/**
 * This implementation is encapsulated as Base64Format in the web.xtclang.org module,
 * but the module is currently in flux, so the implementation is included "inline" here.
 */
module Base64 {
    @Inject Console console;
    void run() {
        String orig  = \|VG8gZXJyIGlzIGh1bWFuLCBidXQgdG8gcmVhbGx5IGZvdWwgdGhpbmdzIHVwIH\
                        |lvdSBuZWVkIGEgY29tcHV0ZXIuCiAgICAtLSBQYXVsIFIuIEVocmxpY2g=
                       ;
        Byte[] bytes = decode(orig);
        String text  = encode(bytes, pad=True);
        assert text == orig;
        console.print($"base64={text}, bytes={bytes}");
    }

    static Byte[] read(Iterator<Char> stream) {
        Int    charLen    = 0;
        charLen := stream.knownSize();
        Byte[] byteBuf    = new Byte[](charLen * 6 / 8);
        Byte   prevBits   = 0;
        Int    prevCount  = 0;
        while (Char ch := stream.next()) {
            if (Byte newBits := isBase64(ch, assertTrash=True)) {
                if (prevCount == 0) {
                    prevBits  = newBits;
                    prevCount = 6;
                } else {
                    byteBuf.add((prevBits << 8-prevCount) | (newBits >> prevCount-2));
                    prevBits   = newBits;
                    prevCount -= 2;
                }
            }
        }

        return byteBuf.freeze(True);
    }

    static void write(Byte[] value, Appender<Char> stream, Boolean pad=False, Int? lineLength=Null) {
        lineLength ?:= Int.MaxValue;

        Int    lineOffset = 0;
        Int    totalChars = 0;
        Byte   prevByte   = 0;
        Int    prevCount  = 0;          // number of leftover bits from the previous byte
        Int    byteOffset = 0;
        Int    byteLength = value.size;
        while (True) {
            // glue together the next six bits, which will create one character of output
            Byte sixBits;
            if (byteOffset >= byteLength) {
                if (prevCount == 0) {
                    break;
                }
                sixBits   = prevByte << 6 - prevCount;
                prevCount = 0;
            } else if (prevCount == 6) {
                sixBits   = prevByte << 6 - prevCount;
                prevCount = 0;
            } else {
                Byte nextByte = value[byteOffset++];
                sixBits    = (prevByte << 6 - prevCount) | (nextByte >> 2 + prevCount);
                prevByte   = nextByte;
                prevCount += 2;
            }

            if (lineOffset >= lineLength) {
                stream.add('\r').add('\n');
                totalChars += lineOffset;
                lineOffset  = 0;
            }

            stream.add(base64(sixBits & 0b111111));
            ++lineOffset;
        }

        if (pad) {
            totalChars += lineOffset;
            for (Int i = 0, Int padCount = 4 - (totalChars & 0b11) & 0b11; i < padCount; ++i) {
                if (lineOffset >= lineLength) {
                    stream.add('\r').add('\n');
                    lineOffset  = 0;
                }

                stream.add('=');
                ++lineOffset;
            }
        }
    }

    static String encode(Byte[] value, Boolean pad=False, Int? lineLength=Null) {
        // calculate buffer size
        Int byteLen = value.size;
        Int charLen = (byteLen * 8 + 5) / 6;
        if (pad) {
            charLen += 4 - (charLen & 0b11) & 0b11;
        }
        if (lineLength != Null) {
            charLen += ((charLen + lineLength - 1) / lineLength - 1).maxOf(0) * 2;
        }

        StringBuffer charBuf = new StringBuffer(charLen);
        write(value, charBuf, pad, lineLength);
        return charBuf.toString();
    }

    static Byte[] decode(String text) {
        Int    charLen    = text.size;
        Byte[] byteBuf    = new Byte[](charLen * 6 / 8);
        Byte   prevBits   = 0;
        Int    prevCount  = 0;
        for (Int offset = 0; offset < charLen; ++offset) {
            if (Byte newBits := isBase64(text[offset], assertTrash=True)) {
                if (prevCount == 0) {
                    prevBits  = newBits;
                    prevCount = 6;
                } else {
                    byteBuf.add((prevBits << 8-prevCount) | (newBits >> prevCount-2));
                    prevBits   = newBits;
                    prevCount -= 2;
                }
            }
        }

        return byteBuf.freeze(True);
    }

    /**
     * Translate a single Base64 character to the least significant 6 bits of a `Byte` value.
     *
     * @param ch  the Base64 character; no pad or newlines allowed
     *
     * @return the value in the range `0 ..< 64`
     */
    static Byte valOf(Char ch) {
        return switch (ch) {
            case 'A'..'Z': (ch - 'A').toUInt8();
            case 'a'..'z': (ch - 'a').toUInt8() + 26;
            case '0'..'9': (ch - '0').toUInt8() + 52;
            case '+': 62;
            case '/': 63;

            case '=':        assert as $"Unexpected padding character in Base64: {ch.quoted()}";
            case '\r', '\n': assert as $"Unexpected newline character in Base64: {ch.quoted()}";
            default:         assert as $"Invalid Base64 character: {ch.quoted()}";
        };
    }

    /**
     * Translate a single Base64 character to the least significant 6 bits of a `Byte` value.
     *
     * @param ch           the character to test if it is Base64
     * @param assertTrash  (optional) pass True to assert on illegal Base64 characters
     *
     * @return the value in the range `0 ..< 64`
     */
    static conditional Byte isBase64(Char ch, Boolean assertTrash=False) {
        return switch (ch) {
            case 'A'..'Z': (True, (ch - 'A').toUInt8());
            case 'a'..'z': (True, (ch - 'a').toUInt8() + 26);
            case '0'..'9': (True, (ch - '0').toUInt8() + 52);
            case '+': (True, 62);
            case '/': (True, 63);

            case '=':           // "pad" sometimes allowed (or required) at end
            case '\r', '\n':    // newlines sometimes allowed (or required)
                False;

            default: assertTrash ? assert as $"Invalid Base64 character: {ch.quoted()}" : False;
        };
    }

    /**
     * Convert the passed byte value to a Base64 character.
     *
     * @param the byte value, which must be in the range `0..63`
     *
     * @return the Base64 character
     */
    static Char base64(Byte byte) {
        return switch (byte) {
            case  0 ..< 26: 'A'+byte;
            case 26 ..< 52: 'a'+(byte-26);
            case 52 ..< 62: '0'+(byte-52);
            case 62: '+';
            case 63: '/';
            default: assert:bounds as $"byte={byte}";
        };
    }
}