html.ts

import htmlEntities from './entities';

const windows1252 = [
	8364,
	129,
	8218,
	402,
	8222,
	8230,
	8224,
	8225,
	710,
	8240,
	352,
	8249,
	338,
	141,
	381,
	143,
	144,
	8216,
	8217,
	8220,
	8221,
	8226,
	8211,
	8212,
	732,
	8482,
	353,
	8250,
	339,
	157,
	382,
	376,
];
const entityPattern = new RegExp(
	`&(#?(?:x[\\w\\d]+|\\d+|${Object.keys(htmlEntities).join('|')}));?`,
	'g'
);

export function decodeCharacterReferences(html: string) {
	return html.replace(entityPattern, (match, entity) => {
		let code;

		// Handle named entities
		if (entity[0] !== '#') {
			code = htmlEntities[entity];
		} else if (entity[1] === 'x') {
			code = parseInt(entity.substring(2), 16);
		} else {
			code = parseInt(entity.substring(1), 10);
		}

		if (!code) {
			return match;
		}

		return String.fromCodePoint(validateCode(code));
	});
}

const NUL = 0;

// some code points are verboten. If we were inserting HTML, the browser would replace the illegal
// code points with alternatives in some cases - since we're bypassing that mechanism, we need
// to replace them ourselves
//
// Source: http://en.wikipedia.org/wiki/Character_encodings_in_HTML#Illegal_characters
function validateCode(code: number) {
	// line feed becomes generic whitespace
	if (code === 10) {
		return 32;
	}

	// ASCII range. (Why someone would use HTML entities for ASCII characters I don't know, but...)
	if (code < 128) {
		return code;
	}

	// code points 128-159 are dealt with leniently by browsers, but they're incorrect. We need
	// to correct the mistake or we'll end up with missing € signs and so on
	if (code <= 159) {
		return windows1252[code - 128];
	}

	// basic multilingual plane
	if (code < 55296) {
		return code;
	}

	// UTF-16 surrogate halves
	if (code <= 57343) {
		return NUL;
	}

	// rest of the basic multilingual plane
	if (code <= 65535) {
		return code;
	}

	// supplementary multilingual plane 0x10000 - 0x1ffff
	if (code >= 65536 && code <= 131071) {
		return code;
	}

	// supplementary ideographic plane 0x20000 - 0x2ffff
	if (code >= 131072 && code <= 196607) {
		return code;
	}

	return NUL;
}