util: New function bitwise_zero().

Signed-off-by: Ben Pfaff <blp@nicira.com>

diff --git a/lib/util.c b/lib/util.c
index 811902a..dfdc51e 100644 (file)
--- a/lib/util.c
+++ b/lib/util.c
@@ -803,6 +803,54 @@ 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.
+ *
+ * If you consider all of 'dst' to be a single unsigned integer in network byte
+ * order, then bit N is the bit with value 2**N.  That is, bit 0 is the bit
+ * with value 1 in dst[dst_len - 1], bit 1 is the bit with value 2, bit 2 is
+ * the bit with value 4, ..., 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.
  *

diff --git a/lib/util.h b/lib/util.h
index c8eb004..d504f2f 100644 (file)
--- 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);

diff --git a/tests/test-util.c b/tests/test-util.c
index 45ea8f2..82d0861 100644 (file)
--- a/tests/test-util.c
+++ b/tests/test-util.c
@@ -102,6 +102,47 @@ check_bitwise_copy(void)
     }
 }
 
+static void
+check_bitwise_zero(void)
+{
+    unsigned int n_loops;
+    int dst_ofs;
+    int n_bits;
+
+    n_loops = 0;
+    for (n_bits = 0; n_bits <= 64; n_bits++) {
+        for (dst_ofs = 0; dst_ofs < 64 - n_bits; dst_ofs++) {
+            ovs_be64 dst = htonll(random_uint64());
+            ovs_be64 orig_dst = dst;
+            ovs_be64 expect;
+
+            if (n_bits == 64) {
+                expect = htonll(0);
+            } else {
+                uint64_t mask = (UINT64_C(1) << n_bits) - 1;
+                expect = orig_dst & ~htonll(mask << dst_ofs);
+            }
+
+            bitwise_zero(&dst, sizeof dst, dst_ofs, n_bits);
+            if (expect != dst) {
+                fprintf(stderr,"bitwise_zero(0x%016"PRIx64",8,%d, %d) "
+                        "yielded 0x%016"PRIx64" "
+                        "instead of the expected 0x%016"PRIx64"\n",
+                        ntohll(orig_dst), dst_ofs,
+                        n_bits,
+                        ntohll(dst), ntohll(expect));
+                abort();
+            }
+
+            n_loops++;
+        }
+    }
+
+    if (n_loops != 64 * (64 + 1) / 2) {
+        abort();
+    }
+}
+
 int
 main(void)
 {
@@ -127,5 +168,7 @@ main(void)
 
     check_bitwise_copy();
 
+    check_bitwise_zero();
+
     return 0;
 }