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Versions: 00 01                                                         
INTERNET-DRAFT                                          Adam M. Costello
draft-ietf-idn-amc-ace-r-00.txt                              2001-Mar-27
Expires 2001-Sep-27

                         AMC-ACE-R version 0.0.0

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 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

    The list of Internet-Draft Shadow Directories can be accessed at

    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


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

    AMC-ACE-R is similar to AMC-ACE-O [AMCACEO00] but is simpler and not
    quite as efficient.

    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


    Base-32 characters
    Encoding and decoding algorithms
    Case sensitivity models
    Comparison with RACE, BRACE, LACE, AltDUDE, AMC-ACE-M, AMC-ACE-O
    Example strings
    Security considerations
    Example implementation


    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-R 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


    AMC-ACE-R is a working name that should be changed if it is adopted.
    (The R merely indicates that it is the eighteenth ACE devised by
    this author.  BRACE was the third.  D-L, N, P, and Q were not worth
    releasing.)  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):

        UTF-D ("D" for "domain names")
        NUDE (Normal Unicode Domain Encoding)

    A name that makes no reference to domain names:

        UTF-37 (there are 37 characters in the output repertoire)


    AMC-ACE-R maps a sequence of Unicode code points to a sequence of
    LDH characters.  The encoder input and decoder output are arrays of
    code points, not characters, bytes, or code units (in particular,
    not UTF-16 surrogates).  Formally, the encoder output and decoder
    input are character strings, not code points, code units, or bytes,
    although implementations will of course need to represent the
    characters somehow, usually as bytes or other code units.

    Each Unicode code point 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.  Unicode code points representing LDH characters
    are encoded as those LDH characters, except that hyphen-minus is
    doubled.  Other Unicode code points 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 variable-length code sequence of one to five
    quintets represents a delta, which is added to a reference point to
    yield a Unicode code point.  There are five reference points, one
    for each code length, three of which continually change during the
    encoding/decoding process.

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 and decoding algorithms

    The algorithms are given below as commented pseudocode.  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-R never needs to right-shift a negative value.
    As in C, "continue" means terminate the current iteration of the
    innermost loop, "break" means terminate the innermost loop, and
    "return" means terminate the current function.

    shared variables:  # All others are local to each function.
      array refpoint[1..5]  # refpoint[k] is for sequences of length k

    function update_refpoints(history[first..latest]):
      # Adapt refpoint[1..3] based on the code points seen so far.
      for k = 1 to 3 do begin
        let b = k << 2
        if latest - first == 1
        then let refpoint[k] = (history[latest] >> b) << b
        else for i = latest - 1 down to first do begin
          if history[i] represents an LDH character then continue
          if (refpoint[k] XOR history[i]) >> b == 0 then break
          if (history[latest] XOR history[i]) >> b == 0 then begin
            let refpoint[k] = (history[latest] >> b) << b

    function encode(input[first..last]):
      let refpoint[1..5] = 0x60, 0, 0, 0, 0x10000
      let output = the empty string
      let literal = false
      for i = first to last do begin
        if input[i] == 0x2D then append two hyphen-minuses to output
        else if input[i] represents an LDH character then begin
          if not literal then append hyphen-minus to output
          let literal = true
          append the character represented by input[i] to output
        else begin
          if literal then append hyphen-minus to output
          let literal = false
          for k = 1 to infinity do begin
            let delta = codepoint - refpoint[k]
            if delta >= 0 and delta >> (4*k) == 0 then break
          extract the k least significant nybbles of delta
          prepend 0 to the last nybble and 1 to the rest
          output base-32 characters corresponding to the quintets
      return output

    function decode(input string):
      let refpoint[1..5] = 0x60, 0, 0, 0, 0x10000
      let output = the empty array
      let literal = false
      while not end-of-input do begin
        if the next character is hyphen-minus then begin
          consume the character
          if the next character is also hyphen-minus
          then consume it and append 0x2D to output
          else toggle literal
        else if literal then consume the character and output it
        else begin
          consume characters and convert them to quintets until
            encountering a quintet beginning with 0
          fail upon encountering a non-base-32 character or end-of-input
          let k = the number of quintets obtained
          strip the first bit of each quintet
          concatenate the resulting nybbles to form delta
          append refpoint[k] + delta to output
      let check = encode(output)
      if check != the input string then fail
      return output

    The comparison at the end of decode() must be case-insensitive
    if ACEs are always compared case-insensitively (which is true of
    domain names), case-sensitive otherwise (see also section "Case
    sensitivity models").  This check is necessary to guarantee the
    uniqueness property, that there cannot be two distinct encoded
    strings representing the same sequence of integers.  This check also
    frees the decoder from having to check for overflow while decoding
    the base-32 characters.


    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 order to use AMC-ACE-R in domain names, the choice of signature
    must be mindful of the requirement in [RFC952] that labels never
    begin or end with hyphen-minus.  Since the raw encoded string
    sometimes begins with a hyphen-minus, the signature must include
    a prefix that does not begin with hyphen-minus.  If the Unicode
    strings are forbidden from ending with hyphen-minus (which seems
    prudent anyway), then the raw encoded string will never end with
    hyphen-minus; otherwise, the signature must include a suffix as well
    as a prefix.

    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 "?---", 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 the last of the base-32 characters that represent
        it, which is guaranteed to be a letter rather than a digit.
        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

        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 should not be very difficult to extend the nameprep algorithm
        [NAMEPREP03] to remember case information; it could be done by
        adding flags to the mapping tables.

    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 may 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

      * 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-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


    In this section we compare AMC-ACE-R and six other ACEs: RACE
    [RACE03], BRACE [BRACE00], LACE [LACE01], AltDUDE [AltDUDE00],
    AMC-ACE-M [AMCACEM00], and AMC-ACE-O [AMCACEO00].  We do not include
    SACE [SACE], UTF-5 [UTF5], UTF-6 [UTF6], or DUDE [DUDE01] in the
    comparison, because SACE appears obviously too complex, UTF-5
    appears obviously too inefficient, UTF-6 can never be more efficient
    than its similarly simple successor DUDE, and DUDE is almost
    identical to AltDUDE.

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

          LACE, AltDUDE: simple but not trivial
        RACE, AMC-ACE-R: less simple
              AMC-ACE-O: moderate
              AMC-ACE-M: fairly complex
                  BRACE: complex

    AMC-ACE-R is similar to AMC-ACE-O, but is considerably simpler
    because it does not calculate the most useful reference points
    beforehand, encode them, and decode them.  Instead, it uses a simple
    heuristic to set the reference points adaptively based on the code
    points that have been seen so far.

    Implementations can be long and straightforward, or short and
    subtle, but for whatever it's worth, here are the code sizes of
    four of the algorithms that were implemented by this author in
    similar styles:

      AltDUDE: 130 lines @@@@@@@@@@@@@@@@@@@
    AMC-ACE-R: 171 lines @@@@@@@@@@@@@@@@@@@@@@@@
    AMC-ACE-O: 232 lines @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
    AMC-ACE-M: 324 lines @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

    (Not counted in the code sizes are blank lines, lines containing
    only comments or only a single brace, and wrapper code for testing.
    BRACE was also implemented by this author, but it was a less general
    implementation, with bounded input and output sizes.)

    If a different implementation style were to alter the code sizes
    additively, or multiplicatively, or a combination thereof, AMC-ACE-O
    would remain about halfway between AltDUDE and AMC-ACE-M, and
    AMC-ACE-R would remain closer to AltDUDE than to AMC-ACE-O.

    Case preservation support:

        AltDUDE, AMC-ACE-M/O/R:  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.  AltDUDE and AMC-ACE-M/O/R do not have this intermediate
    stage, and enforce alignment between the base-32 characters and the
    Unicode characters, which facilitates the case preservation.

    The relative efficiency of the various algorithms is suggested
    by the sizes of the encodings in section "Example strings".  The
    lengths of examples A-K (which are the same sentence translated into
    a languages from a variety of language families using a variety
    of scripts) are shown graphically below for each ACE, scaled by a
    factor of 0.4 so they fit on one line, and sorted so they look like
    a cummulative distribution.  The fictional "Super-ACE" encodes its
    input using whichever of the other seven ACEs is shortest for that

      A Arabic      29 @@@@@@@@@@@@
      B Chinese     31 @@@@@@@@@@@@
      J Taiwanese   31 @@@@@@@@@@@@
      D Hebrew      37 @@@@@@@@@@@@@@@
      H Russian     47 @@@@@@@@@@@@@@@@@@@
      E Hindi       50 @@@@@@@@@@@@@@@@@@@@
      F Japanese    60 @@@@@@@@@@@@@@@@@@@@@@@@
      I Spanish     66 @@@@@@@@@@@@@@@@@@@@@@@@@@
      C Czech       68 @@@@@@@@@@@@@@@@@@@@@@@@@@@
      G Korean      79 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
      K Vietnamese 112 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

      B Chinese     28 @@@@@@@@@@@
      A Arabic      31 @@@@@@@@@@@@
      J Taiwanese   31 @@@@@@@@@@@@
      D Hebrew      39 @@@@@@@@@@@@@@@@
      H Russian     48 @@@@@@@@@@@@@@@@@@@
      E Hindi       52 @@@@@@@@@@@@@@@@@@@@@
      F Japanese    52 @@@@@@@@@@@@@@@@@@@@@
      C Czech       58 @@@@@@@@@@@@@@@@@@@@@@@
      I Spanish     68 @@@@@@@@@@@@@@@@@@@@@@@@@@@
      G Korean      79 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
      K Vietnamese 109 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

      A Arabic      25 @@@@@@@@@@
      B Chinese     26 @@@@@@@@@@
      D Hebrew      33 @@@@@@@@@@@@@
      J Taiwanese   36 @@@@@@@@@@@@@@
      H Russian     38 @@@@@@@@@@@@@@@
      C Czech       43 @@@@@@@@@@@@@@@@@
      F Japanese    49 @@@@@@@@@@@@@@@@@@@@
      E Hindi       58 @@@@@@@@@@@@@@@@@@@@@@@
      I Spanish     59 @@@@@@@@@@@@@@@@@@@@@@@@
      K Vietnamese  81 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
      G Korean      89 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

      B Chinese     24 @@@@@@@@@@
      A Arabic      28 @@@@@@@@@@@
      J Taiwanese   30 @@@@@@@@@@@@
      D Hebrew      32 @@@@@@@@@@@@@
      C Czech       36 @@@@@@@@@@@@@@
      H Russian     40 @@@@@@@@@@@@@@@@
      F Japanese    42 @@@@@@@@@@@@@@@@@
      I Spanish     47 @@@@@@@@@@@@@@@@@@@
      E Hindi       55 @@@@@@@@@@@@@@@@@@@@@@
      K Vietnamese  70 @@@@@@@@@@@@@@@@@@@@@@@@@@@@
      G Korean      89 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

      B Chinese     24 @@@@@@@@@@
      A Arabic      28 @@@@@@@@@@@
      J Taiwanese   30 @@@@@@@@@@@@
      D Hebrew      31 @@@@@@@@@@@@
      C Czech       34 @@@@@@@@@@@@@@
      H Russian     40 @@@@@@@@@@@@@@@@
      F Japanese    41 @@@@@@@@@@@@@@@@
      I Spanish     49 @@@@@@@@@@@@@@@@@@@@
      E Hindi       54 @@@@@@@@@@@@@@@@@@@@@@
      K Vietnamese  69 @@@@@@@@@@@@@@@@@@@@@@@@@@@@
      G Korean      80 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

      B Chinese     22 @@@@@@@@@
      A Arabic      26 @@@@@@@@@@
      J Taiwanese   27 @@@@@@@@@@@
      D Hebrew      33 @@@@@@@@@@@@@
      C Czech       36 @@@@@@@@@@@@@@
      F Japanese    40 @@@@@@@@@@@@@@@@
      H Russian     42 @@@@@@@@@@@@@@@@@
      E Hindi       45 @@@@@@@@@@@@@@@@@@
      I Spanish     48 @@@@@@@@@@@@@@@@@@@
      K Vietnamese  72 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@
      G Korean      78 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

      B Chinese     23 @@@@@@@@@
      J Taiwanese   27 @@@@@@@@@@@
      A Arabic      28 @@@@@@@@@@@
      D Hebrew      31 @@@@@@@@@@@@
      C Czech       34 @@@@@@@@@@@@@@
      H Russian     38 @@@@@@@@@@@@@@@
      F Japanese    42 @@@@@@@@@@@@@@@@@
      I Spanish     48 @@@@@@@@@@@@@@@@@@@
      E Hindi       54 @@@@@@@@@@@@@@@@@@@@@@
      K Vietnamese  69 @@@@@@@@@@@@@@@@@@@@@@@@@@@@
      G Korean      71 @@@@@@@@@@@@@@@@@@@@@@@@@@@@

      B Chinese     22 @@@@@@@@@
      A Arabic      25 @@@@@@@@@@
      J Taiwanese   27 @@@@@@@@@@@
      D Hebrew      31 @@@@@@@@@@@@
      C Czech       34 @@@@@@@@@@@@@@
      H Russian     38 @@@@@@@@@@@@@@@
      F Japanese    40 @@@@@@@@@@@@@@@@
      E Hindi       45 @@@@@@@@@@@@@@@@@@
      I Spanish     47 @@@@@@@@@@@@@@@@@@@
      K Vietnamese  69 @@@@@@@@@@@@@@@@@@@@@@@@@@@@
      G Korean      71 @@@@@@@@@@@@@@@@@@@@@@@@@@@@

             RACE: 610 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
             LACE: 595 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
          AltDUDE: 537 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
        AMC-ACE-R: 493 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
        AMC-ACE-O: 480 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
            BRACE: 469 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
        AMC-ACE-M: 465 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
        Super-ACE: 449 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

    worst cases:
             RACE: 112 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
             LACE: 109 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
          AltDUDE:  89 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
        AMC-ACE-R:  89 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
        AMC-ACE-O:  80 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
            BRACE:  78 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
        AMC-ACE-M:  71 @@@@@@@@@@@@@@@@@@@@@@@@@@@@
        Super-ACE:  71 @@@@@@@@@@@@@@@@@@@@@@@@@@@@

    The totals and worst cases above give more weight to languages
    that produce longer encodings, which arguably yields a good metric
    (because being efficient for easy languages is arguably less
    important than being efficient for difficult languages).  We can
    alternatively give each language equal weight by dividing each
    output length by the corresponding Super-ACE output length.  This
    method yields:

           RACE: 14.9 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
           LACE: 14.5 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
        AltDUDE: 13.0 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
      AMC-ACE-R: 12.0 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
      AMC-ACE-O: 11.8 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
      AMC-ACE-M: 11.4 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
          BRACE: 11.4 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
      Super-ACE: 11.0 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

    worst cases:
           RACE: 2.00 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
           LACE: 1.71 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
        AltDUDE: 1.33 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
      AMC-ACE-R: 1.25 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
      AMC-ACE-O: 1.20 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
      AMC-ACE-M: 1.20 @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
          BRACE: 1.11 @@@@@@@@@@@@@@@@@@@@@@@@@@@@
      Super-ACE: 1.00 @@@@@@@@@@@@@@@@@@@@@@@@@

    No matter which way we average, the results suggest that AltDUDE is
    preferrable to RACE and LACE, because it is no more complex, is more
    efficient, and has better support for case preservation.

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

    AltDUDE, AMC-ACE-R, AMC-ACE-O, and AMC-ACE-M are progressively
    more complex and more efficient, and have equal support for case
    preservation.  The choice depends on how much efficiency is required
    and how much complexity is acceptable.

    The efficiency gaps between AMC-ACE-M, AMC-ACE-O, and AMC-ACE-R are
    mostly due to the Korean (Hangul) string.  Of the 15 characters
    by which the AMC-ACE-M total beats the AMC-ACE-O total, 9 come
    from the Korean string.  Similarly, of the 13 characters by which
    the AMC-ACE-O total beats the AMC-ACE-R total, 9 come from the
    Korean string.  The large increases in complexity from AMC-ACE-R to
    -O to -M yield significant efficiency gains for Korean, but only
    very small gains for the other languages.  More sample strings
    from more languages need to be tried before one can conclude that
    Korean is the only significant beneficiary, but if it is, then this
    author would suggest that AMC-ACE-R is preferable to -O and -M, with
    apologies to Korean speakers.

    That would leave a choice between AltDUDE and AMC-ACE-R, the latter
    being somewhat more complex and somewhat more efficient.

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.  For RACE, LACE, and AltDUDE, 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-* and AltDUDE are abbreviated AMC-* and ADUDE.  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 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.

    (A) 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

        ADUDE:  yueqpcycrcyjhbpznpitjycxf
        BRACE:  28akcjwcmp3ciwb4t3ngd4nbaz
        AMC-R:  ywekhfuhuikwdwefivevjbuiwktr
        AMC-O:  ageekhfuhuiukdefivevjvbuiktr
        AMC-M:  agiekhfuhuiukdefivevjvbuiktr
        RACE:   azceur2fe4ucuq2eivediojrfbfb6
        LACE:   cedeisshiutsqksdircuqnbzgeueuhy
        UTF-16: ??????????????????????????????????
        UTF-8:  ??????????????????????????????????

    (B) 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
        AMC-R:  w87g8nvk6awisp259eupyx2h
        AMC-O:  eqpg8nvk6awisp259eupyx2h
        ADUDE:  w85gvk7g9k2iwf6x9j6x7ju54k
        UTF-8:  ???????????????????????????
        LACE:   azhnn3b2ybea2aml6qau4libmwdq
        RACE:   3bhnmtxmjy5e5qcojbha3c7ujywwlby

    (C) 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-O:  piq-Pro-p-prost-9m-nemluv-6pp-esky
        AMC-M:  g26-Pro-p-prost-9m-nemluv-6pp-esky
        AMC-R:  -Pro-tsp-prost-ttm-nemluv-s8psp-esky
        BRACE:  i32-Pro-u-prost-8y-nemluv-29f3n-esky
        ADUDE:  tActptyctzpctptnhtyrtzfmibtjd3mt8atyitgtitc
        UTF-16: ????????????????????????????????????????????
        LACE:   amaha4tpaeaq2biaobzg643uaearwbyanzsw23dvo3wqcainaqagk43\
        RACE:   ah7xb73s75xq373q75zp6377op7xig77n37wl73n75wp65p7o3762dp\

    (D) 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-O:  afpnqeep8e8jfinaqdb8ijp8cb8ij8k
        AMC-M:  af4nqeep8e8jfinaqdb8ijp8cb8ij8k
        AMC-R:  x7nqeep8e8j7f7inaqdb8ijp8cb8ij8k
        ADUDE:  x5nckajvjpvnpenqpcvjvbevrvdvjvbvd
        BRACE:  27vkyp7bgwmbpfjgc4ynx5nd8xsp5nd9c
        RACE:   axon5vgu3xsotvoy3tin5u6r5dm53ywr5dm6u
        LACE:   cyc5zxwu2to6j2ov3donbxwt2huntxpc2hunt2q
        UTF-8:  ????????????????????????????????????????????
        UTF-16: ????????????????????????????????????????????

    (E) 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-O:  ajeurvjvcmthvjvruipugatfpurmscuivjascunmvcvitfuehvjisc
        AMC-M:  ajhurbvcwmthbhuiwpugitfwpurwmscuibiscunwmvcatfuerbwisc
        AMC-R:  3urvjvcwmthjruiwpugwatfwpurmscuivjascunmvcvitfuewhjwisc
        ADUDE:  3wrtgmzjxnuqgthyfymygxfxiycyewjuktbzjwcuqyhzjkupvbydzqz\
        UTF-16: ???????????????????????????????????????????????????????\
        UTF-8:  ???????????????????????????????????????????????????????\

    (F) Japanese:
        U+306A U+305C U+307F U+3093 U+306A U+65E5 U+672C U+8A9E U+3092
        U+8A71 U+3057 U+3066 U+304F U+308C U+306A U+3044 U+306E U+304B
        (kanji and hiragana)

        UTF-16: ????????????????????????????????????
        BRACE:  ji8nr5zj8uqth7v97mjchakwcg7dqemw88nj5gbe
        AMC-O:  gvagkxnzr3dkx8fzun243q3c24zbxhgwr2nkweqwm
        AMC-R:  vsykxnzr3dkyx8fyzun243q3c24zbxhgwr2nkweqwm
        AMC-M:  bsnkxnzr3dkyx8fyzun243q3c24zbxhgwr2nkweqwm
        ADUDE:  vsskvgud8n9jxx2ru6j875c54sn548d54ugvbuj6d8guqukuf
        LACE:   auyguxd7snvaczpfaftsyamktyatbeqbrjyqqmcxmzhyy2senzfq
        UTF-8:  ??????????????????????????????????????????????????????
        RACE:   3aygumc4gb7tbezqnjs6kzzmrkpdbeukoeyfomdggbhtbdbqniyeimd\

    (G) 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\
        BRACE:  y394qebjusrcndbs82pkvstf96sxufcr7ffr4vbgdwsxufcx8pdktgb\
        LACE:   77atrlgey5mlvkfu4dakzn4mwtsmo5gvlsww3rnuxf6mo5gvotkvzmx\
        RACE:   3datrlgey5mlvkfu4dakzn4mwtsmo5gvlsww3rnuxf6mo5gvotkvzmx\
        AMC-O:  m6hwq6tvi466exi44ia6s4nz2neze7xxn47yp6x5e3znze7xze7xxnu\
        ADUDE:  6txiy79ny53nz79a8wizwwnzzuavyizv3atuuiz2vby27jz66iz8sit\
        AMC-R:  6tvi466ezxi544i5w8a6s4nz2nw8e6zze7xxn47yp6x5e53znze7xze\

    (H) 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)

        ADUDE:  wxRbzjzcjzrzfdmdffigpnnzqrpzpbzqdcazmc
        AMC-M:  aehHgrvfemvgvfgfafvfvdgvcgiwrkhgimjjca
        AMC-R:  wvRqwhfnwdgfqpipfdqcqwawrcvrvqwawdbbvkvi
        AMC-O:  aedRqwhfnwdgfqpipfdqcqwawrwcrqwawdwbwbki
        BRACE:  269xyjvcyafqfdwyr3xfd8z8byi6z39xyi692s7ug2
        RACE:   aq7t4rzvhrbtmnj6hu4d2njthyzd4qcpii7t4qcdifatuoa
        LACE:   dqcd6pshgu6egnrvhy6tqpjvgm7depsaj5bd6psainaucory
        UTF-16: ???????????????????????????????????????????????????????\
        UTF-8:  ???????????????????????????????????????????????????????

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

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

        UTF-8:  Porqu??nopuedensimplementehablarenEspa??ol
        AMC-R:  -Porqu-8j-nopuedensimplementehablarenEspa-9b-ol
        AMC-M:  aa7-Porqu-b-nopuedensimplementehablarenEspa-j-ol
        BRACE:  22x-Porqu-9-nopuedensimplementehablarenEspa-j-ol
        AMC-O:  aaq-Porqu-j-nopuedensimplementehablarenEspa-9b-ol
        ADUDE:  tAtrtpde3n2hbtrftabbmtptketptnjiimtktbpjdqptdthmMtgdtb3\
        RACE:   abyg64troxuw433qovswizloonuw24dmmvwwk3tumvugcytmmfzgk3t\
        LACE:   faaha33sof26s3tpob2wkzdfnzzws3lqnrsw2zloorswqylcnrqxezl\
        UTF-16: ???????????????????????????????????????????????????????\

    (J) 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
        AMC-R:  w87gxstbzuvc6a385psp244kupyx2h
        AMC-O:  eqpgxstbzuvc6a385psp244kupyx2h
        RACE:   3bhnmuaroize5qe6xvha3cvkjywwlby
        LACE:   75hnmuaroize5qe6xvha3cvkjywwlby
        ADUDE:  w85gt86huuudv69c7szp7s5a6w4h6w2hu54k

    (K) Vietnamese:

        <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-O:  aava-Ta-vud-isaoho-vud-kh-9e-ngth-8kj-chi-j-no-b-iti-8k\
        AMC-M:  ada-Ta-ud-isaoho-ud-kh-s9e-ngth-s8kj-chi-j-no-b-iti-s8k\
        AMC-R:  -Ta-vud-isaoho-vud-kh-9e-ngth-8kvsj-chi-vsj-no-b-iti-s8\
        BRACE:  i54-Ta-8-isaoho-ay-kh-29n-ngth-s2xa6i-chi-k-no-2g-iti-2\
        UTF-16: ???????????????????????????????????????????????????????\
        ADUDE:  tEtfvwcvwktktcqhhvwnvwid3n3kjtdtn2cv8dvykmbvyavyhbvyqvy\
        LACE:   aiahiyibamrqmadjonqw62dpaebsgcaannupi3thoruouaidbebqay3\
        RACE:   ap7xj73bep7wt73t75q76377nd7w6i77np7wr77u75xp6z77ot7wr77\

    The next 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).

    (L) 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
        AMC-O:  fb8h-3-e-B-z7we3t7bymwizxtr
        ADUDE:  xdx8whx8tGz7ug863f6s5kuduwxh
        RACE:   3aadgxtuabrh2rer2fiwwukioupq
        LACE:   74adgxtuabrh2rer2fiwwukioupq
        AMC-R:  -3-x8ze-B-z7we3t7bxtymtwizxtr

    (M) <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
        AMC-R:  x52j4e3wiz92qyszf---with--SUPER--MONKEYS
        AMC-O:  fmij4e3wiz92qyszf---with--SUPER--MONKEYS
        BRACE:  uvj7fuaqcahy982xa---with--SUPER--MONKEYS
        ADUDE:  x58jupu8nuy6gt99m-yssctqtptn-tMGFtFtH-tRCBFQtNK
        UTF-16: ????????????????????????????????????????????????
        LACE:   ajnytjablfeac74oafqhkeyafv3qm5difvzxk4dfoiww233onnsxs4y
        RACE:   3bnysw5elfeh7dtaouac2adxabuqa5aanaac2adtab2qa4aamuaheab\

    (N) 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-O:  daf-Hello--Another--Way---p2nq2nyqx2veyuwa
        AMC-M:  bsk-Hello--Another--Way---p2nq2nyqx2veyuwa
        ADUDE:  Ipjad-Qrbtmtnpth-Ftgti-vsue7b7c7c8cy2xkv4ze
        AMC-R:  -Hello--Another--Way---vsxpvs2nxq2nyqx2veyuwa
        UTF-16: ??????????????????????????????????????????????????
        LACE:   ciagqzlmnrxs2ylon52gqzlsfv3wc6jnauyf3dc6rrxacwbuafrea
        RACE:   3aagqadfabwaa3aan4ac2adbabxaa3yaoqagqadfabzaaliao4agcad\

    (O) <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-O:  dagzciex6wmy2vjqw8sm-2
        AMC-M:  bsnzciex6wmy2vjqw8sm-2
        BRACE:  ji96u56uwbhf2wqxnw4s-2
        AMC-R:  vszcyiyex6wmy2vjqw8sm-2
        ADUDE:  vstctkny6urvwzcx2xhz8yfw8vj
        RACE:   3ayhemdigbsfys3iheyg4tqlaaza
        LACE:   74yhemdigbsfys3iheyg4tqlaaza

    (P) 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
        AMC-O:  dag-Maji-h-Koi-xj2m-5-z37cxuwp
        ADUDE:  PnmdvssqvssNegvsva7cvs5qz38hu53r
        AMC-R:  -Maji-vsyh-Koi-vsxj2m-5-z37cxuwp
        RACE:   3aag2adbabvaa2jqm4agwadpabutawjqrmadk6oskjgq
        LACE:   74ag2adbabvaa2jqm4agwadpabutawjqrmadk6oskjgq

    (Q) <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-O:  dapbf4d9n-de-8m9da
        AMC-M:  bs3jp4d9n-de-8m9di
        AMC-R:  vs7bf4d9n-de-8m9d7a
        RACE:   gdi5li7475sp6zpl6pia
        ADUDE:  vs5bezgxrvs3ibvs2qtiud
        UTF-8:  ????????????de?????????
        LACE:   aqyndvnd7qbaazdfamyox46q

    (R) <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-O:  dagxpq5j7e9n6jh
        AMC-M:  bsmfyq5j7e9n6jr
        ADUDE:  vsvpvd7hypuivf4q
        AMC-R:  vsxpyq5j7e9n6jyh
        UTF-8:  ?????????????????????

    The last example is an ASCII string that breaks not only the
    existing rules for host name labels but also the rules proposed in
    [NAMEPREP03] for internationalized domain names.

    (S) -> $1.00 <-

        UTF-8:  -> $1.00 <-
        ADUDE:  -xqtqetftrtqatatn-
        RACE:   aawt4ibegexdambahqwq
        LACE:   bmac2praeqys4mbqea6c2
        AMC-R:  --vquaue-1-q-00-avn--
        UTF-16: ??????????????????????
        AMC-O:  aac--vqae-1-q-00-avn--
        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-R 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, and this work is leveraged for domain names by
    "nameprep" [NAMEPREP03].


    AMC-ACE-R reuses a number of preexisting techniques.

    The basic encoding of integers to nybbles to quintets to base-32
    comes from UTF-5 [UTF5], and the particular variant used here comes
    from AMC-ACE-M [AMCACEM00].

    The idea of avoiding 0, 1, o, and l in base-32 strings was taken
    from SFS [SFS].

    The idea of encoding deltas from reference points was taken from
    RACE (of which the latest version is [RACE03]), which may have
    gotten the idea from Unicode Technical Standard #6 [UTS6].

    The idea of switching between literal mode and base-32 mode comes
    from BRACE [BRACE00].

    The general idea of using the alphabetic case of base-32 characters
    to record the desired case of the Unicode characters was suggested
    by this author, and first applied to the UTF-5-style encoding in
    DUDE (of which the latest version is [DUDE01]).

    The heuristic used to adapt the reference points based on past code
    points is new in AMC-ACE-R.


    [AltDUDE00] Adam Costello, "AltDUDE version 0.0.2", 2001-Mar-19,

    [AMCACEM00] Adam Costello, "AMC-ACE-M version 0.1.0", 2001-Feb-12,

    [AMCACEO00] Adam Costello, "AMC-ACE-O version 0.0.3", 2001-Mar-19,

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

    [DUDE01] Mark Welter, Brian Spolarich, "DUDE: Differential Unicode
    Domain Encoding", 2001-Mar-02, draft-ietf-idn-dude-01.

    [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.

    [NAMEPREP03] Paul Hoffman, Marc Blanchet, "Preparation
    of Internationalized Host Names", 2001-Feb-24,

    [PROVINCIAL] Michael Kaplan, "The 'anyone can be provincial!' page",

    [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)",

    [SFS] David Mazieres et al, "Self-certifying File System",

    [UNICODE] The Unicode Consortium, "The Unicode Standard",

    [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-*.

    [UTS6] Misha Wolf, Ken Whistler, Charles Wicksteed,
    Mark Davis, Asmus Freytag, "Unicode Technical Standard
    #6: A Standard Compression Scheme for Unicode",

    [UTFCONV] Mark Davis, "UTF Converter",


    Adam M. Costello <amc@cs.berkeley.edu>

Example implementation

/* amc-ace-r.c 0.0.0 (2001-Mar-27-Tue)    */
/* Adam M. Costello <amc@cs.berkeley.edu> */

/* This is ANSI C code (C89) implementing AMC-ACE-R version 0.0.*. */

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

#include <limits.h>

enum amc_ace_status {

enum case_sensitivity { case_sensitive, case_insensitive };

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

enum amc_ace_status amc_ace_r_encode(
  unsigned int input_length,
  const u_code_point *input,
  const unsigned char *uppercase_flags,
  unsigned int *output_size,
  char *output );

    /* amc_ace_r_encode() converts Unicode to AMC-ACE-R (without      */
    /* any signature).  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.  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+1, because of */
    /* how the encoding is defined.                                   */

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

    /* amc_ace_r_decode() converts AMC-ACE-R (without any signature)  */
    /* 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.        */
    /* 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>

static int is_ldh(u_code_point codept)
  return codept >  122 ? 0 :
         codept >=  97 ? 1 :
         codept >   90 ? 0 :
         codept >=  65 ? 1 :
         codept >   57 ? 0 :
         codept >=  48 ? 1 :
         codept ==  45      ;

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

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

/* 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 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(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,s1,s2) returns 0 if the strings s1 and s2 */
/* are equal, 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 char *s1, const char *s2 )
  char c1, c2;

  if (case_sensitivity != case_insensitive) return strcmp(s1,s2) != 0;

  for (;;) {
    c1 = *s1;
    c2 = *s2;
    if (c1 >= 65 && c1 <= 90) c1 += 32;
    if (c2 >= 65 && c2 <= 90) c2 += 32;
    if (c1 != c2) return 1;
    if (c1 == 0) return 0;
    ++s1, ++s2;

/* update_refpoints(refpoint,history,latest) updates refpoint[1..3] */
/* based on the history, where history[latest] is the latest code   */
/* point.                                                           */

void update_refpoints( u_code_point *refpoint,
  const u_code_point *history, unsigned int latest )
  unsigned int k, b, i;

  for (k = 1;  k <= 3;  ++k) {
    b = k << 2;
    if (latest == 0) refpoint[k] = (history[0] >> b) << b;
    else for (i = latest;  i-- > 0; ) {
      if (is_ldh(history[i])) continue;
      if ((refpoint[k] ^ history[i]) >> b == 0) break;

      if ((history[latest] ^ history[i]) >> b == 0) {
        refpoint[k] = (history[latest] >> b) << b;

/* Main encode function: */

enum amc_ace_status amc_ace_r_encode(
  unsigned int input_length,
  const u_code_point *input,
  const unsigned char *uppercase_flags,
  unsigned int *output_size,
  char *output )
  unsigned int max_out, next_out, literal, i, k, out;
  u_code_point codept, delta;
  char shift;
  u_code_point refpoint[6] = {0, 0x60, 0, 0, 0, 0x10000};

  max_out = *output_size;
  next_out = 0;
  literal = 0;

  for (i = 0;  i < input_length;  ++i) {
    codept = input[i];
    if (codept > 0x10FFFF) return amc_ace_invalid_input;

    if (codept == 0x2D) {
      /* hyphen-minus is doubled */
      if (max_out - next_out < 1) return amc_ace_big_output;
      output[next_out++] = 0x2D;
      output[next_out++] = 0x2D;
    else if (is_ldh(codept)) {
      /* encode LDH character literally */
      if (max_out - next_out < 1 + !literal) return amc_ace_big_output;
      /* switch to literal mode if necessary: */
      if (!literal) output[next_out++] = 0x2D;
      literal = 1;
      output[next_out++] = codept;
    else {
      /* encode non-LDH character using base-32 */

      shift = uppercase_flags && uppercase_flags[i] ? 32 : 0;
      /* shift will determine the case of the last base-32 digit */

      for (k = 1; ; ++k) {
        delta = codept - refpoint[k];
        if (delta >> (4*k) == 0) break;

      /* We will encode delta as k base-32 digits. */

      if (max_out - next_out < k + literal) return amc_ace_big_output;
      /* switch to base-32 mode if necessary: */
      if (literal) output[next_out++] = 0x2D;
      literal = 0;

      /* Computing the base-32 digits in reverse order is easiest. */
      /* Only the last base-32 digit has the high bit clear.       */

      out = next_out + k - 1;
      output[out] = base32[delta & 0xF] - shift;

      while (out > next_out) {
        delta >>= 4;
        output[--out] = base32[0x10 | (delta & 0xF)];

      next_out += k;

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

/* Main decode function: */

enum amc_ace_status amc_ace_r_decode(
  enum case_sensitivity case_sensitivity,
  char *scratch_space,
  const char *input,
  unsigned int *output_length,
  u_code_point *output,
  unsigned char *uppercase_flags )
  u_code_point q, delta;
  const char *in, *first;
  char c;
  unsigned int next_out, max_out, literal, input_size, scratch_size;
  enum amc_ace_status status;
  u_code_point refpoint[6] = {0, 0x60, 0, 0, 0, 0x10000};

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

  for (c = *in;  c != 0; ) {
    if (c == 45 && in[1] != 45) {
      /* unpaired hyphen-minus toggles mode */
      literal = !literal;
      c = *++in;

    if (max_out - next_out < 1) return amc_ace_big_output;

    if (c == 45) {
      /* double hyphen-minus represents a hyphen-minus */
      if (uppercase_flags) uppercase_flags[next_out] = 0;
      output[next_out] = 45;
      c = *(in += 2);
    else {
      if (literal) {
        /* decode one base-32 code point */
        output[next_out] = c;
        c = *++in;
      else {
        /* Base-32 sequence: */

        delta = 0;
        first = in;

        do {
          q = base32_decode(c);
          if (q == base32_invalid) return amc_ace_invalid_input;
          delta = (delta << 4) | (q & 0xF);
          c = *++in;
        } while (q >> 4 == 1);

        output[next_out] = refpoint[in - first] + delta;
        update_refpoints(refpoint, output, next_out);

      /* case of last digit determines uppercase flag: */
      if (uppercase_flags) uppercase_flags[next_out] = is_AtoZ(in[-1]);


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

  input_size = in - input + 1;
  scratch_size = input_size;
  status = amc_ace_r_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)
     ) 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  /* good for host names */

static void usage(char **argv)
    "%s -e reads big-endian UTF-32 and writes AMC-ACE-R ASCII.\n"
    "%s -d reads AMC-ACE-R 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]);

static void fail(const char *msg)

static const char too_big[] =
  "input or output is too large, recompile with larger limits\n";
static const char invalid_input[] = "invalid input\n";
static const char io_error[] = "I/O error\n";

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

  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];
    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 (ferror(stdin)) fail(io_error);

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

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

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

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

    /* Encode, and output the result: */

    output_size = ace_max_size;
    status = amc_ace_r_encode(input_length, input, uppercase_flags,
                              &output_size, output);
    if (status == amc_ace_invalid_input) fail(invalid_input);
    if (status == amc_ace_big_output) fail(too_big);
    assert(status == amc_ace_success);
    r = fputs(output,stdout);
    if (r == EOF) fail(io_error);
    return EXIT_SUCCESS;

  if (argv[1][1] == 'd') {
    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;

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

    fgets(input, ace_max_size, stdin);
    if (ferror(stdin)) fail(io_error);
    if (!feof(stdin)) fail(too_big);

    /* Decode, and output the result: */

    output_length = unicode_max_length;
    status = amc_ace_r_decode(test_case_sensitivity, scratch, input,
                              &output_length, output, uppercase_flags);
    if (status == amc_ace_invalid_input) fail(invalid_input);
    if (status == amc_ace_big_output) fail(too_big);
    assert(status == amc_ace_success);

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

    return EXIT_SUCCESS;

  return EXIT_SUCCESS;  /* not reached, but quiets compiler warning */

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