// Copyright (c) 2026 Storm Dragon
// Authors: Storm Dragon, Angus Kola
// SPDX-License-Identifier: GPL-3.0-or-later

package ocr

import (
	"context"
	"image"
	"image/color"
)

// Preprocess returns owned pixels in recognition coordinates. Scale uses nearest
// neighbor sampling so recognized word boxes map back through the capture transform.
func Preprocess(ctx context.Context, source image.Image, options Options) (image.Image, error) {
	if source == nil || !valid_dimensions(source.Bounds().Dx(), source.Bounds().Dy()) {
		return nil, ErrInvalidImage
	}
	scale := options.Scale
	if scale == 0 {
		scale = 1
	}
	width, height := source.Bounds().Dx(), source.Bounds().Dy()
	if scale < 1 || scale > 4 || height > MaxPixels/scale || !valid_dimensions(width*scale, height*scale) ||
		options.Threshold < 0 || options.Threshold > 255 {
		return nil, ErrLimit
	}
	if scale == 1 && !options.Grayscale && !options.Invert && !options.BlackWhite {
		return source, nil
	}
	processed := image.NewRGBA(image.Rect(0, 0, width*scale, height*scale))
	for y := 0; y < height*scale; y++ {
		if y%32 == 0 && ctx.Err() != nil {
			return nil, ctx.Err()
		}
		for x := 0; x < width*scale; x++ {
			r, g, b, _ := source.At(source.Bounds().Min.X+x/scale, source.Bounds().Min.Y+y/scale).RGBA()
			red, green, blue := uint8(r>>8), uint8(g>>8), uint8(b>>8)
			gray := uint8((299*uint32(red) + 587*uint32(green) + 114*uint32(blue) + 500) / 1000)
			if options.Grayscale || options.BlackWhite {
				red, green, blue = gray, gray, gray
			}
			if options.Invert {
				red, green, blue = 255-red, 255-green, 255-blue
			}
			if options.BlackWhite {
				value := uint8(0)
				if int(red) >= options.Threshold {
					value = 255
				}
				red, green, blue = value, value, value
			}
			processed.SetRGBA(x, y, color.RGBA{R: red, G: green, B: blue, A: 255})
		}
	}
	return processed, ctx.Err()
}