website/app/internal/gallery/thumbs.go

package gallery

import (
	"image"
	"image/jpeg"
	"io/fs"
	"os"
	"path/filepath"
	"strings"

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

const (
	gridThumbMaxEdge = 320
	heroThumbMaxEdge = 640
	gridThumbQuality = 75
	heroThumbQuality = 78
	thumbDirName     = "thumbs"
	heroDirName      = "hero"
)

// EnsureThumbnails creates grid and hero JPEG derivatives mirroring album folders.
func EnsureThumbnails(imagesDir string) error {
	if err := os.MkdirAll(filepath.Join(imagesDir, thumbDirName), 0o755); err != nil {
		return err
	}
	if err := os.MkdirAll(filepath.Join(imagesDir, heroDirName), 0o755); err != nil {
		return err
	}

	return filepath.WalkDir(imagesDir, func(path string, d fs.DirEntry, err error) error {
		if err != nil {
			return err
		}
		if d.IsDir() {
			if d.Name() == thumbDirName || d.Name() == heroDirName {
				return filepath.SkipDir
			}
			return nil
		}
		if !isJPEG(d.Name()) {
			return nil
		}

		rel, err := filepath.Rel(imagesDir, path)
		if err != nil {
			return err
		}
		rel = filepath.ToSlash(rel)
		if !safeRelPath(rel) {
			return nil
		}

		thumbDst := filepath.Join(imagesDir, thumbDirName, filepath.FromSlash(rel))
		heroDst := filepath.Join(imagesDir, heroDirName, filepath.FromSlash(rel))
		if err := os.MkdirAll(filepath.Dir(thumbDst), 0o755); err != nil {
			return err
		}
		if err := os.MkdirAll(filepath.Dir(heroDst), 0o755); err != nil {
			return err
		}
		if err := writeThumb(path, thumbDst, gridThumbMaxEdge, gridThumbQuality); err != nil {
			return err
		}
		return writeThumb(path, heroDst, heroThumbMaxEdge, heroThumbQuality)
	})
}

// EnsurePriority generates thumbnails for one image before first page load.
func EnsurePriority(imagesDir, relPath string) error {
	if !safeRelPath(relPath) || !isJPEG(filepath.Base(relPath)) {
		return nil
	}
	src := filepath.Join(imagesDir, filepath.FromSlash(relPath))
	thumbDst := filepath.Join(imagesDir, thumbDirName, filepath.FromSlash(relPath))
	heroDst := filepath.Join(imagesDir, heroDirName, filepath.FromSlash(relPath))
	if err := os.MkdirAll(filepath.Dir(thumbDst), 0o755); err != nil {
		return err
	}
	if err := os.MkdirAll(filepath.Dir(heroDst), 0o755); err != nil {
		return err
	}
	if err := writeThumb(src, heroDst, heroThumbMaxEdge, heroThumbQuality); err != nil {
		return err
	}
	return writeThumb(src, thumbDst, gridThumbMaxEdge, gridThumbQuality)
}

func writeThumb(src, dst string, maxEdge, quality int) error {
	srcInfo, err := os.Stat(src)
	if err != nil {
		return err
	}
	if dstInfo, err := os.Stat(dst); err == nil && !dstInfo.ModTime().Before(srcInfo.ModTime()) {
		return nil
	}

	f, err := os.Open(src)
	if err != nil {
		return err
	}
	defer f.Close()

	img, err := jpeg.Decode(f)
	if err != nil {
		return err
	}

	thumb := resizeThumb(img, maxEdge)
	out, err := os.Create(dst)
	if err != nil {
		return err
	}
	defer out.Close()

	return jpeg.Encode(out, thumb, &jpeg.Options{Quality: quality})
}

func resizeThumb(src image.Image, maxEdge int) image.Image {
	b := src.Bounds()
	w, h := b.Dx(), b.Dy()
	if w <= 0 || h <= 0 {
		return src
	}

	scale := float64(maxEdge) / float64(w)
	if h > w {
		scale = float64(maxEdge) / float64(h)
	}
	nw := int(float64(w) * scale)
	nh := int(float64(h) * scale)
	if nw < 1 {
		nw = 1
	}
	if nh < 1 {
		nh = 1
	}
	if nw >= w && nh >= h {
		return src
	}

	dst := image.NewRGBA(image.Rect(0, 0, nw, nh))
	draw.CatmullRom.Scale(dst, dst.Bounds(), src, b, draw.Over, nil)
	return dst
}

func isJPEG(name string) bool {
	ext := strings.ToLower(filepath.Ext(name))
	return ext == ".jpg" || ext == ".jpeg"
}