f32: nicer Affine2D string formatting

Signed-off-by: Egon Elbre <egonelbre@gmail.com>

f32/affine.go

@@ -3,8 +3,8 @@
 package f32
 
 import (
-	"fmt"
 	"math"
+	"strconv"
 )
 
 // Affine2D represents an affine 2D transformation. The zero value of Affine2D
@@ -148,5 +148,25 @@ func (a Affine2D) shear(radiansX, radiansY float32) Affine2D {
 
 func (a Affine2D) String() string {
 	sx, hx, ox, hy, sy, oy := a.Elems()
-	return fmt.Sprintf("[[%f %f %f] [%f %f %f]]", sx, hx, ox, hy, sy, oy)
+
+	// precision 6, one period, negative sign and space per number
+	const prec = 6
+	const charsPerFloat = prec + 2 + 1
+	s := make([]byte, 0, 6*charsPerFloat+6)
+
+	s = append(s, '[', '[')
+	s = strconv.AppendFloat(s, float64(sx), 'g', prec, 32)
+	s = append(s, ' ')
+	s = strconv.AppendFloat(s, float64(hx), 'g', prec, 32)
+	s = append(s, ' ')
+	s = strconv.AppendFloat(s, float64(ox), 'g', prec, 32)
+	s = append(s, ']', ' ', '[')
+	s = strconv.AppendFloat(s, float64(hy), 'g', prec, 32)
+	s = append(s, ' ')
+	s = strconv.AppendFloat(s, float64(sy), 'g', prec, 32)
+	s = append(s, ' ')
+	s = strconv.AppendFloat(s, float64(oy), 'g', prec, 32)
+	s = append(s, ']', ']')
+
+	return string(s)
 }

f32/affine_test.go

@@ -37,6 +37,29 @@ func TestTransformOffset(t *testing.T) {
 	}
 }
 
+func TestString(t *testing.T) {
+	tests := []struct {
+		in  Affine2D
+		exp string
+	}{
+		{
+			in:  NewAffine2D(9, 11, 13, 17, 19, 23),
+			exp: "[[9 11 13] [17 19 23]]",
+		}, {
+			in:  NewAffine2D(29, 31, 37, 43, 47, 53),
+			exp: "[[29 31 37] [43 47 53]]",
+		}, {
+			in:  NewAffine2D(29.142342, 31.4123412, 37.53152, 43.51324213, 47.123412, 53.14312342),
+			exp: "[[29.1423 31.4123 37.5315] [43.5132 47.1234 53.1431]]",
+		},
+	}
+	for _, test := range tests {
+		if test.in.String() != test.exp {
+			t.Errorf("string mismatch: have %q, want %q", test.in.String(), test.exp)
+		}
+	}
+}
+
 func TestTransformScale(t *testing.T) {
 	p := Point{X: 1, Y: 2}
 	s := Point{X: -1, Y: 2}