bitmap: Implement bitmap_ffs()

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

@@ -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;
+}

ccan/bitmap/bitmap.h

@@ -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
  */

ccan/bitmap/test/run-ffs.c

@@ -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();
+}