Add Split function (#128)

* Split function

* Add comment and test

converter/go.mod

@@ -1,6 +1,6 @@
 module converter
 
-go 1.22
+go 1.23
 
 require (
 	github.com/sunshineplan/imgconv v0.0.0-00010101000000-000000000000

go.mod

@@ -1,6 +1,6 @@
 module github.com/sunshineplan/imgconv
 
-go 1.21
+go 1.22
 
 require (
 	github.com/sunshineplan/pdf v1.0.7

split.go

@@ -0,0 +1,78 @@
+package imgconv
+
+import (
+	"errors"
+	"image"
+)
+
+// SplitMode defines the mode in which the image will be split
+type SplitMode int
+
+const (
+	// SplitHorizontalMode splits the image horizontally
+	SplitHorizontalMode SplitMode = iota
+	// SplitVerticalMode splits the image vertically
+	SplitVerticalMode
+)
+
+func split(base image.Rectangle, n int, mode SplitMode) (rects []image.Rectangle) {
+	var width, height int
+	if mode == SplitHorizontalMode {
+		width = base.Dx() / n
+		height = base.Dy()
+	} else {
+		width = base.Dx()
+		height = base.Dy() / n
+	}
+	if width == 0 || height == 0 {
+		return
+	}
+	for i := range n {
+		var r image.Rectangle
+		if mode == SplitHorizontalMode {
+			r = image.Rect(
+				base.Min.X+width*i, base.Min.Y,
+				base.Min.X+width*(i+1), base.Min.Y+height,
+			)
+		} else {
+			r = image.Rect(
+				base.Min.X, base.Min.Y+height*i,
+				base.Min.X+width, base.Min.Y+height*(i+1),
+			)
+		}
+		rects = append(rects, r)
+	}
+	return
+}
+
+// Split splits an image into n smaller images based on the specified split mode.
+// If n is less than 1, or the image cannot be split, it returns an error.
+func Split(base image.Image, n int, mode SplitMode) (imgs []image.Image, err error) {
+	if n < 1 {
+		return nil, errors.New("invalid number of parts: must be at least 1")
+	}
+	if img, ok := base.(interface {
+		SubImage(image.Rectangle) image.Image
+	}); ok {
+		rects := split(base.Bounds(), n, mode)
+		if len(rects) == 0 {
+			return nil, errors.New("failed to split the image: invalid dimensions or n is too large")
+		}
+		for _, rect := range rects {
+			imgs = append(imgs, img.SubImage(rect))
+		}
+	} else {
+		return nil, errors.New("image type does not support SubImage extraction")
+	}
+	return
+}
+
+// SplitHorizontal splits an image into n parts horizontally.
+func SplitHorizontal(base image.Image, n int) ([]image.Image, error) {
+	return Split(base, n, SplitHorizontalMode)
+}
+
+// SplitVertical splits an image into n parts vertically.
+func SplitVertical(base image.Image, n int) ([]image.Image, error) {
+	return Split(base, n, SplitVerticalMode)
+}

split_test.go

@@ -0,0 +1,75 @@
+package imgconv
+
+import (
+	"image"
+	"slices"
+	"testing"
+)
+
+func TestSplit(t *testing.T) {
+	for i, testcase := range []struct {
+		base image.Rectangle
+		n    int
+		mode SplitMode
+		want []image.Rectangle
+	}{
+		{image.Rect(0, 0, 100, 100), 4, SplitHorizontalMode, []image.Rectangle{
+			image.Rect(0, 0, 25, 100),
+			image.Rect(25, 0, 50, 100),
+			image.Rect(50, 0, 75, 100),
+			image.Rect(75, 0, 100, 100),
+		}},
+		{image.Rect(0, 0, 100, 100), 4, SplitVerticalMode, []image.Rectangle{
+			image.Rect(0, 0, 100, 25),
+			image.Rect(0, 25, 100, 50),
+			image.Rect(0, 50, 100, 75),
+			image.Rect(0, 75, 100, 100),
+		}},
+		{image.Rect(100, 100, 200, 200), 4, SplitHorizontalMode, []image.Rectangle{
+			image.Rect(100, 100, 125, 200),
+			image.Rect(125, 100, 150, 200),
+			image.Rect(150, 100, 175, 200),
+			image.Rect(175, 100, 200, 200),
+		}},
+		{image.Rect(100, 100, 200, 200), 4, SplitVerticalMode, []image.Rectangle{
+			image.Rect(100, 100, 200, 125),
+			image.Rect(100, 125, 200, 150),
+			image.Rect(100, 150, 200, 175),
+			image.Rect(100, 175, 200, 200),
+		}},
+	} {
+		if rects := split(testcase.base, testcase.n, testcase.mode); slices.CompareFunc(
+			rects,
+			testcase.want,
+			func(a, b image.Rectangle) int {
+				if a.Eq(b) {
+					return 0
+				}
+				return 1
+			},
+		) != 0 {
+			t.Errorf("#%d wrong split results: want %v, got %v", i, testcase.want, rects)
+		}
+	}
+}
+
+func TestSplitError(t *testing.T) {
+	r := image.Rect(0, 0, 100, 100)
+	img := image.NewNRGBA(r)
+	if _, err := Split(img, 10, SplitHorizontalMode); err != nil {
+		t.Fatal(err)
+	}
+	for i, testcase := range []struct {
+		img image.Image
+		n   int
+	}{
+		{r, 10},
+		{img, 0},
+		{img, 101},
+		{image.NewNRGBA(image.Rectangle{}), 10},
+	} {
+		if _, err := Split(testcase.img, testcase.n, SplitHorizontalMode); err == nil {
+			t.Errorf("#%d want error, got nil", i)
+		}
+	}
+}