bitmap: Implement bitmap_ffs()
authorDavid Gibson <david@gibson.dropbear.id.au>
Thu, 2 Oct 2014 14:15:00 +0000 (00:15 +1000)
committerRusty Russell <rusty@rustcorp.com.au>
Mon, 13 Oct 2014 02:45:44 +0000 (13:15 +1030)
This adds a bitmap_ffs() function to find the first (by the usual bitmap
bit ordering) 1 bit in a bitmap between two given bits.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
ccan/bitmap/bitmap.c
ccan/bitmap/bitmap.h
ccan/bitmap/test/run-ffs.c [new file with mode: 0644]

index a6058117985f8259aee36592993b4fd0e9ab3fce..d812af6a2344f47283f2dd89f358a398cf1cea17 100644 (file)
@@ -58,3 +58,68 @@ void bitmap_fill_range(bitmap *bitmap, unsigned long n, unsigned long m)
        if (m > am)
                BITMAP_WORD(bitmap, m) |= bitmap_bswap(tailmask);
 }
+
+static int bitmap_clz(bitmap_word w)
+{
+#if HAVE_BUILTIN_CLZL
+       return __builtin_clzl(w);
+#else
+       int lz = 0;
+       bitmap_word mask = 1UL << (BITMAP_WORD_BITS - 1);
+
+       while (!(w & mask)) {
+               lz++;
+               mask >>= 1;
+       }
+
+       return lz;
+#endif
+}
+
+unsigned long bitmap_ffs(const bitmap *bitmap,
+                        unsigned long n, unsigned long m)
+{
+       unsigned long an = BIT_ALIGN_UP(n);
+       unsigned long am = BIT_ALIGN_DOWN(m);
+       bitmap_word headmask = -1ULL >> (n % BITMAP_WORD_BITS);
+       bitmap_word tailmask = ~(-1ULL >> (m % BITMAP_WORD_BITS));
+
+       assert(m >= n);
+
+       if (am < an) {
+               bitmap_word w = bitmap_bswap(BITMAP_WORD(bitmap, n));
+
+               w &= (headmask & tailmask);
+
+               return w ? am + bitmap_clz(w) : m;
+       }
+
+       if (an > n) {
+               bitmap_word w = bitmap_bswap(BITMAP_WORD(bitmap, n));
+
+               w &= headmask;
+
+               if (w)
+                       return BIT_ALIGN_DOWN(n) + bitmap_clz(w);
+       }
+
+       while (an < am) {
+               bitmap_word w = bitmap_bswap(BITMAP_WORD(bitmap, an));
+
+               if (w)
+                       return an + bitmap_clz(w);
+
+               an += BITMAP_WORD_BITS;
+       }
+
+       if (m > am) {
+               bitmap_word w = bitmap_bswap(BITMAP_WORD(bitmap, m));
+
+               w &= tailmask;
+
+               if (w)
+                       return am + bitmap_clz(w);
+       }
+
+       return m;
+}
index 6c8d7e4d4593e0f1745704c92761e69121fd25dd..6cbf9a22140655a3f55277edc44a5ff23c06f9e0 100644 (file)
@@ -187,6 +187,9 @@ static inline bool bitmap_empty(const bitmap *bitmap, unsigned long nbits)
        return true;
 }
 
+unsigned long bitmap_ffs(const bitmap *bitmap,
+                        unsigned long n, unsigned long m);
+
 /*
  * Allocation functions
  */
diff --git a/ccan/bitmap/test/run-ffs.c b/ccan/bitmap/test/run-ffs.c
new file mode 100644 (file)
index 0000000..0f6cc4c
--- /dev/null
@@ -0,0 +1,74 @@
+#include <ccan/bitmap/bitmap.h>
+#include <ccan/tap/tap.h>
+#include <ccan/array_size/array_size.h>
+#include <ccan/foreach/foreach.h>
+
+#include <ccan/bitmap/bitmap.c>
+
+int bitmap_sizes[] = {
+       1, 2, 3, 4, 5, 6, 7, 8,
+       16, 17, 24, 32, 33,
+       64, 65, 127, 128, 129,
+       1023, 1024, 1025,
+};
+#define NSIZES ARRAY_SIZE(bitmap_sizes)
+
+#define ok_eq(a, b) \
+       ok((a) == (b), "%s [%u] == %s [%u]", \
+          #a, (unsigned)(a), #b, (unsigned)(b))
+
+static void test_size(int nbits)
+{
+       BITMAP_DECLARE(bitmap, nbits);
+       int i;
+
+       bitmap_zero(bitmap, nbits);
+       ok_eq(bitmap_ffs(bitmap, 0, nbits), nbits);
+
+       for (i = 0; i < nbits; i++) {
+               bitmap_zero(bitmap, nbits);
+               bitmap_set_bit(bitmap, i);
+
+               ok_eq(bitmap_ffs(bitmap, 0, nbits), i);
+               ok_eq(bitmap_ffs(bitmap, i, nbits), i);
+               ok_eq(bitmap_ffs(bitmap, i + 1, nbits), nbits);
+
+               bitmap_zero(bitmap, nbits);
+               bitmap_fill_range(bitmap, i, nbits);
+
+               ok_eq(bitmap_ffs(bitmap, 0, nbits), i);
+               ok_eq(bitmap_ffs(bitmap, i, nbits), i);
+               ok_eq(bitmap_ffs(bitmap, i + 1, nbits), (i + 1));
+               ok_eq(bitmap_ffs(bitmap, nbits - 1, nbits), (nbits - 1));
+
+               if (i > 0) {
+                       ok_eq(bitmap_ffs(bitmap, 0, i), i);
+                       ok_eq(bitmap_ffs(bitmap, 0, i - 1), (i - 1));
+               }
+
+               if (i > 0) {
+                       bitmap_zero(bitmap, nbits);
+                       bitmap_fill_range(bitmap, 0, i);
+
+                       ok_eq(bitmap_ffs(bitmap, 0, nbits), 0);
+                       ok_eq(bitmap_ffs(bitmap, i - 1, nbits), (i - 1));
+                       ok_eq(bitmap_ffs(bitmap, i, nbits), nbits);
+               }
+       }
+}
+
+int main(void)
+{
+       int i;
+
+       /* Too complicated to work out the exact number */
+       plan_no_plan();
+
+       for (i = 0; i < NSIZES; i++) {
+               diag("Testing %d-bit bitmap", bitmap_sizes[i]);
+               test_size(bitmap_sizes[i]);
+       }
+
+       /* This exits depending on whether all tests passed */
+       return exit_status();
+}