# HG changeset patch
# User Pierre-Yves David <pierre-yves.david@octobus.net>
# Date 1511354447 -3600
# Node ID 88f11b9881b259a4623f9eafef22ee4c9d7d558a
# Parent  9a2cc4687cb957087e87ed2df49bcb76f66a5264
cache: introduce a changelogsourcebase class

This abstract class will help code that need a cache tracking the changelog
content (eg: the branchmap cache). The cache key used is the same as what the
branchmap uses.

This code was headed for core during the 4.3 cycle but never made it there. So
we starts with a copy in the evolve repository.

diff -r 9a2cc4687cb9 -r 88f11b9881b2 hgext3rd/evolve/genericcaches.py
--- a/hgext3rd/evolve/genericcaches.py	Wed Nov 22 13:40:05 2017 +0100
+++ b/hgext3rd/evolve/genericcaches.py	Wed Nov 22 13:40:47 2017 +0100
@@ -10,6 +10,7 @@
 import struct
 
 from mercurial import (
+    node,
     util,
 )
 
@@ -125,3 +126,41 @@
     def _deserializecachekey(self, data):
         """read the cachekey from bytes"""
         return self._cachekeystruct.unpack(data)
+
+class changelogsourcebase(incrementalcachebase):
+    """an abstract class for cache sourcing data from the changelog
+
+    For this purpose it use a cache key covering changelog content.
+    The cache key parts are: (tiprev, tipnode)
+    """
+
+    __metaclass__ = abc.ABCMeta
+
+    # default key used for an empty cache
+    emptykey = (0, node.nullid)
+    _cachekeyspec = 'i20s'
+    _cachename = None # used for debug message
+
+    # Useful "public" function (no need to override them)
+
+    def _fetchchangelogdata(self, cachekey, cl):
+        """use a cachekey to fetch incremental data
+
+        Exists as its own method to help subclass to reuse it."""
+        tiprev = len(cl) - 1
+        tipnode = cl.node(tiprev)
+        newkey = (tiprev, tipnode)
+        tiprev = len(cl) - 1
+        if newkey == cachekey:
+            return False, [], newkey
+        keyrev, keynode = cachekey
+        if tiprev < keyrev or cl.node(keyrev) != keynode:
+            revs = ()
+            if len(cl):
+                revs = list(cl.revs(stop=tiprev))
+            return True, revs, newkey
+        else:
+            return False, list(cl.revs(start=keyrev + 1, stop=tiprev)), newkey
+
+    def _fetchupdatedata(self, repo):
+        return self._fetchchangelogdata(self._cachekey, repo.changelog)