To produce base64-encoded story files ready for in-browser play by a Javascript-based interpreter such as Parchment.

§1. The Base64 encoding scheme is defined by the Internet standard RFC 1113. Broadly, the idea is to take a binary stream of bytes, break it into threes, and then convert this into a sequence of four emailable characters. To encode 24 bits in four characters, we need six bits per character, so we need \(2^6 = 64\) characters in all. Since $64 = 26 + 26 + 10 + 2$, we can nearly get there with alphanumeric characters alone, adding just two others — conventionally, plus and forward-slash. That's more or less the whole thing, except that we use an equals sign to indicate incompleteness of the final triplet (which might have only 1 or 2 bytes in it).

RFC 1113 permits white space to be used freely, including in particular line breaks, but we don't avail ourselves. 

inchar32_t *RFC1113_table = U"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";

§2. The encoding routine is as follows. 

void Base64::encode(filename *in_filename, filename *out_filename, text_stream *top, text_stream *tail) {
    FILE *IN = BinaryFiles::open_for_reading(in_filename);
    text_stream TO_struct;
    text_stream *TO = &TO_struct;
    if (STREAM_OPEN_TO_FILE(TO, out_filename, UTF8_ENC) == FALSE)
        BlorbErrors::fatal_fs("can't open base-64 encoded story file for output", out_filename);
    Base64::encode_inner(TO, IN, top, tail);
    BinaryFiles::close(IN);
    STREAM_CLOSE(TO);
}

void Base64::encode_inner(OUTPUT_STREAM, FILE *IN, text_stream *top, text_stream *tail) {
    if (top) WRITE("%S", top);
    while (TRUE) {
        int triplet[3], triplet_size = 0;
        Read the triplet of binary bytes, storing 0 to 3 in the size read2.1;
        if (triplet_size == 0) break;
        int quartet[4];
        Convert triplet to a quartet2.2;
        int i; for (i=0; i<4; i++) PUT(RFC1113_table[quartet[i]]);
        if (triplet_size < 3) break;
    }
    if (tail) WRITE("%S", tail);

}

§2.1. If the file ends in mid-triplet, we pad out with zeros.

Read the triplet of binary bytes, storing 0 to 3 in the size read2.1 = 

    triplet[0] = fgetc(IN);
    if (triplet[0] != EOF) {
        triplet_size++;
        triplet[1] = fgetc(IN);
        if (triplet[1] != EOF) {
            triplet_size++;
            triplet[2] = fgetc(IN);
            if (triplet[2] != EOF)
                triplet_size++;
        }
    }
    int i; for (i=triplet_size; i<3; i++) triplet[i] = 0;

§2.2. Convert triplet to a quartet2.2 = 

    int i; for (i=0; i<4; i++) quartet[i] = 0;
    quartet[0] += (triplet[0] & 0xFC) >> 2;
    quartet[1] += (triplet[0] & 0x03) << 4;
    quartet[1] += (triplet[1] & 0xF0) >> 4;
    quartet[2] += (triplet[1] & 0x0F) << 2;
    quartet[2] += (triplet[2] & 0xC0) >> 6;
    quartet[3] += (triplet[2] & 0x3F) << 0;
    switch (triplet_size) {
        case 1: quartet[2] = 64; quartet[3] = 64; break;
        case 2: quartet[3] = 64; break;
    }