# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1489412864 -3600
# Node ID 3d89317842b9cd3da265603d4facb160a50e577f
# Parent  823020deac339cf696c6ace4d95561a7e3a3141b
Made dependency ordering a generic function for possible use elsewhere.

diff -r 823020deac33 -r 3d89317842b9 common.py
--- a/common.py	Mon Mar 13 01:49:11 2017 +0100
+++ b/common.py	Mon Mar 13 14:47:44 2017 +0100
@@ -987,6 +987,56 @@
 def same(s1, s2):
     return set(s1) == set(s2)
 
+def order_dependencies(all_depends):
+
+    """
+    Produce a dependency ordering for the 'all_depends' mapping. This mapping
+    has the form "A depends on B, C...". The result will order A, B, C, and so
+    on.
+    """
+
+    # Record the reverse dependencies, making a mapping of the form
+    # "B is needed by A", "C is needed by A", and so on.
+
+    usage = {}
+
+    # Record path-based dependencies.
+
+    for key in all_depends.keys():
+        usage[key] = set()
+
+    for key, depends in all_depends.items():
+        for depend in depends:
+            init_item(usage, depend, set)
+            usage[depend].add(key)
+
+    # Produce an ordering by obtaining exposed items (required by items already
+    # processed) and putting them at the start of the list.
+
+    ordered = []
+
+    while usage:
+        have_next = False
+
+        for path, n in usage.items():
+            if not n:
+                ordered.insert(0, path)
+                depends = all_depends.get(path)
+
+                # Reduce usage of the referenced items.
+
+                if depends:
+                    for depend in depends:
+                        usage[depend].remove(path)
+
+                del usage[path]
+                have_next = True
+
+        if not have_next:
+            raise ValueError, usage
+
+    return ordered
+
 # General input/output.
 
 def readfile(filename):
diff -r 823020deac33 -r 3d89317842b9 importer.py
--- a/importer.py	Mon Mar 13 01:49:11 2017 +0100
+++ b/importer.py	Mon Mar 13 14:47:44 2017 +0100
@@ -23,7 +23,7 @@
 from errors import ProgramError
 from os.path import exists, extsep, getmtime, join
 from os import listdir, makedirs, remove
-from common import init_item, readfile, writefile
+from common import init_item, order_dependencies, readfile, writefile
 from modules import CachedModule
 from referencing import Reference
 import inspector
@@ -673,44 +673,10 @@
 
         self.condense_dependencies()
 
-        # Record the number of modules using or depending on each module.
-
-        usage = {}
-
-        # Record path-based dependencies.
-
-        for path in self.depends.keys():
-            usage[path] = set()
-
-        for path, depends in self.depends.items():
-            for origin in depends:
-                init_item(usage, origin, set)
-                usage[origin].add(path)
-
-        # Produce an ordering by obtaining exposed modules (required by modules
-        # already processed) and putting them at the start of the list.
-
-        ordered = []
-
-        while usage:
-            have_next = False
-
-            for path, n in usage.items():
-                if not n:
-                    ordered.insert(0, path)
-                    depends = self.depends.get(path)
-
-                    # Reduce usage of the referenced objects.
-
-                    if depends:
-                        for origin in depends:
-                            usage[origin].remove(path)
-
-                    del usage[path]
-                    have_next = True
-
-            if not have_next:
-                raise ProgramError("Modules with unresolvable dependencies exist: %s" % ", ".join(usage.keys()))
+        try:
+            ordered = order_dependencies(self.depends)
+        except ValueError, exc:
+            raise ProgramError("Modules with unresolvable dependencies exist: %s" % ", ".join(exc.args[0].keys()))
 
         if "__main__" in ordered:
             ordered.remove("__main__")