package utils

import (
	"bytes"
	"encoding/base64"
	"fmt"
	"image"
	"image/gif"
	"image/jpeg"
	"image/png"
	"net/http"

	"golang.org/x/image/draw"
)

// CompressImage takes a base‑64‑encoded image (JPEG, PNG or GIF) and returns
// a base‑64‑encoded version that is at most maxLength in size, or an error.
func CompressImage(b64 string, maxLength int) (string, error) {
	raw, err := base64.StdEncoding.DecodeString(b64)
	if err != nil {
		return "", fmt.Errorf("base64 decode: %w", err)
	}
	if len(raw) <= maxLength {
		return b64, nil // small enough already
	}

	switch mime := http.DetectContentType(raw); mime {
	case "image/gif":
		return compressGIF(raw, maxLength)
	default: // jpeg, png, webp, etc. → treat as raster
		return compressRaster(raw, maxLength)
	}
}

// ---------- Raster path (jpeg / png / single‑frame gif) ----------

func compressRaster(src []byte, maxLength int) (string, error) {
	img, _, err := image.Decode(bytes.NewReader(src))
	if err != nil {
		return "", fmt.Errorf("decode raster: %w", err)
	}

	quality := 95
	for {
		var buf bytes.Buffer
		if err := jpeg.Encode(&buf, img, &jpeg.Options{Quality: quality}); err != nil {
			return "", fmt.Errorf("jpeg encode: %w", err)
		}
		if buf.Len() <= maxLength {
			return base64.StdEncoding.EncodeToString(buf.Bytes()), nil
		}

		if quality > 20 {
			quality -= 5
			continue
		}

		// down‑scale 80%
		b := img.Bounds()
		if b.Dx() < 100 || b.Dy() < 100 {
			return "", fmt.Errorf("cannot compress below %.02fMiB without destroying image", float64(maxLength)/1048576.0)
		}
		dst := image.NewRGBA(image.Rect(0, 0, int(float64(b.Dx())*0.8), int(float64(b.Dy())*0.8)))
		draw.ApproxBiLinear.Scale(dst, dst.Bounds(), img, b, draw.Over, nil)
		img = dst
		quality = 95 // restart ladder
	}
}

// ---------- Animated GIF path ----------

func compressGIF(src []byte, maxLength int) (string, error) {
	g, err := gif.DecodeAll(bytes.NewReader(src))
	if err != nil {
		return "", fmt.Errorf("gif decode: %w", err)
	}

	for {
		var buf bytes.Buffer
		if err := gif.EncodeAll(&buf, g); err != nil {
			return "", fmt.Errorf("gif encode: %w", err)
		}
		if buf.Len() <= maxLength {
			return base64.StdEncoding.EncodeToString(buf.Bytes()), nil
		}

		// down‑scale every frame by 80%
		w, h := g.Config.Width, g.Config.Height
		if w < 100 || h < 100 {
			return "", fmt.Errorf("cannot compress animated GIF below 5 MiB without excessive quality loss")
		}

		nw, nh := int(float64(w)*0.8), int(float64(h)*0.8)
		for i, frm := range g.Image {
			// convert paletted frame → RGBA for scaling
			rgba := image.NewRGBA(frm.Bounds())
			draw.Draw(rgba, rgba.Bounds(), frm, frm.Bounds().Min, draw.Src)

			// scaled destination
			dst := image.NewRGBA(image.Rect(0, 0, nw, nh))
			draw.ApproxBiLinear.Scale(dst, dst.Bounds(), rgba, rgba.Bounds(), draw.Over, nil)

			// quantize back to paletted using default encoder quantizer
			paletted := image.NewPaletted(dst.Bounds(), nil)
			draw.FloydSteinberg.Draw(paletted, paletted.Bounds(), dst, dst.Bounds().Min)

			g.Image[i] = paletted
		}
		g.Config.Width, g.Config.Height = nw, nh
		// loop back and test size again …
	}
}

// ---------- Helpers for precise MIME decodes (optional) ----------

func decode(r *bytes.Reader, mime string) (image.Image, error) {
	switch mime {
	case "image/jpeg":
		return jpeg.Decode(r)
	case "image/png":
		return png.Decode(r)
	case "image/gif":
		// for single‑frame GIFs only – call DecodeAll in caller if animated
		return gif.Decode(r)
	default:
		i, _, err := image.Decode(r)
		return i, err // let the stdlib guess the format
	}
}