1 files changed, 20 insertions, 0 deletions
diff --git a/src/libstddjb/bitarray_count.c b/src/libstddjb/bitarray_count.c
new file mode 100644
index 0000000..724f2b3
--- /dev/null
+++ b/src/libstddjb/bitarray_count.c
@@ -0,0 +1,20 @@
+/* ISC license. */
+
+#include <skalibs/bitarray.h>
+
+#define B0(n) n, n+1, n+1, n+2
+#define B1(n) B0(n), B0(n+1), B0(n+1), B0(n+2)
+#define B2(n) B1(n), B1(n+1), B1(n+1), B1(n+2)
+#define B3(n) B2(n), B2(n+1), B2(n+1), B2(n+2)
+
+unsigned int bitarray_countones (unsigned char const *c, unsigned int n)
+{
+  static unsigned char const table[256] = { B3(0) } ;
+  unsigned int len = bitarray_div8(n) ;
+  unsigned int total = 0 ;
+  register unsigned int i = 0 ;
+  if (n & 7) len-- ;
+  for (; i < len ; i++) total += table[c[i]] ;
+  if (n & 7) total += table[c[i] & ((1 << (n & 7)) - 1)] ;
+  return total ;
+}