{ depot, lib, ... }:
let
inherit (depot.users.sterni.nix)
char
flow
fun
int
string
util
;
/* (Internal) function to determine the amount
bytes left in a UTF-8 byte sequence from the
first byte.
This function will throw if the given first
byte is ill-formed, but will not detect all
cases of ill-formed-ness.
Based on table 3-6. from The Unicode Standard,
Version 13.0, section 3.9.
Type: integer -> integer
*/
byteCount = i: flow.cond [
[ (int.bitAnd i 128 == 0) 1 ]
[ (int.bitAnd i 224 == 192) 2 ]
[ (int.bitAnd i 240 == 224) 3 ]
[ (int.bitAnd i 248 == 240) 4 ]
[ true (builtins.throw "Ill-formed first byte ${int.toHex i}") ]
];
/* (Internal) function to check if a given byte in
an UTF-8 byte sequence is well-formed.
Based on table 3-7. from The Unicode Standard,
Version 13.0, section 3.9.
Type: integer -> integer -> integer -> bool
*/
wellFormedByte =
# first byte's integer value
first:
# byte position as an index starting with 0
pos:
let
defaultRange = int.inRange 128 191;
secondBytePredicate = flow.switch first [
[ (int.inRange 194 223) defaultRange ] # C2..DF
[ 224 (int.inRange 160 191) ] # E0
[ (int.inRange 225 236) defaultRange ] # E1..EC
[ 237 (int.inRange 128 159) ] # ED
[ (int.inRange 238 239) defaultRange ] # EE..EF
[ 240 (int.inRange 144 191) ] # F0
[ (int.inRange 241 243) defaultRange ] # F1..F3
[ 244 (int.inRange 128 143) ] # F4
[ (fun.const true) null ]
];
firstBytePredicate = byte: assert first == byte;
first < 128 || secondBytePredicate != null;
in
# Either ASCII or in one of the byte ranges of Table 3-6.
if pos == 0 then firstBytePredicate
# return predicate according to Table 3-6.
else if pos == 1 then assert secondBytePredicate != null; secondBytePredicate
# 3rd and 4th byte have only one validity rule
else defaultRange;
/* Iteration step for decoding an UTF-8 byte sequence.
It decodes incrementally, i. e. it has to be fed
one byte at a time and then returns either a
new state or a final result.
If the resulting attribute set contains the attribute
result, it is finished and the decoded codepoint is
contained in that attribute. In all other cases,
pass the returned set to step again along with
a new byte. The initial state to pass is the empty
set.
Extra attributes are always passed through, so you
can pass extra state. Be sure not to use result,
pos, code, first or count.
This function will throw with a fairly detailed
message if it encounters ill-formed bytes.
The implementation is based on The Unicode Standard,
Version 13.0, section 3.9, especially table 3-6.
Type: { ... } -> string -> ({ result :: integer, ... } | { ... })
Example: utf8.step {} "f"
=> { result = 102; }
*/
step = { pos ? 0, code ? 0, ... }@args: byte:
let
value = char.ord byte;
# first byte is context for well-formed-ness
first = args.first or value;
count = args.count or (byteCount first);
newCode =
if count == 1
then int.bitAnd 127 first # ascii character
else # multi byte UTF-8 sequence
let
# Calculate the bitmask for extracting the
# codepoint data in the current byte.
# If the codepoint is not ASCII, the bits
# used for codepoint data differ depending
# on the byte position and overall byte
# count. The first byte always ignores
# the (count + 1) most significant bits.
# For all subsequent bytes, the 2 most
# significant bits need to be ignored.
# See also table 3-6.
mask =
if pos == 0
then int.exp 2 (8 - (count + 1)) - 1
else 63;
# UTF-8 uses the 6 least significant bits in all
# subsequent bytes after the first one. Therefore
# We can determine the amount we need to shift
# the current value by the amount of bytes left.
offset = (count - (pos + 1)) * 6;
in
code + (int.bitShiftL (int.bitAnd mask value) offset);
illFormedMsg =
"Ill-formed byte ${int.toHex value} at position ${toString pos} in ${toString count} byte UTF-8 sequence";
in
if !(wellFormedByte first pos value) then builtins.throw illFormedMsg
else if pos + 1 == count
then (builtins.removeAttrs args [ # allow extra state being passed through
"count"
"code"
"pos"
"first"
]) // { result = newCode; }
else (builtins.removeAttrs args [ "result" ]) // {
inherit count first;
code = newCode;
pos = pos + 1;
};
/* Decode an UTF-8 string into a list of codepoints.
Throws if the string is ill-formed UTF-8.
Type: string -> [ integer ]
*/
# TODO(sterni): option to fallback to replacement char instead of failure
decode = s:
let
stringLength = builtins.stringLength s;
iterResult = builtins.genericClosure {
startSet = [
{
key = "start";
stringIndex = -1;
state = {};
codepoint = null;
}
];
operator = { state, stringIndex, ... }:
let
# updated values for current iteration step
newIndex = stringIndex + 1;
newState = step state (builtins.substring newIndex 1 s);
in lib.optional (newIndex < stringLength) {
# unique keys to make genericClosure happy
key = toString newIndex;
# carryover state for the next step
stringIndex = newIndex;
state = newState;
# actual payload for later, steps with value null are filtered out
codepoint = newState.result or null;
};
};
in
# extract all steps that yield a code point into a list
builtins.map (v: v.codepoint) (
builtins.filter (
{ codepoint, stringIndex, state, ... }:
let
# error message in case we are missing bytes at the end of input
earlyEndMsg =
if state ? count && state ? pos
then "Missing ${toString (with state; count - pos)} bytes at end of input"
else "Unexpected end of input";
in
# filter out all iteration steps without a codepoint value
codepoint != null
# if we are at the iteration step of a non-empty input string, throw
# an error if no codepoint was returned, as it indicates an incomplete
# UTF-8 sequence.
|| (stringLength > 0 && stringIndex == stringLength - 1 && throw earlyEndMsg)
) iterResult
);
encodeCodepoint = cp:
let
# Find the amount of bytes needed to encode the given codepoint.
# Note that this doesn't check if the Unicode codepoint is allowed,
# but rather allows all theoretically UTF-8-encodeable ones.
count = flow.switch cp [
[ (int.inRange 0 127) 1 ] # 00000000 0xxxxxxx
[ (int.inRange 128 2047) 2 ] # 00000yyy yyxxxxxx
[ (int.inRange 2048 65535) 3 ] # zzzzyyyy yyxxxxxx
[ (int.inRange 65536 2097151) 4 ] # 000uuuuu zzzzyyyy yyxxxxxx
];
# Extract the bit ranges x, y, z and u from the given codepoint
# according to Table 3-6. from The Unicode Standard, Version 13.0,
# section 3.9. u is split into uh and ul since they are used in
# different bytes in the end.
components = lib.mapAttrs (_: { mask, offset }:
int.bitAnd (int.bitShiftR cp offset) mask
) {
x = {
mask = if count > 1 then 63 else 127;
offset = 0;
};
y = {
mask = if count > 2 then 63 else 31;
offset = 6;
};
z = {
mask = 15;
offset = 12;
};
# u which belongs into the second byte
ul = {
mask = 3;
offset = 16;
};
# u which belongs into the first byte
uh = {
mask = 7;
offset = 18;
};
};
inherit (components) x y z ul uh;
# Finally construct the byte sequence for the given codepoint. This is
# usually done by using the component and adding a few bits as a prefix
# which depends on the length of the sequence. The longer the sequence,
# the further back each component is pushed. To simplify this, we
# always construct a 4 element list and take the last `count` elements.
# Thanks to laziness the bogus values created by this are never evaluated.
#
# Based on table 3-6. from The Unicode Standard,
# Version 13.0, section 3.9.
bytes = lib.sublist (4 - count) count [
# 11110uuu
(uh + 240)
# 10uuzzzz or 1110zzzz
(z + (if count > 3 then 128 + int.bitShiftL ul 4 else 224))
# 10yyyyyy or 110yyyyy
(y + (if count > 2 then 128 else 192))
# 10xxxxxx or 0xxxxxxx
(x + (if count > 1 then 128 else 0))
];
in string.fromBytes bytes;
/* Encode a list of Unicode codepoints into an UTF-8 string.
Type: [ integer ] -> string
*/
encode = lib.concatMapStrings encodeCodepoint;
in {
inherit
encode
decode
step
;
}
