[Base64::] Base64.

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

@ 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.

=
char *RFC1113_table = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";

@ 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 read@>;
		if (triplet_size == 0) break;
		int quartet[4];
		@<Convert triplet to a quartet@>;
		int i; for (i=0; i<4; i++) PUT(RFC1113_table[quartet[i]]);
		if (triplet_size < 3) break;
	}
	if (tail) WRITE("%S", tail);

}

@ 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 read@> =
	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;

@<Convert triplet to a quartet@> =
	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;
	}