The text below is copied from the original 19960914 document
"A structure for constant databases" by D. J. Bernstein.
Other explanations of the format are online from
[Yusuke Shinyama](https://www.unixuser.org/~euske/doc/cdbinternals/index.html),
[Richard James Howe](https://github.com/howerj/cdb),
and
[Wikipedia](https://en.wikipedia.org/wiki/Cdb_(software)).
The cdb64 format uses 64-bit quantities stored in 8 bytes;
conceptually this can handle 16 exabytes,
but the current code limits it to 1 exabyte.

<hr>

    A cdb is an associative array: it maps strings (``keys'') to strings
    (``data'').

    A cdb contains 256 pointers to linearly probed open hash tables. The
    hash tables contain pointers to (key,data) pairs. A cdb is stored in
    a single file on disk:

        +----------------+---------+-------+-------+-----+---------+
        | p0 p1 ... p255 | records | hash0 | hash1 | ... | hash255 |
        +----------------+---------+-------+-------+-----+---------+

    Each of the 256 initial pointers states a position and a length. The
    position is the starting byte position of the hash table. The length
    is the number of slots in the hash table.

    Records are stored sequentially, without special alignment. A record
    states a key length, a data length, the key, and the data.

    Each hash table slot states a hash value and a byte position. If the
    byte position is 0, the slot is empty. Otherwise, the slot points to
    a record whose key has that hash value.

    Positions, lengths, and hash values are 32-bit quantities, stored in
    little-endian form in 4 bytes. Thus a cdb must fit into 4 gigabytes.

    A record is located as follows. Compute the hash value of the key in
    the record. The hash value modulo 256 is the number of a hash table.
    The hash value divided by 256, modulo the length of that table, is a
    slot number. Probe that slot, the next higher slot, and so on, until
    you find the record or run into an empty slot.

    The cdb hash function is ``h = ((h << 5) + h) ^ c'', with a starting
    hash of 5381.