math.big: change the handling of negatives in div_mod() to match gmp and Julia, add tests (#24713)

Author: tankf33der

vlib/math/big/big_test.v

@@ -248,6 +248,20 @@ struct DivModTest {
 
 // vfmt off
 const div_mod_test_data = [
+	DivModTest{0, 7, 0, 0},
+	DivModTest{0, -7, 0, 0},
+	DivModTest{3, 7, 0, 3},
+	DivModTest{-3, -7, 0, -3},
+	DivModTest{-3, 7, 0, -3},
+	DivModTest{3, -7, 0, 3},
+	DivModTest{7, 3, 2, 1},
+	DivModTest{-7, -3, 2, -1},
+	DivModTest{-7, 3, -2, -1},
+	DivModTest{7, -3, -2, 1},
+	DivModTest{8, 2, 4, 0},
+	DivModTest{-8, -2, 4, 0},
+	DivModTest{-8, 2, -4, 0},
+	DivModTest{8, -2, -4, 0},
 	DivModTest{13, 10, 1, 3},
 	DivModTest{13, 9, 1, 4},
 	DivModTest{7, 5, 1, 2},

vlib/math/big/integer.v

@@ -415,38 +415,31 @@ pub fn (multiplicand Integer) * (multiplier Integer) Integer {
 //
 // DO NOT use this method if the divisor has any chance of being 0.
 fn (dividend Integer) div_mod_internal(divisor Integer) (Integer, Integer) {
-	$if debug {
-		assert divisor.signum != 0
-	}
-
-	if dividend.signum == 0 {
-		return zero_int, zero_int
-	}
-	if divisor == one_int {
-		return dividend.clone(), zero_int
-	}
-	if divisor.signum == -1 {
-		q, r := dividend.div_mod_internal(divisor.neg())
-		return q.neg(), r
-	}
-	if dividend.signum == -1 {
-		q, r := dividend.neg().div_mod_internal(divisor)
-		if r.signum == 0 {
-			return q.neg(), zero_int
-		} else {
-			return q.neg() - one_int, divisor - r
-		}
-	}
-	// Division for positive integers
 	mut q := []u32{cap: int_max(1, dividend.digits.len - divisor.digits.len + 1)}
 	mut r := []u32{cap: dividend.digits.len}
+	mut q_signum := 0
+	mut r_signum := 0
+
 	divide_digit_array(dividend.digits, divisor.digits, mut q, mut r)
+	if dividend.signum > 0 && divisor.signum > 0 {
+		q_signum = 1
+		r_signum = 1
+	} else if dividend.signum > 0 && divisor.signum < 0 {
+		q_signum = -1
+		r_signum = 1
+	} else if dividend.signum < 0 && divisor.signum > 0 {
+		q_signum = -1
+		r_signum = -1
+	} else {
+		q_signum = 1
+		r_signum = -1
+	}
 	quotient := Integer{
-		signum: if q.len == 0 { 0 } else { 1 }
+		signum: if q.len == 0 { 0 } else { q_signum }
 		digits: q
 	}
 	remainder := Integer{
-		signum: if r.len == 0 { 0 } else { 1 }
+		signum: if r.len == 0 { 0 } else { r_signum }
 		digits: r
 	}
 	return quotient, remainder