Merge pull request #314 from travisturner/decimal-adjust-precision

Adjust decimal precision if we have decimal places to sacrifice.

pql/decimal.go

@@ -396,6 +396,17 @@ func ParseDecimal(s string) (Decimal, error) {
 		scale = 0
 	}
 
+	// If the mantissa can't be represented by an int64, but it contains
+	// enough decimal places such that we can sacrifice precision, then
+	// we do that. This is an attempt to be compatible with the way
+	// `strconv.ParseFloat` works.
+	if m, s, ok := reducePrecision(sign, mantissa, scale); ok {
+		mantissa = m
+		scale = s
+	} else {
+		return Decimal{}, errors.Errorf("value out of range: %s", mantissa)
+	}
+
 	// We have to use ParseUint here (as opposed to ParseInt) because
 	// math.MinInt64 is a valid value, but its absolute value is not.
 	// So this allows us to handle that one value without overflow, and
@@ -420,6 +431,48 @@ func ParseDecimal(s string) (Decimal, error) {
 	}, nil
 }
 
+// reducePrecision takes a []byte mantissa and scale, and if possible
+// will adjust the mantissa (by reducing precision) until it can be
+// represented by an int64. The returned bool indicates whether the
+// reduction was successful.
+func reducePrecision(sign bool, mantissa []byte, scale int64) ([]byte, int64, bool) {
+	// Trim leading zeros before considering length.
+	var zeroIdx int
+	for i := range mantissa {
+		if mantissa[i] == '0' {
+			zeroIdx++
+		} else {
+			break
+		}
+	}
+	mantissa = mantissa[zeroIdx:]
+
+	// If we zero out the mantissa to an empty
+	// string, that means it's value should be 0.
+	if len(mantissa) == 0 {
+		mantissa = []byte{'0'}
+		return mantissa, scale, true
+	}
+
+	lenMantissa := len(mantissa)
+	maxStr := "9223372036854775807"
+	if sign {
+		maxStr = "9223372036854775808"
+	}
+
+	if lenMantissa <= 18 || (lenMantissa == 19 && string(mantissa) <= maxStr) {
+		return mantissa, scale, true
+	}
+
+	// If we don't have any decimal places to sacrifice,
+	// we can't change anything.
+	if scale <= 0 {
+		return mantissa, scale, false
+	}
+
+	return reducePrecision(sign, mantissa[:len(mantissa)-1], scale-1)
+}
+
 func quotient(d Decimal) int64 {
 	if d.Scale == 0 {
 		return d.Value

pql/decimal_test.go

@@ -67,6 +67,15 @@ func TestDecimal(t *testing.T) {
 			{"9223372036854775807000", pql.Decimal{9223372036854775807, -3}, ""},
 			{"-9223372036854775807000", pql.Decimal{-9223372036854775807, -3}, ""},
 
+			// precision adjustment
+			{"2.666666666666666667", pql.Decimal{2666666666666666667, 18}, ""},
+			{"2.6666666666666666667", pql.Decimal{2666666666666666666, 18}, ""},
+			{"2.6666666666666666666667", pql.Decimal{2666666666666666666, 18}, ""},
+			{"-9.223372036854775808", pql.Decimal{-9223372036854775808, 18}, ""},
+			{"-9.223372036854775809", pql.Decimal{-922337203685477580, 17}, ""},
+			{"9.223372036854775807", pql.Decimal{9223372036854775807, 18}, ""},
+			{"9.223372036854775808", pql.Decimal{922337203685477580, 17}, ""},
+
 			// Error cases.
 			{"", pql.Decimal{}, "decimal string is empty"},
 			{"-", pql.Decimal{}, "decimal string is empty"},
@@ -75,8 +84,9 @@ func TestDecimal(t *testing.T) {
 			{"0.12.3", pql.Decimal{}, "invalid decimal string"},
 			{"--12300", pql.Decimal{}, "invalid syntax"},
 
-			{"922337203685477580.9", pql.Decimal{}, "value out of range"},
-			{"-922337203685477580.9", pql.Decimal{}, "value out of range"},
+			// These are no longer error cases since we introduced precision adjustment.
+			//{"922337203685477580.9", pql.Decimal{}, "value out of range"},
+			//{"-922337203685477580.9", pql.Decimal{}, "value out of range"},
 			{"9223372036854775808000", pql.Decimal{}, "value out of range"},
 			{"-9223372036854775809000", pql.Decimal{}, "value out of range"},
 		}