====== Base64 エンコード/デコード ======

===== 文字列の Base64 エンコード/デコード =====
**[[https://developer.mozilla.org/ja/docs/Web/API/WindowBase64/btoa|encodedData = window.btoa(stringToEncode)]]** - Web API | MDN\\
**[[https://developer.mozilla.org/ja/docs/Web/API/WindowBase64/atob|decodedData = window.atob(encodedData)]]** - Web API | MDN\\
を利用した例は以下のようになる。\\
<WRAP prewrap 100% #js_term>
</WRAP>
<code javascript>
js > btoa('Hello World!')
SGVsbG8gV29ybGQh
js > atob('SGVsbG8gV29ybGQh')
Hello World!
</code>
<WRAP prewrap 100%>
<code javascript>
js > btoa('🐍')
InvalidCharacterError: Failed to execute 'btoa' on 'Window': The string to be encoded contains characters outside of the Latin1 range.
js > btoa(unescape(encodeURIComponent('🐍')))
8J+QjQ==
js > decodeURIComponent(escape(atob('8J+QjQ==')))
🐍
</code>
</WRAP>

===== byteArray を自前でエンコード/デコードするコード =====
<file javascript base64.js>
class Base64 {
    static base64Encode(byteArray) {
        let   base64        = '';
        const encodings     = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/';
        
        const bytes         = new Uint8Array(byteArray);
        const byteLength    = bytes.byteLength;
        const byteRemainder = byteLength % 3;
        const mainLength    = byteLength - byteRemainder;
        
        let a, b, c, d;
        let chunk;
        
        // Main loop deals with bytes in chunks of 3
        for (var i = 0; i < mainLength; i = i + 3) {
            // Combine the three bytes into a single integer
            chunk = (bytes[i] << 16) | (bytes[i + 1] << 8) | bytes[i + 2];
            
            // Use bitmasks to extract 6-bit segments from the triplet
            a = (chunk & 16515072) >> 18; // 16515072 = (2^6 - 1) << 18
            b = (chunk & 258048)   >> 12; // 258048   = (2^6 - 1) << 12
            c = (chunk & 4032)     >>  6; // 4032     = (2^6 - 1) << 6
            d = chunk & 63;               // 63       = 2^6 - 1
            
            // Convert the raw binary segments to the appropriate ASCII encoding
            base64 += encodings[a] + encodings[b] + encodings[c] + encodings[d];
        }
        
        // Deal with the remaining bytes and padding
        if (byteRemainder == 1) {
            chunk = bytes[mainLength];
            
            a = (chunk & 252) >> 2; // 252 = (2^6 - 1) << 2
            
            // Set the 4 least significant bits to zero
            b = (chunk & 3)   << 4; // 3   = 2^2 - 1
            
            base64 += encodings[a] + encodings[b] + '==';
        }
        else if (byteRemainder == 2) {
            chunk = (bytes[mainLength] << 8) | bytes[mainLength + 1];
            
            a = (chunk & 64512) >> 10; // 64512 = (2^6 - 1) << 10
            b = (chunk & 1008)  >>  4; // 1008  = (2^6 - 1) << 4
            
            // Set the 2 least significant bits to zero
            c = (chunk & 15)    <<  2; // 15    = 2^4 - 1
            
            base64 += encodings[a] + encodings[b] + encodings[c] + '=';
        }
        
        return base64;
    }
    
    static base64Decode(base64Str) {
        const _keyStr = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
        // get last chars to see if are valid
        const lkey1 = _keyStr.indexOf(base64Str.charAt(base64Str.length - 1));
        const lkey2 = _keyStr.indexOf(base64Str.charAt(base64Str.length - 2));
        
        let bytes = (base64Str.length / 4) * 3;
        if (lkey1 == 64) bytes--; // padding chars, so skip
        if (lkey2 == 64) bytes--; // padding chars, so skip
        
        let byteArray = [];
        let chr1, chr2, chr3;
        let enc1, enc2, enc3, enc4;
        let i = 0;
        let j = 0;
        
        base64Str = base64Str.replace(/[^A-Za-z0-9\+\/\=]/g, "");
        
        for (i = 0; i < bytes; i += 3) {
            //get the 3 octects in 4 ascii chars
            enc1 = _keyStr.indexOf(base64Str.charAt(j++));
            enc2 = _keyStr.indexOf(base64Str.charAt(j++));
            enc3 = _keyStr.indexOf(base64Str.charAt(j++));
            enc4 = _keyStr.indexOf(base64Str.charAt(j++));
            
            chr1 = (enc1 << 2) | (enc2 >> 4);
            chr2 = ((enc2 & 15) << 4) | (enc3 >> 2);
            chr3 = ((enc3 & 3) << 6) | enc4;
            
            byteArray[i] = chr1;
            if (enc3 != 64) byteArray[i + 1] = chr2;
            if (enc4 != 64) byteArray[i + 2] = chr3;
        }
        
        return byteArray;
    }
}
</file>