35 lines
1.2 KiB
JavaScript
35 lines
1.2 KiB
JavaScript
export function unfinishedBytes(u, len, enc) {
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switch (enc) {
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case 'utf-8': {
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// 0-3
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let p = 0
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while (p < 2 && p < len && (u[len - p - 1] & 0xc0) === 0x80) p++ // go back 0-2 trailing bytes
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if (p === len) return 0 // no space for lead
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const l = u[len - p - 1]
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if (l < 0xc2 || l > 0xf4) return 0 // not a lead
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if (p === 0) return 1 // nothing to recheck, we have only lead, return it. 2-byte must return here
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if (l < 0xe0 || (l < 0xf0 && p >= 2)) return 0 // 2-byte, or 3-byte or less and we already have 2 trailing
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const lower = l === 0xf0 ? 0x90 : l === 0xe0 ? 0xa0 : 0x80
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const upper = l === 0xf4 ? 0x8f : l === 0xed ? 0x9f : 0xbf
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const n = u[len - p]
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return n >= lower && n <= upper ? p + 1 : 0
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}
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case 'utf-16le':
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case 'utf-16be': {
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// 0-3
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let p = 0
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if (len % 2 !== 0) p++ // uneven bytes
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const l = len - p - 1
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if (len - p >= 2) {
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const last = enc === 'utf-16le' ? (u[l] << 8) ^ u[l - 1] : (u[l - 1] << 8) ^ u[l]
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if (last >= 0xd8_00 && last < 0xdc_00) p += 2 // lone lead
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}
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return p
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}
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}
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throw new Error('Unsupported encoding')
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}
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