# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1252774103 -7200
# Node ID 76bd0bcfa998cf54b8de031b82ff1db309699442
# Parent  0cd36671c34eaafa644a3186437ededc50dcd4a9
Attempted to provide cache navigation without slicing the cache all the time.

diff -r 0cd36671c34e -r 76bd0bcfa998 iixr.py
--- a/iixr.py	Sat Sep 12 01:52:23 2009 +0200
+++ b/iixr.py	Sat Sep 12 18:48:23 2009 +0200
@@ -199,8 +199,12 @@
 
     def __init__(self, f):
         File.__init__(self, f)
+        self.reset_cache()
+
+    def reset_cache(self):
         self.cache = ""
         self.cache_length = 0
+        self.cache_start = 0
 
     def read_number(self):
 
@@ -255,30 +259,56 @@
     # Cache-affected methods.
 
     def read(self, n):
-        needed = n - self.cache_length
+        needed = n - (self.cache_length - self.cache_start)
+
+        # Read the needed number of characters, if possible.
+
         if needed > 0:
             s = self.f.read(max(needed, 1000))
             self.cache += s
             self.cache_length += len(s)
 
-        s = self.cache[:n]
-        self.cache = self.cache[n:]
-        self.cache_length = len(self.cache)
+        # Get the end of the requested block.
+
+        next_start = self.cache_start + n
+        s = self.cache[self.cache_start:next_start]
+
+        # Reposition the pointer to the cache.
+
+        self._seek_cache(len(s))
         return s
 
     def tell(self):
-        return self.f.tell() - self.cache_length
+        return self.f.tell() - self.cache_length + self.cache_start
 
     def seek(self, offset):
         current = self.tell()
         self.f.seek(offset)
+
+        # If seeking forward, attempt to navigate the cache.
+
         if offset >= current:
-            discarded = offset - current
-            self.cache = self.cache[discarded:]
+            self._seek_cache(offset - current)
+        else:
+            self.reset_cache()
+
+    def _seek_cache(self, delta):
+        next_start = self.cache_start + delta
+
+        if next_start >= len(self.cache):
+            self.reset_cache()
+
+        # If the cache is too big, resize it.
+
+        elif next_start > 1000:
+            self.cache = self.cache[next_start:]
             self.cache_length = len(self.cache)
+            self.cache_start = 0
+
+        # Otherwise, just reference the next part of the cache.
+
         else:
-            self.cache = ""
-            self.cache_length = 0
+            self.cache_start = next_start
 
 class FileOpener: