+/*
+ * The entries in a tree are ordered in the _path_ order,
+ * which means that a directory entry is ordered by adding
+ * a slash to the end of it.
+ *
+ * So a directory called "a" is ordered _after_ a file
+ * called "a.c", because "a/" sorts after "a.c".
+ */
+#define TREE_UNORDERED (-1)
+#define TREE_HAS_DUPS (-2)
+
+static int verify_ordered(struct tree_entry_list *a, struct tree_entry_list *b)
+{
+ int len1 = strlen(a->name);
+ int len2 = strlen(b->name);
+ int len = len1 < len2 ? len1 : len2;
+ unsigned char c1, c2;
+ int cmp;
+
+ cmp = memcmp(a->name, b->name, len);
+ if (cmp < 0)
+ return 0;
+ if (cmp > 0)
+ return TREE_UNORDERED;
+
+ /*
+ * Ok, the first <len> characters are the same.
+ * Now we need to order the next one, but turn
+ * a '\0' into a '/' for a directory entry.
+ */
+ c1 = a->name[len];
+ c2 = b->name[len];
+ if (!c1 && !c2)
+ /*
+ * git-write-tree used to write out a nonsense tree that has
+ * entries with the same name, one blob and one tree. Make
+ * sure we do not have duplicate entries.
+ */
+ return TREE_HAS_DUPS;
+ if (!c1 && a->directory)
+ c1 = '/';
+ if (!c2 && b->directory)
+ c2 = '/';
+ return c1 < c2 ? 0 : TREE_UNORDERED;
+}
+