hbitmap: add hbitmap_merge

We add a bitmap merge operation to assist in error cases
where we wish to combine two bitmaps together.

This is algorithmically O(bits) provided HBITMAP_LEVELS remains
constant. For a full bitmap on a 64bit machine:
sum(bits/64^k, k, 0, HBITMAP_LEVELS) ~= 1.01587 * bits

We may be able to improve running speed for particularly sparse
bitmaps by using iterators, but the running time for dense maps
will be worse.

We present the simpler solution first, and we can refine it later
if needed.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1429314609-29776-8-git-send-email-jsnow@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

2 files changed, 46 insertions, 0 deletions

diff --git a/include/qemu/hbitmap.h b/include/qemu/hbitmap.h
index 550d7ce2c3..6cb2d0e6a7 100644
--- a/include/qemu/hbitmap.h
+++ b/include/qemu/hbitmap.h
@@ -65,6 +65,19 @@ struct HBitmapIter {
 HBitmap *hbitmap_alloc(uint64_t size, int granularity);
 
 /**
+ * hbitmap_merge:
+ * @a: The bitmap to store the result in.
+ * @b: The bitmap to merge into @a.
+ * @return true if the merge was successful,
+ *         false if it was not attempted.
+ *
+ * Merge two bitmaps together.
+ * A := A (BITOR) B.
+ * B is left unmodified.
+ */
+bool hbitmap_merge(HBitmap *a, const HBitmap *b);
+
+/**
  * hbitmap_empty:
  * @hb: HBitmap to operate on.
  *
diff --git a/util/hbitmap.c b/util/hbitmap.c
index 5b78613400..150d6e98b7 100644
--- a/util/hbitmap.c
+++ b/util/hbitmap.c
@@ -399,3 +399,36 @@ HBitmap *hbitmap_alloc(uint64_t size, int granularity)
     hb->levels[0][0] |= 1UL << (BITS_PER_LONG - 1);
     return hb;
 }
+
+/**
+ * Given HBitmaps A and B, let A := A (BITOR) B.
+ * Bitmap B will not be modified.
+ *
+ * @return true if the merge was successful,
+ *         false if it was not attempted.
+ */
+bool hbitmap_merge(HBitmap *a, const HBitmap *b)
+{
+    int i;
+    uint64_t j;
+
+    if ((a->size != b->size) || (a->granularity != b->granularity)) {
+        return false;
+    }
+
+    if (hbitmap_count(b) == 0) {
+        return true;
+    }
+
+    /* This merge is O(size), as BITS_PER_LONG and HBITMAP_LEVELS are constant.
+     * It may be possible to improve running times for sparsely populated maps
+     * by using hbitmap_iter_next, but this is suboptimal for dense maps.
+     */
+    for (i = HBITMAP_LEVELS - 1; i >= 0; i--) {
+        for (j = 0; j < a->sizes[i]; j++) {
+            a->levels[i][j] |= b->levels[i][j];
+        }
+    }
+
+    return true;
+}