package imageprocessor

import (
	"bytes"
	"image"
	"image/color"
	"image/png"
	"math"
	"testing"
)

// TestImageProcessor_FitCoverServesThinSource verifies that FitCover serves
// a thin source at the requested size. An 8192x1 source scaled to cover
// 8192x8192 is 8192 times as long as the output on its long side. The centre
// of an 8192x1 source with the proportions of 1x8192 is 1/8192 of a pixel
// wide. Each case is also turned on its side.
func TestImageProcessor_FitCoverServesThinSource(t *testing.T) {
	t.Parallel()

	tests := []struct {
		name                    string
		inputWidth, inputHeight int
		size                    Size
	}{
		{"8192x1 at 8192x8192", 8192, 1, Size{Width: 8192, Height: 8192}},
		{"8192x1 at 1x8192", 8192, 1, Size{Width: 1, Height: 8192}},
		{"1x8192 at 8192x8192", 1, 8192, Size{Width: 8192, Height: 8192}},
		{"1x8192 at 8192x1", 1, 8192, Size{Width: 8192, Height: 1}},
	}

	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			t.Parallel()

			input := createTestJPEG(t, tt.inputWidth, tt.inputHeight)

			result, err := New(Params{}).Process(t.Context(),
				bytes.NewReader(input),
				&Request{Size: tt.size, Format: FormatJPEG, FitMode: FitCover})
			if err != nil {
				t.Fatalf("Process() error = %v", err)
			}

			defer func() { _ = result.Content.Close() }()

			t.Logf("Process() made %dx%d", result.Width, result.Height)

			if result.Width != tt.size.Width || result.Height != tt.size.Height {
				t.Errorf("Process() made %dx%d, want %dx%d",
					result.Width, result.Height, tt.size.Width, tt.size.Height)
			}
		})
	}
}

// createBandsPNG creates a PNG of three equal bands, red, green and blue,
// side by side when it is wider than tall and top to bottom otherwise.
func createBandsPNG(t *testing.T, width, height int) []byte {
	t.Helper()

	bands := []color.RGBA{
		{R: 255, A: 255},
		{G: 255, A: 255},
		{B: 255, A: 255},
	}

	img := image.NewRGBA(image.Rect(0, 0, width, height))
	for y := range height {
		for x := range width {
			band := 3 * y / height
			if width > height {
				band = 3 * x / width
			}

			img.Set(x, y, bands[band])
		}
	}

	return encodePNG(t, img)
}

// TestImageProcessor_FitCoverCropsTheCentre verifies that FitCover keeps the
// centre of the source: a 300x100 source of red, green and blue bands side
// by side, or a 100x300 one with them top to bottom, comes out at 50x50 as
// its green band.
func TestImageProcessor_FitCoverCropsTheCentre(t *testing.T) {
	t.Parallel()

	tests := []struct {
		name                    string
		inputWidth, inputHeight int
	}{
		{"bands side by side", 300, 100},
		{"bands top to bottom", 100, 300},
	}

	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			t.Parallel()

			input := createBandsPNG(t, tt.inputWidth, tt.inputHeight)

			result, err := New(Params{}).Process(t.Context(),
				bytes.NewReader(input),
				&Request{
					Size:    Size{Width: 50, Height: 50},
					Format:  FormatPNG,
					FitMode: FitCover,
				})
			if err != nil {
				t.Fatalf("Process() error = %v", err)
			}

			defer func() { _ = result.Content.Close() }()

			output, err := png.Decode(result.Content)
			if err != nil {
				t.Fatalf("failed to decode output: %v", err)
			}

			bounds := output.Bounds()
			t.Logf("Process() made %dx%d", bounds.Dx(), bounds.Dy())

			if bounds.Dx() != 50 || bounds.Dy() != 50 {
				t.Fatalf("Process() made %dx%d, want 50x50",
					bounds.Dx(), bounds.Dy())
			}

			// Resampling may blend a little of the bands beside the
			// green one into the output's edges.
			for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
				for x := bounds.Min.X; x < bounds.Max.X; x++ {
					r, g, b, _ := output.At(x, y).RGBA()
					if r>>8 > 64 || g>>8 < 192 || b>>8 > 64 {
						t.Fatalf("output pixel %d,%d is %d,%d,%d, want green",
							x, y, r>>8, g>>8, b>>8)
					}
				}
			}
		})
	}
}

// The greys of createSquarePNG, and the grey halfway between them.
const (
	backgroundGrey = 192
	squareGrey     = 64
	halfwayGrey    = (backgroundGrey + squareGrey) / 2
)

// createSquarePNG creates a grey PNG side pixels square with a darker grey
// square squareSide pixels wide in its centre.
func createSquarePNG(t *testing.T, side, squareSide int) []byte {
	t.Helper()

	first := (side - squareSide) / 2
	last := first + squareSide - 1

	img := image.NewGray(image.Rect(0, 0, side, side))
	for y := range side {
		for x := range side {
			grey := uint8(backgroundGrey)
			if x >= first && x <= last && y >= first && y <= last {
				grey = squareGrey
			}

			img.SetGray(x, y, color.Gray{Y: grey})
		}
	}

	return encodePNG(t, img)
}

// squareLength returns how long the square is along greys, the greys of a
// row or column of the output that crosses it: the distance between the
// first and the last point where greys passes halfwayGrey, or 0 when there
// is no such point. Resampling blurs the square's edges over several
// pixels, and each point is placed between the two pixels on either side
// of it in proportion to their greys, so the length is measured to a
// fraction of a pixel.
func squareLength(greys []float64) float64 {
	first, last := -1.0, -1.0

	for i := 1; i < len(greys); i++ {
		before, after := greys[i-1], greys[i]
		if (before > halfwayGrey) == (after > halfwayGrey) {
			continue
		}

		point := float64(i-1) + (before-halfwayGrey)/(before-after)
		if first < 0 {
			first = point
		}

		last = point
	}

	return last - first
}

// TestImageProcessor_FitCoverKeepsProportions verifies that FitCover scales
// both sides of the source by the same amount: a square in the centre of
// the source comes out square. Its width and height are measured along the
// output's middle row and column.
func TestImageProcessor_FitCoverKeepsProportions(t *testing.T) {
	t.Parallel()

	tests := []struct {
		name             string
		side, squareSide int
		size             Size
	}{
		{"16x16 at 100x60", 16, 8, Size{Width: 100, Height: 60}},
		{"16x16 at 60x100", 16, 8, Size{Width: 60, Height: 100}},
		{"10x10 at 100x35", 10, 2, Size{Width: 100, Height: 35}},
		{"10x10 at 35x100", 10, 2, Size{Width: 35, Height: 100}},
		{"64x64 at 300x100", 64, 16, Size{Width: 300, Height: 100}},
		{"64x64 at 100x300", 64, 16, Size{Width: 100, Height: 300}},
	}

	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			t.Parallel()

			input := createSquarePNG(t, tt.side, tt.squareSide)

			result, err := New(Params{}).Process(t.Context(),
				bytes.NewReader(input),
				&Request{Size: tt.size, Format: FormatPNG, FitMode: FitCover})
			if err != nil {
				t.Fatalf("Process() error = %v", err)
			}

			defer func() { _ = result.Content.Close() }()

			output, err := png.Decode(result.Content)
			if err != nil {
				t.Fatalf("failed to decode output: %v", err)
			}

			bounds := output.Bounds()
			if bounds.Dx() != tt.size.Width || bounds.Dy() != tt.size.Height {
				t.Fatalf("Process() made %dx%d, want %dx%d",
					bounds.Dx(), bounds.Dy(), tt.size.Width, tt.size.Height)
			}

			row := make([]float64, bounds.Dx())
			for x := range row {
				r, _, _, _ := output.At(x, bounds.Dy()/2).RGBA()
				row[x] = float64(r >> 8)
			}

			column := make([]float64, bounds.Dy())
			for y := range column {
				r, _, _, _ := output.At(bounds.Dx()/2, y).RGBA()
				column[y] = float64(r >> 8)
			}

			width, height := squareLength(row), squareLength(column)
			t.Logf("square is %.2f wide and %.2f high", width, height)

			if width == 0 || math.Abs(width-height) > 0.5 {
				t.Errorf("square is %.2f wide and %.2f high, want it square",
					width, height)
			}
		})
	}
}