Claude Opus 5andbdeshi
6447a995d1
The memo held one entry per picture ever rendered, for the life of the process. Correct for one author's laptop, wrong for what this engine is meant to be: a server that runs for months and serves a whole site to many readers. Least-recently-used rather than clearing when full, because a site has pages nobody opens for months and a front page opened every minute — discarding wholesale throws away exactly the entries about to be asked for again. A map into a recency-ordered list: reads promote, evictions take the back, both constant time. 1024 entries is a few hundred kilobytes. Generous enough that a normal site never evicts, bounded enough that no site can grow the process without limit. The number is a constant and not a setting, because a knob with one user is a knob nobody asked for. Eviction, replacement and the bound are tested, including under -race, since requests are concurrent and the store is shared. Latent item cleared. ext 1975/2000.
242 lines
8.0 KiB
Go
242 lines
8.0 KiB
Go
package shortcodes
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import (
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"bytes"
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"crypto/sha256"
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"encoding/hex"
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"fmt"
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"image"
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_ "image/gif"
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"image/jpeg"
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"image/png"
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"io/fs"
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"log/slog"
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"os"
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"path"
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"path/filepath"
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"strconv"
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"strings"
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"golang.org/x/image/draw"
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_ "golang.org/x/image/webp"
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"khosra/internal/content"
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"khosra/internal/render"
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)
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// widths are the derivative sizes offered to a browser. Three, spanning phone to desktop: enough for srcset
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// to matter and few enough that the startup pass stays quick. They become a setting when someone wants a
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// different set, not before.
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var widths = []int{480, 960, 1440}
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// Derive writes every missing derivative for the pictures under content/, returning how many it made.
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//
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// Ahead of the request, never during one: resampling takes long enough to be felt, and there is no page cache
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// to hide it (ADR-0042). Idempotent, because a derivative is named after its source's content — so a rerun
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// stats and skips, and an edited picture simply has a different name. Originals are only ever read.
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func Derive(siteFS fs.FS, cacheDir string) (int, error) {
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if siteFS == nil || cacheDir == "" {
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return 0, nil
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}
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if err := os.MkdirAll(cacheDir, 0o755); err != nil {
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return 0, fmt.Errorf("derivative cache %s: %w", cacheDir, err)
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}
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made := 0
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err := fs.WalkDir(siteFS, "content", func(p string, d fs.DirEntry, err error) error {
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if err != nil || d.IsDir() || !derivable(p) {
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return err
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}
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if strings.Contains(p, "/"+content.ExtrasDir+"/") {
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// Extras are supporting material, not published pictures: deriving sizes nothing references would
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// only slow startup down (content-model.md).
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return nil
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}
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data, err := fs.ReadFile(siteFS, p)
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if err != nil {
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slog.Error("skipping unreadable picture", "path", p, "err", err)
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return nil
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}
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n, err := derive(data, p, cacheDir)
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if err != nil {
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// One unreadable picture must not stop a site from starting (ADR-0029).
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slog.Error("skipping picture", "path", p, "err", err)
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return nil
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}
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made += n
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return nil
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})
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if err != nil {
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return made, fmt.Errorf("walk for pictures: %w", err)
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}
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return made, nil
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}
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// derive writes the derivatives one picture is missing.
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func derive(data []byte, name, cacheDir string) (int, error) {
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src, _, err := image.Decode(bytes.NewReader(data))
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if err != nil {
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return 0, err
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}
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made := 0
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for _, w := range widths {
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if src.Bounds().Dx() <= w {
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// Never upscale: a bigger file that looks worse is not a derivative worth having.
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continue
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}
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file := filepath.Join(cacheDir, derivedName(data, name, w))
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if _, err := os.Stat(file); err == nil {
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continue
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}
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if err := write(file, scale(src, w), name); err != nil {
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return made, err
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}
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made++
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}
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return made, nil
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}
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// scale resamples to a target width, keeping the aspect ratio.
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//
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// CatmullRom because the alternative in the standard library is nearest neighbour, which is visibly wrong on
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// photographs — the reason ADR-0040 accepted a dependency at all.
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func scale(src image.Image, width int) image.Image {
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b := src.Bounds()
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height := b.Dy() * width / b.Dx()
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out := image.NewRGBA(image.Rect(0, 0, width, height))
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draw.CatmullRom.Scale(out, out.Bounds(), src, b, draw.Over, nil)
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return out
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}
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// write encodes an image beside its siblings in the cache, atomically.
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//
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// Through a temporary file and a rename, so a derivative is either absent or complete: a half-written one
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// would be served as a broken image and, being content-named, never regenerated.
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func write(file string, img image.Image, source string) error {
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tmp, err := os.CreateTemp(filepath.Dir(file), ".khosra-*")
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if err != nil {
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return err
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}
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defer os.Remove(tmp.Name())
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if err := encode(tmp, img, source); err != nil {
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tmp.Close()
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return err
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}
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if err := tmp.Close(); err != nil {
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return err
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}
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return os.Rename(tmp.Name(), file)
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}
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// encode writes PNG for sources that may carry transparency and JPEG for the rest, which is also what
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// derivedName spells in the extension.
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func encode(w *os.File, img image.Image, source string) error {
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if lossless(source) {
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return png.Encode(w, img)
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}
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return jpeg.Encode(w, img, &jpeg.Options{Quality: 82})
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}
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// inspected remembers what each picture is.
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//
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// Measured, not assumed: without this the same bytes were read, hashed and decoded on every request, costing
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// ~102µs per picture — a twelve-picture gallery spent 1.2ms of its 1.24ms doing work it had already done
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// (BenchmarkGalleryPage in internal/web). conventions.md allows a cache in the render path once a benchmark
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// asks for one, and this is the smallest thing the benchmark asks for.
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//
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// Keyed by name, size and modification time, so an edited picture is inspected again rather than remembered
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// wrongly, and bounded, so a long-running process serving a large site cannot grow without limit (ADR-0073).
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var inspected = newMemo(remembered)
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// Picture describes one image for a fragment: where to fetch it, what a browser may choose instead, and the
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// intrinsic size, so a page reserves the right box before any bytes arrive.
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//
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// Src stays the original: a derivative is an optimisation, and a browser that ignores srcset still gets the
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// picture the author put there (ADR-0042).
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func picture(origin render.Origin, file string) (render.Picture, bool) {
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if origin.Files == nil || !showable(file) {
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return render.Picture{}, false
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}
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// The author's file is the picture, whatever the engine can make of it. An SVG has no decoder here and an
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// AVIF has none anywhere, so both are rendered as they are, without a size or a srcset — dropping them
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// would delete content because the engine cannot optimise it.
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p := render.Picture{Src: file}
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if !derivable(file) {
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return p, true
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}
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name := path.Join(origin.Dir, file)
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info, err := fs.Stat(origin.Files, name)
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if err != nil {
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slog.Error("picture unreadable", "path", name, "err", err)
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return p, true
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}
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key := fmt.Sprintf("%s\x00%d\x00%d", name, info.Size(), info.ModTime().UnixNano())
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if known, is := inspected.get(key); is {
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return known, true
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}
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data, err := fs.ReadFile(origin.Files, name)
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if err != nil {
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slog.Error("picture unreadable", "path", name, "err", err)
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return p, true
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}
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cfg, _, err := image.DecodeConfig(bytes.NewReader(data))
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if err != nil {
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slog.Warn("picture kept but not sized: cannot decode", "path", name, "err", err)
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return p, true
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}
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p.Width, p.Height = cfg.Width, cfg.Height
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var sources []string
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for _, w := range widths {
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if cfg.Width <= w {
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continue
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}
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sources = append(sources, content.DerivedURL(derivedName(data, name, w))+" "+strconv.Itoa(w)+"w")
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}
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if len(sources) > 0 {
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// The original closes the set at its own width, so a wide viewport still has the best file to pick.
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sources = append(sources, file+" "+strconv.Itoa(cfg.Width)+"w")
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p.Srcset = strings.Join(sources, ", ")
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}
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inspected.put(key, p)
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return p, true
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}
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// derivedName is the cache filename for one source at one width: the source's content hash, so an edit
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// changes the name and nothing stale is ever served, plus the width and the encoding's extension.
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func derivedName(data []byte, source string, width int) string {
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sum := sha256.Sum256(data)
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ext := ".jpg"
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if lossless(source) {
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ext = ".png"
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}
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return hex.EncodeToString(sum[:8]) + "-w" + strconv.Itoa(width) + ext
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}
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// lossless reports whether a source may carry transparency, which decides the derivative's encoding.
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func lossless(source string) bool {
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switch strings.ToLower(path.Ext(source)) {
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case ".png", ".gif", ".webp":
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return true
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}
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return false
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}
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// showable reports whether a browser can display the file: what belongs in a gallery, whether or not this
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// engine can do anything clever with it.
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func showable(name string) bool {
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switch strings.ToLower(path.Ext(name)) {
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case ".jpg", ".jpeg", ".png", ".gif", ".webp", ".svg", ".avif":
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return true
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}
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return false
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}
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// derivable reports whether the file can be decoded and resampled here. SVG needs no resizing and AVIF has no
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// decoder available, so neither gets derivatives (ADR-0040).
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func derivable(name string) bool {
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switch strings.ToLower(path.Ext(name)) {
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case ".jpg", ".jpeg", ".png", ".gif", ".webp":
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return true
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}
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return false
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}
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