From 03a2bd6777f1fb2502963c46f51a485d42bb595c Mon Sep 17 00:00:00 2001
From: Michael Haddon <recruitment.mhaddon@gmail.com>
Date: Thu, 4 Dec 2025 21:43:57 +0000
Subject: [PATCH] fix negative zero from multiplication/division by zero bigint

---
 bigint.go      |  6 ++---
 bigint_test.go | 68 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 71 insertions(+), 3 deletions(-)

diff --git a/bigint.go b/bigint.go
index 6271a40..3e2cd40 100644
--- a/bigint.go
+++ b/bigint.go
@@ -325,7 +325,7 @@ func addInline(xVal, yVal uint64, xNeg, yNeg bool) (zVal uint64, zNeg, ok bool)
 //gcassert:inline
 func mulInline(xVal, yVal uint64, xNeg, yNeg bool) (zVal uint64, zNeg, ok bool) {
 	hi, lo := bits.Mul64(xVal, yVal)
-	neg := xNeg != yNeg
+	neg := xNeg != yNeg && lo != 0
 	overflow := hi != 0
 	return lo, neg, !overflow
 }
@@ -336,7 +336,7 @@ func quoInline(xVal, yVal uint64, xNeg, yNeg bool) (quoVal uint64, quoNeg, ok bo
 		return 0, false, false
 	}
 	quo := xVal / yVal
-	neg := xNeg != yNeg
+	neg := xNeg != yNeg && quo != 0
 	return quo, neg, true
 }
 
@@ -346,7 +346,7 @@ func remInline(xVal, yVal uint64, xNeg, yNeg bool) (remVal uint64, remNeg, ok bo
 		return 0, false, false
 	}
 	rem := xVal % yVal
-	return rem, xNeg, true
+	return rem, xNeg && rem != 0, true
 }
 
 ///////////////////////////////////////////////////////////////////////////////
diff --git a/bigint_test.go b/bigint_test.go
index 92a025f..0334643 100644
--- a/bigint_test.go
+++ b/bigint_test.go
@@ -1094,6 +1094,74 @@ func TestBigIntMulRangeZ(t *testing.T) {
 	}
 }
 
+func TestBigIntNoNegativeZero(t *testing.T) {
+	zero := NewBigInt(0)
+	negOne := NewBigInt(-1)
+	one := NewBigInt(1)
+	negTwo := NewBigInt(-2)
+
+	t.Run("Mul", func(t *testing.T) {
+		// (-1) * 0 should be 0, not negative zero
+		var result BigInt
+		result.Mul(negOne, zero)
+		if result.Sign() != 0 {
+			t.Errorf("(-1) * 0: got Sign() = %d, want 0", result.Sign())
+		}
+		if result.Cmp(zero) != 0 {
+			t.Errorf("(-1) * 0: got Cmp(0) = %d, want 0", result.Cmp(zero))
+		}
+
+		// 0 * (-1) should be 0, not negative zero
+		result.Mul(zero, negOne)
+		if result.Sign() != 0 {
+			t.Errorf("0 * (-1): got Sign() = %d, want 0", result.Sign())
+		}
+	})
+
+	t.Run("Quo", func(t *testing.T) {
+		// 0 / (-1) should be 0, not negative zero
+		var result BigInt
+		result.Quo(zero, negOne)
+		if result.Sign() != 0 {
+			t.Errorf("0 / (-1): got Sign() = %d, want 0", result.Sign())
+		}
+		if result.Cmp(zero) != 0 {
+			t.Errorf("0 / (-1): got Cmp(0) = %d, want 0", result.Cmp(zero))
+		}
+
+		// 1 / (-2) should be 0, not negative zero
+		result.Quo(one, negTwo)
+		if result.Sign() != 0 {
+			t.Errorf("1 / (-2): got Sign() = %d, want 0", result.Sign())
+		}
+	})
+
+	t.Run("Rem", func(t *testing.T) {
+		// (-2) % 2 should be 0, not negative zero
+		var result BigInt
+		two := NewBigInt(2)
+		result.Rem(negTwo, two)
+		if result.Sign() != 0 {
+			t.Errorf("(-2) %% 2: got Sign() = %d, want 0", result.Sign())
+		}
+		if result.Cmp(zero) != 0 {
+			t.Errorf("(-2) %% 2: got Cmp(0) = %d, want 0", result.Cmp(zero))
+		}
+	})
+
+	t.Run("QuoRem", func(t *testing.T) {
+		// 0 / (-1) via QuoRem
+		var quo, rem BigInt
+		quo.QuoRem(zero, negOne, &rem)
+		if quo.Sign() != 0 {
+			t.Errorf("QuoRem(0, -1): quo Sign() = %d, want 0", quo.Sign())
+		}
+		if rem.Sign() != 0 {
+			t.Errorf("QuoRem(0, -1): rem Sign() = %d, want 0", rem.Sign())
+		}
+	})
+}
+
 func TestBigIntBinomial(t *testing.T) {
 	var z BigInt
 	for _, test := range []struct {