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80 lines
2.7 KiB
OpenEdge ABL
Executable file
80 lines
2.7 KiB
OpenEdge ABL
Executable file
[Base64::] Base64.
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To produce base64-encoded story files ready for in-browser play by
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a Javascript-based interpreter such as Parchment.
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@ The Base64 encoding scheme is defined by the Internet standard RFC 1113.
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Broadly, the idea is to take a binary stream of bytes, break it into
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threes, and then convert this into a sequence of four emailable
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characters. To encode 24 bits in four characters, we need six bits per
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character, so we need $2^6 = 64$ characters in all. Since $64 = 26 +
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26 + 10 + 2$, we can nearly get there with alphanumeric characters alone,
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adding just two others -- conventionally, plus and forward-slash.
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That's more or less the whole thing, except that we use an equals sign
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to indicate incompleteness of the final triplet (which might have only
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1 or 2 bytes in it).
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RFC 1113 permits white space to be used freely, including in particular
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line breaks, but we don't avail ourselves.
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=
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char *RFC1113_table = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
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@ The encoding routine is as follows.
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=
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void Base64::encode(filename *in_filename, filename *out_filename, text_stream *top, text_stream *tail) {
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FILE *IN = BinaryFiles::open_for_reading(in_filename);
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text_stream TO_struct;
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text_stream *TO = &TO_struct;
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if (STREAM_OPEN_TO_FILE(TO, out_filename, UTF8_ENC) == FALSE)
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BlorbErrors::fatal_fs("can't open base-64 encoded story file for output", out_filename);
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Base64::encode_inner(TO, IN, top, tail);
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BinaryFiles::close(IN);
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STREAM_CLOSE(TO);
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}
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void Base64::encode_inner(OUTPUT_STREAM, FILE *IN, text_stream *top, text_stream *tail) {
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if (top) WRITE("%S", top);
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while (TRUE) {
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int triplet[3], triplet_size = 0;
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@<Read the triplet of binary bytes, storing 0 to 3 in the size read@>;
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if (triplet_size == 0) break;
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int quartet[4];
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@<Convert triplet to a quartet@>;
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int i; for (i=0; i<4; i++) PUT(RFC1113_table[quartet[i]]);
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if (triplet_size < 3) break;
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}
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if (tail) WRITE("%S", tail);
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}
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@ If the file ends in mid-triplet, we pad out with zeros.
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@<Read the triplet of binary bytes, storing 0 to 3 in the size read@> =
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triplet[0] = fgetc(IN);
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if (triplet[0] != EOF) {
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triplet_size++;
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triplet[1] = fgetc(IN);
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if (triplet[1] != EOF) {
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triplet_size++;
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triplet[2] = fgetc(IN);
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if (triplet[2] != EOF)
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triplet_size++;
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}
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}
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int i; for (i=triplet_size; i<3; i++) triplet[i] = 0;
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@<Convert triplet to a quartet@> =
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int i; for (i=0; i<4; i++) quartet[i] = 0;
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quartet[0] += (triplet[0] & 0xFC) >> 2;
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quartet[1] += (triplet[0] & 0x03) << 4;
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quartet[1] += (triplet[1] & 0xF0) >> 4;
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quartet[2] += (triplet[1] & 0x0F) << 2;
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quartet[2] += (triplet[2] & 0xC0) >> 6;
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quartet[3] += (triplet[2] & 0x3F) << 0;
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switch (triplet_size) {
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case 1: quartet[2] = 64; quartet[3] = 64; break;
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case 2: quartet[3] = 64; break;
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}
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