INTERNET-DRAFT Adam M. Costello
draft-costello-idn-amc-ace-z-00.txt 2001-Jul-11
Expires 2002-Jan-11
AMC-ACE-Z version 0.2.1
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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to the author at amc@cs.berkeley.edu, or to the idn working
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Abstract
AMC-ACE-Z is a simple and efficient ASCII-Compatible Encoding (ACE)
designed for use with Internationalized Domain Names [IDN] [IDNA].
It transforms a Unicode string [UNICODE] into a string of characters
allowed in hostname labels (ASCII letters, digits, and hyphens)
and back again. AMC-ACE-Z is an instance of Bootstring that uses
particular parameter values appropriate for IDNA and uses an IDNA
signature prefix. Bootstring allows a string of basic code points
to uniquely represent any string of code points drawn from a larger
set. This document specifies Bootstring and the parameter values
for AMC-ACE-Z.
Contents
1. Introduction
2. Terminology
3. Bootstring description
3.1 Basic code point segregation
3.2 Insertion unsort coding
3.3 Generalized variable-length integers
3.4 Bias adaptation
4. Bootstring parameters
5. Parameter values for AMC-ACE-Z
6. Bootstring algorithms
6.1 Bias adaptation function
6.2 Decoding procedure
6.3 Encoding procedure
7. AMC-ACE-Z example strings
8. Security considerations
9. References
A. Author contact information
B. Mixed-case annotation
C. Sample implementation
1. Introduction
The IDNA draft [IDNA] describes an architecture for supporting
internationalized domain names. Each label of a domain name may
begin with a special prefix, in which case the remainder of the
label is an ASCII-Compatible Encoding (ACE) of a Unicode string
satisfying certain constraints. For the details of the constraints,
see [IDNA] and [NAMEPREP]. The prefix has not yet been specified,
but see http://www.i-d-n.net/ for prefixes to be used for testing
and experimentation.
Bootstring has been designed to have the following features:
* Completeness: Every extended string (sequence of arbitrary code
points) can be represented by a basic string (sequence of basic
code points). Restrictions on what strings are allowed, and on
length, may be imposed by higher layers.
* Uniqueness: Every extended string maps to at most one basic
string.
* Reversibility: Any extended string mapped to a basic string can
be recovered from that basic string.
* Efficient encoding: The ratio of extended string length to
basic string length is small. This is important in the context
of domain names because RFC 1034 [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; Bootstring aims at a good balance between them.
* Readability: Basic code points appearing in the extended
string are represented as themselves in the basic string. This
comes for free because it makes the encoding more efficient on
average.
In addition, AMC-ACE-Z can support an optional feature described in
appendix B "Mixed-case annotation".
AMC-ACE-Z is a working name that should be changed if it is adopted.
(The Z merely indicates that it is the twenty-sixth ACE devised by
this author. Most were not worth releasing.)
2. Terminology
The key words "must", "shall", "required", "should", "recommended",
and "may" in this document are to be interpreted as described in RFC
2119 [RFC2119].
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.
The operators div and mod perform integer division; (x div y) is the
quotient of x divided by y, discarding the remainder, and (x mod y)
is the remainder, so (x div y) * y + (x mod y) == x. Bootstring
uses these operators only with nonnegative operands, so the quotient
and remainder are always nonnegative.
The ?: operator is a conditional; (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.
The "break" statement jumps out of the innermost loop (as in C).
3. Bootstring description
Bootstring represents an arbitrary sequence of code points (the
"extended string") as a sequence of basic code points (the
"basic string"). This section describes the representation.
Section 6 "Bootstring algorithms" presents the algorithms as
pseudocode. There is also commented C code in appendix C "Sample
implementation".
3.1 Basic code point segregation
All basic code points appearing in the extended string are
represented literally at the beginning of the basic string, in their
original order, followed by a delimiter if (and only if) the number
of basic code points is nonzero. The delimiter is a particular
basic code point, which never appears in the remainder of the basic
string. The decoder can therefore find the end of the literal
portion (if there is one) by scanning for the last delimiter.
3.2 Insertion unsort coding
The remainder of the basic string (after the last delimiter if there
is one) represents a sequence of nonnegative integral deltas as
generalized variable-length integers, described in section 3.3. The
meaning of the deltas is best understood in terms of the decoder.
The decoder builds the extended string incrementally. Initially,
the extended string is a copy of the literal portion of the basic
string (excluding the last delimiter). Each delta causes the
decoder to insert a code point into the extended string according
to the following procedure. There are two state variables: a
code point n, and an index i that ranges from zero (which is the
first position of the extended string) to the current length of
the extended string (which refers to a potential position beyond
the current end). The decoder advances the state monotonically
(never returning to an earlier state) by taking steps only upward.
Each step increments i, except when i already equals the length
of the extended string, in which case a step resets i to zero
and increments n. For each delta (in order), the decoder takes
delta steps upward, then inserts the value n into the extended
string at position i, then increments i (to skip over the code
point just inserted). (An implementation should not take each
step individually, but should insead use division and remainder
calculations to advance by delta steps all at once.)
The encoder's main task is to derive the sequence of deltas that
will cause the decoder to construct the desired string. It can do
this by repeatedly scanning the extended string for the next code
point that the decoder would need to insert, and counting the number
of steps the decoder would need to take, mindful of the fact that
the decoder will be stepping over only those code points that have
already been inserted. Section 6.3 "Encoding procedure" gives a
precise algorithm.
3.3 Generalized variable-length integers
In a conventional integer representation the base is the number of
distinct symbols for digits, whose values are 0 through base-1. Let
digit_0 denote the least significant digit, digit_1 the next least
significant, and so on. The value represented is the sum over j of
digit_j * w(j), where w(j) = base^j is the weight (scale factor)
for position j. For example, in the base 8 integer 437, the digits
are 7, 3, and 4, and the weights are 1, 8, and 64, so the value is
7 + 3*8 + 4*64 = 287. This representation has two disadvantages:
First, there are multiple encodings of each value (because there
can be extra zeros in the most significant positions), which
is inconvenient when unique encodings are required. Second,
the integer is not self-delimiting, so if multiple integers are
concatenated the boundaries between them are lost.
The generalized variable-length representation solves these two
problems. The digit values are still 0 through base-1, but now
the integer is self-delimiting by means of thresholds t(j), each
of which is in the range 0 through base-1. Exactly one digit, the
most significant, satisfies digit_j < t(j). Therefore, if several
integers are concatenated, it is easy to separate them, starting
with the first if they are little-endian (least significant digit
first), or starting with the last if they are big-endian (most
significant digit first). As before, the value is the sum over j of
digit_j * w(j), but the weights are different:
w(0) = 1
w(j) = w(j-1) * (base - t(j-1)) for j > 0
For example, consider the little-endian sequence of base 8 digits
734251... Suppose the thresholds are 2, 3, 5, 5, 5, 5... This
implies that the weights are 1, 1*(8-2) = 6, 6*(8-3) = 30, 30*(8-5)
= 90, 90*(8-5) = 270, and so on. 7 is not less than 2, and 3 is
not less than 3, but 4 is less than 5, so 4 must be the last digit.
The value of 734 is 7*1 + 3*6 + 4*30 = 145. The next integer is
251, with value 2*1 + 5*6 + 1*30 = 62. Decoding this representation
is very similar to decoding a conventional integer: Start with a
current value of N = 0 and a weight w = 1. Fetch the next digit d
and increase N by d * w. If d is less than the current threshold
(t) then stop, otherwise increase w by a factor of (base - t),
update t for the next position, and repeat.
Encoding this representation is similar to encoding a conventional
integer: If N < t then output one digit for N and stop, otherwise
output the digit for t + ((N - t) mod (base - t)), then replace N
with (N - t) div (base - t), update t for the next position, and
repeat.
For any particular set of values of t(j), there is exactly one
generalized variable-length representation of each nonnegative
integral value.
Bootstring uses little-endian ordering so that the deltas can be
separated starting with the first. The t(j) values are defined in
terms of the constants base, tmin, and tmax, and a state variable
called bias:
t(j) = base * (j + 1) - bias,
clamped to the range tmin through tmax
(The clamping means that if the formula yields a value less than
tmin or greater than tmax, then t(j) = tmin or tmax, respectively.)
These t(j) values cause the representation to favor integers within
a particular range determined by the bias.
3.4 Bias adaptation
After each delta is encoded or decoded, bias is set for the next
delta as follows:
1. Delta is scaled in order to avoid overflow in the next step:
let delta = delta div 2
But when this is the very first delta, the divisor is not 2, but
instead a constant called damp. This compensates for the fact
that the second delta is usually much smaller than the first.
2. Delta is increased to compensate for the fact that the next
delta will be inserting into a longer string:
let delta = delta + (delta div numpoints)
numpoints is the total number of code points encoded/decoded so
far (including the one corresponding to this delta itself, and
including the basic code points).
3. Delta is repeatedly divided until it falls within a threshold,
to predict the minimum number of digits needed to represent the
next delta:
while delta > ((base - tmin) * tmax) div 2
do let delta = delta div (base - tmin)
4. The bias is set:
let bias =
(base * the number of divisions performed in step 3) +
(((base - tmin + 1) * delta) div (delta + skew))
The motivation for this procedure is that the current delta provides
a hint about the likely size of the next delta, and so t(j) is
set to tmax for the more significant digits starting with the one
expected to be last, tmin for the less significant digits up through
the one expected to be third-last, and somewhere between tmin and
tmax for the digit expected to be second-last (balancing the hope of
the expected-last digit being unnecessary against the danger of it
being insufficient).
4. Bootstring parameters
Given a set of basic code points, one must be chosen as the
delimiter. The base is the number of distinguishable basic code
points remaining. They must be associated with the values in the
range 0 through base-1. In some cases multiple code points must
represent the same value; for example, uppercase and lowercase
versions of a letter must be equivalent if basic strings are
case-insensitive.
The initial value of n should be the minimum non-basic code point
that is allowed in extended strings.
The remaining five parameters (tmin, tmax, skew, damp, and the
initial value of bias) must satisfy the following constraints:
0 <= tmin <= tmax <= base-1
skew >= 1
damp >= 2
initial_bias mod base <= base - tmin
Provided the constraints are satisfied, these five parameters affect
efficiency but not correctness. They should be chosen empirically.
If support for mixed-case annotation is desired (see appendix B),
make sure that the code points corresponding to 0 through tmax-1 all
have both uppercase and lowercase forms.
5. Parameter values for AMC-ACE-Z
AMC-ACE-Z uses the following values for the Bootstring parameters:
base = 36
tmin = 1
tmax = 26
skew = 38
damp = 700
initial_bias = 72
initial_n = U+00A1
In AMC-ACE-Z, code points are Unicode code points [UNICODE], that
is, integers in the range 0..10FFFF, but not D800..DFFF, which are
reserved for use by UTF-16. The basic code points, along with their
values, are:
U+002D (-) = delimiter
41..5A (A-Z) = 0 to 25, respectively
61..7A (a-z) = 0 to 25, respectively
30..39 (0-9) = 26 to 35, respectively
Using hyphen-minus as the delimiter implies that the ACE can end
with a hyphen-minus only if the Unicode string consists entirely
of basic code points, but IDNA forbids such strings from being
ACE-encoded. And since IDNA prepends a prefix that does not begin
with a hyphen-minus, AMC-ACE-Z conforms to the RFC 952 requirement
that hostname labels neither begin nor end with a hyphen-minus
[RFC952].
A decoder must recognize the letters in both uppercase and lowercase
forms (including mixtures of both forms). An encoder should output
only uppercase forms or only lowercase forms, unless it uses
mixed-case annotation (see appendix B).
Presumably most users will not manually copy ACEs by writing or
typing them (as opposed to letting computers do it via cut & paste),
but those that do will need to be alert to the potential visual
ambiguity between the following sets of characters:
G 6
I l 1
O 0
S 5
U V
Z 2
Such ambiguities are usually resolved by context, but in an ACE
there is no context apparent to humans.
6. Bootstring algorithms
6.1 Bias adaptation function
function adapt(delta,numpoints,firsttime):
let delta = delta div (firsttime ? damp : 2)
let delta = delta + (delta div numpoints)
let k = 0
while delta > ((base - tmin) * tmax) div 2
do let delta = delta div (base - tmin) and let k = k + base
return k + (((base - tmin + 1) * delta) div (delta + skew))
6.2 Decoding procedure
let n = initial_n
let i = 0
let bias = initial_bias
let output = an empty string indexed from 0
search the input for the last delimiter (do not consume the input)
if one is found that is not at the very beginning then consume all
preceeding code points, copy them to output, consume the delimiter
while the input is not exhausted do begin
let oldi = i
let w = 1
for k = base to infinity in steps of base do begin
consume a code point, fail on end-of-input or invalid code point
let digit = the code point's value
let i = i + digit * w, fail on overflow
let t = k <= bias ? tmin : k - bias > tmax ? tmax : k - bias
if digit < t then break
let w = w * (base - t), fail on overflow
end
let bias = adapt(i - oldi, length(output) + 1, oldi == 0)
let n = n + i div (length(output) + 1), fail on overflow
let i = i mod (length(output) + 1)
if n is a basic code point then fail # see Note1 below
insert n into output at position i
increment i
end
Note1: The check for whether n is a basic code point can be omitted
if initial_n exceeds all basic code points (which is true for
AMC-ACE-Z), because n only increases from initial_n.
Because the decoder state can only advance monotonically, and there
is only one representation of any delta, there is therefore only
one encoded string that can represent a given sequence of integers.
The only error conditions are invalid code points, unexpected
end-of-input, overflow (attempts to compute values that exceed the
maximum value of an integer variable), and basic code points encoded
using deltas instead of appearing literally. If the decoder fails
on these errors as shown above, then it cannot produce the same
output for two distinct inputs, and hence it need not re-encode its
output to verify that it matches the input.
The assignment of t, where t is clamped to the range tmin through
tmax, does not handle the case where bias < k < bias + tmin, but
that is impossible because of the way bias is computed and because
of the constraints on the parameters.
If the programming language does not provide overflow detection,
the following technique can be used. Suppose A, B, and C are
representable nonnegative integers and C is nonzero. Then A + B
overflows if and only if B > maxint - A, and A + (B * C) overflows
if and only if B > (maxint - A) div C. See appendix C "Sample
implementation" for demonstrations of this technique in AMC-ACE-Z.
6.3 Encoding procedure
let n = initial_n
let delta = 0
let bias = initial_bias
let h = b = the number of basic code points in the input
copy them to the output in order, followed by a delimiter if b > 0
if the input contains a non-basic code point < n then fail
while h < length(input) do begin
let m = the minimum non-basic code point >= n in the input # Note2
let delta = delta + (m - n) * (h + 1), fail on overflow
let n = m
for each integer m in the input (in order) do begin
if m is a basic code point # see Note2 below
then increment delta, fail on overflow, and continue
if m < n then increment delta, fail on overflow
if m == n then begin
let q = delta
for k = base to infinity in steps of base do begin
let t = k <= bias ? tmin : k - bias > tmax ? tmax : k - bias
if q < t then break
output the code point for digit t + ((q - t) mod (base - t))
let q = (q - t) div (base - t)
end
output the code point for digit q
let bias = adapt(delta, h + 1, h == b)
let delta = 0
increment h
end
end
increment delta and n
end
Note2: There are two places in the main loop where the encoder
checks whether a code point is basic. If initial_n exceeds all
basic code points (which is true for AMC-ACE-Z) then m and n can
never be basic code points, and the logic can be simplified.
The checks for overflow are necessary to avoid producing invalid
output when the input contains very large values or is very long.
Wider integer variables can handle more extreme inputs. For
AMC-ACE-Z, 26-bit unsigned integers are sufficient, because in
IDNA code points are limited 0..10FFFF and ACEs are limited to 59
characters (excluding the prefix).
The increment of delta at the bottom of the outer loop cannot
overflow because delta < length(input) before the increment, and
length(input) is already assumed to be representable. The increment
of n could overflow, but only if h == length(input), in which case
the procedure is finished anyway.
7. AMC-ACE-Z example strings
In the AMC-ACE-Z encodings below, the IDNA signature prefix is not
shown. AMC-ACE-Z is abbreviated AMC-Z. 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 ?" (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-Z: gfbpdaj6bu4bxfgehfvwxn
(B) Chinese (simplified):
u+4ED6 u+4EEC u+4E3A u+4EC0 u+4E48 u+4E0D u+8BF4 u+4E2D u+6587
AMC-Z: kgqwcrb4cv8a8dqg056pqjye
(C) Czech: Proprostnemluvesky
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-Z: Proprostnemluvesky-xgb24dma41a
(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-Z: 6cbcagdahymbxekheh6e0a7fei0b
(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-Z: k0baa7eci9glrd9b2ae1bj0hfcgg6iyaf8o0a1dig0cd
(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-Z: p7jok5ay5dzabd5bym9f0cm5685rrjetr6pdxa
(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-Z: c89aomsvi5e83db1d2a355cv1e0vak1dwrv93d5xbh15a0dt30a5jps\
d879ccm6fea98c
(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-Z: d0abfaaepdrnnbgefbaDotcwatmq2g4l
(I) Spanish: PorqunopuedensimplementehablarenEspaol
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-Z: PorqunopuedensimplementehablarenEspaol-nkc56a
(J) Taiwanese:
u+4ED6 u+5011 u+7232 u+4EC0 u+9EBD u+4E0D u+8AAA u+4E2D u+6587
AMC-Z: kgqwctvzc91f659drss3x8bo0yb
(K) Vietnamese:
Tisaohkhngthch\
nitingVit
U+0054 u+1EA1 u+0069 u+0073 u+0061 u+006F u+0068 u+1ECD u+006B
u+0068 u+00F4 u+006E u+0067 u+0074 u+0068 u+1EC3 u+0063 u+0068
u+1EC9 u+006E u+00F3 u+0069 u+0074 u+0069 u+1EBF u+006E u+0067
U+0056 u+0069 u+1EC7 u+0074
AMC-Z: TisaohkhngthchnitingVit-xvbr8268qyxafd2f1b9g
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) 3B
u+0033 u+5E74 U+0042 u+7D44 u+91D1 u+516B u+5148 u+751F
AMC-Z: 3B-2t4c5e180e575a65lsy2b
(M) -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-Z: -with-SUPER-MONKEYS-us48ag80a8qai00g7n9n
(N) Hello-Another-Way-
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-Z: Hello-Another-Way--it3qua05auwb3674vfr0b
(O) 2
u+3072 u+3068 u+3064 u+5C4B u+6839 u+306E u+4E0B u+0032
AMC-Z: 2-y7tlzr9756bt3uc0v
(P) MajiKoi5
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-Z: MajiKoi5-q03gue6qz075azm5e
(Q) de
u+30D1 u+30D5 u+30A3 u+30FC u+0064 u+0065 u+30EB u+30F3 u+30D0
AMC-Z: de-pd4avhby1noc0d
(R)
u+305D u+306E u+30B9 u+30D4 u+30FC u+30C9 u+3067
AMC-Z: f8juau41awczczp
8. 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-Z 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].
References
[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)", 2001-Jun-16, draft-ietf-idn-idna-02.
[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.
[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.
[UNICODE] The Unicode Consortium, "The Unicode Standard",
http://www.unicode.org/unicode/standard/standard.html.
A. Author contact information
Adam M. Costello
University of California, Berkeley
http://www.cs.berkeley.edu/~amc/
B. Mixed-case annotation
In order to use AMC-ACE-Z to represent case-insensitive strings,
higher layers need to case-fold the strings prior to AMC-ACE-Z
encoding. The encoded string can, however, use mixed case as an
annotation telling how to convert the original folded string into a
mixed-case string for display purposes.
Basic code points are represented literally, and can therefore use
mixed case directly. Each non-basic code point is represented by
a delta, which is represented by a sequence of basic code points,
the last of which provides the annotation. If it is uppercase,
it is a suggestion to map the non-basic code point to uppercase
(if possible); if it is lowercase, it is a suggestion to map the
non-basic code point to lowercase (if possible).
AMC-ACE-Z encoders and decoders are not required to support these
annotations, and higher layers need not use them.
C. Sample implementation
/******************************************/
/* amc-ace-z.c 0.2.1 (2001-Jul-11-Wed) */
/* Adam M. Costello */
/******************************************/
/* This is ANSI C code (C89) implementing AMC-ACE-Z version 0.2.x. */
/************************************************************/
/* Public interface (would normally go in its own .h file): */
#include
enum amc_ace_status {
amc_ace_success,
amc_ace_bad_input, /* Input is invalid. */
amc_ace_big_output, /* Output would exceed the space provided. */
amc_ace_overflow /* Input requires wider integers to process. */
};
#if UINT_MAX >= (1 << 26) - 1
typedef unsigned int amc_ace_z_uint;
#else
typedef unsigned long amc_ace_z_uint;
#endif
enum amc_ace_status amc_ace_z_encode(
amc_ace_z_uint input_length,
const amc_ace_z_uint input[],
const unsigned char uppercase_flags[],
amc_ace_z_uint *output_size,
char output[] );
/* amc_ace_z_encode() converts Unicode to AMC-ACE-Z (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 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. */
enum amc_ace_status amc_ace_z_decode(
const char input[],
amc_ace_z_uint *output_length,
amc_ace_z_uint output[],
unsigned char uppercase_flags[] );
/* amc_ace_z_decode() converts AMC-ACE-Z (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 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
/*** Bootstring parameters for AMC-ACE-Z ***/
enum { base = 36, tmin = 1, tmax = 26, skew = 38, damp = 700,
initial_bias = 72, initial_n = 0xA1, delimiter = 0x2D };
/* encode_digit(d) returns the basic code point whose value */
/* (when used for representing integers) is d, which must be */
/* in the range 0 to base-1. The lowercase form is used. */
static char encode_digit(amc_ace_z_uint d)
{
return d + 22 + 75 * (d < 26);
/* 0..25 map to ASCII a..z */
/* 26..35 map to ASCII 0..9 */
}
/* decode_digit(cp) returns the numeric value of a basic code point */
/* (for use in representing integers) in the range 0 to base-1, or */
/* base if cp is the delimiter, or base+1 otherwise. */
static amc_ace_z_uint decode_digit(amc_ace_z_uint cp)
{
return cp - 48 < 10 ? cp - 22 : cp - 65 < 26 ? cp - 65 :
cp - 97 < 26 ? cp - 97 : cp == delimiter ? base : base + 1;
}
/*** Useful constants ***/
/* maxint is the maximum value of an amc_ace_z_uint variable: */
static const amc_ace_z_uint maxint = -1;
/* lobase and cutoff are used in the calculation of bias: */
enum { lobase = base - tmin, cutoff = lobase * tmax / 2 };
/*** Main encode function ***/
enum amc_ace_status amc_ace_z_encode(
amc_ace_z_uint input_length,
const amc_ace_z_uint input[],
const unsigned char uppercase_flags[],
amc_ace_z_uint *output_size,
char output[] )
{
amc_ace_z_uint n, delta, h, b, out, max_out, bias, j, m, q, k, t;
char shift;
/* Initialize the state: */
n = initial_n;
delta = out = 0;
max_out = *output_size;
bias = initial_bias;
/* Handle the basic code points, and make sure */
/* that all code points < n are basic code points: */
for (j = 0; j < input_length; ++j) {
if (decode_digit(input[j]) <= base) {
if (max_out - out < 2) return amc_ace_big_output;
output[out++] = input[j];
}
else if (input[j] < n) return amc_ace_bad_input;
}
h = b = out;
/* h is the number of code points that have been handled, b is the */
/* number of basic code points, and out is the number of characters */
/* that have been output. */
if (b > 0) output[out++] = delimiter;
/* Main encoding loop: */
while (h < input_length) {
/* All non-basic code points < n have been */
/* handled already. Find the next larger one: */
for (m = maxint, j = 0; j < input_length; ++j) {
/* not needed for AMC-ACE-Z: */
/* if (decode_digit(input[j]) <= base) continue; */
if (input[j] >= n && input[j] < m) m = input[j];
}
/* Increase delta enough to advance the decoder's */
/* state to , but guard against overflow: */
if (m - n > (maxint - delta) / (h + 1)) return amc_ace_overflow;
delta += (m - n) * (h + 1);
n = m;
for (j = 0; j < input_length; ++j) {
/* Not needed for AMC-ACE-Z: */
#if 0
if (decode_digit(input[j]) <= base) {
if (++delta == 0) return amc_ace_overflow;
continue;
}
#endif
if (input[j] < n && ++delta == 0) return amc_ace_overflow;
if (input[j] == n) {
/* Represent delta as a generalized variable-length integer: */
for (q = delta, k = base; ; k += base) {
if (out >= max_out) return amc_ace_big_output;
t = k <= bias ? tmin : k - bias >= tmax ? tmax : k - bias;
if (q < t) break;
output[out++] = encode_digit(t + (q - t) % (base - t));
q = (q - t) / (base - t);
}
shift = uppercase_flags && uppercase_flags[j] ? 32 : 0;
/* shift controls the case of the terminal character: */
output[out++] = encode_digit(q) - shift;
/* Adapt the bias: */
delta = h == b ? delta / damp : delta >> 1;
delta += delta / (h + 1);
for (bias = 0; delta > cutoff; bias += base) delta /= lobase;
bias += (lobase + 1) * delta / (delta + skew);
delta = 0;
++h;
}
}
++delta, ++n;
}
/* Append the null terminator: */
if (out >= max_out) return amc_ace_big_output;
output[out++] = 0;
*output_size = out;
return amc_ace_success;
}
/*** Main decode function ***/
enum amc_ace_status amc_ace_z_decode(
const char input[],
amc_ace_z_uint *output_length,
amc_ace_z_uint output[],
unsigned char uppercase_flags[] )
{
amc_ace_z_uint n, out, i, oldi, max_out, bias, w, k, delta, digit, t;
const char *in, *p;
/* Initialize the state: */
n = initial_n;
out = i = 0;
max_out = *output_length;
bias = initial_bias;
/* Handle the basic code points: Let p point to the last */
/* delimiter, or to the start if there is none, then copy */
/* everything before p to the output. */
for (p = in = input; *in; ++in) if (*in == delimiter) p = in;
if (p - input > max_out) return amc_ace_big_output;
for (in = input; in < p; ++in) {
if (uppercase_flags) uppercase_flags[out] = *in >= 65 && *in <= 90;
output[out++] = *in;
}
/* Main decoding loop: Start just after p if any basic code */
/* points were copied; start at the beginning otherwise. */
for (in = p > input ? p + 1 : input; *in != 0; ++out) {
/* in points to the next character to be consumed, and */
/* out is the number of code points in the output array. */
/* Decode a generalized variable-length integer into delta, */
/* which gets added to i. The overflow checking is easier */
/* if we increase i as we go, then subtract off its starting */
/* value at the end to obtain delta. */
for (oldi = i, w = 1, k = base; ; k += base) {
digit = decode_digit(*in++);
if (digit >= base) return amc_ace_bad_input;
if (digit > (maxint - i) / w) return amc_ace_overflow;
i += digit * w;
t = k <= bias ? tmin : k - bias >= tmax ? tmax : k - bias;
if (digit < t) break;
if (w > maxint / (base - t)) return amc_ace_overflow;
w *= (base - t);
}
/* Adapt the bias: */
delta = oldi == 0 ? i / damp : (i - oldi) >> 1;
delta += delta / (out + 1);
for (bias = 0; delta > cutoff; bias += base) delta /= lobase;
bias += (lobase + 1) * delta / (delta + skew);
/* i was supposed to wrap around from out+1 to 0, */
/* incrementing n each time, so we'll fix that now: */
if (i / (out + 1) > maxint - n) return amc_ace_overflow;
n += i / (out + 1);
i %= (out + 1);
/* Insert n at position i of the output: */
/* not needed for AMC-ACE-Z: */
/* if (decode_digit(n) <= base) return amc_ace_invalid_input; */
if (out >= max_out) return amc_ace_big_output;
if (uppercase_flags) {
memmove(uppercase_flags + i + 1, uppercase_flags + i, out - i);
/* Case of last character determines uppercase flag: */
uppercase_flags[i] = in[-1] >= 65 && in[-1] <= 90;
}
memmove(output + i + 1, output + i, (out - i) * sizeof *output);
output[i++] = n;
}
*output_length = out;
return amc_ace_success;
}
/******************************************************************/
/* Wrapper for testing (would normally go in a separate .c file): */
#include
#include
#include
#include
/* 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
};
static void usage(char **argv)
{
fprintf(stderr,
"%s -e reads code points and writes an AMC-ACE-Z string.\n"
"%s -d reads an AMC-ACE-Z 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-Z 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 overflow[] = "arithmetic overflow\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') {
amc_ace_z_uint 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 > (amc_ace_z_uint)-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_z_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);
if (status == amc_ace_overflow) fail(overflow);
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], *pp;
amc_ace_z_uint output[unicode_max_length];
unsigned char uppercase_flags[unicode_max_length];
unsigned int input_length, output_length, i;
/* Read the AMC-ACE-Z 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_z_decode(input, &output_length,
output, uppercase_flags);
if (status == amc_ace_bad_input) fail(invalid_input);
if (status == amc_ace_big_output) fail(too_big);
if (status == amc_ace_overflow) fail(overflow);
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 */
}
INTERNET-DRAFT expires 2002-Jan-11