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// SPDX-License-Identifier: MPL-2.0
// (c) Hare authors <https://harelang.org>
export type decoder = struct {
offs: size,
src: []u8,
};
// Initializes a new UTF-8 decoder.
export fn decode(src: []u8) decoder = decoder {
src = src,
offs = 0,
};
const masks: [2][8]u8 = [
[0x3f, 0x3f, 0x3f, 0x3f, 0x3f, 0x3f, 0x3f, 0x3f],
[0x7f, 0x1f, 0x0f, 0x0f, 0x0f, 0x07, 0x07, 0x07],
];
// Returns the next rune from a decoder. done is returned when there are no
// remaining codepoints.
export fn next(d: *decoder) (rune | done | more | invalid) = {
if (d.offs == len(d.src)) {
return done;
};
// from https://github.com/skeeto/scratch/blob/master/parsers/utf8_decode.c
// See https://bjoern.hoehrmann.de/utf-8/decoder/dfa/
// and https://nullprogram.com/blog/2020/12/31/ for an explanation of
// the algorithm.
let next = 0, state = 0;
let r = 0u32;
for (d.offs < len(d.src); d.offs += 1) {
next = table[state][d.src[d.offs]];
r = r << 6 | d.src[d.offs] & masks[(state - 1): uint >> 31][next & 7];
if (next <= 0) {
d.offs += 1;
return if (next == 0) r: rune else invalid;
};
state = next;
};
return more;
};
// Returns the previous rune from a decoder. done is returned when there are no
// previous codepoints.
export fn prev(d: *decoder) (rune | done | more | invalid) = {
if (d.offs == 0) {
return done;
};
let n = d.offs;
d.offs -= 1;
for (d.offs < len(d.src); d.offs -= 1) {
if (table[0][d.src[d.offs]] != -1) {
let t = d.offs;
defer d.offs = t;
let r = next(d);
return if (n != d.offs || r is more) invalid else r;
};
if (n - d.offs == 4) {
// Too many continuation bytes in a row
return invalid;
};
};
return more;
};
@test fn decode() void = {
const input: [_]u8 = [
0xE3, 0x81, 0x93, 0xE3, 0x82, 0x93, 0xE3, 0x81,
0xAB, 0xE3, 0x81, 0xA1, 0xE3, 0x81, 0xAF, 0x00,
];
assert(validate(input) is void);
const expected = ['こ', 'ん', 'に', 'ち', 'は', '\0'];
let decoder = decode(input);
for (let i = 0z; i < len(expected); i += 1) {
match (next(&decoder)) {
case (invalid | more | done) =>
abort();
case let r: rune =>
assert(r == expected[i]);
};
};
assert(next(&decoder) is done);
assert(decoder.offs == len(decoder.src));
for (let i = 0z; i < len(expected); i += 1) {
match (prev(&decoder)) {
case (invalid | more | done) =>
abort();
case let r: rune =>
assert(r == expected[len(expected) - i - 1]);
};
};
assert(prev(&decoder) is done);
const inv: [_]u8 = [0xA0, 0xA1];
decoder = decode(inv);
assert(next(&decoder) is invalid);
decoder.offs = 2;
assert(prev(&decoder) is more);
assert(validate(inv) is invalid);
const incomplete: [_]u8 = [0xE3, 0x81];
decoder = decode(incomplete);
assert(next(&decoder) is more);
decoder.offs = 2;
assert(prev(&decoder) is invalid);
assert(validate(incomplete) is invalid);
const surrogate: [_]u8 = [0xED, 0xA0, 0x80];
decoder = decode(surrogate);
assert(next(&decoder) is invalid);
decoder.offs = 3;
assert(prev(&decoder) is invalid);
assert(validate(surrogate) is invalid);
const overlong: [_]u8 = [0xF0, 0x82, 0x82, 0xAC];
decoder = decode(overlong);
assert(next(&decoder) is invalid);
decoder.offs = 4;
assert(prev(&decoder) is invalid);
assert(validate(overlong) is invalid);
const badcont: [_]u8 = [0xC2, 0xFF];
decoder = decode(badcont);
assert(next(&decoder) is invalid);
assert(validate(badcont) is invalid);
const extracont: [_]u8 = [0xC2, 0xA3, 0x95];
decoder = decode(extracont);
decoder.offs = 3;
assert(prev(&decoder) is invalid);
assert(validate(extracont) is invalid);
const maxinrange: [_]u8 = [0xF4, 0x8F, 0xBF, 0xBF];
decoder = decode(maxinrange);
match (next(&decoder)) {
case let r: rune =>
assert(r == 0x10FFFFu32: rune);
case => abort();
};
decoder.offs = 4;
match (prev(&decoder)) {
case let r: rune =>
assert(r == 0x10FFFFu32: rune);
case => abort();
};
const minoutofrange: [_]u8 = [0xF5, 0x94, 0x80, 0x80];
decoder = decode(minoutofrange);
assert(next(&decoder) is invalid);
decoder.offs = 4;
assert(prev(&decoder) is invalid);
};
// Returns void if a given byte slice contains only valid UTF-8 sequences,
// otherwise returns invalid.
export fn validate(src: []u8) (void | invalid) = {
let state = 0;
for (let i = 0z; i < len(src) && state >= 0; i += 1) {
state = table[state][src[i]];
};
return if (state == 0) void else invalid;
};
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