[Python-Dev] [patch #100889] a smaller unicode name database

Fredrik Lundh Fredrik Lundh" <effbot@telia.com
Sun, 16 Jul 2000 20:40:46 +0200

bill wrote:
> Smaller data sizes are definitely cool, but as MAL mentioned you =
> include the genration code, nor an explanation about how the stuff =

brute force ;-)

seriously, it's a pretty simple design.  here's an overview:

    The unicodenames database consists of two parts: a name
    database which maps character codes to names, and a code
    database, mapping names to codes.

* The Name Database (getname)

    First, the 10538 text strings are split into 42193 words,
    and combined into a 4949-word lexicon (a 29k byte array).

    Each word is given a unique index number (common words get
    lower numbers), and there's a "lexicon offset" table mapping
    from numbers to words (10k).

    To get back to the original text strings, I use a "phrase
    book".  For each original string, the phrase book stores a a
    list of word numbers.  Numbers 0-127 are stored in one byte,
    higher numbers (less common words) use two bytes.  At this
    time, about 65% of the words can be represented by a single
    byte.  The result is a 56k array.

    The final data structure is an offset table, which maps code
    points to phrase book offsets.  Instead of using one big
    table, I split each code point into a "page number" and a
    "line number" on that page.

      offset =3D line[ (page[code>>SHIFT]<<SHIFT) + (code&MASK) ]

    Since the unicode space is sparsely populated, it's possible
    to split the code so that lots of pages gets no contents.  I
    use a brute force search to find the optimal SHIFT value.

    In the current database, the page table has 1024 entries
    (SHIFT is 6), and there are 199 unique pages in the line
    table.  The total size of the offset table is 26k.

* The code database (getcode)

    For the code table, I use a straight-forward hash table to store
    name to code mappings.  It's basically the same implementation
    as in Python's dictionary type, but a different hash algorithm.
    The table lookup loop simply uses the name database to check
    for hits.

    In the current database, the hash table is 32k.