// Package registry looks up the digest a tag points at in a container
// registry, using HEAD requests only.
package registry

import (
	"context"
	"fmt"

	"github.com/google/go-containerregistry/pkg/authn"
	"github.com/google/go-containerregistry/pkg/name"
	"github.com/google/go-containerregistry/pkg/v1/remote"

	"github.com/shcizo/package-updater/internal/imageref"
)

// Client looks up tag digests with HEAD requests only. Docker Hub counts a
// GET of a manifest as a pull against the rate limit and a HEAD as nothing,
// so Client never issues a manifest GET.
type Client struct {
	keychain  authn.Keychain
	userAgent string
}

// New returns a Client that finds credentials the way the Docker CLI does:
// in $DOCKER_CONFIG/config.json, else ~/.docker/config.json. Registries
// without an entry are accessed anonymously. userAgent is added to the
// User-Agent header of every request.
func New(userAgent string) *Client {
	return &Client{keychain: authn.DefaultKeychain, userAgent: userAgent}
}

// Digest returns the digest that "repo:tag" points at, as the registry
// reports it: for a multi-arch image that is the digest of the index, the
// same one `docker inspect` shows in RepoDigests. A digest suffix in ref is
// ignored. It sends HEAD /v2/<repo>/manifests/<tag> and nothing else to the
// manifest endpoint; it does not fall back to GET when the registry omits
// the digest header.
func (c *Client) Digest(ctx context.Context, ref string) (string, error) {
	p := imageref.Parse(ref)
	if p.Tag == "" {
		return "", fmt.Errorf("image %q has no tag", ref)
	}
	r, err := name.ParseReference(p.Repo + ":" + p.Tag)
	if err != nil {
		return "", fmt.Errorf("parse %s: %w", ref, err)
	}
	desc, err := remote.Head(r,
		remote.WithContext(ctx),
		remote.WithAuthFromKeychain(c.keychain),
		remote.WithUserAgent(c.userAgent))
	if err != nil {
		return "", fmt.Errorf("head %s: %w", r, err)
	}
	return desc.Digest.String(), nil
}