package world

import (
	"image"
	"image/color"

	"github.com/fogleman/gg"
)

// PrimitiveDrawer is a low-level drawing backend used by the world renderer.
//
// The renderer is responsible for all torus logic, viewport/margin logic,
// coordinate projection, and primitive duplication. This interface only accepts
// final canvas pixel coordinates and exposes the minimum drawing operations
// needed to build and render paths.
//
// AddPoint, AddLine, and AddCircle append geometry to the current path.
// They do not render by themselves. The caller must finalize the path by
// calling Stroke or Fill.
//
// Save and Restore are intended for temporary local state changes such as
// clipping, colors, line width, or dash settings. After Restore, the outer
// drawing state must be visible again.
type PrimitiveDrawer interface {
	// Save stores the current drawing state.
	Save()

	// Restore restores the most recently saved drawing state.
	Restore()

	// ResetClip clears the current clipping region completely.
	ResetClip()

	// ClipRect intersects the current clipping region with the given rectangle
	// in canvas pixel coordinates.
	ClipRect(x, y, w, h float64)

	// SetStrokeColor sets the color used by Stroke.
	SetStrokeColor(c color.Color)

	// SetFillColor sets the color used by Fill.
	SetFillColor(c color.Color)

	// SetLineWidth sets the line width used by Stroke.
	SetLineWidth(width float64)

	// SetDash sets the dash pattern used by Stroke.
	// Passing no values clears the current dash pattern.
	SetDash(dashes ...float64)

	// SetDashOffset sets the dash phase used by Stroke.
	SetDashOffset(offset float64)

	// AddPoint appends a point marker centered at (x, y) with radius r
	// to the current path in canvas pixel coordinates.
	AddPoint(x, y, r float64)

	// AddLine appends a line segment to the current path in canvas pixel coordinates.
	AddLine(x1, y1, x2, y2 float64)

	// AddCircle appends a circle to the current path in canvas pixel coordinates.
	AddCircle(cx, cy, r float64)

	// Stroke renders the current path using the current stroke state.
	Stroke()

	// Fill renders the current path using the current fill state.
	Fill()

	// CopyShift shifts backing pixels by (dx,dy). Newly exposed areas become transparent/undefined;
	// caller is expected to ClearRectTo() the dirty areas before drawing.
	CopyShift(dx, dy int)

	// Clear operations must NOT change clip state.
	ClearAllTo(bg color.Color)
	ClearRectTo(x, y, w, h int, bg color.Color)
}

// ggClipRect stores one clip rectangle in canvas pixel coordinates.
// GGDrawer replays these rectangles on Restore because gg.Context Push/Pop
// do not restore clip masks the way this package expects.
type ggClipRect struct {
	x, y float64
	w, h float64
}

// GGDrawer is a PrimitiveDrawer implementation backed by gg.Context.
//
// It intentionally does not perform any world logic. It only forwards already
// projected canvas coordinates to gg while additionally maintaining a clip stack
// compatible with this package's Save/Restore contract.
type GGDrawer struct {
	DC *gg.Context

	clips     []ggClipRect
	clipStack [][]ggClipRect

	// scratch is a reusable buffer for CopyShift to avoid allocations.
	scratch *image.RGBA
}

// Save stores the current gg state and the current logical clip stack.
func (d *GGDrawer) Save() {
	d.DC.Push()

	snapshot := append([]ggClipRect(nil), d.clips...)
	d.clipStack = append(d.clipStack, snapshot)
}

// Restore restores the previous gg state and rebuilds the outer clip state.
//
// gg.Context.Pop restores most state from the stack, but its clip mask handling
// does not match this package's expected Save/Restore semantics. To preserve the
// contract, GGDrawer explicitly resets the clip and replays the previously saved
// clip rectangles after Pop.
func (d *GGDrawer) Restore() {
	if len(d.clipStack) == 0 {
		panic("GGDrawer: Restore without matching Save")
	}

	snapshot := d.clipStack[len(d.clipStack)-1]
	d.clipStack = d.clipStack[:len(d.clipStack)-1]

	d.DC.Pop()

	d.clips = append([]ggClipRect(nil), snapshot...)
	d.DC.ResetClip()
	for _, clip := range d.clips {
		d.DC.DrawRectangle(clip.x, clip.y, clip.w, clip.h)
		d.DC.Clip()
	}
}

// ResetClip clears the current clipping region and the logical clip stack
// for the active state frame.
func (d *GGDrawer) ResetClip() {
	d.DC.ResetClip()
	d.clips = nil
}

// ClipRect intersects the current clipping region with the given rectangle
// and records it so the clip can be reconstructed after Restore.
func (d *GGDrawer) ClipRect(x, y, w, h float64) {
	d.DC.DrawRectangle(x, y, w, h)
	d.DC.Clip()

	d.clips = append(d.clips, ggClipRect{x: x, y: y, w: w, h: h})
}

// SetStrokeColor sets the stroke color by installing a solid stroke pattern.
func (d *GGDrawer) SetStrokeColor(c color.Color) {
	d.DC.SetStrokeStyle(gg.NewSolidPattern(c))
}

// SetFillColor sets the fill color by installing a solid fill pattern.
func (d *GGDrawer) SetFillColor(c color.Color) {
	d.DC.SetFillStyle(gg.NewSolidPattern(c))
}

// SetLineWidth sets the line width used for stroking.
func (d *GGDrawer) SetLineWidth(width float64) {
	d.DC.SetLineWidth(width)
}

// SetDash sets the dash pattern used for stroking.
func (d *GGDrawer) SetDash(dashes ...float64) {
	d.DC.SetDash(dashes...)
}

// SetDashOffset sets the dash phase used for stroking.
func (d *GGDrawer) SetDashOffset(offset float64) {
	d.DC.SetDashOffset(offset)
}

// AddPoint appends a point marker to the current path.
func (d *GGDrawer) AddPoint(x, y, r float64) {
	d.DC.DrawPoint(x, y, r)
}

// AddLine appends a line segment to the current path.
func (d *GGDrawer) AddLine(x1, y1, x2, y2 float64) {
	d.DC.DrawLine(x1, y1, x2, y2)
}

// AddCircle appends a circle to the current path.
func (d *GGDrawer) AddCircle(cx, cy, r float64) {
	d.DC.DrawCircle(cx, cy, r)
}

// Stroke renders the current path using the current stroke state.
func (d *GGDrawer) Stroke() {
	d.DC.Stroke()
}

// Fill renders the current path using the current fill state.
func (d *GGDrawer) Fill() {
	d.DC.Fill()
}

// CopyShift shifts the backing RGBA image by (dx, dy) pixels.
// It clears newly exposed areas to transparent.
func (d *GGDrawer) CopyShift(dx, dy int) {
	if dx == 0 && dy == 0 {
		return
	}

	img, ok := d.DC.Image().(*image.RGBA)
	if !ok || img == nil {
		panic("GGDrawer.CopyShift: backing image is not *image.RGBA")
	}

	b := img.Bounds()
	w := b.Dx()
	h := b.Dy()
	if w <= 0 || h <= 0 {
		return
	}

	adx := abs(dx)
	ady := abs(dy)
	if adx >= w || ady >= h {
		// Everything shifts out of bounds => just clear.
		for i := range img.Pix {
			img.Pix[i] = 0
		}
		return
	}

	// Prepare scratch with the same bounds.
	if d.scratch == nil || d.scratch.Bounds().Dx() != w || d.scratch.Bounds().Dy() != h {
		d.scratch = image.NewRGBA(b)
	} else {
		// Clear scratch to transparent.
		for i := range d.scratch.Pix {
			d.scratch.Pix[i] = 0
		}
	}

	// Compute source/destination rectangles.
	dstX0 := 0
	dstY0 := 0
	srcX0 := 0
	srcY0 := 0
	if dx > 0 {
		dstX0 = dx
	} else {
		srcX0 = -dx
	}
	if dy > 0 {
		dstY0 = dy
	} else {
		srcY0 = -dy
	}

	copyW := w - max(dstX0, srcX0)
	copyH := h - max(dstY0, srcY0)
	if copyW <= 0 || copyH <= 0 {
		for i := range img.Pix {
			img.Pix[i] = 0
		}
		return
	}

	// Copy row-by-row (RGBA, 4 bytes per pixel).
	for row := 0; row < copyH; row++ {
		srcY := srcY0 + row
		dstY := dstY0 + row

		srcOff := srcY*img.Stride + srcX0*4
		dstOff := dstY*d.scratch.Stride + dstX0*4
		n := copyW * 4

		copy(d.scratch.Pix[dstOff:dstOff+n], img.Pix[srcOff:srcOff+n])
	}

	// Swap buffers by copying scratch into img.
	// (We keep img pointer stable for gg.Context.)
	copy(img.Pix, d.scratch.Pix)
}

func (d *GGDrawer) ClearAllTo(bg color.Color) {
	img, ok := d.DC.Image().(*image.RGBA)
	if !ok || img == nil {
		panic("GGDrawer.ClearAllTo: backing image is not *image.RGBA")
	}

	r, g, b, a := bg.RGBA()
	// Convert from 16-bit range to 8-bit.
	R := byte(r >> 8)
	G := byte(g >> 8)
	B := byte(b >> 8)
	A := byte(a >> 8)

	p := img.Pix
	for i := 0; i+3 < len(p); i += 4 {
		p[i+0] = R
		p[i+1] = G
		p[i+2] = B
		p[i+3] = A
	}
}

func (d *GGDrawer) ClearRectTo(x, y, w, h int, bg color.Color) {
	if w <= 0 || h <= 0 {
		return
	}

	img, ok := d.DC.Image().(*image.RGBA)
	if !ok || img == nil {
		panic("GGDrawer.ClearRectTo: backing image is not *image.RGBA")
	}

	bounds := img.Bounds()
	x0 := max(x, bounds.Min.X)
	y0 := max(y, bounds.Min.Y)
	x1 := min(x+w, bounds.Max.X)
	y1 := min(y+h, bounds.Max.Y)
	if x0 >= x1 || y0 >= y1 {
		return
	}

	r, g, b, a := bg.RGBA()
	R := byte(r >> 8)
	G := byte(g >> 8)
	B := byte(b >> 8)
	A := byte(a >> 8)

	rowBytes := (x1 - x0) * 4
	for yy := y0; yy < y1; yy++ {
		off := yy*img.Stride + x0*4
		for i := 0; i < rowBytes; i += 4 {
			img.Pix[off+i+0] = R
			img.Pix[off+i+1] = G
			img.Pix[off+i+2] = B
			img.Pix[off+i+3] = A
		}
	}
}