Add AddDecimal support to pql

This allows us to add two Decimals together as long
as the result doesn't overflow int64 either in value or in scale.

pql/decimal.go

@@ -4,6 +4,7 @@ package pql
 import (
 	"fmt"
 	"math"
+	"math/big"
 	"strconv"
 	"strings"
 
@@ -79,6 +80,40 @@ func MinMax(scale int64) (Decimal, Decimal) {
 	return min, max
 }
 
+// AddDecimal adds a and b together and returns a new Decimal with the computed sum.
+//
+// If the Scale of a and b don't match, the returned Decimal will have the
+// smallest Scale needed to precisely represent the sum.
+func AddDecimal(a, b Decimal) (Decimal, bool) {
+	av, bv := big.NewInt(a.Value), big.NewInt(b.Value)
+
+	// if the scales dont match,
+	// we add zeros to the end of the one with the smaller scale until they match
+	// or we overflow
+	// then we add the values and return the decimal
+	as, bs := a.Scale, b.Scale
+	var ok bool
+	if a.Scale > b.Scale {
+		av, bv = bv, av
+		as, bs = bs, as
+	}
+
+	for as < bs {
+		av = av.Mul(av, big.NewInt(10))
+		as++
+	}
+
+	av = av.Add(av, bv)
+	ret, ok := av.Int64(), av.IsInt64()
+	if !ok {
+		return Decimal{}, ok
+	}
+	return Decimal{
+		Value: ret,
+		Scale: as,
+	}, ok
+}
+
 // LessThan returns true if d < d2.
 func (d Decimal) LessThan(d2 Decimal) bool {
 	return d.lessThan(d2, false)
@@ -388,7 +423,6 @@ func ParseDecimal(s string) (Decimal, error) {
 	} 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

pql/decimal_test.go

@@ -3,6 +3,7 @@ package pql_test
 
 import (
 	"encoding/json"
+	"fmt"
 	"reflect"
 	"strings"
 	"testing"
@@ -247,3 +248,86 @@ func TestDecimal(t *testing.T) {
 		})
 	})
 }
+
+type testAddDecimalCase struct {
+	a     pql.Decimal
+	b     pql.Decimal
+	exp   pql.Decimal
+	notOk bool
+}
+
+func TestAddDecimal(t *testing.T) {
+	toDecimal := func(a interface{}) pql.Decimal {
+		switch ac := a.(type) {
+		case string:
+			return mustParse(t, ac)
+		case pql.Decimal:
+			return ac
+		default:
+			t.Fatalf("cannot support type %T", ac)
+		}
+		return pql.Decimal{}
+	}
+
+	newTestAddDecimalCase := func(a, b, exp interface{}, notOk bool) testAddDecimalCase {
+		return testAddDecimalCase{
+			a:     toDecimal(a),
+			b:     toDecimal(b),
+			exp:   toDecimal(exp),
+			notOk: notOk,
+		}
+	}
+
+	tests := []testAddDecimalCase{
+		newTestAddDecimalCase("40.37", "40.37", "80.74", false),
+		newTestAddDecimalCase("18.5", "9.25", "27.75", false),
+		newTestAddDecimalCase("18.50", "9.25", "27.75", false),
+		newTestAddDecimalCase("-18.50", "-9.25", "-27.75", false),
+		newTestAddDecimalCase("-40.37", "40.37", "0", false),
+		newTestAddDecimalCase("-50.38", "-50.38", "-100.76", false),
+		newTestAddDecimalCase("-40.381", "-40.38", "-80.761", false),
+		newTestAddDecimalCase("-40.3700000000000000000001", "-50.38", "-90.7500000000000000000001", false),
+		newTestAddDecimalCase("10.37000000000000001", "-10", "0.37000000000000001", false),
+		// this is a weird one. because we reduce precision when we parse strings, we
+		// expect the value to be smaller than it really should be if you did
+		// the math yourself.
+		//
+		// there's not really a good way around this unless we use math/big.Int to represent
+		// our Value/Scale.
+		//
+		// but if we did this, to quote seebs: "i suspect that
+		// our performance would tank so badly by the time we were doing >63-bit
+		// numbers that there's no real-world benefit to us."
+		newTestAddDecimalCase("10.370000000000000001", "-10", "0.37", false),
+		newTestAddDecimalCase("-9223372036854775807", "-9223372036854775807", pql.Decimal{}, true),
+		newTestAddDecimalCase("9223372036854775807", "9223372036854775807", pql.Decimal{}, true),
+		newTestAddDecimalCase(pql.Decimal{Value: 0, Scale: 0}, pql.Decimal{Value: 1, Scale: 4}, pql.Decimal{Value: 1, Scale: 4}, false),
+	}
+
+	for i, test := range tests {
+		t.Run(fmt.Sprintf("%d", i), func(t *testing.T) {
+			if got, ok := pql.AddDecimal(test.a, test.b); !got.EqualTo(test.exp) {
+				t.Logf("%#v + %#v, expected %#v, got %#v", test.a, test.b, test.exp, got)
+				t.Errorf("%v + %v, expected %v, got %v", test.a, test.b, test.exp, got)
+			} else if !ok != test.notOk {
+				t.Errorf("expected %v, got %v", test.notOk, ok)
+			}
+
+			if got, ok := pql.AddDecimal(test.b, test.a); !got.EqualTo(test.exp) {
+				t.Logf("%#v + %#v, expected %#v, got %#v", test.a, test.b, test.exp, got)
+				t.Errorf("%v + %v, expected %v, got %v", test.b, test.a, test.exp, got)
+			} else if !ok != test.notOk {
+				t.Errorf("expected %v, got %v", test.notOk, ok)
+			}
+		})
+	}
+}
+
+func mustParse(t *testing.T, num string) pql.Decimal {
+	t.Helper()
+	d, err := pql.ParseDecimal(num)
+	if err != nil {
+		t.Fatalf("unexpected error parsing %s to pql.Decimal: %v", num, err)
+	}
+	return d
+}