// Package checksum defines the content hash used to spot duplicate photos and
// the backfill that fills it in for photos uploaded before it existed.
//
// The hash is sha256 over the untouched original bytes, hex encoded, stored in
// photos_photos.checksum. Duplicate scope is one gallery — the unique index is
// on (gallery_id, checksum) — so the same image may still live in several
// galleries, each with its own row and its own stored object.
package checksum

import (
	"crypto/sha256"
	"encoding/hex"
	"hash"
	"io"
)

// New returns a fresh hasher. Upload hashes as it spools the body, so it needs
// the writer rather than a finished reader.
func New() hash.Hash { return sha256.New() }

// Format renders a hasher's digest the way it is stored.
func Format(h hash.Hash) string { return hex.EncodeToString(h.Sum(nil)) }

// Sum reads r to EOF and returns its digest.
func Sum(r io.Reader) (string, error) {
	h := New()
	if _, err := io.Copy(h, r); err != nil {
		return "", err
	}
	return Format(h), nil
}