package video

import (
	"context"
	"fmt"
	"github.com/disintegration/imaging"
	"image"
	"image/png"
	"io"
	"os"
	"os/exec"
	"strings"
	"time"
)

var (
	ProcessingDeadline = 2 * time.Second
	// 75 concurrent ffmpeg processes are enough to swap the box out on their
	// own, bomb or no bomb.
	semaphore = make(chan struct{}, 16)
)

func generatePreview(ctx context.Context, inp string, out string) error {
	// Reject decompression bombs before ffmpeg expands a frame into memory.
	if err := checkGeometry(ctx, inp); err != nil {
		return err
	}
	// -max_alloc caps a single ffmpeg allocation: a 1080p frame needs ~8 MB, a
	// 16383x16383 bomb needs ~1 GB. Second line of defence behind checkGeometry,
	// and it works even when the probe told us nothing.
	cmd := exec.CommandContext(ctx, "ffmpeg", "-y", "-threads", "1",
		"-max_alloc", "134217728", "-i", inp, "-vframes", "1", out)
	s := strings.Builder{}
	cmd.Stdout = &s
	cmd.Stderr = &s
	err := cmd.Run()
	if err != nil {
		fmt.Println(s.String())
	}
	return err
}

func GeneratePreview(ctx context.Context, inp string, out string) error {
	select {
	case semaphore <- struct{}{}:
		ctx, cancel := context.WithTimeout(ctx, ProcessingDeadline)
		defer func() {
			cancel()
			<-semaphore
		}()
		return generatePreview(ctx, inp, out)
	case <-ctx.Done():
		return ctx.Err()
	}
}

func Blur(inp string, out string) error {
	f, err := os.Open(inp)
	if err != nil {
		return err
	}
	defer f.Close()

	// Same guard on the decode side: imaging.Blur keeps several copies of the
	// bitmap alive, so an oversized preview is an OOM all by itself.
	cfg, _, err := image.DecodeConfig(f)
	if err != nil {
		return err
	}
	if cfg.Width*cfg.Height > MaxPixels {
		return fmt.Errorf("preview %dx%d is too large to blur", cfg.Width, cfg.Height)
	}
	if _, err := f.Seek(0, io.SeekStart); err != nil {
		return err
	}

	img, _, err := image.Decode(f)
	if err != nil {
		return err
	}
	outimg := imaging.Blur(img, 5)
	outfile, err := os.Create(out)
	if err != nil {
		return err
	}
	defer outfile.Close()
	return png.Encode(outfile, outimg)
}