templater: provide loop counter as "index" keyword

This was originally written for JSON templating where we would have to be
careful to not add extra comma, but seems generally useful.

Inner loop started by % operator has its own counter.

mercurial/cmdutil.py

@@ -8,6 +8,7 b''
 from __future__ import absolute_import
 
 import errno
+import itertools
 import os
 import re
 import tempfile
@@ -1452,6 +1453,7 b' class changeset_templater(changeset_prin'
             self.t = formatter.maketemplater(ui, 'changeset', tmpl,
                                              cache=defaulttempl)
 
+        self._counter = itertools.count()
         self.cache = {}
 
         # find correct templates for current mode
@@ -1490,6 +1492,7 b' class changeset_templater(changeset_prin'
         props['ctx'] = ctx
         props['repo'] = self.repo
         props['ui'] = self.repo.ui
+        props['index'] = next(self._counter)
         props['revcache'] = {'copies': copies}
         props['cache'] = self.cache
 

mercurial/formatter.py

@@ -103,6 +103,7 b' baz: foo, bar'
 
 from __future__ import absolute_import
 
+import itertools
 import os
 
 from .i18n import _
@@ -338,6 +339,7 b' class templateformatter(baseformatter):'
         self._topic = topic
         self._t = gettemplater(ui, topic, opts.get('template', ''),
                                cache=templatekw.defaulttempl)
+        self._counter = itertools.count()
         self._cache = {}  # for templatekw/funcs to store reusable data
     def context(self, **ctxs):
         '''insert context objects to be used to render template keywords'''
@@ -350,6 +352,7 b' class templateformatter(baseformatter):'
         props = {}
         if 'ctx' in self._item:
             props.update(templatekw.keywords)
+        props['index'] = next(self._counter)
         # explicitly-defined fields precede templatekw
         props.update(self._item)
         if 'ctx' in self._item:

mercurial/templatekw.py

@@ -7,6 +7,7 b''
 
 from __future__ import absolute_import
 
+from .i18n import _
 from .node import hex, nullid
 from . import (
     encoding,
@@ -422,6 +423,12 b' def showgraphnode(repo, ctx, **args):'
     else:
         return 'o'
 
+@templatekeyword('index')
+def showindex(**args):
+    """Integer. The current iteration of the loop. (0 indexed)"""
+    # just hosts documentation; should be overridden by template mapping
+    raise error.Abort(_("can't use index in this context"))
+
 @templatekeyword('latesttag')
 def showlatesttag(**args):
     """List of strings. The global tags on the most recent globally

mercurial/templater.py

@@ -411,8 +411,9 b' def runmap(context, mapping, data):'
             else:
                 raise error.ParseError(_("%r is not iterable") % d)
 
-    for v in diter:
+    for i, v in enumerate(diter):
         lm = mapping.copy()
+        lm['index'] = i
         if isinstance(v, dict):
             lm.update(v)
             lm['originalnode'] = mapping.get('node')

tests/test-command-template.t

@@ -2683,6 +2683,16 b' Pass generator object created by templat'
   $ hg log -l 1 --template '{if(author, author)|user}\n'
   test
 
+Test index keyword:
+
+  $ hg log -l 2 -T '{index + 10}{files % " {index}:{file}"}\n'
+  10 0:a 1:b 2:fifth 3:fourth 4:third
+  11 0:a
+
+  $ hg branches -T '{index} {branch}\n'
+  0 default
+  1 foo
+
 Test diff function:
 
   $ hg diff -c 8