# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1297644837 -3600
# Node ID 52bfb08d5b2f539118a72a3bbc8a4eb20649f194
# Parent  cf159c14882af2b676ca9c5d6580f62940e97573
Fix the index bisection and attempt to prevent unnecessary seeking and scanning.

diff -r cf159c14882a -r 52bfb08d5b2f iixr/terms.py
--- a/iixr/terms.py	Mon Feb 14 00:44:57 2011 +0100
+++ b/iixr/terms.py	Mon Feb 14 01:53:57 2011 +0100
@@ -381,18 +381,38 @@
         self.reader = reader
         self.index_reader = index_reader
         self.records = list(index_reader)
+        self.terms = [t for t, dp in self.records]
+
+        # Cache the last term and positions.
+
+        self.last_term = None
+        self.last_positions = None
 
     def go_to_term(self, term):
 
+        """
+        Return the 'term' and positions or nearest following term and positions.
+        """
+
+        if self.last_term == term:
+            return self.last_term, self.last_positions
+
         # Get the record providing a term less than or equal to the requested
         # term, getting the first entry if no such records exist.
 
-        i = max(0, bisect_right(self.records, (term, None)) - 1)
-        t, offset = self.records[i]
+        after = bisect_right(self.terms, term)
+        before = max(0, after - 1)
+
+        t, offset = self.records[before]
+        terms_after = self.terms[after:]
 
         # Seek to the corresponding record in the information file.
+        # Only do this if the term is more quickly reached by seeking. 
 
-        self.reader.seek(offset)
+        if term <= t or self.last_term is None or term <= self.last_term or \
+            self.last_term < t or terms_after and terms_after[0] <= self.last_term:
+
+            self.reader.seek(offset)
 
         # Where the found term is equal or greater, just read the positions for
         # the index entry.
@@ -426,7 +446,8 @@
         return self
 
     def next(self):
-        return self.reader.next()
+        self.last_term, self.last_positions = t = self.reader.next()
+        return t
 
     def close(self):
         if self.reader is not None: