Fix callgrind output compression

callgrind format allows to compress repeated strings into one.  yappi was using
the syntax for compression but did not compress much.  This change makes yappi
actually compress repeated strings.

kcachegrind merges functions with the same compressed ID into one (as if they
were inlined) even if they were defined in different modules. Therefore this
change assigns different IDs to different functions with the same name.

yappi.py

@@ -4,6 +4,7 @@
 
  Sumer Cip 2014
 """
+import itertools
 import os
 import sys
 import _yappi
@@ -14,6 +15,7 @@
 except ImportError:
     from threading import get_ident     # Python 3
 
+from collections import defaultdict
 from contextlib import contextmanager
 
 class YappiError(Exception): pass
@@ -648,26 +650,43 @@ def _save_as_CALLGRIND(self, path):
 
         lines = [header]
 
-        # add function definitions
-        file_ids = ['']
-        func_ids = ['']
+        # Each function has a distinct number even if its name already has a
+        # number because kcachegrind merges functions with the same number.
+        numbers_seq = itertools.count()
+        func_defs = lambda: defaultdict(lambda: defaultdict(lambda: next(numbers_seq)))
+        modules_seq = enumerate(iter(func_defs, None))
+        modules = defaultdict(lambda: next(modules_seq))
+        # modules = {'file.py': [module_index, {'func': {line: func_index}}]}
+        fl = lambda x: modules[x.module][0]
+        fn = lambda x: modules[x.module][1][x.name][x.lineno]
+
+        # enumerate modules and functions
         for func_stat in self:
-            file_ids += [ 'fl=(%d) %s' % (func_stat.index, func_stat.module) ]
-            func_ids += [ 'fn=(%d) %s %s:%s' % (func_stat.index, func_stat.name, func_stat.module, func_stat.lineno) ]
+            fn(func_stat)
+            for child in func_stat.children:
+                fn(child)
 
-        lines += file_ids + func_ids
+        # add function definitions
+        for module in sorted(modules):
+            lines += ['', 'fl=(%d) %s' % (modules[module][0], module)]
+            for func, defs in sorted(modules[module][1].items()):
+                suffix = ''
+                for line in sorted(defs):
+                    if len(defs) > 1:  # disambiguate redefined functions
+                        suffix = ' +%d' % line
+                    lines += ['fn=(%d) %s%s' % (defs[line], func, suffix)]
 
         # add stats for each function we have a record of
         for func_stat in self:
             func_stats = [ '',
-                           'fl=(%d)' % func_stat.index,
-                           'fn=(%d)' % func_stat.index]
+                           'fl=(%d)' % fl(func_stat),
+                           'fn=(%d)' % fn(func_stat)]
             func_stats += [ '%s %s' % (func_stat.lineno, int(func_stat.tsub * 1e6)) ]
 
             # children functions stats
             for child in func_stat.children:
-                func_stats += [ 'cfl=(%d)' % child.index,
-                                'cfn=(%d)' % child.index,
+                func_stats += [ 'cfl=(%d)' % fl(child),
+                                'cfn=(%d)' % fn(child),
                                 'calls=%d 0' % child.ncall,
                                 '0 %d' % int(child.ttot * 1e6)
                                 ]