{ depot, lib, ... }:
let
# TODO(sterni): encode
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.
Throws if the first byte is invalid.
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;
in
# The first byte is either ASCII which requires no checks
# or we automatically check it when we check the subsequent
# bytes. The downside is that this may generate bad error
# messages in very rare cases.
if pos == 0
then lib.const true
else if pos > 1 # 3rd and 4th byte have only one validity rule
then defaultRange
else assert pos == 1; 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)
(builtins.throw "Invalid first byte ${int.toHex first}")
]
];
/* 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
iter = { codes ? [], ... }@args: byte:
let
res = step args byte;
in
# foldl' forceValues the calculate value only at the end
# this makes the thunk grow large enough to cause a stack
# overflow with sufficiently large strings. To avoid this
# we always deepSeq the result which also keeps memory
# usage of decode reasonable.
builtins.deepSeq res
(if res ? result
then res // {
codes = codes ++ [ res.result ];
}
else res);
iterResult =
builtins.foldl' iter {} (string.toChars s);
earlyEndMsg =
if iterResult ? count && iterResult ? pos
then "Missing ${toString (with iterResult; count - pos)} bytes at end of input"
else "Unexpected end of input";
in
if iterResult ? result
then iterResult.codes
else builtins.throw earlyEndMsg;
/* Decodes an UTF-8 string, but doesn't throw on error.
Instead it returns null.
Type: string -> ( [ integer ] | null)
*/
decodeSafe = s:
let
res = builtins.tryEval (decode s);
in
if res.success
then res.value
else null;
in {
inherit
decode
decodeSafe
step
;
}
