From 0cada0bdfec099cd2ce0c9f2fb1334289a1a598b Mon Sep 17 00:00:00 2001
From: nknordeen <nknordeen@gmail.com>
Date: Sat, 9 Jan 2021 21:15:12 -0700
Subject: [PATCH] optimize NewFromString a bit (#198)

* optimize NewFromString a bit

* add benchmark for large number

Co-authored-by: Nicholas Nordeen <nnordeen@fanatics.com>
---
 decimal.go            | 44 ++++++++++++++++++++++++++++++++-----------
 decimal_bench_test.go | 41 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 74 insertions(+), 11 deletions(-)

diff --git a/decimal.go b/decimal.go
index 75f845e..22f49e5 100644
--- a/decimal.go
+++ b/decimal.go
@@ -147,23 +147,45 @@ func NewFromString(value string) (Decimal, error) {
 		exp = expInt
 	}
 
-	parts := strings.Split(value, ".")
-	if len(parts) == 1 {
+	pIndex := -1
+	vLen := len(value)
+	for i := 0; i < vLen; i++ {
+		if value[i] == '.' {
+			if pIndex > -1 {
+				return Decimal{}, fmt.Errorf("can't convert %s to decimal: too many .s", value)
+			}
+			pIndex = i
+		}
+	}
+
+	if pIndex == -1 {
 		// There is no decimal point, we can just parse the original string as
 		// an int
 		intString = value
-	} else if len(parts) == 2 {
-		intString = parts[0] + parts[1]
-		expInt := -len(parts[1])
-		exp += int64(expInt)
 	} else {
-		return Decimal{}, fmt.Errorf("can't convert %s to decimal: too many .s", value)
+		if pIndex+1 < vLen {
+			intString = value[:pIndex] + value[pIndex+1:]
+		} else {
+			intString = value[:pIndex]
+		}
+		expInt := -len(value[pIndex+1:])
+		exp += int64(expInt)
 	}
 
-	dValue := new(big.Int)
-	_, ok := dValue.SetString(intString, 10)
-	if !ok {
-		return Decimal{}, fmt.Errorf("can't convert %s to decimal", value)
+	var dValue *big.Int
+	// strconv.ParseInt is faster than new(big.Int).SetString so this is just a shortcut for strings we know won't overflow
+	if len(intString) <= 18 {
+		parsed64, err := strconv.ParseInt(intString, 10, 64)
+		if err != nil {
+			return Decimal{}, fmt.Errorf("can't convert %s to decimal", value)
+		}
+		dValue = big.NewInt(parsed64)
+	} else {
+		dValue = new(big.Int)
+		_, ok := dValue.SetString(intString, 10)
+		if !ok {
+			return Decimal{}, fmt.Errorf("can't convert %s to decimal", value)
+		}
 	}
 
 	if exp < math.MinInt32 || exp > math.MaxInt32 {
diff --git a/decimal_bench_test.go b/decimal_bench_test.go
index b505eb0..fd3ddb6 100644
--- a/decimal_bench_test.go
+++ b/decimal_bench_test.go
@@ -1,6 +1,7 @@
 package decimal
 
 import (
+	"fmt"
 	"math"
 	"math/rand"
 	"sort"
@@ -183,3 +184,43 @@ func BenchmarkDecimal_IsInteger(b *testing.B) {
 		d.IsInteger()
 	}
 }
+
+func BenchmarkDecimal_NewFromString(b *testing.B) {
+	count := 72
+	prices := make([]string, 0, count)
+	for i := 1; i <= count; i++ {
+		prices = append(prices, fmt.Sprintf("%d.%d", i*100, i))
+	}
+
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		for _, p := range prices {
+			d, err := NewFromString(p)
+			if err != nil {
+				b.Log(d)
+				b.Error(err)
+			}
+		}
+	}
+}
+
+func BenchmarkDecimal_NewFromString_large_number(b *testing.B) {
+	count := 72
+	prices := make([]string, 0, count)
+	for i := 1; i <= count; i++ {
+		prices = append(prices, "9323372036854775807.9223372036854775807")
+	}
+
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		for _, p := range prices {
+			d, err := NewFromString(p)
+			if err != nil {
+				b.Log(d)
+				b.Error(err)
+			}
+		}
+	}
+}
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