add Round: returns the float32/float64 of rounding half away from the…

… specified precision

add Truncate: returns the float32/float64 of the specified precision

math.go

@@ -1,8 +1,10 @@
 package lo
 
 import (
+	"fmt"
 	"github.com/samber/lo/internal/constraints"
 	"math"
+	"strconv"
 )
 
 // Range creates an array of numbers (positive and/or negative) with given length.
@@ -110,30 +112,26 @@ func MeanBy[T any, R constraints.Float | constraints.Integer](collection []T, it
 func Round[T float64 | float32](f T, n ...int) T {
 	var nn = 3
 	if len(n) > 0 {
-		nnn := n[0]
-		if nnn >= 0 {
-			nn = nnn
-			if nn > 15 {
-				nn = 15
-			}
+		nn = n[0]
+		if nn < 0 || nn > 15 {
+			panic("Round() precision must be between 0 and 15")
 		}
 	}
-	pow10N := math.Pow10(nn)
-	return T(math.Round(float64(f)*pow10N) / pow10N)
+	r, _ := strconv.ParseFloat(fmt.Sprintf("%.*f", nn, f), 64)
+	return T(r)
 }
 
 // Truncate returns the float32/float64 of the specified precision
 func Truncate[T float64 | float32](f T, n ...int) T {
 	var nn = 3
 	if len(n) > 0 {
-		nnn := n[0]
-		if nnn >= 0 {
-			nn = nnn
-			if nn > 15 {
-				nn = 15
-			}
+		nn = n[0]
+		if nn < 0 || nn > 15 {
+			panic("Truncate() precision must be between 0 and 15")
 		}
 	}
 	pow10N := math.Pow10(nn)
-	return T(math.Floor(float64(f)*pow10N) / pow10N)
+	integer, fractional := math.Modf(float64(f))
+	r, _ := strconv.ParseFloat(fmt.Sprintf("%.*f", nn, integer+math.Trunc(fractional*pow10N)/pow10N), 64)
+	return T(r)
 }

math_example_test.go

@@ -90,31 +90,44 @@ func ExampleRound() {
 	result2 := Round(1.23456, 2)
 	result3 := Round(1.23456, 3)
 	result4 := Round(1.23456, 7)
+	result5 := Round(1.234999999999999, 15)
+	result6 := Round(1.234999999999999, 7)
 
 	fmt.Printf("%v\n", result1)
 	fmt.Printf("%v\n", result2)
 	fmt.Printf("%v\n", result3)
 	fmt.Printf("%v\n", result4)
+	fmt.Printf("%v\n", result5)
+	fmt.Printf("%v\n", result6)
+
 	// Output:
 	// 1.235
 	// 1.23
 	// 1.235
 	// 1.23456
+	// 1.235
+	// 1.235
 }
 
 func ExampleTruncate() {
 	result1 := Truncate(1.23456)
 	result2 := Truncate(1.23456, 2)
 	result3 := Truncate(1.23456, 4)
 	result4 := Truncate(1.23456, 7)
+	result5 := Truncate(1.2349999999999999, 15)
+	result6 := Truncate(1.2349999999999999, 7)
 
 	fmt.Printf("%v\n", result1)
 	fmt.Printf("%v\n", result2)
 	fmt.Printf("%v\n", result3)
 	fmt.Printf("%v\n", result4)
+	fmt.Printf("%v\n", result5)
+	fmt.Printf("%v\n", result6)
 	// Output:
 	// 1.234
 	// 1.23
 	// 1.2345
 	// 1.23456
+	// 1.234999999999999
+	// 1.2349999
 }

math_test.go

@@ -137,8 +137,8 @@ func TestRound(t *testing.T) {
 	result3 := Round(1.23456, 2)
 	result4 := Round(1.23456, 3)
 	result5 := Round(1.23456, 7)
-	result6 := Round(1.23456, 16)
-	result7 := Round(1.23456, -1)
+	result6 := Round(1.23456, 15)
+	result7 := Round(1.23456789, 7)
 	result8 := Round(1.23456, 0)
 	result9 := Round(1.00000000001, 5)
 
@@ -148,7 +148,7 @@ func TestRound(t *testing.T) {
 	is.Equal(result4, 1.235)
 	is.Equal(result5, 1.23456)
 	is.Equal(result6, 1.23456)
-	is.Equal(result7, 1.235)
+	is.Equal(result7, 1.2345679)
 	is.Equal(result8, 1.0)
 	is.Equal(result9, 1.0)
 }
@@ -162,8 +162,8 @@ func TestTruncate(t *testing.T) {
 	result3 := Truncate(1.23456, 2)
 	result4 := Truncate(1.23456, 3)
 	result5 := Truncate(1.23456, 7)
-	result6 := Truncate(1.23456, 16)
-	result7 := Truncate(1.23456, -1)
+	result6 := Truncate(1.23456, 15)
+	result7 := Truncate(1.23456789, 7)
 	result8 := Truncate(1.23456, 0)
 	result9 := Truncate(1.00000000001, 5)
 
@@ -173,7 +173,7 @@ func TestTruncate(t *testing.T) {
 	is.Equal(result4, 1.234)
 	is.Equal(result5, 1.23456)
 	is.Equal(result6, 1.23456)
-	is.Equal(result7, 1.234)
+	is.Equal(result7, 1.2345678)
 	is.Equal(result8, 1.0)
 	is.Equal(result9, 1.0)
 }