diff --git a/include/mcl/vint.hpp b/include/mcl/vint.hpp
index d29cb4e8..25a80fb2 100644
--- a/include/mcl/vint.hpp
+++ b/include/mcl/vint.hpp
@@ -22,6 +22,17 @@
 
 namespace mcl {
 
+// return the max size in x[0..n)
+inline int getRealSize(const Unit *x, int n)
+{
+	assert(n > 0);
+	while (n > 0) {
+		if (x[n - 1]) return n;
+		n--;
+	}
+	return 1;
+}
+
 /**
 	signed integer with variable length
 */
@@ -31,13 +42,12 @@ class Vint {
 	static const int invalidVar = -2147483647 - 1; // abs(invalidVar) is not defined
 	static const size_t N = maxUnitSize * 2 + 1;
 private:
-	Unit buf_[N];
+	Unit buf_[N]; // assume buf_[size_ - 1] != 0 unless the value is zero
 	size_t size_;
 	bool isNeg_;
-	void trim(size_t n)
+	void trim()
 	{
-		assert(n > 0);
-		int i = (int)n - 1;
+		int i = (int)size_ - 1;
 		for (; i > 0; i--) {
 			if (buf_[i]) {
 				size_ = i + 1;
@@ -75,7 +85,7 @@ class Vint {
 			c = bint::addUnit(dst + yn, n, c);
 		}
 		dst[xn] = c;
-		z.trim(xn + 1);
+		z.trim();
 	}
 	static void uadd1(Vint& z, const Unit *x, size_t xn, Unit y)
 	{
@@ -86,7 +96,7 @@ class Vint {
 		}
 		if (z.buf_ != x) bint::copyN(z.buf_, x, xn);
 		z.buf_[zn - 1] = bint::addUnit(z.buf_, xn, y);
-		z.trim(zn);
+		z.trim();
 	}
 	static void usub1(Vint& z, const Unit *x, size_t xn, Unit y)
 	{
@@ -99,7 +109,7 @@ class Vint {
 		Unit c = bint::subUnit(dst, xn, y);
 		(void)c;
 		assert(!c);
-		z.trim(zn);
+		z.trim();
 	}
 	static void usub(Vint& z, const Unit *x, size_t xn, const Unit *y, size_t yn)
 	{
@@ -117,7 +127,7 @@ class Vint {
 			c = bint::subUnit(dst, n, c);
 		}
 		assert(!c);
-		z.trim(xn);
+		z.trim();
 	}
 	static void _add(Vint& z, const Vint& x, bool xNeg, const Vint& y, bool yNeg)
 	{
@@ -187,13 +197,13 @@ class Vint {
 		}
 		Unit *xx = (Unit*)CYBOZU_ALLOCA(sizeof(Unit) * xn);
 		bint::copyN(xx, x, xn);
-		Unit *qq = q ? &q->buf_[0] : 0;
-		size_t rn = bint::div(qq, qn, xx, xn, &y[0], yn);
+		Unit *qq = q ? q->buf_ : 0;
+		size_t rn = bint::div(qq, qn, xx, xn, y, yn);
 		r.copy(xx, rn);
+		r.trim();
 		if (q) {
-			q->trim(qn);
+			q->trim();
 		}
-		r.trim(rn);
 	}
 	/*
 		@param x [inout] x <- d
@@ -362,7 +372,7 @@ class Vint {
 		if (!*pb) {
 			return;
 		}
-		trim(unitSize);
+		trim();
 	}
 	/*
 		set [0, max) randomly
@@ -375,7 +385,7 @@ class Vint {
 		size_ = n;
 		rg.read(pb, buf_, n * sizeof(Unit));
 		if (!*pb) return;
-		trim(n);
+		trim();
 		*this %= max;
 	}
 	/*
@@ -459,7 +469,7 @@ class Vint {
 			buf_[q] |= mask;
 		} else {
 			buf_[q] &= ~mask;
-			trim(q + 1);
+			trim();
 		}
 	}
 	/*
@@ -470,17 +480,10 @@ class Vint {
 	*/
 	void setStr(bool *pb, const char *str, int base = 0)
 	{
-		// allow twice size of MCL_MAX_BIT_SIZE because of multiplication
-		const size_t maxN = (MCL_MAX_BIT_SIZE * 2 + UnitBitSize - 1) / UnitBitSize;
-		if (!setSize(maxN)) {
-			*pb = false;
-			return;
-		}
-		isNeg_ = false;
 		size_t len = strlen(str);
-		size_t n = fp::strToArray(&isNeg_, &buf_[0], maxN, str, len, base);
-		if (n == 0) return;
-		trim(n);
+		size_t n = fp::strToArray(&isNeg_, buf_, N, str, len, base);
+		if (n == 0 || !setSize(n)) return;
+		trim();
 		*pb = true;
 	}
 	static int compare(const Vint& x, const Vint& y)
@@ -543,7 +546,7 @@ class Vint {
 		size_t zn = xn + yn;
 		if (!z.setSize(zn)) return;
 		bint::mulNM(z.buf_, x.buf_, xn, y.buf_, yn);
-		z.trim(zn);
+		z.trim();
 		z.isNeg_ = x.isNeg_ ^ y.isNeg_;
 	}
 	static void sqr(Vint& y, const Vint& x)
@@ -565,7 +568,7 @@ class Vint {
 		if (!z.setSize(zn)) return;
 		z.buf_[zn - 1] = bint::mulUnitN(z.buf_, x.buf_, y, xn);
 		z.isNeg_ = x.isNeg_;
-		z.trim(zn);
+		z.trim();
 	}
 	static void divu1(Vint& q, const Vint& x, Unit y)
 	{
@@ -609,9 +612,9 @@ class Vint {
 		int r;
 		if (q) {
 			q->isNeg_ = xNeg ^ yNeg;
-			if (!q->setSize(xn)) return 0;
 			r = (int)bint::divUnit(q->buf_, x.buf_, xn, absY);
-			q->trim(xn);
+			q->setSize(xn);
+			q->trim();
 		} else {
 			r = (int)bint::modUnit(x.buf_, xn, absY);
 		}
@@ -653,12 +656,12 @@ class Vint {
 	}
 	static Unit udivModu1(Vint *q, const Vint& x, Unit y)
 	{
-		assert(!x.isNeg_);
+		assert(!x.isNeg_ && y != 0);
 		size_t xn = x.size();
-		if (q && !q->setSize(N)) return 0;
+		if (q) q->setSize(xn);
 		Unit r = bint::divUnit(q ? q->buf_ : 0, x.buf_, xn, y);
 		if (q) {
-			q->trim(xn);
+			q->trim();
 			q->isNeg_ = false;
 		}
 		return r;
@@ -693,15 +696,11 @@ class Vint {
 		char buf[1024];
 		size_t n = fp::local::loadWord(buf, sizeof(buf), is);
 		if (n == 0) return;
-		const size_t maxN = 384 / (sizeof(MCL_SIZEOF_UNIT) * 8);
-		if (!setSize(maxN)) {
-			*pb = false;
-			return;
-		}
 		isNeg_ = false;
-		n = fp::strToArray(&isNeg_, buf_, maxN, buf, n, ioMode);
+		n = fp::strToArray(&isNeg_, buf_, N, buf, n, ioMode);
 		if (n == 0) return;
-		trim(n);
+		size_ = n;
+		trim();
 		*pb = true;
 	}
 	// logical left shift (copy sign)
@@ -710,10 +709,10 @@ class Vint {
 		assert(shiftBit <= MCL_MAX_BIT_SIZE * 2); // many be too big
 		size_t xn = x.size();
 		size_t yn = xn + (shiftBit + UnitBitSize - 1) / UnitBitSize;
-		if (!y.setSize(yn)) return;
 		bint::shiftLeft(y.buf_, x.buf_, shiftBit, xn);
 		y.isNeg_ = x.isNeg_;
-		y.trim(yn);
+		y.size_ = yn;
+		y.trim();
 	}
 	// logical right shift (copy sign)
 	static void shr(Vint& y, const Vint& x, size_t shiftBit)
@@ -725,10 +724,10 @@ class Vint {
 			return;
 		}
 		size_t yn = xn - shiftBit / UnitBitSize;
-		y.setSize(yn);
-		bint::shiftRight(&y.buf_[0], &x.buf_[0], shiftBit, xn);
+		bint::shiftRight(y.buf_, x.buf_, shiftBit, xn);
 		y.isNeg_ = x.isNeg_;
-		y.trim(yn);
+		y.size_ = yn;
+		y.trim();
 	}
 	static void neg(Vint& y, const Vint& x)
 	{
@@ -750,34 +749,29 @@ class Vint {
 	static void orBit(Vint& z, const Vint& x, const Vint& y)
 	{
 		assert(!x.isNeg_ && !y.isNeg_);
-		const Vint *px = &x, *py = &y;
-		if (x.size() < y.size()) {
-			fp::swap_(px, py);
+		size_t min = x.size_, max = y.size_;
+		const Unit *src = y.buf_;
+		if (x.size_ > y.size_) {
+			min = y.size_;
+			max = x.size_;
+			src = x.buf_;
 		}
-		size_t xn = px->size();
-		size_t yn = py->size();
-		assert(xn >= yn);
-		z.setSize(xn);
-		for (size_t i = 0; i < yn; i++) {
+		for (size_t i = 0; i < min; i++) {
 			z.buf_[i] = x.buf_[i] | y.buf_[i];
 		}
-		bint::copyN(z.buf_ + yn, px->buf_ + yn, xn - yn);
-		z.trim(xn);
+		bint::copyN(z.buf_ + min, src + min, max - min);
+		z.size_ = max;
+		z.isNeg_ = false;
 	}
 	static void andBit(Vint& z, const Vint& x, const Vint& y)
 	{
 		assert(!x.isNeg_ && !y.isNeg_);
-		const Vint *px = &x, *py = &y;
-		if (x.size() < y.size()) {
-			fp::swap_(px, py);
-		}
-		size_t yn = py->size();
-		assert(px->size() >= yn);
-		z.setSize(yn);
-		for (size_t i = 0; i < yn; i++) {
+		size_t zn = fp::min_(x.size_, y.size_);
+		for (size_t i = 0; i < zn; i++) {
 			z.buf_[i] = x.buf_[i] & y.buf_[i];
 		}
-		z.trim(yn);
+		z.size_ = zn;
+		z.trim();
 	}
 	static void orBitu1(Vint& z, const Vint& x, Unit y)
 	{
diff --git a/test/vint_test.cpp b/test/vint_test.cpp
index 998fdccc..fdf07c8a 100644
--- a/test/vint_test.cpp
+++ b/test/vint_test.cpp
@@ -813,6 +813,7 @@ CYBOZU_TEST_AUTO(shift)
 		y >>= i;
 		CYBOZU_TEST_EQUAL(y, z);
 	}
+
 	for (int i = 0; i < 3; i++) {
 		Vint::shr(y, x, i * UnitBitSize);
 		Vint::pow(s, Vint(2), i * UnitBitSize);
@@ -1291,8 +1292,15 @@ CYBOZU_TEST_AUTO(andOr)
 	Vint z;
 	z = x & y;
 	CYBOZU_TEST_EQUAL(z, Vint("1209221003550923564822922"));
+	z.clear();
+	z = y & x;
+	CYBOZU_TEST_EQUAL(z, Vint("1209221003550923564822922"));
+	z.clear();
 	z = x | y;
 	CYBOZU_TEST_EQUAL(z, Vint("29348220482094820948208435244134352108849315802"));
+	z.clear();
+	z = y | x;
+	CYBOZU_TEST_EQUAL(z, Vint("29348220482094820948208435244134352108849315802"));
 #ifndef MCL_AVOID_EXCEPTION_TEST
 //	CYBOZU_TEST_EXCEPTION(Vint("-2") | Vint("5"), cybozu::Exception);
 //	CYBOZU_TEST_EXCEPTION(Vint("-2") & Vint("5"), cybozu::Exception);