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Fix UTF-32 surrogate handling in toWellFormed #53

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Fix UTF-32 surrogate handling in toWellFormed #53

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1d07b6a
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Copilot Jan 28, 2026
9305e6b
Fix UTF-32 surrogate handling bug
Copilot Jan 28, 2026
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14 changes: 3 additions & 11 deletions fallback/utf32.js
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Original file line number Diff line number Diff line change
Expand Up @@ -51,18 +51,10 @@ export function toWellFormed(u32) {
for (let i = 0; i < length; i++) {
const x = u32[i]
if (x >= 0xd8_00) {
// In UTF-32, all surrogate codepoints (0xD800-0xDFFF) are invalid
// Unlike UTF-16, there are no surrogate pairs in UTF-32
if (x < 0xe0_00) {
// An unexpected trail or a lead at the very end of input
if (x > 0xdb_ff || i + 1 >= length) {
u32[i] = replacementCodepoint
} else {
const next = u32[i + 1] // Process valid pairs immediately
if (next < 0xdc_00 || next >= 0xe0_00) {
u32[i] = replacementCodepoint
} else {
i++ // consume next
}
}
u32[i] = replacementCodepoint
} else if (x >= 0x11_00_00) {
// also fix out-of-range in the same pass, both are unlikely
u32[i] = replacementCodepoint
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