Fix number conversion for Go 1.26 (#1864)

Co-authored-by: Copilot <[email protected]>

Author: jakebailey

internal/jsnum/jsnum.go

@@ -41,31 +41,33 @@ func isNonFinite(x float64) bool {
 }
 
 // https://tc39.es/ecma262/2024/multipage/abstract-operations.html#sec-touint32
-func (n Number) toUint32() uint32 {
+func (x Number) toUint32() uint32 {
+	// The only difference between ToUint32 and ToInt32 is the interpretation of the bits.
+	return uint32(x.toInt32())
+}
+
+// https://tc39.es/ecma262/2024/multipage/abstract-operations.html#sec-toint32
+func (n Number) toInt32() int32 {
 	x := float64(n)
-	// Fast path: if the number is the range (-2^31, 2^32), i.e. an SMI,
+
+	// Fast path: if the number is in the range (-2^31, 2^32), i.e. an SMI,
 	// then we don't need to do any special mapping.
 	if smi := int32(x); float64(smi) == x {
-		return uint32(smi)
+		return smi
 	}
 
-	// If the number is non-finite (NaN, +Inf, -Inf; exp=0x7FF), it maps to zero.
+	// 2. If number is not finite or number is either +0𝔽 or -0𝔽, return +0𝔽.
+	// Zero was covered by the test above.
 	if isNonFinite(x) {
 		return 0
 	}
 
-	// Otherwise, take x modulo 2^32, mapping positive numbers
-	// to [0, 2^32) and negative numbers to (-2^32, -0.0].
+	// Let int be truncate(ℝ(number)).
+	x = math.Trunc(x)
+	// Let int32bit be int modulo 2**32.
 	x = math.Mod(x, 1<<32)
-
-	// Convert to uint32, which will wrap negative numbers.
-	return uint32(x)
-}
-
-// https://tc39.es/ecma262/2024/multipage/abstract-operations.html#sec-toint32
-func (x Number) toInt32() int32 {
-	// The only difference between ToUint32 and ToInt32 is the interpretation of the bits.
-	return int32(x.toUint32())
+	// If int32bit ≥ 2**31, return 𝔽(int32bit - 2**32); otherwise return 𝔽(int32bit).
+	return int32(int64(x))
 }
 
 func (x Number) toShiftCount() uint32 {

internal/jsnum/jsnum_test.go

@@ -109,18 +109,18 @@ func TestBitwiseNOT(t *testing.T) {
 	t.Parallel()
 
 	tests := []struct {
-		input, want Number
+		got, want Number
 	}{
-		{-2147483649, Number(2147483647).BitwiseNOT()},
-		{-4294967296, Number(0).BitwiseNOT()},
-		{2147483648, Number(-2147483648).BitwiseNOT()},
-		{4294967296, Number(0).BitwiseNOT()},
+		{Number(-2147483649).BitwiseNOT(), Number(2147483647).BitwiseNOT()},
+		{Number(-4294967296).BitwiseNOT(), Number(0).BitwiseNOT()},
+		{Number(2147483648).BitwiseNOT(), Number(-2147483648).BitwiseNOT()},
+		{Number(4294967296).BitwiseNOT(), Number(0).BitwiseNOT()},
 	}
 
 	for _, test := range tests {
-		t.Run(test.input.String(), func(t *testing.T) {
+		t.Run(test.got.String(), func(t *testing.T) {
 			t.Parallel()
-			assertEqualNumber(t, test.input.BitwiseNOT(), test.want)
+			assertEqualNumber(t, test.got, test.want)
 		})
 	}
 }