package imgcache

import (
	"bytes"
	"context"
	"errors"
	"fmt"
	"image"
	"image/gif"
	"image/jpeg"
	"image/png"
	"io"

	"github.com/disintegration/imaging"
	"golang.org/x/image/webp"
)

// MaxInputDimension is the maximum allowed width or height for input images.
// Images larger than this are rejected to prevent DoS via decompression bombs.
const MaxInputDimension = 8192

// ErrInputTooLarge is returned when input image dimensions exceed MaxInputDimension.
var ErrInputTooLarge = errors.New("input image dimensions exceed maximum")

// ImageProcessor implements the Processor interface using pure Go libraries.
type ImageProcessor struct{}

// NewImageProcessor creates a new image processor.
func NewImageProcessor() *ImageProcessor {
	return &ImageProcessor{}
}

// Process transforms an image according to the request.
func (p *ImageProcessor) Process(
	_ context.Context,
	input io.Reader,
	req *ImageRequest,
) (*ProcessResult, error) {
	// Read input
	data, err := io.ReadAll(input)
	if err != nil {
		return nil, fmt.Errorf("failed to read input: %w", err)
	}

	// Decode image
	img, format, err := p.decode(data)
	if err != nil {
		return nil, fmt.Errorf("failed to decode image: %w", err)
	}

	// Get original dimensions
	bounds := img.Bounds()
	origWidth := bounds.Dx()
	origHeight := bounds.Dy()

	// Determine target dimensions
	targetWidth := req.Size.Width
	targetHeight := req.Size.Height

	// If both are 0, keep original size
	if targetWidth == 0 && targetHeight == 0 {
		targetWidth = origWidth
		targetHeight = origHeight
	}

	// Resize if needed
	if targetWidth != origWidth || targetHeight != origHeight {
		img = p.resize(img, targetWidth, targetHeight, req.FitMode)
	}

	// Determine output format
	outputFormat := req.Format
	if outputFormat == FormatOriginal || outputFormat == "" {
		outputFormat = p.formatFromString(format)
	}

	// Encode to target format
	output, err := p.encode(img, outputFormat, req.Quality)
	if err != nil {
		return nil, fmt.Errorf("failed to encode: %w", err)
	}

	finalBounds := img.Bounds()

	return &ProcessResult{
		Content:       io.NopCloser(bytes.NewReader(output)),
		ContentLength: int64(len(output)),
		ContentType:   ImageFormatToMIME(outputFormat),
		Width:         finalBounds.Dx(),
		Height:        finalBounds.Dy(),
	}, nil
}

// SupportedInputFormats returns MIME types this processor can read.
func (p *ImageProcessor) SupportedInputFormats() []string {
	return []string{
		string(MIMETypeJPEG),
		string(MIMETypePNG),
		string(MIMETypeGIF),
		string(MIMETypeWebP),
	}
}

// SupportedOutputFormats returns formats this processor can write.
func (p *ImageProcessor) SupportedOutputFormats() []ImageFormat {
	return []ImageFormat{
		FormatJPEG,
		FormatPNG,
		FormatGIF,
		// WebP encoding not supported in pure Go, will fall back to PNG
	}
}

// decode decodes image data into an image.Image.
func (p *ImageProcessor) decode(data []byte) (image.Image, string, error) {
	r := bytes.NewReader(data)

	// Try standard formats first
	img, format, err := image.Decode(r)
	if err == nil {
		return img, format, nil
	}

	// Try WebP
	r.Reset(data)

	img, err = webp.Decode(r)
	if err == nil {
		return img, "webp", nil
	}

	return nil, "", fmt.Errorf("unsupported image format")
}

// resize resizes the image according to the fit mode.
func (p *ImageProcessor) resize(img image.Image, width, height int, fit FitMode) image.Image {
	switch fit {
	case FitCover:
		// Resize and crop to fill exact dimensions
		return imaging.Fill(img, width, height, imaging.Center, imaging.Lanczos)

	case FitContain:
		// Resize to fit within dimensions, maintaining aspect ratio
		return imaging.Fit(img, width, height, imaging.Lanczos)

	case FitFill:
		// Resize to exact dimensions (may distort)
		return imaging.Resize(img, width, height, imaging.Lanczos)

	case FitInside:
		// Same as contain, but only shrink
		bounds := img.Bounds()
		origW := bounds.Dx()
		origH := bounds.Dy()

		if origW <= width && origH <= height {
			return img // Already fits
		}

		return imaging.Fit(img, width, height, imaging.Lanczos)

	case FitOutside:
		// Resize so smallest dimension fits, may exceed target on other dimension
		bounds := img.Bounds()
		origW := bounds.Dx()
		origH := bounds.Dy()

		scaleW := float64(width) / float64(origW)
		scaleH := float64(height) / float64(origH)

		scale := scaleW
		if scaleH > scaleW {
			scale = scaleH
		}

		newW := int(float64(origW) * scale)
		newH := int(float64(origH) * scale)

		return imaging.Resize(img, newW, newH, imaging.Lanczos)

	default:
		// Default to cover
		return imaging.Fill(img, width, height, imaging.Center, imaging.Lanczos)
	}
}

const defaultQuality = 85

// encode encodes an image to the specified format.
func (p *ImageProcessor) encode(img image.Image, format ImageFormat, quality int) ([]byte, error) {
	if quality <= 0 {
		quality = defaultQuality
	}

	var buf bytes.Buffer

	switch format {
	case FormatJPEG:
		err := jpeg.Encode(&buf, img, &jpeg.Options{Quality: quality})
		if err != nil {
			return nil, err
		}

	case FormatPNG:
		err := png.Encode(&buf, img)
		if err != nil {
			return nil, err
		}

	case FormatGIF:
		err := gif.Encode(&buf, img, nil)
		if err != nil {
			return nil, err
		}

	case FormatWebP:
		// Pure Go doesn't have WebP encoder, fall back to PNG
		// TODO: Add WebP encoding support via external library
		err := png.Encode(&buf, img)
		if err != nil {
			return nil, err
		}

	case FormatAVIF:
		// AVIF not supported in pure Go, fall back to JPEG
		err := jpeg.Encode(&buf, img, &jpeg.Options{Quality: quality})
		if err != nil {
			return nil, err
		}

	default:
		return nil, fmt.Errorf("unsupported output format: %s", format)
	}

	return buf.Bytes(), nil
}

// formatFromString converts a format string to ImageFormat.
func (p *ImageProcessor) formatFromString(format string) ImageFormat {
	switch format {
	case "jpeg":
		return FormatJPEG
	case "png":
		return FormatPNG
	case "gif":
		return FormatGIF
	case "webp":
		return FormatWebP
	default:
		return FormatJPEG
	}
}