10FFFF.  Subtract
                      0x10000 and encode the difference as five base-32
                      characters:

                          1xxxx 1xxxx 1xxxx 1xxxx [0xxxx]

Decoding procedure

    The details of the decoding procedure are implied by the encoding
    procedure.  The overall sequence of steps is as follows.

     1) Undo the encoder's step 6:  From the first few base-32
        characters, determine whether narrow or wide style is used, and
        determine the offsets.

     2) Set the mode to base-32.  For each remaining input character, use
        the first of the following cases that applies:

         2.1) The character is a hyphen-minus, and the following
              character is also a hyphen-minus.  Consume them both and
              output a hyphen-minus.

         2.2) The character is a hyphen-minus.  Consume it and toggle
              the mode flag.

         2.3) The current mode is literal.  Consume the input character
              and output it.

         2.4) Interpret the input character and up to four of its
              successors as base-32.  Consume characters until one is
              found whose value has the form 0xxxx.  That is the one
              that carries the upper/lowercase information.  Remember
              the length of the code.  If the length is one and wide
              style is being used, consume two more characters.
              Decode the base-32 characters into an integer, add the
              appropriate offset (which depends on the remembered code
              length), and output the Unicode character corresponding to
              the resulting code point.

              If the case-flexible or case-preserving model is being
              used (see section "Case sensitivity models"), the decoder
              must either perform the case conversion as it is decoding,
              or construct a separate record of the case information to
              accompany the output string.

     3) Before returning the output (be it a string or a string plus
        case information), the decoder must invoke the encoder on it,
        and compare the result to the input string.  The comparison
        must be case-sensitive if the case-sensitive or case-flexible
        model is being used, case-insensitive if the case-insensitive
        or case-preserving model is being used.  If the two strings do
        not match, it is an error.  This check is necessary to guarantee
        the uniqueness property (there cannot be two distinct encoded
        strings representing the same Unicode string).

    If the decoder at any time encounters an unexpected character, or
    unexpected end of input, then the input is invalid.

Signature

    The issue of how to distinguish ACE strings from unencoded strings
    is largely orthogonal to the encoding scheme itself, and is
    therefore not specified here.  In the context of domain name labels,
    a standard prefix and/or suffix (chosen to be unlikely to occur
    naturally) would presumably be attached to ACE labels.  (In that
    case, it would probably be good to forbid the encoding of Unicode
    strings that appear to match the signature, to avoid confusing
    humans about whether they are looking at a Unicode string or an ACE
    string.)

    In order to use AMC-ACE-M in domain names, the choice of signature
    must be mindful of the requirement in [RFC952] that labels never
    begin or end with hyphen-minus.  The raw encoded string will never
    begin with a hyphen-minus, and will end with a hyphen-minus iff the
    Unicode string ends with a hyphen-minus.  The easiest solution is
    to use a suffix as the signature.  Alternatively, if the Unicode
    strings were forbidden from ending with a hyphen-minus, a prefix
    could be used.

    It appears that "---" is extremely rare in domain names; among the
    four-character prefixes of all the second-level domains under .com,
    .net, and .org, "---" never appears at all.  Therefore, perhaps the
    signature should be of the form ?--- (prefix) or ---? (suffix),
    where ? could be "u" for Unicode, or "i" for internationalized, or
    "a" for ACE, or maybe "q" or "z" because they are rare.

Case sensitivity models

    The higher layer must choose one of the following four models.

    Models suitable for domain names:

      * Case-insensitive:  Before a string is encoded, all its non-LDH
        characters must be case-folded so that any strings differing
        only in case become the same string (for example, strings could
        be forced to lowercase).  Folding LDH characters is optional.
        The case of base-32 characters and literal-mode characters is
        arbitrary and not significant.  Comparisons between encoded
        strings must be case-insensitive.  The original case of non-LDH
        characters cannot be recovered from the encoded string.

      * Case-preserving:  The case of the Unicode characters is not
        considered significant, but it can be preserved and recovered,
        just like in non-internationalized host names.  Before a string
        is encoded, all its non-LDH characters must be case-folded
        as in the previous model.  LDH characters are naturally able
        to retain their case attributes because they are encoded
        literally.  The case attribute of a non-LDH character is
        recorded in one of the base-32 characters that represent
        it (section "Encoding procedure" tells which one).  If the
        base-32 character is uppercase, it means the Unicode character
        is caseless or should be forced to uppercase after being
        decoded (which is a no-op if the case folding already forces
        to uppercase).  If the base-32 character is lowercase, it
        means the Unicode character is caseless or should be forced to
        lowercase after being decoded (which is a no-op if the case
        folding already forces to lowercase).  The case of the other
        base-32 characters in a multi-quintet encoding is arbitrary
        and not significant.  Only uppercase and lowercase attributes
        can be recorded, not titlecase.  Comparisons between encoded
        strings must be case-insensitive, and are equivalent to
        case-insensitive comparisons between the Unicode strings.  The
        intended mixed-case Unicode string can be recovered as long as
        the encoded characters are unaltered, but altering the case of
        the encoded characters is not harmful--it merely alters the case
        of the Unicode characters, and such a change is not considered
        significant.

        In this model, the input to the encoder and the output of the
        decoder can be the unfolded Unicode string (in which case the
        encoder and decoder are responsible for performing the case
        folding and recovery), or can be the folded Unicode string
        accompanied by separate case information (in which case the
        higher layer is responsible for performing the case folding and
        recovery).  Whichever layer performs the case recovery must
        first verify that the Unicode string is properly folded, to
        guarantee the uniqueness of the encoding.

        It is easy to extend the nameprep algorithm [NAMEPREP02] to
        remember case information.  It merely requires an additional
        bit to be associated with each output code point in the mapping
        table.

    The case-insensitive and case-preserving models are interoperable.
    If a domain name passes from a case-preserving entity to a
    case-insensitive entity, the case information will be lost, but
    the domain name will still be equivalent.  This phenomenon already
    occurs with non-internationalized domain names.

    Models unsuitable for domain names, but possibly useful in other
    contexts:

      * Case-sensitive:  Unicode strings may contain both uppercase and
        lowercase characters, which are not folded.  Base-32 characters
        must be lowercase.  Comparisons between encoded strings must be
        case-sensitive.

      * Case-flexible:  Like case-preserving, except that the choice
        of whether the case of the Unicode characters is considered
        significant is deferred.  Therefore, base-32 characters must
        be lowercase, except for those used to indicate uppercase
        Unicode characters.  Comparisons between encoded strings may be
        case-sensitive or case-insensitive, and such comparisons are
        equivalent to the corresponding comparisons between the Unicode
        strings.

Comparison with RACE, BRACE, LACE, and DUDE

    In this section we compare AMC-ACE-M and four other ACEs: RACE
    [RACE03], BRACE [BRACE00], LACE [LACE01], and Extended DUDE
    [DUDE00].  We do not include SACE [SACE], UTF-5 [UTF5], or UTF-6
    [UTF6] in the comparison, because SACE appears obviously too
    complex, UTF-5 appears obviously too inefficient, and UTF-6 can
    never be more efficient than its similarly simple successor, DUDE.

    Case preservation support:

        DUDE, AMC-ACE-M:  all characters
                  BRACE:  only the letters A-Z, a-z
             RACE, LACE:  none

    RACE, BRACE, and LACE transform the Unicode string to an
    intermediate bit string, then into a base-32 string, so there is no
    particular alignment between the base-32 characters and the Unicode
    characters.  DUDE and AMC-ACE-M do not have this intermediate stage,
    and enforce alignment between the base-32 characters and the Unicode
    characters, which facilitates the case preservation.

    Complexity is hard to measure.  This author would subjectively
    describe the complexity of the algorithms as:

        RACE, LACE, DUDE: fairly simple but not trivial
               AMC-ACE-M: moderate
                   BRACE: complex

    The complexity of AMC-ACE-M is in the number of rules, but the
    individual rules are not very complex, and they are generally
    non-interacting.

    The relative efficiency of the various algorithms is suggested
    by the sizes of the encodings in section "Example strings".  For
    each ACE there is a graph below showing a horizontal bar for
    each example string, representing the ACE length divided by the
    minimum length among all the ACEs for that example string (so the
    ratio is at least 1).  Example R is excluded because it violates
    nameprep [NAMEPREP02].  The other example strings all use different
    languages, except that there are several Japanese examples.  To
    avoid skewing the results, each graph collapses all the Japanese
    ratios into a single bar representing the median ratio.  A ratio r
    is represented by a bar of length r/0.04 characters.  Since the bar
    will always be at least 1/0.04 = 25 characters long, we show the
    first 25 characters as "O" and the rest as "@". The bars are sorted
    so that the graph looks like a cummulative distribution.  Each bar
    is labeled with the language of the corresponding example string.
    (The difference between the Chinese and Taiwanese strings is that
    the former uses simplified characters.)

        RACE:
          Hindi       OOOOOOOOOOOOOOOOOOOOOOOOO@@@
          Korean      OOOOOOOOOOOOOOOOOOOOOOOOO@@@
          Arabic      OOOOOOOOOOOOOOOOOOOOOOOOO@@@@
          Taiwanese   OOOOOOOOOOOOOOOOOOOOOOOOO@@@@
          Hebrew      OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@
          Russian     OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@@
          Japanese    OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@@@
          Spanish     OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@@@@@
          Chinese     OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@@@@@@
          Vietnamese  OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@@@@@@@@@@@@
          Czech       OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@@@@@@@@@@@@@@@@@@@@@

        LACE:
          Korean      OOOOOOOOOOOOOOOOOOOOOOOOO@@@
          Hindi       OOOOOOOOOOOOOOOOOOOOOOOOO@@@@
          Taiwanese   OOOOOOOOOOOOOOOOOOOOOOOOO@@@@
          Arabic      OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@@
          Hebrew      OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@@
          Chinese     OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@@@
          Japanese    OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@@@
          Russian     OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@@@
          Spanish     OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@@@@@@
          Vietnamese  OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@@@@@@@@@@
          Czech       OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@@@@@@@@@@@@@@

        DUDE:
          Russian     OOOOOOOOOOOOOOOOOOOOOOOOO
          Arabic      OOOOOOOOOOOOOOOOOOOOOOOOO
          Hebrew      OOOOOOOOOOOOOOOOOOOOOOOOO@@
          Vietnamese  OOOOOOOOOOOOOOOOOOOOOOOOO@@@@
          Chinese     OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@
          Japanese    OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@
          Korean      OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@@
          Spanish     OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@@
          Czech       OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@@@
          Hindi       OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@@@
          Taiwanese   OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@@@@

        AMC-ACE-M:
          Czech       OOOOOOOOOOOOOOOOOOOOOOOOO
          Hebrew      OOOOOOOOOOOOOOOOOOOOOOOOO
          Japanese    OOOOOOOOOOOOOOOOOOOOOOOOO
          Korean      OOOOOOOOOOOOOOOOOOOOOOOOO
          Russian     OOOOOOOOOOOOOOOOOOOOOOOOO
          Spanish     OOOOOOOOOOOOOOOOOOOOOOOOO
          Taiwanese   OOOOOOOOOOOOOOOOOOOOOOOOO
          Vietnamese  OOOOOOOOOOOOOOOOOOOOOOOOO
          Chinese     OOOOOOOOOOOOOOOOOOOOOOOOO@
          Arabic      OOOOOOOOOOOOOOOOOOOOOOOOO@@@
          Hindi       OOOOOOOOOOOOOOOOOOOOOOOOO@@@@@

        BRACE:
          Chinese     OOOOOOOOOOOOOOOOOOOOOOOOO
          Hindi       OOOOOOOOOOOOOOOOOOOOOOOOO
          Japanese    OOOOOOOOOOOOOOOOOOOOOOOOO
          Spanish     OOOOOOOOOOOOOOOOOOOOOOOOO
          Taiwanese   OOOOOOOOOOOOOOOOOOOOOOOOO
          Arabic      OOOOOOOOOOOOOOOOOOOOOOOOO@
          Czech       OOOOOOOOOOOOOOOOOOOOOOOOO@
          Vietnamese  OOOOOOOOOOOOOOOOOOOOOOOOO@
          Hebrew      OOOOOOOOOOOOOOOOOOOOOOOOO@@
          Korean      OOOOOOOOOOOOOOOOOOOOOOOOO@@
          Russian     OOOOOOOOOOOOOOOOOOOOOOOOO@@@

    These results suggest that DUDE is preferrable to RACE and LACE,
    because it has similar simplicity, better support for case
    preservation, and is somewhat more efficient.

    The results also suggest that AMC-ACE-M is preferrable to BRACE,
    because it has similar efficiency, better support for case
    preservation, and is simpler.

    DUDE and AMC-ACE-M have equal support for case preservation, but
    AMC-ACE-M offers significantly better efficiency, at the cost of
    significantly greater complexity, so choosing between them entails a
    value judgement.

Example strings

    In the ACE encodings below, signatures (like "bq--" for RACE) are
    not shown.  Non-LDH characters in the Unicode string are forced to
    lowercase before being encoded using BRACE, RACE, and LACE.  For
    RACE and LACE, the letters A-Z are likewise forced to lowercase.
    UTF-8 and UTF-16 are included for length comparisons, with non-ASCII
    bytes shown as "?". AMC-ACE-M is abbreviated AMC-M.  Backslashes
    show where line breaks have been inserted in ACE strings too long
    for one line.  The RACE and LACE encodings are courtesy of Mark
    Davis's online UTF converter [UTFCONV] (slightly modified to remove
    the length restrictions).

    The first several examples are all names of Japanese music artists,
    song titles, and TV programs, just because the author happens to
    have them handy (but Japanese is useful for providing examples
    of single-row text, two-row text, ideographic text, and various
    mixtures thereof).

    (A) 3<nen>B<gumi><kinpachi><sensei>  (Japanese TV program title)

        <nen>              = U+5E74                       (kanji)
        <gumi>             = U+7D44                       (kanji)
        <kinpachi><sensei> = U+91D1 U+516B U+5148 U+751F  (kanji)

        UTF-16: ????????????????
        UTF-8:  3???B???????????????
        AMC-M:  utk-3-8ze-B-hkenqtymwifi9
        BRACE:  u-3-ygj-b-ynb6gjc7pp4k5p5w
        DUDE:   j3le74G062nd44p1d1l16bk8n51f
        RACE:   3aadgxtuabrh2rer2fiwwukioupq
        LACE:   74adgxtuabrh2rer2fiwwukioupq

    (B) <amuro><namie>-with-SUPER-MONKEYS  (Japanese music group name)

        <amuro><namie> = U+5B89 U+5BA4 U+5948 U+7F8E U+6075  (kanji)

        UTF-8:  ??????????????????-with-SUPER-MONKEYS
        AMC-M:  u5m2j4etwif6q2zf---with--SUPER--MONKEYS
        BRACE:  uvj7fuaqcahy982xa---with--SUPER--MONKEYS
        DUDE:   lb89q4p48nf8em075-g077m9n4m8-N3LGM5N2-MdVURLN9J
        UTF-16: ????????????????????????????????????????????????
        LACE:   ajnytjablfeac74oafqhkeyafv3qm5difvzxk4dfoiww233onnsxs4y
        RACE:   3bnysw5elfeh7dtaouac2adxabuqa5aanaac2adtab2qa4aamuaheab\
                nabwqa3yanyagwadfab4qa4y

    (C) Hello-Another-Way-<sorezore><no><basho>  (Japanese song title)

        <sorezore><no> = U+305D U+308C U+305E U+308C U+306E  (hiragana)
        <basho>        = U+5834 U+6240                       (kanji)

        UTF-8:  Hello-Another-Way-?????????????????????
        BRACE:  ji7-Hello--Another--Way---v3jhaefvd2ufj62
        AMC-M:  bsk-Hello--Another--Way---p2nq2nyqx2veyuwa
        DUDE:   M8lssv-Huvn4m8ln2-Nm1n9-j05docleocmel834m240
        UTF-16: ??????????????????????????????????????????????????
        LACE:   ciagqzlmnrxs2ylon52gqzlsfv3wc6jnauyf3dc6rrxacwbuafrea
        RACE:   3aagqadfabwaa3aan4ac2adbabxaa3yaoqagqadfabzaaliao4agcad\
                zaawtaxjqrqyf4memgbxfqndcia

    (D) <hitotsu><yane><no><shita>2  (Japanese TV program title)

        <hitotsu> = U+3072 U+3068 U+3064  (hiragana)
        <yane>    = U+5C4B U+6839         (kanji)
        <no>      = U+306E                (hiragana)
        <shita>   = U+4E0B                (kanji)

        UTF-16: ????????????????
        UTF-8:  ?????????????????????2
        AMC-M:  bsnzciex6wmy2vjqw8sm-2
        BRACE:  ji96u56uwbhf2wqxnw4s-2
        DUDE:   j072m8klc4bm839j06eke0bg032
        RACE:   3ayhemdigbsfys3iheyg4tqlaaza
        LACE:   74yhemdigbsfys3iheyg4tqlaaza

    (E) Maji<de>Koi<suru>5<byou><mae> (Japanese song title)

        <de>        = U+3067         (hiragana)
        <suru>      = U+3059 U+308B  (hiragana)
        <byou><mae> = U+79D2 U+524D  (kanji)

        UTF-8:  Maji???Koi??????5??????
        UTF-16: ??????????????????????????
        AMC-M:  bsm-Maji-r-Koi-b2m-5-z37cxuwp
        BRACE:  ji8-Maji-g-Koi-qe7x-5-wx7p6ma
        DUDE:   Mdhqpj067G06bvpj059obg035n9d2l24d
        RACE:   3aag2adbabvaa2jqm4agwadpabutawjqrmadk6oskjgq
        LACE:   74ag2adbabvaa2jqm4agwadpabutawjqrmadk6oskjgq

    (F) <pafii>de<runba>  (Japanese song title)

        <pafii> = U+30D1 U+30D5 U+30A3 U+30FC  (katakana)
        <runba> = U+30EB U+30F3 U+30D0         (katakana)

        UTF-16: ??????????????
        BRACE:  3iu8pazt-de-pygi
        AMC-M:  bs3jp4d9n-de-8m9di
        RACE:   gdi5li7475sp6zpl6pia
        DUDE:   j0d1lq3vcg064lj0ebv3t0
        UTF-8:  ????????????de?????????
        LACE:   aqyndvnd7qbaazdfamyox46q

    (G) <sono><supiido><de>  (Japanese song title)

        <sono>    = U+305D U+306E                (hiragana)
        <supiido> = U+30B9 U+30D4 U+30FC U+30C9  (katakana)
        <de>      = U+3067                       (hiragana)

        RACE:   gbow5oou7tewo
        UTF-16: ??????????????
        BRACE:  bidprdmp9wt7mi
        LACE:   a4yf23vz2t6mszy
        AMC-M:  bsmfyq5j7e9n6jr
        DUDE:   j05dmer9t4vcs9m7
        UTF-8:  ?????????????????????

    The next several examples are all translations of the sentence "Why
    can't they just speak in <language>?" (courtesy of Michael Kaplan's
    "provincial" page [PROVINCIAL]).  Word breaks and punctuation have
    been removed, as is often done in domain names.

    (H) Arabic (Egyptian):
        U+0644 U+064A U+0647 U+0645 U+0627 U+0628 U+062A U+0643 U+0644
        U+0645 U+0648 U+0634 U+0639 U+0631 U+0628 U+064A U+061F

        DUDE:   m44qnli7oqk3kloj4phi8kahf
        BRACE:  28akcjwcmp3ciwb4t3ngd4nbaz
        AMC-M:  agiekhfuhuiukdefivevjvbuiktr
        RACE:   azceur2fe4ucuq2eivediojrfbfb6
        LACE:   cedeisshiutsqksdircuqnbzgeueuhy
        UTF-16: ??????????????????????????????????
        UTF-8:  ??????????????????????????????????

    (I) Chinese (simplified):
        U+4ED6 U+4EEC U+4E3A U+4EC0 U+4E48 U+4E0D U+8BF4 U+4E2D U+6587

        UTF-16: ??????????????????
        BRACE:  kgcqqsgp26i5h4zn7req5i
        AMC-M:  uqj7g8nvk6awispn9wupdnh
        DUDE:   ked6ucjas0k8gdobf4ke2dm587
        UTF-8:  ???????????????????????????
        LACE:   azhnn3b2ybea2aml6qau4libmwdq
        RACE:   3bhnmtxmjy5e5qcojbha3c7ujywwlby

    (J) Czech: Pro<ccaron>prost<ecaron>nemluv<iacute><ccaron>esky

        <ccaron> = U+010D
        <ecaron> = U+011B
        <iacute> = U+00ED

        UTF-8:  Pro??prost??nemluv????esky
        AMC-M:  g26-Pro-p-prost-9m-nemluv-6pp-esky
        BRACE:  i32-Pro-u-prost-8y-nemluv-29f3n-esky
        DUDE:   N0imfh0dg70imfn3kh1bg6eltsn5mudh0dg65n3mbn9
        UTF-16: ????????????????????????????????????????????
        LACE:   amaha4tpaeaq2biaobzg643uaearwbyanzsw23dvo3wqcainaqagk43\
                lpe
        RACE:   ah7xb73s75xq373q75zp6377op7xig77n37wl73n75wp65p7o3762dp\
                7mx7xh73l754q

    (K) Hebrew:
        U+05DC U+05DE U+05D4 U+05D4 U+05DD U+05E4 U+05E9 U+05D5 U+05D8
        U+05DC U+05D0 U+05DE U+05D3 U+05D1 U+05E8 U+05D9 U+05DD U+05E2
        U+05D1 U+05E8 U+05D9 U+05EA

        AMC-M:  af4nqeep8e8jfinaqdb8ijp8cb8ij8k
        DUDE:   ldcukktu4pt5osgujhu8t9tu2t1u8t9ua
        BRACE:  27vkyp7bgwmbpfjgc4ynx5nd8xsp5nd9c
        RACE:   axon5vgu3xsotvoy3tin5u6r5dm53ywr5dm6u
        LACE:   cyc5zxwu2to6j2ov3donbxwt2huntxpc2hunt2q
        UTF-8:  ????????????????????????????????????????????
        UTF-16: ????????????????????????????????????????????

    (L) Hindi:
        U+092F U+0939 U+0932 U+094B U+0917 U+0939 U+093F U+0928 U+094D
        U+0926 U+0940 U+0915 U+094D U+092F U+094B U+0902 U+0928 U+0939
        U+0940 U+0902 U+092C U+094B U+0932 U+0938 U+0915 U+0924 U+0947
        U+0939 U+0948 U+0902  (Devanagari)

        BRACE:  2b7xtenqdr7zc6uma2pmcz7ibage237kdemicnk9gei32
        RACE:   bextsmslc44t6kcnezabktjpjmbcqokaaiwewmrycuseookiai
        LACE:   dyes6ojsjmltspzijuteafknf5fqekbziabcyszshaksirzzjaba
        AMC-M:  ajhurbvcwmthbhuiwpugitfwpurwmscuibiscunwmvcatfuerbwisc
        DUDE:   p2fj9ikbh7j9vi8kdi6k0h5kdifkbg2i8j9k0g2ickbj2oh5i4k7j9k\
                8g2
        UTF-16: ???????????????????????????????????????????????????????\
                ?????
        UTF-8:  ???????????????????????????????????????????????????????\
                ???????????????????????????????????

    (M) Korean:
        U+C138 U+ACC4 U+C758 U+BAA8 U+B4E0 U+C0AC U+B78C U+B4E4 U+C774
        U+D55C U+AD6D U+C5B4 U+B97C U+C774 U+D574 U+D55C U+B2E4 U+BA74
        U+C5BC U+B9C8 U+B098 U+C88B U+C744 U+AE4C  (Hangul syllables)

        UTF-16: ????????????????????????????????????????????????
        UTF-8:  ???????????????????????????????????????????????????????\
                ?????????????????
        AMC-M:  yhxcj2w6exiaxi68acfn92n68ezehk6xypdpwam6zehmwhk648eavwd\
                p6aqi23ieemweywn
        BRACE:  y394qebjusrcndbs82pkvstf96sxufcr7ffr4vbgdwsxufcx8pdktgb\
                gmnsqydmk7im56arju6pt82
        LACE:   77atrlgey5mlvkfu4dakzn4mwtsmo5gvlsww3rnuxf6mo5gvotkvzmx\
                exj2mlpfzzcyjrsely5ck4ta
        RACE:   3datrlgey5mlvkfu4dakzn4mwtsmo5gvlsww3rnuxf6mo5gvotkvzmx\
                exj2mlpfzzcyjrsely5ck4ta
        DUDE:   s138qcc4s758raa8ke0s0acr78cke4s774t55cqd6ds5b4r97cs774t\
                574lcr2e4q74s5bcr9c8g98s88bn44qe4c

    (N) Russian:
        U+041F U+043E U+0447 U+0435 U+043C U+0443 U+0436 U+0435 U+043E
        U+043D U+0438 U+043D U+0435 U+0433 U+043E U+0432 U+043E U+0440
        U+044F U+0442 U+043F U+043E U+0440 U+0443 U+0441 U+0441 U+043A
        U+0438  (Cyrillic)

        DUDE:   K3fuk7j5sk3j6lutotljuiuk0vijfuk0jhhjao
        AMC-M:  aehHgrvfemvgvfgfafvfvdgvcgiwrkhgimjjca
        BRACE:  269xyjvcyafqfdwyr3xfd8z8byi6z39xyi692s7ug2
        RACE:   aq7t4rzvhrbtmnj6hu4d2njthyzd4qcpii7t4qcdifatuoa
        LACE:   dqcd6pshgu6egnrvhy6tqpjvgm7depsaj5bd6psainaucory
        UTF-16: ???????????????????????????????????????????????????????\
                ???
        UTF-8:  ???????????????????????????????????????????????????????
                ???

    (O) Spanish: Porqu<eacute>nopuedensimplementehablarenEspa<ntilde>ol

        <eacute> = U+00E9
        <ntilde> = U+00F1

        UTF-8:  Porqu??nopuedensimplementehablarenEspa??ol
        AMC-M:  aa7-Porqu-b-nopuedensimplementehablarenEspa-j-ol
        BRACE:  22x-Porqu-9-nopuedensimplementehablarenEspa-j-ol
        DUDE:   N0mfn2hlu9mevn0lm5klun3m9tn0mcltlun4m5ohishn2m5uLn3gm1v\
                1mfs
        RACE:   abyg64troxuw433qovswizloonuw24dmmvwwk3tumvugcytmmfzgk3t\
                fonygd4lpnq
        LACE:   faaha33sof26s3tpob2wkzdfnzzws3lqnrsw2zloorswqylcnrqxezl\
                omvzxayprn5wa
        UTF-16: ???????????????????????????????????????????????????????\
                ?????????????????????????

    (P) Taiwanese:
        U+4ED6 U+5011 U+7232 U+4EC0 U+9EBD U+4E0D U+8AAA U+4E2D U+6587

        UTF-16: ??????????????????
        UTF-8:  ???????????????????????????
        AMC-M:  uqj7g2tbgtu6a385pspnxkupdnh
        BRACE:  kgcqui49gatc2wyrn8y7cndgte9
        RACE:   3bhnmuaroize5qe6xvha3cvkjywwlby
        LACE:   75hnmuaroize5qe6xvha3cvkjywwlby
        DUDE:   ked6l011n232kec0pebdke0doaaake2dm587

    (Q) Vietnamese:
        Ta<dotbelow>isaoho<dotbelow>kh<ocirc>ngth<ecirc><hookabove>chi\
        <hookabove>no<acute>iti<ecirc><acute>ngVi<ecirc><dotbelow>t

        <dotbelow>  = U+0323
        <ocirc>     = U+00F4
        <ecirc>     = U+00EA
        <hookabove> = U+0309
        <acute>     = U+0301

        UTF-8:  Ta??isaoho??kh??ngth????chi??no??iti????ngVi????t
        AMC-M:  ada-Ta-ud-isaoho-ud-kh-s9e-ngth-s8kj-chi-j-no-b-iti-s8k\
                b-ngVi-s8kud-t
        BRACE:  i54-Ta-8-isaoho-ay-kh-29n-ngth-s2xa6i-chi-k-no-2g-iti-2\
                9c29-ngVi-25p48-t
        UTF-16: ???????????????????????????????????????????????????????\
                ?????????????????????
        DUDE:   N4m1j23g69n3m1vovj23g6bov4menn4m8uaj09g63opj09g6evj01g6\
                9n4m9uaj01g6enN6m9uaj23g74
        LACE:   aiahiyibamrqmadjonqw62dpaebsgcaannupi3thoruouaidbebqay3\
                ineaqgcicabxg6aidaecaa2lunhvacaybauag4z3wnhvacazdaeahi
        RACE:   ap7xj73bep7wt73t75q76377nd7w6i77np7wr77u75xp6z77ot7wr77\
                kbh7wh73i75uqt73o75xqd73j752p62p75ia763x7m77xn73j77vch7\
                3u

    The last example is an ASCII string that breaks not only the
    existing rules for host name labels but also the rules proposed in
    [NAMEPREP02] for internationalized domain names.

    (R) -> $1.00 <-

        UTF-8:  -> $1.00 <-
        DUDE:   -jei0kj1iej0gi0jc-
        RACE:   aawt4ibegexdambahqwq
        LACE:   bmac2praeqys4mbqea6c2
        UTF-16: ??????????????????????
        AMC-M:  aae--vqae-1-q-00-avn--
        BRACE:  229--t2b4-1-w-00-i9i--

Security considerations

    Users expect each domain name in DNS to be controlled by a single
    authority.  If a Unicode string intended for use as a domain label
    could map to multiple ACE labels, then an internationalized domain
    name could map to multiple ACE domain names, each controlled by
    a different authority, some of which could be spoofs that hijack
    service requests intended for another.  Therefore AMC-ACE-M is
    designed so that each Unicode string has a unique encoding.

    However, there can still be multiple Unicode representations of the
    "same" text, for various definitions of "same".  This problem is
    addressed to some extent by the Unicode standard under the topic
    of canonicalization, but some text strings may be misleading or
    ambiguous to humans when used as domain names, such as strings
    containing dots, slashes, at-signs, etc.  These issues are being
    further studied under the topic of "nameprep" [NAMEPREP02].

References

    [ACEID01] Yoshiro Yoneya, Naomasa Maruyama, "Proposal for
    a determining process of ACE identifier", 2000-Dec-19,
    draft-ietf-idn-aceid-01.

    [BRACE00] Adam Costello, "BRACE: Bi-mode Row-based
    ASCII-Compatible Encoding for IDN version 0.1.2", 2000-Sep-19,
    draft-ietf-idn-brace-00.

    [DUDE00] Brian Spolarich, Mark Welter, "DUDE: Differential Unicode
    Domain Encoding", 2000-Nov-21, draft-ietf-idn-dude-00.

    [IDN] Internationalized Domain Names (IETF working group),
    http://www.i-d-n.net/, idn@ops.ietf.org.

    [LACE01] Paul Hoffman, Mark Davis, "LACE: Length-based ASCII
    Compatible Encoding for IDN", 2001-Jan-05, draft-ietf-idn-lace-01.

    [NAMEPREP02] Paul Hoffman, Marc Blanchet, "Preparation
    of Internationalized Host Names", 2001-Jan-17,
    draft-ietf-idn-nameprep-02.

    [PROVINCIAL] Michael Kaplan, "The 'anyone can be provincial!' page",
    http://www.trigeminal.com/samples/provincial.html.

    [RACE03] Paul Hoffman, "RACE: Row-based ASCII Compatible Encoding
    for IDN", 2000-Nov-28, draft-ietf-idn-race-03.

    [RFC952] K. Harrenstien, M. Stahl, E. Feinler, "DOD Internet Host
    Table Specification", 1985-Oct, RFC 952.

    [RFC1034] P. Mockapetris, "Domain Names - Concepts and Facilities",
    1987-Nov, RFC 1034.

    [RFC1123] Internet Engineering Task Force, R. Braden (editor),
    "Requirements for Internet Hosts -- Application and Support",
    1989-Oct, RFC 1123.

    [SACE] Dan Oscarsson, "Simple ASCII Compatible Encoding (SACE)",
    draft-ietf-idn-sace-*.

    [UNICODE] The Unicode Consortium, "The Unicode Standard",
    http://www.unicode.org/unicode/standard/standard.html.

    [UTF5] James Seng, Martin Duerst, Tin Wee Tan, "UTF-5, a
    Transformation Format of Unicode and ISO 10646", draft-jseng-utf5-*.

    [UTF6] Mark Welter, Brian W. Spolarich, "UTF-6 - Yet Another
    ASCII-Compatible Encoding for IDN", draft-ietf-idn-utf6-*.

    [UTFCONV] Mark Davis, "UTF Converter",
    http://www.macchiato.com/unicode/convert.html.

Author

    Adam M. Costello <amc@cs.berkeley.edu>
    http://www.cs.berkeley.edu/~amc/


Example implementation


/******************************************/
/* amc-ace-m.c 0.1.0 (2001-Feb-12-Mon)    */
/* Adam M. Costello <amc@cs.berkeley.edu> */
/******************************************/

/* This is ANSI C code implementing AMC-ACE-M version 0.1.*. */


/************************************************************/
/* Public interface (would normally go in its own .h file): */

#include <limits.h>

enum amc_ace_status {
  amc_ace_success,
  amc_ace_invalid_input,
  amc_ace_output_too_big
};

enum case_sensitivity { case_sensitive, case_insensitive };

#if UINT_MAX >= 0x10FFFF
typedef unsigned int u_code_point;
#else
typedef unsigned long u_code_point;
#endif

int amc_ace_m_encode(
  unsigned int input_length,
  const u_code_point *input,
  const unsigned char *uppercase_flags,
  unsigned int *output_size,
  unsigned char *output );

    /* amc_ace_m_encode() converts Unicode to AMC-ACE-M.  The input  */
    /* must be represented as an array of Unicode code points        */
    /* (not code units; surrogate pairs are not allowed), and the    */
    /* output will be represented as null-terminated ASCII.  The     */
    /* input_length is the number of code points in the input.  The  */
    /* output_size is an in/out argument: the caller must pass       */
    /* in the maximum number of characters that may be output        */
    /* (including the terminating null), and on successful return    */
    /* it will contain the number of characters actually output      */
    /* (including the terminating null, so it will be one more than  */
    /* strlen() would return, which is why it is called output_size  */
    /* rather than output_length).  The uppercase_flags array must   */
    /* hold input_length boolean values, where nonzero means the     */
    /* corresponding Unicode character should be forced to uppercase */
    /* after being decoded, and zero means it is caseless or should  */
    /* be forced to lowercase.  Alternatively, uppercase_flags may   */
    /* be a null pointer, which is equivalent to all zeros.  The     */
    /* letters a-z and A-Z are always encoded literally, regardless  */
    /* of the corresponding flags.  The encoder always outputs       */
    /* lowercase base-32 characters except when nonzero values       */
    /* of uppercase_flags require otherwise, so the encoder is       */
    /* compatible with any of the case models.  The return value     */
    /* may be any of the amc_ace_status values defined above; if     */
    /* not amc_ace_success, then output_size and output may contain  */
    /* garbage.  On success, the encoder will never need to write an */
    /* output_size greater than input_length*5+6, because of how the */
    /* encoding is defined.                                          */

int amc_ace_m_decode(
  enum case_sensitivity case_sensitivity,
  unsigned char *scratch_space,
  const unsigned char *input,
  unsigned int *output_length,
  u_code_point *output,
  unsigned char *uppercase_flags );

    /* amc_ace_m_decode() converts AMC-ACE-M to Unicode.  The input   */
    /* must be represented as null-terminated ASCII, and the output   */
    /* will be represented as an array of Unicode code points.        */
    /* The case_sensitivity argument influences the check on the      */
    /* well-formedness of the input string; it must be case_sensitive */
    /* if case-sensitive comparisons are allowed on encoded strings,  */
    /* case_insensitive otherwise (see also section "Case sensitivity */
    /* models" of the AMC-ACE-M specification).  The scratch_space    */
    /* must point to space at least as large as the input, which will */
    /* get overwritten (this allows the decoder to avoid calling      */
    /* malloc()).  The output_length is an in/out argument: the       */
    /* caller must pass in the maximum number of code points that     */
    /* may be output, and on successful return it will contain the    */
    /* actual number of code points output.  The uppercase_flags      */
    /* array must have room for at least output_length values, or it  */
    /* may be a null pointer if the case information is not needed.   */
    /* A nonzero flag indicates that the corresponding Unicode        */
    /* character should be forced to uppercase by the caller, while   */
    /* zero means it is caseless or should be forced to lowercase.    */
    /* The letters a-z and A-Z are output already in the proper case, */
    /* but their flags will be set appropriately so that applying the */
    /* flags would be harmless.  The return value may be any of the   */
    /* amc_ace_status values defined above; if not amc_ace_success,   */
    /* then output_length, output, and uppercase_flags may contain    */
    /* garbage.  On success, the decoder will never need to write     */
    /* an output_length greater than the length of the input (not     */
    /* counting the null terminator), because of how the encoding is  */
    /* defined.                                                       */


/**********************************************************/
/* Implementation (would normally go in its own .c file): */

#include <string.h>

/* Character utilities: */

/* is_ldh(codept) returns 1 if the code point represents an LDH   */
/* character (ASCII letter, digit, or hyphen-minus), 0 otherwise. */

static int is_ldh(u_code_point codept)
{
  if (codept ==  45) return 1;
  if (codept <   48) return 0;
  if (codept <=  57) return 1;
  if (codept <   65) return 0;
  if (codept <=  90) return 1;
  if (codept <   97) return 0;
  if (codept <= 122) return 1;
  return 0;
}

/* is_AtoZ(c) returns 1 if c is an         */
/* uppercase ASCII letter, zero otherwise. */

static unsigned char is_AtoZ(unsigned char c)
{
  return c >= 65 && c <= 90;
}

/* special_row_offset[n] holds the offset of the       */
/* bottom of special row 0xD8 + n, where n is in 0..7. */

static u_code_point special_row_offset[] =
  { 0x0020, 0x005B, 0x007B, 0x00A0, 0x00C0, 0x00DF, 0x0134, 0x0270 };

/* base32[n] is the lowercase base-32 character representing  */
/* the number n from the range 0 to 31.  Note that we cannot  */
/* use string literals for ASCII characters because an ANSI C */
/* compiler does not necessarily use ASCII.                   */

static const unsigned char base32[] = {
  97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107,     /* a-k */
  109, 110,                                               /* m-n */
  112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122,  /* p-z */
  50, 51, 52, 53, 54, 55, 56, 57                          /* 2-9 */
};

/* base32_decode(c) returns the value of a base-32 character, in the */
/* range 0 to 31, or the constant base32_invalid if c is not a valid */
/* base-32 character.                                                */

enum { base32_invalid = 32 };

static unsigned int base32_decode(unsigned char c)
{
  if (c < 50) return base32_invalid;
  if (c <= 57) return c - 26;
  if (c < 97) c += 32;
  if (c < 97 || c == 108 || c == 111 || c > 122) return base32_invalid;
  return c - 97 - (c > 108) - (c > 111);
}

/* unequal(case_sensitivity,a1,a2,n) returns 0 if the arrays   */
/* a1 and a2 are equal in the first n positions, 1 otherwise.  */
/* If case_sensitivity is case_insensitive, then ASCII A-Z are */
/* considered equal to a-z respectively.                       */

static int unequal(
  enum case_sensitivity case_sensitivity,
  const unsigned char *a1,
  const unsigned char *a2,
  unsigned int n )
{
  const unsigned char *end;
  unsigned char c1, c2;

  if (case_sensitivity != case_insensitive) return memcmp(a1,a2,n);

  for (end = a1 + n;  a1 < end;  ++a1, ++a2) {
    c1 = *a1;
    c2 = *a2;
    if (c1 >= 65 && c1 <= 90) c1 += 32;
    if (c2 >= 65 && c2 <= 90) c2 += 32;
    if (c1 != c2) return 1;
  }

  return 0;
}


/* Encoder: */

int amc_ace_m_encode(
  unsigned int input_length,
  const u_code_point *input,
  const unsigned char *uppercase_flags,
  unsigned int *output_size,
  unsigned char *output )
{
  unsigned int literal, wide;  /* boolean */
  u_code_point codept, n, diff, morebits;
  u_code_point A, B, C, offsetA, offsetB, offsetC, offset;
  const u_code_point *input_end, *p, *pp;
  unsigned int count, max, next_in, next_out, max_out, codelen, i;
  unsigned char c;

  input_end = input + input_length;

  /* 1) Verify that only valid code points appear: */

  for (p = input;  p < input_end;  ++p) {
    if (*p >> 11 == 0x1B || *p > 0x10FFFF) return amc_ace_invalid_input;
  }

  /* 2) Determine the most populous row: B and offsetB */

  /* first check the special rows: */

  B = 0xD8;
  offsetB = special_row_offset[0];
  max = 0;

  for (n = 0;  n < 8;  ++n) {
    offset = special_row_offset[n];
    count = 0;

    for (p = input;  p < input_end;  ++p) {
      if (*p - offset <= 0xFF && !is_ldh(*p)) ++count;
    }

    if (count > max) {
      B = 0xD8 + n;
      offsetB = offset;
      max = count;
    }
  }

  /* now check the regular rows: */

  for (pp = input;  pp < input_end;  ++pp) {
    n = *pp >> 8;
    count = 0;

    for (p = input;  p < input_end;  ++p) {
      if (*p >> 8 == n && !is_ldh(*p)) ++count;
    }

    if (count > max || (count == max && n < B)) {
      B = n;
      offsetB = n << 8;
      max = count;
    }
  }

  /* 3) Determine the most populous 16-window: A and offsetA */

  A = 0;
  max = 0;

  for (n = 0;  n <= 0x1F;  ++n) {
    offset = ((offsetB >> 3) + n) << 3;
    count = 0;

    for (p = input;  p < input_end;  ++p) {
      if (*p - offset <= 0xF && !is_ldh(*p)) ++count;
    }

    if (count > max) {
      A = n;
      offsetA = offset;
      max = count;
    }
  }

  /* 4) Determine the most populous 20k-window: C */

  C = 0;
  max = 0;

  for (pp = input;  pp < input_end;  ++pp) {
    count = 0;
    n = *pp >> 11;
    offset = n << 11;

    for (p = input;  p < input_end;  ++p) {
      if (*p - offset <= 0x4FFF && !is_ldh(*p)) ++count;

      if (count > max || (count == max && n < C)) {
        C = n;
        max = count;
      }
    }
  }

  /* 5) Determine the style to use: wide or narrow */

  /* if narrow style were used: */

  offsetC = (offsetB >> 12) << 12;
  count = 3 + (B > 0xFF);

  for (p = input;  p < input_end;  ++p) {
    if (is_ldh(*p)) { }
    else if (*p - offsetA <= 0xF) count += 1;
    else if (*p - offsetB <= 0xFF) count += 2;
    else if (*p - offsetC <= 0xFFF) count += 3;
    else if (*p <= 0xFFFF) count += 4;
    else count += 5;
  }

  max = count;

  /* if wide style were used: */

  offsetC = C << 11;
  count =  B <= 0xFF && C <= 0x1F ?  3 :  5;

  for (p = input;  p < input_end;  ++p) {
    if (is_ldh(*p)) { }
    else if (*p - offsetB <= 0xFF) count += 2;
    else if (*p - offsetC <= 0x4FFF) count += 3;
    else if (*p <= 0xFFFF) count += 4;
    else count += 5;
  }

  wide = (count < max);

  /* 6) Initialize offsetC, and encode the style and offsets: */

  max_out = *output_size;
  next_out = 0;

  if (wide) {
    offsetC = C << 11;

    if (B <= 0xFF && C <= 0x1F) {
      if (max_out - next_out < 3) return amc_ace_output_too_big;
      output[next_out++] = base32[0x10 | (B >> 5)];
      output[next_out++] = base32[B & 0x1F];
      output[next_out++] = base32[C];
    }
    else {
      if (max_out - next_out < 5) return amc_ace_output_too_big;
      output[next_out++] = base32[0x18 | (B >> 10)];
      output[next_out++] = base32[(B >> 5) & 0x1F];
      output[next_out++] = base32[B & 0x1F];
      output[next_out++] = base32[C >> 5];
      output[next_out++] = base32[C & 0x1F];
    }
  }
  else {
    offsetC = (offsetB >> 12) << 12;

    if (B <= 0xFF) {
      if (max_out - next_out < 3) return amc_ace_output_too_big;
      output[next_out++] = base32[B >> 5];
      output[next_out++] = base32[B & 0x1F];
    }
    else {
      if (max_out - next_out < 4) return amc_ace_output_too_big;
      output[next_out++] = base32[8 | (B >> 10)];
      output[next_out++] = base32[(B >> 5) & 0x1F];
      output[next_out++] = base32[B & 0x1F];
    }

    output[next_out++] = base32[A];
  }

  /* 7) Main encoding loop: */

  literal = 0;

  for (next_in = 0;  next_in < input_length;  ++next_in) {
    codept = input[next_in];

    if (codept == 45 /* hyphen-minus */) {
      /* case 7.1 */
      if (max_out - next_out < 2) return amc_ace_output_too_big;
      output[next_out++] = 45;
      output[next_out++] = 45;
      continue;
    }

    if (is_ldh(codept)) {
      /* case 7.2 */
      if (!literal) {
        if (max_out - next_out < 1) return amc_ace_output_too_big;
        output[next_out++] = 45;
        literal = 1;
      }

      if (max_out - next_out < 1) return amc_ace_output_too_big;
      output[next_out++] = codept;
      continue;
    }

    /* case 7.3 */

    if (literal) {
      if (max_out - next_out < 1) return amc_ace_output_too_big;
      output[next_out++] = 45;
      literal = 0;
    }

    if (!wide) {
      diff = codept - offsetA;

      if (diff <= 0xF) {
        /* case 7.3.1 */
        codelen = 1;
        goto encoder_base32_bottom;
      }
    }

    diff = codept - offsetB;

    if (diff <= 0xFF) {
      /* case 7.3.2 */
      codelen = 2;
      goto encoder_base32_bottom;
    }

    diff = codept - offsetC;

    if (diff <= 0xFFF) {
      /* case 7.3.3 */
      codelen = 3;
      goto encoder_base32_bottom;
    }

    if (wide) {
      diff = codept - offsetC - 0x1000;

      if (diff <= 0x3FFF) {
        /* case 7.3.4 */
        codelen = 1;
        morebits = diff & 0x3FF;
        diff >>= 10;
        goto encoder_base32_bottom;
      }
    }

    if (codept <= 0xFFFF) {
      /* case 7.3.5 */
      diff = codept;
      codelen = 4;
      goto encoder_base32_bottom;
    }

    /* case 7.3.6 */
    diff = codept - 0x10000;
    codelen =  5;

  encoder_base32_bottom: /* output diff as n base-32 digits: */
    if (max_out - next_out < codelen) return amc_ace_output_too_big;
    i = codelen - 1;
    c = base32[diff & 0xF];
    if (uppercase_flags && uppercase_flags[next_in]) c -= 32;
    output[next_out + i] = c;

    while (i > 0) {
      diff >>= 4;
      output[next_out + --i] = base32[0x10 | (diff & 0xF)];
    }

    next_out += codelen;

    if (wide && codelen == 1) {
      /* case 7.3.4 */
      if (max_out - next_out < 2) return amc_ace_output_too_big;
      output[next_out++] = base32[morebits >> 5];
      output[next_out++] = base32[morebits & 0x1F];
    }
  }

  /* null terminator: */
  if (max_out - next_out < 1) return amc_ace_output_too_big;
  output[next_out++] = 0;
  *output_size = next_out;
  return amc_ace_success;
}


/* Decoder: */

int amc_ace_m_decode(
  enum case_sensitivity case_sensitivity,
  unsigned char *scratch_space,
  const unsigned char *input,
  unsigned int *output_length,
  u_code_point *output,
  unsigned char *uppercase_flags )
{
  unsigned int literal, wide, large;  /* boolean */
  const unsigned char *next_in;
  unsigned char c;
  unsigned int next_out, max_out, codelen, input_size, scratch_size;
  u_code_point q, B, offsets[6], diff, offset;
  enum amc_ace_status status;

  /* 1) Decode the style and offsets: */

  next_in = input;
  q = base32_decode(*next_in++);
  if (q == base32_invalid) return amc_ace_invalid_input;
  wide = q >> 4;
  large = (q >> 3) & 1;
  B = q & 7;
  q = base32_decode(*next_in++);
  if (q == base32_invalid) return amc_ace_invalid_input;
  B = (B << 5) | q;

  if (large) {
    q = base32_decode(*next_in++);
    if (q == base32_invalid) return amc_ace_invalid_input;
    B = (B << 5) | q;
  }

  /* offsets[codelen] is for base-32 codes with codelen characters */
  /* (not counting the extra two in wide-style 0xxxx xxxxx xxxxx)  */

  offsets[2] = B >> 3 == 0x1B ? special_row_offset[B & 7] : B << 8;
  q = base32_decode(*next_in++);
  if (q == base32_invalid) return amc_ace_invalid_input;

  if (!wide) {
    offsets[1] = ((offsets[2] >> 3) + q) << 3;
    offsets[3] = (offsets[2] >> 12) << 12;
  }
  else {
    offset = q << 11;

    if (large) {
      q = base32_decode(*next_in++);
      if (q == base32_invalid) return amc_ace_invalid_input;
      offset = (offset << 5) | q;
    }

    offsets[3] = offset;
    offsets[1] = offset + 0x1000;
  }

  offsets[4] = 0;
  offsets[5] = 0x10000;

  /* 2) Main decoding loop: */

  max_out = *output_length;
  next_out = 0;
  literal = 0;

  for (;;) {
    c = *next_in++;
    if (!c) break;

    if (c == 45 /* hyphen-minus */) {
      if (*next_in == 45) {
        /* case 2.1: "--" decodes to "-" */
        ++next_in;
        if (max_out - next_out < 1) return amc_ace_output_too_big;
        if (uppercase_flags) uppercase_flags[next_out] = 0;
        output[next_out++] = 45;
        continue;
      }

      /* case 2.2: unpaired hyphen-minus toggles mode */
      literal = !literal;
      continue;
    }

    if (!is_ldh(c)) return amc_ace_invalid_input;
    if (max_out - next_out < 1) return amc_ace_output_too_big;

    if (literal) {
      /* case 2.3: literal letter/digit */
      if (uppercase_flags) uppercase_flags[next_out] = is_AtoZ(c);
      output[next_out++] = c;
      continue;
    }

    /* case 2.4: base-32 sequence */

    diff = 0;
    codelen = 1;

    for (;;) {
      q = base32_decode(c);
      if (q == base32_invalid) return amc_ace_invalid_input;
      diff = (diff << 4) | (q & 0xF);
      if ((q & 0x10) == 0) break;
      if (++codelen > 5) return amc_ace_invalid_input;
      c = *next_in++;
    }

    /* Now codelen is the number of input characters read, */
    /* and c is the character holding the uppercase flag.  */

    if (wide && codelen == 1) {
      q = base32_decode(*next_in++);
      if (q == base32_invalid) return amc_ace_invalid_input;
      diff = (diff << 5) | q;
      q = base32_decode(*next_in++);
      if (q == base32_invalid) return amc_ace_invalid_input;
      diff = (diff << 5) | q;
    }

    offset = offsets[codelen];
    if (uppercase_flags) uppercase_flags[next_out] = is_AtoZ(c);
    output[next_out++] = offset + diff;
  }

  /* 3) Re-encode the output and compare to the input: */

  input_size = next_in - input;
  scratch_size = input_size;
  status = amc_ace_m_encode(next_out, output, uppercase_flags,
                            &scratch_size, scratch_space);
  if (status != amc_ace_success ||
      scratch_size != input_size ||
      unequal(case_sensitivity, scratch_space, input, input_size)
     ) return amc_ace_invalid_input;
  *output_length = next_out;
  return amc_ace_success;
}


/******************************************************************/
/* Wrapper for testing (would normally go in a separate .c file): */

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

/* For testing, we'll just set some compile-time limits rather than */
/* use malloc(), and set a compile-time option rather than using a  */
/* command-line option.                                             */

enum {
  unicode_max_length = 256,
  ace_max_size = 256,
  test_case_sensitivity = case_insensitive
};


static void usage(char **argv)
{
  fprintf(stderr,
    "%s -e reads big-endian UTF-32 and writes AMC-ACE-M ASCII.\n"
    "%s -d reads AMC-ACE-M ASCII and writes big-endian UTF-32.\n"
    "UTF-32 is extended: bit 31 is used as force-to-uppercase flag.\n"
    , argv[0], argv[0]);
  exit(EXIT_FAILURE);
}


static void fail(const char *msg)
{
  fputs(msg,stderr);
  exit(EXIT_FAILURE);
}

static const char too_large[] =
  "input or output is too large, recompile with larger limits\n";

static const char invalid_input[] = "invalid input\n";

int main(int argc, char **argv)
{
  enum amc_ace_status status;

  if (argc != 2) usage(argv);
  if (argv[1][0] != '-') usage(argv);
  if (argv[1][2] != '\0') usage(argv);

  if (argv[1][1] == 'e') {
    u_code_point input[unicode_max_length];
    unsigned char uppercase_flags[unicode_max_length];
    unsigned char output[ace_max_size];
    unsigned int input_length, output_size;
    int c0, c1, c2, c3;

    /* Read the UTF-32 input string: */

    input_length = 0;

    for (;;) {
      c0 = getchar();
      c1 = getchar();
      c2 = getchar();
      c3 = getchar();

      if (c1 == EOF || c2 == EOF || c3 == EOF) {
        if (c0 != EOF) fail("input not a multiple of 4 bytes\n");
        break;
      }

      if (input_length == unicode_max_length) fail(too_large);

      if ((c0 != 0 && c0 != 0x80)
          || c1 < 0 || c1 > 0x10
          || c2 < 0 || c2 > 0xFF
          || c3 < 0 || c3 > 0xFF ) {
        fail(invalid_input);
      }

      input[input_length] = ((u_code_point) c1 << 16) |
                            ((u_code_point) c2 <<  8) | (u_code_point) c3;
      uppercase_flags[input_length] = (c0 >> 7);
      ++input_length;
    }

    /* Encode, and output the result: */

    output_size = ace_max_size;
    status = amc_ace_m_encode(input_length, input, uppercase_flags,
                              &output_size, output);
    if (status == amc_ace_invalid_input) fail(invalid_input);
    if (status == amc_ace_output_too_big) fail(too_large);
    assert(status == amc_ace_success);
    fputs((char *) output, stdout);
    return EXIT_SUCCESS;
  }

  if (argv[1][1] == 'd') {
    unsigned char input[ace_max_size], scratch[ace_max_size];
    u_code_point output[unicode_max_length], codept;
    unsigned char uppercase_flags[unicode_max_length];
    unsigned int output_length, i;
    size_t n;

    /* Read the AMC-ACE-M ASCII input string: */

    n = fread(input, 1, ace_max_size, stdin);
    if (n == ace_max_size) fail(too_large);
    input[n] = 0;

    /* Decode, and output the result: */

    output_length = unicode_max_length;
    status = amc_ace_m_decode(test_case_sensitivity, scratch, input,
                              &output_length, output, uppercase_flags);
    if (status == amc_ace_invalid_input) fail(invalid_input);
    if (status == amc_ace_output_too_big) fail(too_large);
    assert(status == 0);

    for (i = 0;  i < output_length;  ++i) {
      putchar(uppercase_flags[i] ? 0x80 : 0);
      codept = output[i];
      putchar(codept >> 16);
      putchar((codept >> 8) & 0xFF);
      putchar(codept & 0xFF);
    }

    return EXIT_SUCCESS;
  }

  usage(argv);
  return EXIT_SUCCESS;  /* not reached, but quiets a compiler warning */
}



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