lib/decimal: follow-up for e6bad5174f

- Add a benchmark for CalbirateAndScale.
- Reduce the decimal multipliers table size from 256Kb to 192bytes.
- Use more clear naming for variables.

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/pull/5672

lib/decimal/decimal.go

@@ -7,20 +7,6 @@ import (
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/fastnum"
 )
 
-const tableLen = 32767
-
-var table = func() []int64 {
-	out := make([]int64, 0, tableLen)
-	var cur int64 = 10
-	for i := 1; i <= tableLen; i++ {
-		out = append(out, cur)
-		if cur*10 > 0 {
-			cur *= 10
-		}
-	}
-	return out
-}()
-
 // CalibrateScale calibrates a and b with the corresponding exponents ae, be
 // and returns the resulting exponent e.
 func CalibrateScale(a []int64, ae int16, b []int64, be int16) (e int16) {
@@ -49,29 +35,49 @@ func CalibrateScale(a []int64, ae int16, b []int64, be int16) (e int16) {
 		}
 	}
 	upExp -= downExp
+
 	if upExp > 0 {
-		times := table[upExp-1]
+		m := getDecimalMultiplier(uint16(upExp))
 		for i, v := range a {
 			if isSpecialValue(v) {
 				// Do not take into account special values.
 				continue
 			}
-			a[i] = v * times
+			a[i] = v * m
 		}
 	}
 	if downExp > 0 {
-		times := table[downExp-1]
-		for i, v := range b {
-			if isSpecialValue(v) {
-				// Do not take into account special values.
-				continue
+		if downExp > 18 {
+			for i, v := range b {
+				if isSpecialValue(v) {
+					// Do not take into account special values.
+					continue
+				}
+				b[i] = 0
+			}
+		} else {
+			m := getDecimalMultiplier(uint16(downExp))
+			for i, v := range b {
+				if isSpecialValue(v) {
+					// Do not take into account special values.
+					continue
+				}
+				b[i] = v / m
 			}
-			b[i] = v / times
 		}
 	}
 	return be + downExp
 }
 
+func getDecimalMultiplier(exp uint16) int64 {
+	if exp >= uint16(len(decimalMultipliers)) {
+		return 1
+	}
+	return decimalMultipliers[exp]
+}
+
+var decimalMultipliers = []int64{1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6, 1e7, 1e8, 1e9, 1e10, 1e11, 1e12, 1e13, 1e14, 1e15, 1e16, 1e17, 1e18}
+
 // ExtendFloat64sCapacity extends dst capacity to hold additionalItems
 // and returns the extended dst.
 func ExtendFloat64sCapacity(dst []float64, additionalItems int) []float64 {

lib/decimal/decimal_timing_test.go

@@ -7,6 +7,28 @@ import (
 	"testing"
 )
 
+func BenchmarkCalbirateScale(b *testing.B) {
+	aSrc := []int64{1, 2, 3, 4, 5, 67, 8, 9, 12, 324, 29, -34, -94, -84, -34, 0, 2, 3}
+	bSrc := []int64{2, 3, 4, 5, 67, 8, 9, 12, 324, 29, -34, -94, -84, -34, 0, 2, 3, 1}
+	const aScale = 1
+	const bScale = 2
+	const scaleExpected = 1
+
+	b.ReportAllocs()
+	b.SetBytes(int64(len(aSrc)))
+	b.RunParallel(func(pb *testing.PB) {
+		var a, b []int64
+		for pb.Next() {
+			a = append(a[:0], aSrc...)
+			b = append(b[:0], bSrc...)
+			scale := CalibrateScale(a, aScale, b, bScale)
+			if scale != scaleExpected {
+				panic(fmt.Errorf("unexpected scale; got %d; want %d", scale, scaleExpected))
+			}
+		}
+	})
+}
+
 func BenchmarkAppendDecimalToFloat(b *testing.B) {
 	b.Run("RealFloat", func(b *testing.B) {
 		benchmarkAppendDecimalToFloat(b, testVA, vaScale)