78 lines
2.1 KiB
TypeScript
78 lines
2.1 KiB
TypeScript
import { Context, Recogniser } from '.';
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import match, { Match } from '../match';
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export default class Utf8 implements Recogniser {
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name() {
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return 'UTF-8';
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}
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match(det: Context): Match | null {
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let hasBOM = false,
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numValid = 0,
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numInvalid = 0,
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trailBytes = 0,
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confidence;
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const input = det.fRawInput;
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if (
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det.fRawLength >= 3 &&
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(input[0] & 0xff) == 0xef &&
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(input[1] & 0xff) == 0xbb &&
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(input[2] & 0xff) == 0xbf
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) {
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hasBOM = true;
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}
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// Scan for multi-byte sequences
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for (let i = 0; i < det.fRawLength; i++) {
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const b = input[i];
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if ((b & 0x80) == 0) continue; // ASCII
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// Hi bit on char found. Figure out how long the sequence should be
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if ((b & 0x0e0) == 0x0c0) {
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trailBytes = 1;
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} else if ((b & 0x0f0) == 0x0e0) {
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trailBytes = 2;
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} else if ((b & 0x0f8) == 0xf0) {
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trailBytes = 3;
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} else {
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numInvalid++;
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if (numInvalid > 5) break;
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trailBytes = 0;
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}
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// Verify that we've got the right number of trail bytes in the sequence
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for (;;) {
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i++;
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if (i >= det.fRawLength) break;
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if ((input[i] & 0xc0) != 0x080) {
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numInvalid++;
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break;
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}
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if (--trailBytes == 0) {
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numValid++;
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break;
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}
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}
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}
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// Cook up some sort of confidence score, based on presense of a BOM
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// and the existence of valid and/or invalid multi-byte sequences.
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confidence = 0;
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if (hasBOM && numInvalid == 0) confidence = 100;
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else if (hasBOM && numValid > numInvalid * 10) confidence = 80;
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else if (numValid > 3 && numInvalid == 0) confidence = 100;
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else if (numValid > 0 && numInvalid == 0) confidence = 80;
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else if (numValid == 0 && numInvalid == 0)
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// Plain ASCII.
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confidence = 10;
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else if (numValid > numInvalid * 10)
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// Probably corrupt utf-8 data. Valid sequences aren't likely by chance.
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confidence = 25;
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else return null;
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return match(det, this, confidence);
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}
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}
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