// Command xfwm4gen renders the Das OS xfwm4 window decorations.
//
// xfwm4 themes are a directory of image slices plus a themerc file. Drawing
// ~90 slices by hand is error-prone, so they are generated from one palette:
//
//	xfwm4gen -variant light -out build/DasOS/xfwm4
package main

import (
	"flag"
	"fmt"
	"image"
	"image/color"
	"image/png"
	"log"
	"math"
	"os"
	"path/filepath"
	"strings"
)

const (
	titleHeight  = 32
	frameWidth   = 3
	cornerRadius = 8.0
	cornerWidth  = 12
	buttonWidth  = 30
	circleRadius = 11.0
	glyphHalf    = 4.0
	strokeWidth  = 1.5
	samples      = 4 // supersampling per axis, gives 16 samples per pixel
)

type palette struct {
	titleBg, border, separator, glyph, text color.NRGBA
}

type variant struct {
	active, inactive                  palette
	hover, pressed                    color.NRGBA
	closeHover, closePressed, onClose color.NRGBA
}

var variants = map[string]variant{
	"light": {
		active: palette{
			titleBg: rgb(0xe9ecf1), border: rgb(0xb4bcc9), separator: rgb(0xd3d8e1),
			glyph: rgb(0x2e3440), text: rgb(0x1f2430),
		},
		inactive: palette{
			titleBg: rgb(0xf4f5f8), border: rgb(0xccd2db), separator: rgb(0xe2e5eb),
			glyph: rgb(0x9aa1ad), text: rgb(0x8a909c),
		},
		hover: rgb(0xd5dae3), pressed: rgb(0xc3cad6),
		closeHover: rgb(0xe0464c), closePressed: rgb(0xc2353b), onClose: rgb(0xffffff),
	},
	"dark": {
		active: palette{
			titleBg: rgb(0x2b2f38), border: rgb(0x0e1014), separator: rgb(0x1d2026),
			glyph: rgb(0xe3e6ec), text: rgb(0xeceff4),
		},
		inactive: palette{
			titleBg: rgb(0x24272e), border: rgb(0x15171b), separator: rgb(0x1b1d22),
			glyph: rgb(0x7c8391), text: rgb(0x8a909c),
		},
		hover: rgb(0x3b404b), pressed: rgb(0x4a505d),
		closeHover: rgb(0xe0464c), closePressed: rgb(0xc2353b), onClose: rgb(0xffffff),
	},
}

func rgb(v uint32) color.NRGBA {
	return color.NRGBA{R: uint8(v >> 16), G: uint8(v >> 8), B: uint8(v), A: 0xff}
}

func hex(c color.NRGBA) string {
	return fmt.Sprintf("#%02x%02x%02x", c.R, c.G, c.B)
}

// shader returns the colour of a point in pixel coordinates; transparent outside the shape.
type shader func(x, y float64) color.NRGBA

// render rasterises a shader with supersampling, averaging in premultiplied space.
func render(w, h int, fn shader) *image.NRGBA {
	img := image.NewNRGBA(image.Rect(0, 0, w, h))
	const n = samples * samples
	for py := 0; py < h; py++ {
		for px := 0; px < w; px++ {
			var r, g, b, a float64
			for sy := 0; sy < samples; sy++ {
				for sx := 0; sx < samples; sx++ {
					c := fn(float64(px)+(float64(sx)+0.5)/samples, float64(py)+(float64(sy)+0.5)/samples)
					ca := float64(c.A) / 255
					r += float64(c.R) * ca
					g += float64(c.G) * ca
					b += float64(c.B) * ca
					a += ca
				}
			}
			if a == 0 {
				continue
			}
			img.SetNRGBA(px, py, color.NRGBA{
				R: uint8(math.Round(r / a)), G: uint8(math.Round(g / a)), B: uint8(math.Round(b / a)),
				A: uint8(math.Round(a / n * 255)),
			})
		}
	}
	return img
}

// titleRow is the colour of the title bar at height y: top border, fill, separator above the client.
func titleRow(p palette, y float64) color.NRGBA {
	switch {
	case y < 1:
		return p.border
	case y >= titleHeight-1:
		return p.separator
	default:
		return p.titleBg
	}
}

// insideRounded reports whether (x, y) lies in a shape whose top-left corner is
// rounded with radius r at origin (ox, oy) and which extends right and down.
func insideRounded(x, y, ox, oy, r float64) bool {
	if x < ox || y < oy {
		return false
	}
	if x < ox+r && y < oy+r {
		return math.Hypot(x-(ox+r), y-(oy+r)) <= r
	}
	return true
}

func topLeft(p palette) shader {
	return func(x, y float64) color.NRGBA {
		switch {
		case !insideRounded(x, y, 0, 0, cornerRadius):
			return color.NRGBA{}
		case !insideRounded(x, y, 1, 1, cornerRadius-1):
			return p.border
		case x < frameWidth:
			return p.titleBg
		default:
			return titleRow(p, y)
		}
	}
}

func mirror(fn shader, w int) shader {
	return func(x, y float64) color.NRGBA { return fn(float64(w)-x, y) }
}

func leftEdge(p palette) shader {
	return func(x, _ float64) color.NRGBA {
		if x < 1 {
			return p.border
		}
		return p.titleBg
	}
}

func bottomEdge(p palette) shader {
	return func(_, y float64) color.NRGBA {
		if y >= frameWidth-1 {
			return p.border
		}
		return p.titleBg
	}
}

func bottomLeft(p palette) shader {
	return func(x, y float64) color.NRGBA {
		if x < 1 || y >= frameWidth-1 {
			return p.border
		}
		return p.titleBg
	}
}

type segment struct{ x1, y1, x2, y2 float64 }

// glyph is a button symbol drawn around the button centre.
type glyph struct {
	lines []segment
	ring  float64 // stroked circle radius, 0 for none
	dot   float64 // filled circle radius, 0 for none
}

func square(x1, y1, x2, y2 float64) []segment {
	return []segment{{x1, y1, x2, y1}, {x2, y1, x2, y2}, {x2, y2, x1, y2}, {x1, y2, x1, y1}}
}

var glyphs = func() map[string]glyph {
	const g = glyphHalf
	return map[string]glyph{
		"close":            {lines: []segment{{-g, -g, g, g}, {-g, g, g, -g}}},
		"hide":             {lines: []segment{{-g, 3, g, 3}}},
		"maximize":         {lines: square(-g, -g, g, g)},
		"maximize-toggled": {lines: append(square(-g, -g+2.5, g-2.5, g), segment{-g + 2.5, -g, g, -g}, segment{g, -g, g, g - 2.5})},
		"menu":             {lines: []segment{{-g, -3, g, -3}, {-g, 0, g, 0}, {-g, 3, g, 3}}},
		"shade":            {lines: []segment{{-g, 2, 0, -2}, {0, -2, g, 2}}},
		"shade-toggled":    {lines: []segment{{-g, -2, 0, 2}, {0, 2, g, -2}}},
		"stick":            {ring: g},
		"stick-toggled":    {dot: g},
	}
}()

func distToSegment(px, py float64, s segment) float64 {
	dx, dy := s.x2-s.x1, s.y2-s.y1
	t := ((px-s.x1)*dx + (py-s.y1)*dy) / (dx*dx + dy*dy)
	t = math.Max(0, math.Min(1, t))
	return math.Hypot(px-(s.x1+t*dx), py-(s.y1+t*dy))
}

func (g glyph) covers(x, y float64) bool {
	for _, s := range g.lines {
		if distToSegment(x, y, s) <= strokeWidth/2 {
			return true
		}
	}
	r := math.Hypot(x, y)
	if g.ring > 0 && math.Abs(r-g.ring) <= strokeWidth/2 {
		return true
	}
	return g.dot > 0 && r <= g.dot
}

func button(p palette, g glyph, circle, ink color.NRGBA) shader {
	cx, cy := buttonWidth/2.0, titleHeight/2.0
	return func(x, y float64) color.NRGBA {
		dx, dy := x-cx, y-cy
		switch {
		case g.covers(dx, dy):
			return ink
		case circle.A != 0 && math.Hypot(dx, dy) <= circleRadius:
			return circle
		default:
			return titleRow(p, y)
		}
	}
}

type piece struct {
	name string
	w, h int
	fn   shader
}

func pieces(v variant) []piece {
	var out []piece
	for _, state := range []string{"active", "inactive"} {
		p := v.active
		if state == "inactive" {
			p = v.inactive
		}
		out = append(out,
			piece{"top-left-" + state, cornerWidth, titleHeight, topLeft(p)},
			piece{"top-right-" + state, cornerWidth, titleHeight, mirror(topLeft(p), cornerWidth)},
			piece{"left-" + state, frameWidth, 1, leftEdge(p)},
			piece{"right-" + state, frameWidth, 1, mirror(leftEdge(p), frameWidth)},
			piece{"bottom-" + state, 1, frameWidth, bottomEdge(p)},
			piece{"bottom-left-" + state, frameWidth, frameWidth, bottomLeft(p)},
			piece{"bottom-right-" + state, frameWidth, frameWidth, mirror(bottomLeft(p), frameWidth)},
		)
		for i := 1; i <= 5; i++ {
			out = append(out, piece{fmt.Sprintf("title-%d-%s", i, state), 1, titleHeight,
				func(_, y float64) color.NRGBA { return titleRow(p, y) }})
		}
	}
	for name, g := range glyphs {
		hover, pressed, ink := v.hover, v.pressed, v.active.glyph
		if name == "close" {
			hover, pressed, ink = v.closeHover, v.closePressed, v.onClose
		}
		out = append(out,
			piece{name + "-active", buttonWidth, titleHeight, button(v.active, g, color.NRGBA{}, v.active.glyph)},
			piece{name + "-inactive", buttonWidth, titleHeight, button(v.inactive, g, color.NRGBA{}, v.inactive.glyph)},
			piece{name + "-prelight", buttonWidth, titleHeight, button(v.active, g, hover, ink)},
			piece{name + "-pressed", buttonWidth, titleHeight, button(v.active, g, pressed, ink)},
		)
	}
	return out
}

func themerc(v variant) string {
	return strings.Join([]string{
		"# Generated by xfwm4gen, do not edit.",
		"active_text_color=" + hex(v.active.text),
		"inactive_text_color=" + hex(v.inactive.text),
		"button_offset=4",
		"button_spacing=0",
		"full_width_title=true",
		"maximized_offset=0",
		"show_app_icon=true",
		"title_alignment=center",
		"title_horizontal_offset=0",
		"title_shadow_active=false",
		"title_shadow_inactive=false",
		"title_vertical_offset_active=0",
		"title_vertical_offset_inactive=0",
		"",
	}, "\n")
}

func writePNG(path string, img image.Image) error {
	f, err := os.Create(path)
	if err != nil {
		return err
	}
	if err := png.Encode(f, img); err != nil {
		f.Close()
		return fmt.Errorf("encode %s: %w", path, err)
	}
	return f.Close()
}

func generate(v variant, dir string) error {
	if err := os.MkdirAll(dir, 0o755); err != nil {
		return err
	}
	for _, p := range pieces(v) {
		if err := writePNG(filepath.Join(dir, p.name+".png"), render(p.w, p.h, p.fn)); err != nil {
			return err
		}
	}
	return os.WriteFile(filepath.Join(dir, "themerc"), []byte(themerc(v)), 0o644)
}

func main() {
	name := flag.String("variant", "light", "colour variant: light or dark")
	out := flag.String("out", "", "output directory (the theme's xfwm4 folder)")
	flag.Parse()

	v, ok := variants[*name]
	if !ok || *out == "" {
		flag.Usage()
		os.Exit(2)
	}
	if err := generate(v, *out); err != nil {
		log.Fatalf("xfwm4gen: %v", err)
	}
}