// Package template reads a painted world map and turns it into the fields the geology solve needs.
//
// The map is an image; the legend beside it says what each colour means. The image is cylindrical: X
// wraps, Y does not, so the left and right edges are the same meridian and the top and bottom rows are
// the poles. Nothing here knows how big the world is - that is the planet package's job. This package
// only answers "what did the author paint here".
//
// The one rule that governs the whole design is Docs/Terrain-Next.md 3.2: paint the uplift, never the
// height. A painted heightmap is handed to a solver that erodes it into something else and throws away
// the drainage network, which is the reason the generator exists. So a class carries an uplift rate and
// an erodibility, and the solve makes the terrain.
package template

import (
	"bufio"
	"fmt"
	"image"
	"os"

	// Registered for image.Decode. JPEG is here because the first template anyone painted was a JPEG;
	// PNG is what a template should be, because JPEG bleeds colour across every class boundary and the
	// classifier then has to clean up after it.
	_ "image/jpeg"
	_ "image/png"
)

// DecodeRGB reads an image and returns tightly packed 8-bit RGB, three bytes a pixel, row-major.
//
// field.ReadHeightmap cannot be used for this and deliberately is not extended: it decodes PNG only, and
// its fallback branch collapses colour to luma, which is right for a DEM and destroys a painted map -
// two different classes can share a luma and here several nearly do.
func DecodeRGB(path string) (px []uint8, w, h int, err error) {
	px, _, w, h, err = decode(path, false)
	return px, w, h, err
}

// DecodeRGBA is DecodeRGB with the alpha channel kept alongside.
//
// It exists for the annotation layer and only for it. A class template is opaque by definition - every pixel
// is some class - so throwing alpha away there costs nothing. An overlay is the opposite: it is a transparent
// sheet with strokes on it, most of it is nothing, and "nothing" is exactly what alpha records. An image with
// no alpha comes back fully opaque, which is the right reading of a flattened export.
func DecodeRGBA(path string) (px, alpha []uint8, w, h int, err error) {
	return decode(path, true)
}

func decode(path string, wantAlpha bool) (px, alpha []uint8, w, h int, err error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, nil, 0, 0, err
	}
	defer f.Close()

	img, _, err := image.Decode(bufio.NewReaderSize(f, 1<<20))
	if err != nil {
		return nil, nil, 0, 0, fmt.Errorf("%s: %w", path, err)
	}

	b := img.Bounds()
	w, h = b.Dx(), b.Dy()
	if w <= 0 || h <= 0 {
		return nil, nil, 0, 0, fmt.Errorf("%s: empty image", path)
	}
	px = make([]uint8, w*h*3)
	if wantAlpha {
		alpha = make([]uint8, w*h)
		for i := range alpha {
			alpha[i] = 255
		}
	}

	// The fast paths matter: a 7738x3761 template is 29 million pixels, and going through the At()
	// interface for every one of them costs seconds rather than milliseconds.
	switch src := img.(type) {
	case *image.RGBA:
		// Premultiplied: the RGB bytes are already scaled by alpha, so a half-transparent red reads as a
		// darker red. Nothing here un-multiplies it, because every consumer that cares about alpha treats a
		// non-opaque pixel as blank and never looks at its colour.
		for y := 0; y < h; y++ {
			row := src.Pix[(y+b.Min.Y-src.Rect.Min.Y)*src.Stride:]
			off := (b.Min.X - src.Rect.Min.X) * 4
			for x := 0; x < w; x++ {
				o := (y*w + x) * 3
				px[o], px[o+1], px[o+2] = row[off+x*4], row[off+x*4+1], row[off+x*4+2]
				if alpha != nil {
					alpha[y*w+x] = row[off+x*4+3]
				}
			}
		}
	case *image.NRGBA:
		for y := 0; y < h; y++ {
			row := src.Pix[(y+b.Min.Y-src.Rect.Min.Y)*src.Stride:]
			off := (b.Min.X - src.Rect.Min.X) * 4
			for x := 0; x < w; x++ {
				o := (y*w + x) * 3
				px[o], px[o+1], px[o+2] = row[off+x*4], row[off+x*4+1], row[off+x*4+2]
				if alpha != nil {
					alpha[y*w+x] = row[off+x*4+3]
				}
			}
		}
	case *image.YCbCr:
		// What image/jpeg returns.
		for y := 0; y < h; y++ {
			for x := 0; x < w; x++ {
				yi := src.YOffset(b.Min.X+x, b.Min.Y+y)
				ci := src.COffset(b.Min.X+x, b.Min.Y+y)
				r, g, bl := ycbcrToRGB(src.Y[yi], src.Cb[ci], src.Cr[ci])
				o := (y*w + x) * 3
				px[o], px[o+1], px[o+2] = r, g, bl
			}
		}
	default:
		for y := 0; y < h; y++ {
			for x := 0; x < w; x++ {
				r, g, bl, a := img.At(b.Min.X+x, b.Min.Y+y).RGBA()
				o := (y*w + x) * 3
				px[o], px[o+1], px[o+2] = uint8(r>>8), uint8(g>>8), uint8(bl>>8)
				if alpha != nil {
					alpha[y*w+x] = uint8(a >> 8)
				}
			}
		}
	}
	return px, alpha, w, h, nil
}

// ycbcrToRGB is image/color's conversion, inlined so the YCbCr path does not allocate a color.Color per
// pixel. Same arithmetic, same rounding.
func ycbcrToRGB(y, cb, cr uint8) (uint8, uint8, uint8) {
	yy := int32(y) * 0x10101
	cb1 := int32(cb) - 128
	cr1 := int32(cr) - 128

	r := yy + 91881*cr1
	if uint32(r)&0xff000000 == 0 {
		r >>= 16
	} else {
		r = ^(r >> 31) & 0xffff >> 8
	}
	g := yy - 22554*cb1 - 46802*cr1
	if uint32(g)&0xff000000 == 0 {
		g >>= 16
	} else {
		g = ^(g >> 31) & 0xffff >> 8
	}
	b := yy + 116130*cb1
	if uint32(b)&0xff000000 == 0 {
		b >>= 16
	} else {
		b = ^(b >> 31) & 0xffff >> 8
	}
	return uint8(r), uint8(g), uint8(b)
}