python xapian存储结构

import xapian
import posixpath
def get_db_path():
    XAPIAN_ROOT = '/tmp/'
    xapian_user_database_path = posixpath.join(XAPIAN_ROOT, u'user_index')
    return xapian_user_database_path
def add_document(database, words):
    doc = xapian.Document()
    for w in words:
        doc.add_term(w)
    database.add_document(doc)
def build_index():
    user_database = xapian.WritableDatabase(get_db_path(), xapian.DB_CREATE_OR_OPEN)
    words = ['a', 'b', 'c']
    add_document(user_database, words)
def search(words, offset=0, length=10):
    user_database = xapian.Database(get_db_path())
    enquire = xapian.Enquire(user_database)
    query = xapian.Query(xapian.Query.OP_AND, words)
    enquire.set_query(query)
    return enquire.get_mset(int(offset), int(length))
def _show_q_results(matches):
    print '%i results found.' % matches.get_matches_estimated()
    print 'Results 1 - %i:' % matches.size()
    for match in matches:
        print '%i: %i%% docid=%i [%s]' % (match.rank + 1,
                                          match.percent,
                                          match.docid,
                                          match.document.get_data()
                                          )
if __name__ == '__main__':
    #index 
    build_index()
    
    #search
    _show_q_results(search(['a','b']))

from @brass_table.cc
/* A B-tree comprises (a) a base file, containing essential information (Block
   size, number of the B-tree root block etc), (b) a bitmap, the Nth bit of the
   bitmap being set if the Nth block of the B-tree file is in use, and (c) a
   file DB containing the B-tree proper. The DB file is divided into a sequence
   of equal sized blocks, numbered 0, 1, 2 ... some of which are free, some in
   use. Those in use are arranged in a tree.
   Each block, b, has a structure like this:
     R L M T D o1 o2 o3 ... oN <gap> [item] .. [item] .. [item] ...
     <---------- D ----------> <-M->
   And then,
   R = REVISION(b)  is the revision number the B-tree had when the block was
                    written into the DB file.
   L = GET_LEVEL(b) is the level of the block, which is the number of levels
                    towards the root of the B-tree structure. So leaf blocks
                    have level 0 and the one root block has the highest level
                    equal to the number of levels in the B-tree.
   M = MAX_FREE(b)  is the size of the gap between the end of the directory and
                    the first item of data. (It is not necessarily the maximum
                    size among the bits of space that are free, but I can&#39;t
                    think of a better name.)
   T = TOTAL_FREE(b)is the total amount of free space left in b.
   D = DIR_END(b)   gives the offset to the end of the directory.
   o1, o2 ... oN are a directory of offsets to the N items held in the block.
   The items are key-tag pairs, and as they occur in the directory are ordered
   by the keys.
   An item has this form:
           I K key x C tag
             <--K-->
           <------I------>
   A long tag presented through the API is split up into C tags small enough to
   be accommodated in the blocks of the B-tree. The key is extended to include
   a counter, x, which runs from 1 to C. The key is preceded by a length, K,
   and the whole item with a length, I, as depicted above.