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Versions: 00                                                            
INTERNET-DRAFT                                          Adam M. Costello
draft-ietf-idn-amc-ace-v-00.txt                              2001-May-31
Expires 2001-Nov-30

                         AMC-ACE-V 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-v

Abstract

    AMC-ACE-V is a reversible transformation from a sequence of Unicode
    [UNICODE] code points to a sequence of letters, digits, and hyphens
    (LDH characters).  AMC-ACE-V could be used as an ASCII-Compatible
    Encoding (ACE) for internationalized domain names [IDN] [IDNA].

    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 hyphens to be
    unsafe, a different character could be used to play its role, like
    underscore.)

Contents

    Features
    Name
    Terminology
    Description
    Base-32 characters
    Encoding and decoding algorithms
    Signature
    Mixed-case annotation
    Comparison with other ACEs
    Example strings
    Security considerations
    Acknowledgements
    References
    Author
    Example implementation

Features

    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.

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

    Reversibility:  Any Unicode string mapped to an LDH string can be
    recovered from that LDH string.

    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-V aims at a reasonable balance between them, with
    slightly more emphasis on efficiency.

    Mixed-case annotation:  Even if the Unicode string has been
    case-folded prior to encoding, it is possible to used mixed case
    in the encoded string as an annotation telling how to convert the
    folded Unicode string into a mixed-case Unicode string for display
    purposes.  This feature is optional; see section "Mixed-case
    annotation".

    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-V is a working name that should be changed if it is adopted.
    (The V merely indicates that it is the twenty-second ACE devised by
    this author.  BRACE was the third.  Most 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:

        UniHost
        NUDE (Normal Unicode Domain Encoding)
        UTF-D ("D" for "domain names")
        UTF-37 (there are 37 characters in the output repertoire)

Terminology

    LDH characters are the letters A-Z and a-z, the digits 0-9, and
    hyphen-minus.

    A quartet is a sequence of four bits (also known as a nibble or
    nybble).

    A quintet is a sequence of five bits.

    Hexadecimal values are shown preceeded by "0x".  For example, 0x60
    is decimal 96.

    As in the Unicode Standard [UNICODE], Unicode code points are
    denoted by "U+" followed by four to six hexadecimal digits, while a
    range of code points is denoted by two hexadecimal numbers separated
    by "..", with no prefixes.

    "x..y" means the range of integers x through y inclusive.

    "x << y" means x left-shifted by y bits (equivalent to x times
    2 to the power y), and "x >> y" means x right-shifted by y bits
    (equivalent to x divided by 2 to the power y, discarding the
    remainder).  These operations are used only with nonnegative
    integral values.

    "x ? y : z" means "y if x is true, z if x is false".  It is just
    like "if x then y else z" except that y and z are expressions rather
    than statements.

Description

    AMC-ACE-V represents a sequence of Unicode code points as a sequence
    of LDH characters, although implementations will also need to
    represent the LDH characters somehow, typically as ASCII octets.
    The encoder input and decoder output are arrays of Unicode code
    points (integral values in the range 0..10FFFF, but not D800..DFFF,
    which are reserved for use by UTF-16).

    This section describes the representation.  Section "Encoding
    and decoding algorithms" presents the algorithms as commented
    pseudocode.  There is also commented C code in section "Example
    implementation".

    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 code points are encoded as one or more LDH
    characters using base-32, in which each character of the encoded
    string represents a quintet according to the table in section
    "Base-32 characters".  A mode change is indicated by an unpaired
    hyphen-minus.  A pair of consecutive hyphen-minuses represents a
    hyphen-minus and does not change the mode.

    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 is also an active style, either 0
    or 1.  In style 0 there are five reference points, one for each code
    length, and the delta is represented by the lowest four bits of each
    quintet.  The highest bit of each quintet is 1, except for the last
    quintet, where it is 0, allowing the decoder to detect the end of
    the sequence.

    Style 0 code sequences:
        delta from reference point 1: 0xxxx
        delta from reference point 2: 1xxxx 0xxxx
        delta from reference point 3: 1xxxx 1xxxx 0xxxx
        delta from reference point 4: 1xxxx 1xxxx 1xxxx 0xxxx
        delta from reference point 5: 1xxxx 1xxxx 1xxxx 1xxxx 0xxxx

    Style 1 is the same as style 0 except that the single-quintet
    sequence (0xxxx) is not used, and instead a three-quintet sequence
    (0xxxx xxxxx xxxxx) represents a delta from the third reference
    point plus 0x1000, effectively increasing the range of deltas that
    can be used with the third reference point.

    Style 1 code sequences:
        delta from reference point 2: 1xxxx 0xxxx
        delta from reference point 3: 1xxxx 1xxxx 0xxxx
        delta from ref.pt.3 + 0x1000: 0xxxx xxxxx xxxxx
        delta from reference point 4: 1xxxx 1xxxx 1xxxx 0xxxx
        delta from reference point 5: 1xxxx 1xxxx 1xxxx 1xxxx 0xxxx

    For each reference point, the delta can range from 0 to some maximum
    value determined by the available bits in the code sequence, so
    each reference point is the bottom of a window of code points.  The
    maximum delta for each window depends on the style:

    Style 0 maximum deltas:
       window 1: 0xF
       window 2: 0xFF
       window 3: 0xFFF
       window 4: 0xFFFF
       window 5: 0xFFFFF

    Style 1 maximum deltas:
       window 2: 0xFF
       window 3: 0x4FFF
       window 4: 0xFFFF
       window 5: 0xFFFFF

    A code point is encoded as an offset into one of the windows of the
    active style, the smallest window that contains it.

    Reference points 4 and 5 are fixed at 0 and 0x10000 respectively,
    for both styles, so that windows 4 and 5 always cover the entire
    Unicode code space 0..10FFFF.  The other five windows (windows 1 to
    3 of style 0, and windows 2 and 3 of style 1) and the active style
    are updated whenever a code point n has been encoded or decoded in
    base-32 mode, using following heuristic.

    The active style is:
        set to 0 if n is in style 0 window 1,
        set to 1 if n is in none of style 0 windows 1..3,
        unchanged otherwise.

    For reference points 1..3 of style 0, and reference points 2 and 3
    of style 1 (in that order), a new value for the reference point is
    considered.  The new value is computed as follows:

    Reference point 1 of style 0 set to:
        n rounded down to a multiple of 8.

    Reference point 2 of both styles is set to:
        0xA0 if n is in A0..17F,
        n rounded down to a multiple of 0x100 otherwise.

    Reference point 3 of style 0 is set to:
        0x4E00 if n is in 3000..9FFF,
        n rounded down to a multiple of 0x800 otherwise.

    Reference point 3 of style 1 is set to:
        0x4E00 if n is in 3000..9FFF,
        0x8800 if n is in A000..D7FF,
        n rounded down to a multiple of 0x1000 otherwise.

    The new value is evaluated against the existing value by counting
    the number of base-32 characters that would be used to encode all
    the non-LDH code points that have been encoded/decoded so far.  If
    using new reference point value would result in a larger total than
    keeping the existing value, the existing value is kept, otherwise
    the reference point is changed to the new value (before the next
    reference point is reconsidered).

    The initial values of the state variables are:

                             mode:  base-32
                     active style:  0
        style 0 reference point 1:  0xE0
        style 0 reference point 2:  0xA0
        style 0 reference point 3:  0
        style 0 reference point 4:  0
        style 0 reference point 5:  0x10000
        style 1 reference point 2:  0
        style 1 reference point 3:  0
        style 1 reference point 4:  0
        style 1 reference point 5:  0x10000

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 (including mixtures of both forms).
    An encoder should output only lowercase forms or only uppercase
    forms unless it uses the feature described in section "Mixed-case
    annotation").

Encoding and decoding algorithms

    All ordering of bits, quartets, and quintets is big-endian (most
    significant first).  When subroutines alter variables that are
    passed in as arguments, those changes are seen by the caller after
    the subroutine returns.

    procedure initialize(refpoint,style,literal):
      let refpoint[0][1..5] = (0xE0, 0xA0, 0, 0, 0x10000)
      let refpoint[1][2..5] = (         0, 0, 0, 0x10000)
      let style = 0
      let literal = false

    function classify(s,n):
      # Compute the number of base-32 characters required to encode
      # code point n using style s.
      constant maxdelta[0][1..5] = (0xF, 0xFF,  0xFFF, 0xFFFF, 0xFFFFF)
      constant maxdelta[1][2..5] = (     0xFF, 0x4FFF, 0xFFFF, 0xFFFFF)
      if n is the code point of an LDH character then return 0
      for k = 1 + s to infinity
      do if n - refpoint[s][k] is in 0..maxdelta[s][k] then return k

    procedure update(refpoint, style, history[first..latest]):
      # Update the active style and reference points based on the
      # history of code points seen so far.
      let n = history[latest]
      # Compute the active style:
      let k = classify(0,n);
      let style =  k == 1 ? 0 : k >= 4 ? 1 : style
      # Compute the new candidate reference points:
      let newrp[1] = (n >> 3) << 3
      let newrp[2] = n is in A0..17F ? 0xA0 : (n >> 8) << 8
      # newrp[3] depends on the style.
      for s = 0 to 1 do begin
        let newrp[3] = s == 1 and n is in A000..D7FF ? 0x8800 :
          n is in 3000..9FFF ? 0x4E00 : (n >> (11+s)) << (11+s)
        for k = 1 + s to 3 do begin
          # Count the number of base-32 characters that would be used to
          # encode the history using the old and new reference point.
          let oldrp = refpoint[s][k]
          let oldsum = newsum = 0
          for i = first to latest do begin
            let refpoint[s][k] = oldrp
            let oldsum = oldsum + classify(s, history[i])
            let refpoint[s][k] = newrp[k]
            let newsum = newsum + classify(s, history[i])
          end
          # If the new reference point is worse, don't use it:
          if newsum > oldsum then let refpoint[s][k] = oldrp
        end
      end

    procedure encode(input[first..last]):
      initialize(refpoint,style,literal)
      for i = first to last do begin
        # Check code point range to avoid array bounds errors later:
        if input[i] is not in 0..10FFFF then fail
        # 0x2D is always encoded as two hyphen-minuses, otherwise
        # classify() tells which encoding to use.
        let k = classify(style, input[i])
        if input[i] == 0x2D then output two hyphen-minuses
        else if k == 0 then begin
          # Letter/digit is encoded literally, so get into literal mode.
          if not literal then output hyphen-minus
          let literal = true
          output the character represented by input[i]
        end
        else begin
          # Non-LDH code point is encoded as k base-32 digits,
          # so get into base-32 mode.
          if literal then output hyphen-minus
          let literal = false
          let delta = input[i] - refpoint[style][k]
          # Check for the extended delta of style 1 window 3:
          if k == 3 and delta >= 0x1000
          then represent (delta - 0x1000) in base 32 as three quintets
          else begin
            # Normal case, four bits per quintet:
            represent delta in base 16 as k quartets
            prepend 0 to the last quartet and 1 to each of the others
          end
          output a base-32 character corresponding to each quintet
          update(refpoint, style, input[first..i])
        end
      end

    procedure decode(input string):
      initialize(refpoint,style,literal)
      let history = the empty array
      while the input string is not exhausted do begin
        read the next character into c
        # Unpaired hyphen-minus toggles the mode:
        if c is hyphen-minus and the next character is not
        then read the next character into c and toggle literal
        # Double hyphen-minus represents 0x2D:
        if c is hyphen-minus
        then read the next character and append 0x2D to history
        else if literal then append the code point of c to history
        else begin
          # Decode a base-32 sequence.
          convert c to a quintet
          while a quintet beginning with 0 has not been seen
          do read and convert up to four more characters
          concatenate the lowest four bits of each quintet to form delta
          # Check for the extended delta of style 1 window 3:
          if style == 1 and there was only one quintet then begin
            read two characters and convert them to two more quintets
            concatenate delta and the two quintets to form a new delta
            let delta = delta + 0x1000
          end
          append refpoint[number of quintets decoded] + delta to history
          update(refpoint,style,history)
        end
      end
      # Enforce the uniqueness of the encoding:
      encode history and compare it to the input string
      fail if they are not equal
      output history

    The decoder must always be prepared for premature end-of-input or
    invalid input characters, and must either fail immediately or forge
    ahead and let the comparison at the end fail.  The comparison must
    be case-insensitive if ACEs are always compared case-insensitively
    (which is true of domain names), case-sensitive otherwise.  This
    check is necessary to guarantee the uniqueness property (there
    cannot be two distinct encoded strings representing the same
    sequence of integers).  (If the decoder is one step of a larger
    decoding process, it may be possible to defer the re-encoding and
    comparison to the end of that larger decoding process.)

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 order to use AMC-ACE-V 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.

Mixed-case annotation

    In order to use AMC-ACE-V to represent case-insensitive Unicode
    strings, higher layers need to case-fold the Unicode strings prior
    to AMC-ACE-V encoding.  The encoded string can, however, use
    mixed-case base-32 (rather than all-lowercase or all-uppercase
    as recommended in section "Base-32 characters") as an annotation
    telling how to convert the folded Unicode string into a mixed-case
    Unicode string for display purposes.

    Each non-LDH code point is represented by a sequence of quintets,
    one of which always begins with 0.  When window 3 is used and delta
    exceeds 0xFFF, the first quintet always begins with 0; in all
    other cases, the last quintet always begins with 0.  The base-32
    character representing this quintet is always a letter (as opposed
    to a digit).  If the letter is uppercase, it is a suggestion that
    the Unicode character be mapped to uppercase (if possible); if the
    letter is lowercase, it is a suggestion that the Unicode character
    be mapped to lowercase (if possible).

    AMC-ACE-V encoders and decoders are not required to support these
    annotations, and higher layers need not use them.

Comparison with other ACEs

    Please refer to the comparison in [AMCACEW].

Example strings

    In the ACE encodings below, no signatures are shown.  AMC-ACE-V is
    abbreviated AMC-V.  Backslashes show where line breaks have been
    inserted in strings too long for one line.

    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
        AMC-V:  ywekhfuhuiukdefivevjvbuiktr

    (B) Chinese (simplified):
        u+4ED6 u+4EEC u+4E3A u+4EC0 u+4E48 u+4E0D u+8BF4 u+4E2D u+6587
        AMC-V:  w87g8nvk6awispmrwupb6h

    (C) Czech: Pro<ccaron>prost<ecaron>nemluv<iacute><ccaron>esky
        U+0050 u+0072 u+006F u+010D u+0070 u+0072 u+006F u+0073 u+0074
        u+011B u+006E u+0065 u+006D u+006C u+0075 u+0076 u+00ED u+010D
        u+0065 u+0073 u+006B u+0079
        AMC-V:  -Pro-yp-prost-zm-nemluv-wpyp-esky

    (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-V:  x7ng7eep8e8jfinaqdb8ijp8cb8ij8k

    (E) Hindi (Devanagari):
        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
        AMC-V:  3urvjvcwmthjruiwpugwatfwpurwmscuivjiscunwmkvitfuewhvjwi\
                sc

    (F) Japanese (kanji and hiragana):
        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
        AMC-V:  vsykxnzr3dykb9fcjnme83cmdtxhygwr2nykweyqwm

    (G) Korean (Hangul syllables):
        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
        AMC-V:  6tvifgem42ixihhakfnh6nhhem5wrk6fmpmpwim6zermwrk6gzeivwm\
                p6iqige2nemm4efun

    (H) Russian (Cyrillic):
        U+043F 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
        AMC-V:  wvRgrvfnmvgfqpipfdqcqwawrwcrqwawdwbwbka

    (I) Spanish: Porqu<eacute>nopuedensimplementehablarenEspa<ntilde>ol
        U+0050 u+006F u+0072 u+0071 u+0075 u+00E9 u+006E u+006F u+0070
        u+0075 u+0065 u+0064 u+0065 u+006E u+0073 u+0069 u+006D u+0070
        u+006C u+0065 u+006D u+0065 u+006E u+0074 u+0065 u+0068 u+0061
        u+0062 u+006C u+0061 u+0072 u+0065 u+006E U+0045 u+0073 u+0070
        u+0061 u+00F1 u+006F u+006C
        AMC-V:  -Porqu-j-nopuedensimplementehablarenEspa-j-ol

    (J) Taiwanese:
        u+4ED6 u+5011 u+7232 u+4EC0 u+9EBD u+4E0D u+8AAA u+4E2D u+6587
        AMC-V:  w87gutbfbus6a385psspmfkupb6h

    (K) Vietnamese:
        Ta<dotbelow>isaoho<dotbelow>kh<ocirc>ngth<ecirc><hookabove>chi\
        <hookabove>no<acute>iti<ecirc><acute>ngVi<ecirc><dotbelow>t
        U+0054 u+0061 u+0323 u+0069 u+0073 u+0061 u+006F u+0068 u+006F
        u+0323 u+006B u+0068 u+00F4 u+006E u+0067 u+0074 u+0068 u+00EA
        u+0309 u+0063 u+0068 u+0069 u+0309 u+006E u+006F u+0301 u+0069
        u+0074 u+0069 u+00EA u+0301 u+006E u+0067 U+0056 u+0069 u+00EA
        u+0323 u+0074
        AMC-V:  -Ta-vud-isaoho-d-kh-s9e-ngth-s8ksj-chi-sj-no-sb-iti-csb\
                -ngVi-cud-t

    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>
        u+0033 u+5E74 U+0042 u+7D44 u+91D1 u+516B u+5148 u+751F
        AMC-V:  -3-x8ze-B-h4en8tvymwif29

    (M) <amuro><namie>-with-SUPER-MONKEYS
        u+5B89 u+5BA4 u+5948 u+7F8E u+6075 u+002D u+0077 u+0069 u+0074
        u+0068 u+002D U+0053 U+0055 U+0050 U+0045 U+0052 u+002D U+004D
        U+004F U+004E U+004B U+0045 U+0059 U+0053
        AMC-V:  x52j4e5wiinqavx---with--SUPER--MONKEYS

    (N) Hello-Another-Way-<sorezore><no><basho>
        U+0048 u+0065 u+006C u+006C u+006F u+002D U+0041 u+006E u+006F
        u+0074 u+0068 u+0065 u+0072 u+002D U+0057 u+0061 u+0079 u+002D
        u+305D u+308C u+305E u+308C u+306E u+5834 u+6240
        AMC-V:  -Hello--Another--Way---vsxp2nxq2nyq4vebca

    (O) <hitotsu><yane><no><shita>2
        u+3072 u+3068 u+3064 u+5C4B u+6839 u+306E u+4E0B u+0032
        AMC-V:  vszcyiye8wmct3yqssm-2

    (P) Maji<de>Koi<suru>5<byou><mae>
        U+004D u+0061 u+006A u+0069 u+3067 U+004B u+006F u+0069 u+3059
        u+308B u+0035 u+79D2 u+524D
        AMC-V:  -Maji-vsyh-Koi-xj2m-5-g8uwwp

    (Q) <pafii>de<runba>
        u+30D1 u+30D5 u+30A3 u+30FC u+0064 u+0065 u+30EB u+30F3 u+30D0
        AMC-V:  vs7b7f4d9n-de-8m9d7a

    (R) <sono><supiido><de>
        u+305D u+306E u+30B9 u+30D4 u+30FC u+30C9 u+3067
        AMC-V:  vsxpyq5j7e9n6jyh

    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 <-
        u+002D u+003E u+0020 u+0024 u+0031 u+002E u+0030 u+0030 u+0020
        u+003C u+002D
        AMC-V:  --svquae-1-q-00-avn--

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-V 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].

Acknowledgements

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

    The basic encoding of integers to quartets to quintets to base-32
    comes from UTF-5 [UTF5], and the particular variant used here comes
    from AMC-ACE-M [AMCACEM], as does the "wide style" (style 1).

    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 [BRACE].

    The general idea of using the alphabetic case of base-32 characters
    to indicate 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 style and reference points based on
    past code points is new in AMC-ACE-V.

References

    [AltDUDE] Adam Costello, "AltDUDE version 0.0.3", 2001-May-27,
    update of draft-ietf-idn-altdude-00, latest version at
    http://www.cs.berkeley.edu/~amc/charset/altdude.

    [AMCACEM] Adam Costello, "AMC-ACE-M version 0.1.4", 2001-Apr-01,
    update of draft-ietf-idn-amc-ace-m-00, latest version at
    http://www.cs.berkeley.edu/~amc/charset/amc-ace-m.

    [AMCACEO] Adam Costello, "AMC-ACE-O version 0.0.5", 2001-May-27,
    update of draft-ietf-idn-amc-ace-o-00, latest version at
    http://www.cs.berkeley.edu/~amc/charset/amc-ace-o.

    [AMCACER] Adam Costello, "AMC-ACE-R version 0.2.1",
    2001-May-31, draft-ietf-idn-amc-ace-r-01, latest version at
    http://www.cs.berkeley.edu/~amc/charset/amc-ace-r.

    [AMCACEW] Adam Costello, "AMC-ACE-W version 0.1.0",
    2001-May-31, draft-ietf-idn-amc-ace-w-00, latest version at
    http://www.cs.berkeley.edu/~amc/charset/amc-ace-w.

    [BRACE] Adam Costello, "BRACE: Bi-mode Row-based
    ASCII-Compatible Encoding for IDN version 0.1.2",
    2000-Sep-19, draft-ietf-idn-brace-00, version at
    http://www.cs.berkeley.edu/~amc/charset/brace.

    [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.

    [IDNA] Patrik Faltstrom, Paul Hoffman, "Internationalizing Host
    Names In Applications (IDNA)", draft-ietf-idn-idna-01.

    [NAMEPREP03] Paul Hoffman, Marc Blanchet, "Preparation
    of Internationalized Host Names", 2001-Feb-24,
    draft-ietf-idn-nameprep-03.

    [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.

    [SFS] David Mazieres et al, "Self-certifying File System",
    http://www.fs.net/.

    [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-*.

    [UTS6] Misha Wolf, Ken Whistler, Charles Wicksteed,
    Mark Davis, Asmus Freytag, "Unicode Technical Standard
    #6: A Standard Compression Scheme for Unicode",
    http://www.unicode.org/unicode/reports/tr6/.

Author

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

Example implementation


/******************************************/
/* amc-ace-v.c 0.1.0 (2001-May-31-Thu)    */
/* Adam M. Costello <amc@cs.berkeley.edu> */
/******************************************/

/* This is ANSI C code (C89) implementing AMC-ACE-V 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_bad_input,
  amc_ace_big_output  /* Output would exceed the space provided. */
};

enum case_sensitivity { case_sensitive, case_insensitive };

#if UINT_MAX >= 0x1FFFFF
typedef unsigned int u_code_point;
#else
typedef unsigned long u_code_point;
#endif

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

    /* amc_ace_v_encode() converts Unicode to AMC-ACE-V (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_v_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_v_decode() converts AMC-ACE-V (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>

/* base32[q] is the lowercase base-32 character representing  */
/* the number q 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;
  }
}

/* classify(refpoint,s,n) returns 0 if n represents an LDH character, */
/* else the index of the window containing n.  Window k starts at     */
/* refpoint[s][k] and spans 1 << (4*k) code points, or 0x5000 if s    */
/* is 1 and k is 3.  The style s must be 0 or 1.  Encoding an offset  */
/* into window k requires k base-32 digits.                           */

static unsigned int classify(
  u_code_point refpoint[2][6], unsigned int s, u_code_point n )
{
  unsigned int k;
  const u_code_point max[2][6] = { {0,0xF,0xFF, 0xFFF,0xFFFF,0xFFFFF},
                                   {0,  0,0xFF,0x4FFF,0xFFFF,0xFFFFF} };

  if ( n <= 122 && ( n >= 97 || n == 45 ||
       (n >= 48 && n <= 57) || (n >= 65 && n <= 90) ) ) return 0;
  for (k = 1 + s; ; ++k) if (n - refpoint[s][k] <= max[s][k]) return k;
}

/* update(refpoint,style,history,latest) updates                */
/* refpoint[0..1][1..3] and *style based on history[0..latest]. */

static void update( u_code_point refpoint[2][6], unsigned int *style,
  const u_code_point history[], unsigned int latest )
{
  unsigned int n, k, s, oldsum, newsum, i;
  u_code_point oldrp, newrp[4];

  n = history[latest];

  /* Update the style: */
  k = classify(refpoint,0,n);
  *style =  k == 1 ? 0 : k >= 4 ? 1 : *style;

  /* Compute the new candidate reference points: */
  newrp[1] = (n >> 3) << 3;
  newrp[2] = n - 0xA0 < 0xE0 ? 0xA0 : (n >> 8) << 8;
  /* newrp[3] depends on the style. */

  for (s = 0;  s <= 1;  ++s) {
    newrp[3] = s == 1 && n - 0xA000 < 0x3800 ? 0x8800 :
      n - 0x3000 < 0x7000 ? 0x4E00 : (n >> (11+s)) << (11+s);

    for (k = 1 + s;  k <= 3;  ++k) {
      /* Count the number of base-32 characters that would be */
      /* used to encode history[0..latest] using the old and  */
      /* new reference point.                                 */

      oldrp = refpoint[s][k];
      oldsum = newsum = 0;

      for (i = 0;  i <= latest;  ++i) {
        refpoint[s][k] = oldrp;
        oldsum += classify(refpoint, s, history[i]);
        refpoint[s][k] = newrp[k];
        newsum += classify(refpoint, s, history[i]);
      }

      /* If the new reference point is worse, don't use it: */
      if (newsum > oldsum) refpoint[s][k] = oldrp;
    }
  }
}

/* Main encode function: */

enum amc_ace_status amc_ace_v_encode(
  unsigned int input_length,
  const u_code_point input[],
  const unsigned char uppercase_flags[],
  unsigned int *output_size,
  char output[] )
{
  unsigned int style, literal, max_out, in, out, k, j;
  u_code_point n, delta;
  char shift;

  /* Initialize the state: */

  u_code_point refpoint[2][6] = { {0, 0xE0, 0xA0, 0, 0, 0x10000},
                                  {0,    0,    0, 0, 0, 0x10000}  };

  style = literal = 0;
  max_out = *output_size;

  for (in = out = 0;  in < input_length;  ++in) {

    /* At the start of each iteration, in and out are the number of */
    /* items already input/output, or equivalently, the indices of  */
    /* the next items to be input/output.                           */

    n = input[in];
    /* Check the code point range to avoid array bounds errors later: */
    if (n > 0x10FFFF) return amc_ace_bad_input;

    /* 0x2D is always encoded as two hyphen-minuses,     */
    /* otherwise classify() tells which encoding to use. */

    k = classify(refpoint,style,n);

    if (n == 0x2D) {
      /* Hyphen-minus is doubled. */
      if (max_out - out < 2) return amc_ace_big_output;
      output[out++] = 0x2D;
      output[out++] = 0x2D;
    }
    else if (k == 0) {
      /* Encode a letter/digit literally. */
      if (max_out - out < 1 + !literal) return amc_ace_big_output;
      /* Switch to literal mode if necessary: */
      if (!literal) output[out++] = 0x2D;
      literal = 1;
      output[out++] = n;
    }
    else {
      /* Encode a non-LDH character as k base-32 digits. */

      if (max_out - out < k + literal) return amc_ace_big_output;
      /* Switch to base-32 mode if necessary: */
      if (literal) output[out++] = 0x2D;
      literal = 0;
      shift = uppercase_flags && uppercase_flags[in] ? 32 : 0;
      delta = n - refpoint[style][k];

      /* Check for the extended delta of style 1 window 3: */

      if (k == 3 && delta >= 0x1000) {
        /* The top 16k of window 3 is encoded as 0xxxx xxxxx xxxxx. */
        delta -= 0x1000;
        output[out++] = base32[delta >> 10] - shift;
        output[out++] = base32[(delta >> 5) & 0x1F];
        output[out++] = base32[delta & 0x1F];
      }
      else {
        /* Each quintet has the form 1xxxx except the last is 0xxxx. */
        /* Computing the base-32 digits in reverse order is easiest. */

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

        for (j = 2;  j <= k;  ++j) {
          delta >>= 4;
          output[out - j] = base32[0x10 | (delta & 0xF)];
        }
      }

      update(refpoint, &style, input, in);
    }
  }

  /* Append the null terminator: */
  if (max_out - out < 1) return amc_ace_big_output;
  output[out++] = 0;

  *output_size = out;
  return amc_ace_success;
}

/* Main decode function: */

enum amc_ace_status amc_ace_v_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;
  char c;
  unsigned int style, literal, max_out, in, out, k, scratch_size;
  enum amc_ace_status status;

  /* Initialize the state: */

  u_code_point refpoint[2][6] = { {0, 0xE0, 0xA0, 0, 0, 0x10000},
                                  {0,    0,    0, 0, 0, 0x10000}  };

  style = literal = 0;
  max_out = *output_length;

  for (c = input[in = 0], out = 0;  c != 0;  c = input[++in], ++out) {

    /* At the start of each iteration, in and out are the number of   */
    /* items already input/output, or equivalently, the indices of    */
    /* the next items to be input/output. c is the same as input[in]  */
    /* except when "extra" characters have been consumed (see below). */

    if (c == 0x2D && input[in + 1] != 0x2D) {
      /* Unpaired hyphen-minus toggles mode. */
      literal = !literal;
      c = input[++in];
    }

    if (max_out - out < 1) return amc_ace_big_output;

    if (c == 0x2D) {
      /* Double hyphen-minus represents a hyphen-minus. */
      ++in;
      output[out] = 0x2D;
    }
    else {
      if (literal) output[out] = c;
      else {
        /* Decode a base-32 sequence.                  */
        /* First decode quintets until 0xxxx is found: */

        for (delta = 0, k = 1;  ;  c = input[++in], ++k) {
          q = base32_decode(c);
          if (q == base32_invalid || k > 5) return amc_ace_bad_input;
          delta = (delta << 4) | (q & 0xF);
          if (q >> 4 == 0) break;
        }

        if (style == 1 && k == 1) {
          /* Style 1 has no window 1, so it must be the extended */
          /* delta of window 3, encoded as 0xxxx xxxxx xxxxx.    */
          /* Consume the two "extra" characters:                 */

          for (;  k < 3;  ++k) {
            q = base32_decode(input[++in]);
            if (q == base32_invalid) return amc_ace_bad_input;
            delta = (delta << 5) | q;
          }

          delta += 0x1000;
        }

        output[out] = refpoint[style][k] + delta;
        update(refpoint, &style, output, out);
      }
    }

    /* Case of last non-extra character determines uppercase flag: */
    if (uppercase_flags) uppercase_flags[out] = c >= 65 && c <= 90;
  }

  /* Enforce the uniqueness of the encoding by re-encoding */
  /* the output and comparing the result to the input:     */

  scratch_size = ++in;
  status = amc_ace_v_encode(out, output, uppercase_flags,
                            &scratch_size, scratch_space);
  if (status != amc_ace_success || scratch_size != in ||
      unequal(case_sensitivity, scratch_space, input)
     ) return amc_ace_bad_input;

  *output_length = 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
                          /* suitable for host names */
};


static void usage(char **argv)
{
  fprintf(stderr,
    "%s -e reads code points and writes an AMC-ACE-V string.\n"
    "%s -d reads an AMC-ACE-V string and writes code points.\n"
    "Input and output are plain text in the native character set.\n"
    "Code points are in the form u+hex separated by whitespace.\n"
    "An AMC-ACE-V string is a newline-terminated sequence of LDH\n"
    "characters (without any signature).\n"
    "The case of the u in u+hex is the 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_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";

/* The following string is used to convert LDH      */
/* characters between ASCII and the native charset: */

static const char ldh_ascii[] =
  "................"
  "................"
  ".............-.."
  "0123456789......"
  ".ABCDEFGHIJKLMNO"
  "PQRSTUVWXYZ....."
  ".abcdefghijklmno"
  "pqrstuvwxyz";


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

  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 long codept;
    unsigned char uppercase_flags[unicode_max_length];
    char output[ace_max_size], uplus[3];
    unsigned int input_length, output_size, i;

    /* Read the input code points: */

    input_length = 0;

    for (;;) {
      r = scanf("%2s%lx", uplus, &codept);
      if (ferror(stdin)) fail(io_error);
      if (r == EOF || r == 0) break;

      if (r != 2 || uplus[1] != '+' || codept > (u_code_point)-1) {
        fail(invalid_input);
      }

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

      if (uplus[0] == 'u') uppercase_flags[input_length] = 0;
      else if (uplus[0] == 'U') uppercase_flags[input_length] = 1;
      else fail(invalid_input);

      input[input_length++] = codept;
    }

    /* Encode: */

    output_size = ace_max_size;
    status = amc_ace_v_encode(input_length, input, uppercase_flags,
                              &output_size, output);
    if (status == amc_ace_bad_input) fail(invalid_input);
    if (status == amc_ace_big_output) fail(too_big);
    assert(status == amc_ace_success);

    /* Convert to native charset and output: */

    for (p = output;  *p != 0;  ++p) {
      i = *p;
      assert(i <= 122 && ldh_ascii[i] != '.');
      *p = ldh_ascii[i];
    }

    r = puts(output);
    if (r == EOF) fail(io_error);
    return EXIT_SUCCESS;
  }

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

    /* Read the AMC-ACE-V input string and convert to ASCII: */

    fgets(input, ace_max_size, stdin);
    if (ferror(stdin)) fail(io_error);
    if (feof(stdin)) fail(invalid_input);
    input_length = strlen(input);
    if (input[input_length - 1] != '\n') fail(too_big);
    input[--input_length] = 0;

    for (p = input;  *p != 0;  ++p) {
      pp = strchr(ldh_ascii, *p);
      if (pp == 0) fail(invalid_input);
      *p = pp - ldh_ascii;
    }

    /* Decode: */

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

    /* Output the result: */

    for (i = 0;  i < output_length;  ++i) {
      r = printf("%s+%04lX\n",
                 uppercase_flags[i] ? "U" : "u",
                 (unsigned long) output[i] );
      if (r < 0) fail(io_error);
    }

    return EXIT_SUCCESS;
  }

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



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