diff --git a/bwt.c b/bwt.c
index 7b37fe5..47b06e2 100644
--- a/bwt.c
+++ b/bwt.c
@@ -161,7 +161,7 @@ void bwt_2occ(const bwt_t *bwt, bwtint_t k, bwtint_t l, ubyte_t c, bwtint_t *ok,
 void bwt_occ4(const bwt_t *bwt, bwtint_t k, bwtint_t cnt[4])
 {
 	bwtint_t l, j, x;
-	uint32_t *p;
+	uint32_t *p, tmp;
 	if (k == (bwtint_t)(-1)) {
 		memset(cnt, 0, 4 * sizeof(bwtint_t));
 		return;
@@ -171,9 +171,10 @@ void bwt_occ4(const bwt_t *bwt, bwtint_t k, bwtint_t cnt[4])
 	memcpy(cnt, p, 4 * sizeof(bwtint_t));
 	p += sizeof(bwtint_t);
 	j = k >> 4 << 4;
-	for (l = k / OCC_INTERVAL * OCC_INTERVAL, x = 0; l < j; l += 16, ++p)
+	for (l = k & ~OCC_INTV_MASK, x = 0; l < j; l += 16, ++p)
 		x += __occ_aux4(bwt, *p);
-	x += __occ_aux4(bwt, *p & ~((1U<<((~k&15)<<1)) - 1)) - (~k&15);
+	tmp = *p & ~((1U<<((~k&15)<<1)) - 1);
+	x += __occ_aux4(bwt, tmp) - (~k&15);
 	cnt[0] += x&0xff; cnt[1] += x>>8&0xff; cnt[2] += x>>16&0xff; cnt[3] += x>>24;
 }
 
@@ -188,7 +189,7 @@ void bwt_2occ4(const bwt_t *bwt, bwtint_t k, bwtint_t l, bwtint_t cntk[4], bwtin
 		bwt_occ4(bwt, l, cntl);
 	} else {
 		bwtint_t i, j, x, y;
-		uint32_t *p;
+		uint32_t *p, tmp;
 		if (k >= bwt->primary) --k; // because $ is not in bwt
 		if (l >= bwt->primary) --l;
 		p = bwt_occ_intv(bwt, k);
@@ -196,14 +197,16 @@ void bwt_2occ4(const bwt_t *bwt, bwtint_t k, bwtint_t l, bwtint_t cntk[4], bwtin
 		p += sizeof(bwtint_t);
 		// prepare cntk[]
 		j = k >> 4 << 4;
-		for (i = k / OCC_INTERVAL * OCC_INTERVAL, x = 0; i < j; i += 16, ++p)
+		for (i = k & ~OCC_INTV_MASK, x = 0; i < j; i += 16, ++p)
 			x += __occ_aux4(bwt, *p);
 		y = x;
-		x += __occ_aux4(bwt, *p & ~((1U<<((~k&15)<<1)) - 1)) - (~k&15);
+		tmp = *p & ~((1U<<((~k&15)<<1)) - 1);
+		x += __occ_aux4(bwt, tmp) - (~k&15);
 		// calculate cntl[] and finalize cntk[]
 		j = l >> 4 << 4;
 		for (; i < j; i += 16, ++p) y += __occ_aux4(bwt, *p);
-		y += __occ_aux4(bwt, *p & ~((1U<<((~l&15)<<1)) - 1)) - (~l&15);
+		tmp = *p & ~((1U<<((~l&15)<<1)) - 1);
+		y += __occ_aux4(bwt, tmp) - (~l&15);
 		memcpy(cntl, cntk, 4 * sizeof(bwtint_t));
 		cntk[0] += x&0xff; cntk[1] += x>>8&0xff; cntk[2] += x>>16&0xff; cntk[3] += x>>24;
 		cntl[0] += y&0xff; cntl[1] += y>>8&0xff; cntl[2] += y>>16&0xff; cntl[3] += y>>24;
diff --git a/bwt.h b/bwt.h
index e06329a..ab5aecd 100644
--- a/bwt.h
+++ b/bwt.h
@@ -30,8 +30,10 @@
 
 #include <stdint.h>
 
-// requirement: (OCC_INTERVAL%16 == 0); please DO NOT change this line
-#define OCC_INTERVAL 0x80
+// requirement: (OCC_INTERVAL%16 == 0); please DO NOT change this line because some part of the code assume OCC_INTERVAL=0x80
+#define OCC_INTV_SHIFT 7
+#define OCC_INTERVAL   (1LL<<OCC_INTV_SHIFT)
+#define OCC_INTV_MASK  (OCC_INTERVAL - 1)
 
 #ifndef BWA_UBYTE
 #define BWA_UBYTE