LukasMeine / vigilance

var token = '%[a-f0-9]{2}';

var singleMatcher = new RegExp(token, 'gi');

var multiMatcher = new RegExp('(' + token + ')+', 'gi');

function decodeComponents(components, split) {

	try {

		// Try to decode the entire string first

		return decodeURIComponent(components.join(''));

		// Do nothing

	}

	if (components.length === 1) {

		return components;

	}

	split = split || 1;

	// Split the array in 2 parts

	var left = components.slice(0, split);

	var right = components.slice(split);

	return Array.prototype.concat.call([], decodeComponents(left), decodeComponents(right));

}

function decode(input) {

	try {

		return decodeURIComponent(input);

	} catch (err) {

		var tokens = input.match(singleMatcher);

		for (var i = 1; i < tokens.length; i++) {

			input = decodeComponents(tokens, i).join('');

			tokens = input.match(singleMatcher);

		}

		return input;

	}

}

function customDecodeURIComponent(input) {

	// Keep track of all the replacements and prefill the map with the `BOM`

	var replaceMap = {

		'%FE%FF': '\uFFFD\uFFFD',

		'%FF%FE': '\uFFFD\uFFFD'

	};

	var match = multiMatcher.exec(input);

	while (match) {

		try {

			// Decode as big chunks as possible

			replaceMap[match[0]] = decodeURIComponent(match[0]);

		} catch (err) {

			var result = decode(match[0]);

			if (result !== match[0]) {

				replaceMap[match[0]] = result;

			}

		}

		match = multiMatcher.exec(input);

	}

	// Add `%C2` at the end of the map to make sure it does not replace the combinator before everything else

	replaceMap['%C2'] = '\uFFFD';

	var entries = Object.keys(replaceMap);

	for (var i = 0; i < entries.length; i++) {

		// Replace all decoded components

		var key = entries[i];

		input = input.replace(new RegExp(key, 'g'), replaceMap[key]);

	}

	return input;

}