source: imaps-frontend/node_modules/es-abstract/2018/AdvanceStringIndex.js

'use strict';

var isInteger = require('../helpers/isInteger');
var isLeadingSurrogate = require('../helpers/isLeadingSurrogate');
var isTrailingSurrogate = require('../helpers/isTrailingSurrogate');
var MAX_SAFE_INTEGER = require('../helpers/maxSafeInteger');

var $TypeError = require('es-errors/type');

var $charCodeAt = require('call-bind/callBound')('String.prototype.charCodeAt');

// https://262.ecma-international.org/6.0/#sec-advancestringindex

module.exports = function AdvanceStringIndex(S, index, unicode) {
        if (typeof S !== 'string') {
                throw new $TypeError('Assertion failed: `S` must be a String');
        }
        if (!isInteger(index) || index < 0 || index > MAX_SAFE_INTEGER) {
                throw new $TypeError('Assertion failed: `length` must be an integer >= 0 and <= 2**53');
        }
        if (typeof unicode !== 'boolean') {
                throw new $TypeError('Assertion failed: `unicode` must be a Boolean');
        }
        if (!unicode) {
                return index + 1;
        }
        var length = S.length;
        if ((index + 1) >= length) {
                return index + 1;
        }

        var first = $charCodeAt(S, index);
        if (!isLeadingSurrogate(first)) {
                return index + 1;
        }

        var second = $charCodeAt(S, index + 1);
        if (!isTrailingSurrogate(second)) {
                return index + 1;
        }

        return index + 2;
};