Fix incorrect calculation of decimal modulo (#258)

Author: mwoss

decimal.go

@@ -552,7 +552,7 @@ func (d Decimal) Div(d2 Decimal) Decimal {
 	return d.DivRound(d2, int32(DivisionPrecision))
 }
 
-// QuoRem does divsion with remainder
+// QuoRem does division with remainder
 // d.QuoRem(d2,precision) returns quotient q and remainder r such that
 //   d = d2 * q + r, q an integer multiple of 10^(-precision)
 //   0 <= r < abs(d2) * 10 ^(-precision) if d>=0
@@ -628,7 +628,7 @@ func (d Decimal) DivRound(d2 Decimal, precision int32) Decimal {
 
 // Mod returns d % d2.
 func (d Decimal) Mod(d2 Decimal) Decimal {
-	quo := d.Div(d2).Truncate(0)
+	quo := d.DivRound(d2, -d.exp+1).Truncate(0)
 	return d.Sub(d2.Mul(quo))
 }
 

decimal_test.go

@@ -2131,14 +2131,18 @@ func TestDecimal_Mod(t *testing.T) {
 	}
 
 	inputs := map[Inp]string{
-		Inp{"3", "2"}:                     "1",
-		Inp{"3451204593", "2454495034"}:   "996709559",
-		Inp{"24544.95034", ".3451204593"}: "0.3283950433",
-		Inp{".1", ".1"}:                   "0",
-		Inp{"0", "1.001"}:                 "0",
-		Inp{"-7.5", "2"}:                  "-1.5",
-		Inp{"7.5", "-2"}:                  "1.5",
-		Inp{"-7.5", "-2"}:                 "-1.5",
+		Inp{"3", "2"}:                           "1",
+		Inp{"3451204593", "2454495034"}:         "996709559",
+		Inp{"9999999999", "1275"}:               "324",
+		Inp{"9999999999.9999998", "1275.49"}:    "239.2399998",
+		Inp{"24544.95034", "0.3451204593"}:      "0.3283950433",
+		Inp{"0.499999999999999999", "0.25"}:     "0.249999999999999999",
+		Inp{"0.989512958912895912", "0.000001"}: "0.000000958912895912",
+		Inp{"0.1", "0.1"}:                       "0",
+		Inp{"0", "1.001"}:                       "0",
+		Inp{"-7.5", "2"}:                        "-1.5",
+		Inp{"7.5", "-2"}:                        "1.5",
+		Inp{"-7.5", "-2"}:                       "-1.5",
 	}
 
 	for inp, res := range inputs {