[ovs-dev] [eviction 02/12] util: New function bitwise_zero().

[Ben Pfaff]

Signed-off-by: Ben Pfaff <blp at nicira.com>
---
 lib/util.c |   46 +++++++++++++++++++++++++++++++++++++++++++++-
 lib/util.h |    4 +++-
 2 files changed, 48 insertions(+), 2 deletions(-)

diff --git a/lib/util.c b/lib/util.c
index 7114d9f..e52c526 100644
--- a/lib/util.c
+++ b/lib/util.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2008, 2009, 2010, 2011 Nicira Networks.
+ * Copyright (c) 2008, 2009, 2010, 2011, 2012 Nicira Networks.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -799,6 +799,50 @@ bitwise_copy(const void *src_, unsigned int src_len, unsigned int src_ofs,
     }
 }
 
+/* Zeros the 'n_bits' bits starting from bit 'dst_ofs' in 'dst'.  'dst' is
+ * 'dst_len' bytes long.  Bit 0 of 'dst' is the bit with value 1 in dst[dst_len
+ * - 1], bit 1 is the bit with value 2, bit 8 is the bit with value 1 in
+ * dst[dst_len - 2], and so on.
+ *
+ * Required invariant:
+ *   dst_ofs + n_bits <= dst_len * 8
+ */
+void
+bitwise_zero(void *dst_, unsigned int dst_len, unsigned dst_ofs,
+             unsigned int n_bits)
+{
+    uint8_t *dst = dst_;
+
+    if (!n_bits) {
+        return;
+    }
+
+    dst += dst_len - (dst_ofs / 8 + 1);
+    dst_ofs %= 8;
+
+    if (dst_ofs) {
+        unsigned int chunk = MIN(n_bits, 8 - dst_ofs);
+
+        *dst &= ~(((1 << chunk) - 1) << dst_ofs);
+
+        n_bits -= chunk;
+        if (!n_bits) {
+            return;
+        }
+
+        dst--;
+    }
+
+    while (n_bits >= 8) {
+        *dst-- = 0;
+        n_bits -= 8;
+    }
+
+    if (n_bits) {
+        *dst &= ~((1 << n_bits) - 1);
+    }
+}
+
 /* Copies the 'n_bits' low-order bits of 'value' into the 'n_bits' bits
  * starting at bit 'dst_ofs' in 'dst', which is 'dst_len' bytes long.  Bit 0 of
  * 'dst' is the bit with value 1 in dst[dst_len - 1], bit 1 is the bit with
diff --git a/lib/util.h b/lib/util.h
index c8eb004..d504f2f 100644
--- a/lib/util.h
+++ b/lib/util.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2008, 2009, 2010, 2011 Nicira Networks.
+ * Copyright (c) 2008, 2009, 2010, 2011, 2012 Nicira Networks.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -211,6 +211,8 @@ bool is_all_ones(const uint8_t *, size_t);
 void bitwise_copy(const void *src, unsigned int src_len, unsigned int src_ofs,
                   void *dst, unsigned int dst_len, unsigned int dst_ofs,
                   unsigned int n_bits);
+void bitwise_zero(void *dst_, unsigned int dst_len, unsigned dst_ofs,
+                  unsigned int n_bits);
 void bitwise_put(uint64_t value,
                  void *dst, unsigned int dst_len, unsigned int dst_ofs,
                  unsigned int n_bits);
-- 
1.7.2.5