Base38 and FourCC Codes

Both of these encode a four-character string such as "JPEG" as a uint32_t value. Computers can compare two integer values faster than they can compare two arbitrary strings.

Both schemes maintain ordering: if two four-character strings s and t satisfy (s < t), and those strings have valid numerical encodings, then the numerical values also satisfy (encoding(s) < encoding(t)).

FourCC

FourCC codes are not specific to Wuffs. For example, the AVI multimedia container format can hold various sub-formats, such as “H264” or “YV12”, distinguished in the overall file format by their FourCC code.

The FourCC encoding is the straightforward sequence of each character's ASCII encoding. The FourCC code for "JPEG" is 0x4A504547, since 'J' is 0x4A, 'P' is 0x50, etc. This is essentially 8 bits for each character, 32 bits overall. The big-endian representation of this number is exactly the ASCII (and UTF-8) string "JPEG".

Other FourCC documentation sometimes use a little-endian convention, so that the {0x4A, 0x50, 0x45, 0x47} bytes on the wire for "JPEG" corresponds to the number 0x4745504A (little-endian) instead of 0x4A504547 (big-endian). Wuffs uses the big-endian interpretation, as it maintains ordering.

Base38

Base38 is a tighter encoding than FourCC, fitting four characters into 21 bits instead of 32 bits. This is achieved by using a smaller alphabet of 38 possible values (space, 0-9, ? or a-z), so that it cannot distinguish between e.g. an upper case ‘X’ and a lower case ‘x’. There's also the happy coincidence that 38 ** 4 = 0x1FD110 = 2085136 is slightly smaller than 2 ** 21 = 0x200000 = 2097152.

The base38 encoding of "JPEG" is 0x122FF6, which is 1191926, which is ((21 * (38 ** 3)) + (27 * (38 ** 2)) + (16 * (38 ** 1)) + (18 * (38 ** 0))).

Using only 21 bits means that we can use base38 values to partition the set of possible uint32_t values into file-format specific enumerations. Each package (i.e. Wuffs implementation of a specific file format) can define up to 1024 different values in their own namespace, without conflicting with other packages (assuming that there aren't e.g. two "JPEG" Wuffs packages in the same library). The conventional uint32_t packing is:

• Bit 31 (1 bit) is reserved (zero).
• Bits 10 ..= 30 (21 bits) are the base38 value, shifted by 10.
• Bits 0 ..= 9 (10 bits) are the enumeration value.

For example:

• Quirk values use this ((base38 << 10) | enumeration) scheme.
• Tokens assign 21 out of 64 bits for a Base38 value.