ArnaudBuchholz/gpf-js

/**
 * @file btoa polyfill
 * @since 0.2.9
 */
/*#ifndef(UMD)*/
"use strict";
/*global _GPF_12_BITS*/ // 12
/*global _GPF_18_BITS*/ // 18
/*global _GPF_1_BYTE*/ // 1
/*global _GPF_2_BYTES*/ // 2
/*global _GPF_6_BITS*/ // 6
/*global _GPF_6_BITS_MASK*/ // 63
/*global _GPF_START*/ // 0
/*global _gpfBase64*/ // Base64 encoding string
/*global _gpfMaxUnsignedByte*/ // 255
/*exported _gpfBtoa*/ // btoa polyfill
/*#endif*/

var _GPF_BTOA_MAX_PADDING = 3,
    _GPF_BTOA_INDEX_INCREMENT = 3;

function _gpfBtoaCheck (stringToEncode, index) {
    var value = stringToEncode.charCodeAt(index);
    if (value > _gpfMaxUnsignedByte) {
        throw new TypeError("The string to be encoded contains characters outside of the Latin1 range.");
    }
    return value;
}

function _gpfBtoaRead (input, from) {
    var index = from,
        a,
        b,
        c;
    a = _gpfBtoaCheck(input, index++);
    b = _gpfBtoaCheck(input, index++);
    c = _gpfBtoaCheck(input, index++);
    return a << _GPF_2_BYTES | b << _GPF_1_BYTE | c;
}

function _gpfBtoaEncodeChar (bitmap, shift, mask) {
    return _gpfBase64.charAt(bitmap >> shift & mask);
}

function _gpfBtoaEncode (bitmap) {
    return _gpfBtoaEncodeChar(bitmap, _GPF_18_BITS, _GPF_6_BITS_MASK)
        + _gpfBtoaEncodeChar(bitmap, _GPF_12_BITS, _GPF_6_BITS_MASK)
        + _gpfBtoaEncodeChar(bitmap, _GPF_6_BITS, _GPF_6_BITS_MASK)
        + _gpfBtoaEncodeChar(bitmap, _GPF_START, _GPF_6_BITS_MASK);
}

function _gpfBtoa (stringToEncode) {
    var index = 0,
        result = "",
        rest = stringToEncode.length % _GPF_BTOA_MAX_PADDING; // To determine the final padding
    for (; index < stringToEncode.length; index += _GPF_BTOA_INDEX_INCREMENT) {
        result += _gpfBtoaEncode(_gpfBtoaRead(stringToEncode, index));
    }
    // If there's need of padding, replace the last 'A's with equal signs
    if (rest) {
        return result.slice(_GPF_START, rest - _GPF_BTOA_MAX_PADDING) + "===".substring(rest);
    }
    return result;
}

/*#ifndef(UMD)*/

gpf.internals._gpfBtoa = _gpfBtoa;

/*#endif*/