bitmap: New function bitmap_scan() and macro BITMAP_FOR_EACH_1.

bitmap_scan() can be optimized significantly for the case of a sparse
bitmap but it doesn't seem worth the additional overhead of writing a test
unless and until we show that it's a useful optimization in practice.

lib/bitmap.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2008 Nicira Networks.
+ * Copyright (c) 2008, 2009 Nicira Networks.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -53,3 +53,20 @@ bitmap_equal(const unsigned long *a, const unsigned long *b, size_t n)
     }
     return true;
 }
+
+/* Scans 'bitmap' from bit offset 'start' to 'end', excluding 'end' itself.
+ * Returns the bit offset of the lowest-numbered bit set to 1, or 'end' if
+ * all of the bits are set to 0. */
+size_t
+bitmap_scan(const unsigned long int *bitmap, size_t start, size_t end)
+{
+    /* XXX slow */
+    size_t i;
+
+    for (i = start; i < end; i++) {
+        if (bitmap_is_set(bitmap, i)) {
+            break;
+        }
+    }
+    return i;
+}

lib/bitmap.h

@@ -79,5 +79,10 @@ bitmap_set(unsigned long *bitmap, size_t offset, bool value)
 void bitmap_set_multiple(unsigned long *, size_t start, size_t count,
                          bool value);
 bool bitmap_equal(const unsigned long *, const unsigned long *, size_t n);
+size_t bitmap_scan(const unsigned long int *, size_t start, size_t end);
+
+#define BITMAP_FOR_EACH_1(IDX, SIZE, BITMAP) \
+    for ((IDX) = bitmap_scan(BITMAP, 0, SIZE); (IDX) < (SIZE); \
+         (IDX) = bitmap_scan(BITMAP, (IDX) + 1, SIZE))
 
 #endif /* bitmap.h */