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;
}