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fix(webidl): wrap negative values in ConvertToInt via two's complement #5107

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fix(webidl): wrap negative values in ConvertToInt via two's complement #5107

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11 changes: 10 additions & 1 deletion lib/web/webidl/index.js
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Original file line number Diff line number Diff line change
Expand Up @@ -268,7 +268,16 @@ webidl.util.ConvertToInt = function (V, bitLength, signedness, flags) {
x = webidl.util.IntegerPart(x)

// 10. Set x to x modulo 2^bitLength.
x = x % Math.pow(2, bitLength)
// Use the ECMA-262 "modulo" definition whose result has the sign of the
// divisor, so negative inputs wrap via two's complement instead of being
// returned as-is. Normalize a potential -0 to +0.
const y = Math.pow(2, bitLength)
x = x % y
if (x < 0) {
x += y
} else if (x === 0) {
x = 0
}

// 11. If signedness is "signed" and x ≥ 2^(bitLength − 1),
// then return x − 2^bitLength.
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13 changes: 12 additions & 1 deletion test/webidl/util.js
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Expand Up @@ -72,7 +72,7 @@ test('webidl.util.ConvertToInt(V)', () => {
assert.equal(ConvertToInt(-max - 1, 64, 'signed'), -max, 'signed neg')

assert.equal(ConvertToInt(max + 1, 64, 'unsigned'), max + 1, 'unsigned pos')
assert.equal(ConvertToInt(-max - 1, 64, 'unsigned'), -max - 1, 'unsigned neg')
assert.equal(ConvertToInt(-max - 1, 64, 'unsigned'), 2 ** 64 - max, 'unsigned neg wraps to two\'s complement')

for (const signedness of ['signed', 'unsigned']) {
assert.equal(ConvertToInt(Infinity, 64, signedness), 0)
Expand Down Expand Up @@ -144,4 +144,15 @@ test('webidl.util.ConvertToInt(V)', () => {
-(2 ** 31),
'32-bit signed EnforceRange lower bound'
)

// Negative inputs must wrap via two's complement instead of being returned
// as-is. These mirror the corresponding typed-array behavior, e.g.
// new Uint8Array([-3])[0] === 253, new Int8Array([-200])[0] === 56.
assert.equal(ConvertToInt(-3, 8, 'unsigned'), 253, '8-bit unsigned wraps -3 to 253')
assert.equal(ConvertToInt(-1, 32, 'unsigned'), 4294967295, '32-bit unsigned wraps -1')
assert.equal(ConvertToInt(-200, 8, 'signed'), 56, '8-bit signed wraps -200 to 56')
assert.ok(
Object.is(ConvertToInt(-256, 8, 'unsigned'), 0),
'modulo wrap of negative multiple of 2^bitLength returns +0, not -0'
)
})
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