khosra/internal/ext/shortcodes/images.go

package shortcodes

import (
	"bytes"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"image"
	_ "image/gif"
	"image/jpeg"
	"image/png"
	"io/fs"
	"log/slog"
	"os"
	"path"
	"path/filepath"
	"strconv"
	"strings"

	"golang.org/x/image/draw"
	_ "golang.org/x/image/webp"

	"khosra/internal/content"
	"khosra/internal/render"
)

// widths are the derivative sizes offered to a browser. Three, spanning phone to desktop: enough for srcset
// to matter and few enough that the startup pass stays quick. They become a setting when someone wants a
// different set, not before.
var widths = []int{480, 960, 1440}

// Derive writes every missing derivative for the pictures under content/, returning how many it made.
//
// Ahead of the request, never during one: resampling takes long enough to be felt, and there is no page cache
// to hide it (ADR-0042). Idempotent, because a derivative is named after its source's content — so a rerun
// stats and skips, and an edited picture simply has a different name. Originals are only ever read.
func Derive(siteFS fs.FS, cacheDir string) (int, error) {
	if siteFS == nil || cacheDir == "" {
		return 0, nil
	}
	if err := os.MkdirAll(cacheDir, 0o755); err != nil {
		return 0, fmt.Errorf("derivative cache %s: %w", cacheDir, err)
	}
	made := 0
	err := fs.WalkDir(siteFS, "content", func(p string, d fs.DirEntry, err error) error {
		if err != nil || d.IsDir() || !derivable(p) {
			return err
		}
		data, err := fs.ReadFile(siteFS, p)
		if err != nil {
			slog.Error("skipping unreadable picture", "path", p, "err", err)
			return nil
		}
		n, err := derive(data, p, cacheDir)
		if err != nil {
			// One unreadable picture must not stop a site from starting (ADR-0029).
			slog.Error("skipping picture", "path", p, "err", err)
			return nil
		}
		made += n
		return nil
	})
	if err != nil {
		return made, fmt.Errorf("walk for pictures: %w", err)
	}
	return made, nil
}

// derive writes the derivatives one picture is missing.
func derive(data []byte, name, cacheDir string) (int, error) {
	src, _, err := image.Decode(bytes.NewReader(data))
	if err != nil {
		return 0, err
	}
	made := 0
	for _, w := range widths {
		if src.Bounds().Dx() <= w {
			// Never upscale: a bigger file that looks worse is not a derivative worth having.
			continue
		}
		file := filepath.Join(cacheDir, derivedName(data, name, w))
		if _, err := os.Stat(file); err == nil {
			continue
		}
		if err := write(file, scale(src, w), name); err != nil {
			return made, err
		}
		made++
	}
	return made, nil
}

// scale resamples to a target width, keeping the aspect ratio.
//
// CatmullRom because the alternative in the standard library is nearest neighbour, which is visibly wrong on
// photographs — the reason ADR-0040 accepted a dependency at all.
func scale(src image.Image, width int) image.Image {
	b := src.Bounds()
	height := b.Dy() * width / b.Dx()
	out := image.NewRGBA(image.Rect(0, 0, width, height))
	draw.CatmullRom.Scale(out, out.Bounds(), src, b, draw.Over, nil)
	return out
}

// write encodes an image beside its siblings in the cache, atomically.
//
// Through a temporary file and a rename, so a derivative is either absent or complete: a half-written one
// would be served as a broken image and, being content-named, never regenerated.
func write(file string, img image.Image, source string) error {
	tmp, err := os.CreateTemp(filepath.Dir(file), ".khosra-*")
	if err != nil {
		return err
	}
	defer os.Remove(tmp.Name())
	if err := encode(tmp, img, source); err != nil {
		tmp.Close()
		return err
	}
	if err := tmp.Close(); err != nil {
		return err
	}
	return os.Rename(tmp.Name(), file)
}

// encode writes PNG for sources that may carry transparency and JPEG for the rest, which is also what
// derivedName spells in the extension.
func encode(w *os.File, img image.Image, source string) error {
	if lossless(source) {
		return png.Encode(w, img)
	}
	return jpeg.Encode(w, img, &jpeg.Options{Quality: 82})
}

// Picture describes one image for a fragment: where to fetch it, what a browser may choose instead, and the
// intrinsic size, so a page reserves the right box before any bytes arrive.
//
// Src stays the original: a derivative is an optimisation, and a browser that ignores srcset still gets the
// picture the author put there (ADR-0042).
func picture(origin render.Origin, file string) (render.Picture, bool) {
	if origin.Files == nil || !showable(file) {
		return render.Picture{}, false
	}
	// The author's file is the picture, whatever the engine can make of it. An SVG has no decoder here and an
	// AVIF has none anywhere, so both are rendered as they are, without a size or a srcset — dropping them
	// would delete content because the engine cannot optimise it.
	p := render.Picture{Src: file}
	if !derivable(file) {
		return p, true
	}
	name := path.Join(origin.Dir, file)
	data, err := fs.ReadFile(origin.Files, name)
	if err != nil {
		slog.Error("picture unreadable", "path", name, "err", err)
		return p, true
	}
	cfg, _, err := image.DecodeConfig(bytes.NewReader(data))
	if err != nil {
		slog.Warn("picture kept but not sized: cannot decode", "path", name, "err", err)
		return p, true
	}
	p.Width, p.Height = cfg.Width, cfg.Height
	var sources []string
	for _, w := range widths {
		if cfg.Width <= w {
			continue
		}
		sources = append(sources, content.DerivedURL(derivedName(data, name, w))+" "+strconv.Itoa(w)+"w")
	}
	if len(sources) > 0 {
		// The original closes the set at its own width, so a wide viewport still has the best file to pick.
		sources = append(sources, file+" "+strconv.Itoa(cfg.Width)+"w")
		p.Srcset = strings.Join(sources, ", ")
	}
	return p, true
}

// derivedName is the cache filename for one source at one width: the source's content hash, so an edit
// changes the name and nothing stale is ever served, plus the width and the encoding's extension.
func derivedName(data []byte, source string, width int) string {
	sum := sha256.Sum256(data)
	ext := ".jpg"
	if lossless(source) {
		ext = ".png"
	}
	return hex.EncodeToString(sum[:8]) + "-w" + strconv.Itoa(width) + ext
}

// lossless reports whether a source may carry transparency, which decides the derivative's encoding.
func lossless(source string) bool {
	switch strings.ToLower(path.Ext(source)) {
	case ".png", ".gif", ".webp":
		return true
	}
	return false
}

// showable reports whether a browser can display the file: what belongs in a gallery, whether or not this
// engine can do anything clever with it.
func showable(name string) bool {
	switch strings.ToLower(path.Ext(name)) {
	case ".jpg", ".jpeg", ".png", ".gif", ".webp", ".svg", ".avif":
		return true
	}
	return false
}

// derivable reports whether the file can be decoded and resampled here. SVG needs no resizing and AVIF has no
// decoder available, so neither gets derivatives (ADR-0040).
func derivable(name string) bool {
	switch strings.ToLower(path.Ext(name)) {
	case ".jpg", ".jpeg", ".png", ".gif", ".webp":
		return true
	}
	return false
}