bitf.h: some pedantry to properly round up bitfield index sizes

src/bitf.h

@@ -50,10 +50,12 @@ struct bitf {
 	bitf_word_t	bits[];
 };
 
+// devide with round up
+#define divceil(n, d) (((n) + ((d) - 1)) / (d))
 #define wsz sizeof(bitf_word_t)
 #define wbits (wsz * 8)
 #define index_thr (wbits * 8)
-#define bitf_words(bits) ((((bits + 7) / 8) + wsz - 1) / wsz)
+#define bitf_words(bits) (divceil(bits, wbits))
 #define bitf_sz1(bits) (sizeof(struct bitf) + bitf_words(bits) * wsz)
 
 static inline size_t
@@ -63,8 +65,7 @@ bitf_sz(size_t bits)
 
 	sz = bitf_sz1(bits);
 	while (bits > index_thr) {
-		bits /= wbits;
-		bits++;
+		bits = divceil(bits, wbits);
 		sz += bitf_sz1(bits);
 	}
 	return (sz);
@@ -94,8 +95,7 @@ bitf_init(void *p, size_t bits, size_t space)
 	bitf->nbits = bits;
 
 	if (bits > index_thr) {
-		bits /= wbits;
-		bits++;
+		bits = divceil(bits, wbits);
 		pp = p;
 		pp += sz;
 		space -= sz;