// Package registry looks up the digest a tag points at in a container
// registry and lists the tags of a repository. Manifests are only ever
// requested with HEAD.
package registry

import (
	"context"
	"errors"
	"fmt"
	"log/slog"
	"net"
	"net/http"
	"net/url"
	"regexp"
	"strings"

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

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

// Client looks up tag digests and lists tags. 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. The only GETs it sends are the /v2/ ping and
// tags/list, which Docker Hub does not count as a pull.
type Client struct {
	keychain  authn.Keychain
	userAgent string
	log       *slog.Logger
}

// 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. log receives the full error of a
// failed lookup, which Digest itself returns only in a sanitised form.
func New(userAgent string, log *slog.Logger) *Client {
	return &Client{keychain: authn.DefaultKeychain, userAgent: userAgent, log: log}
}

// 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)
	}
	// Credentials are resolved here, not by ggcr, so that a broken credential
	// helper keeps its own message: that text comes from the local config.
	auth, err := c.keychain.Resolve(r.Context())
	if err != nil {
		return "", fmt.Errorf("lookup %s: %w", r, err)
	}
	desc, err := remote.Head(r,
		remote.WithContext(ctx),
		remote.WithAuth(auth),
		remote.WithUserAgent(c.userAgent))
	if err != nil {
		return "", c.lookupError("lookup", r.String(), ref, err)
	}
	return desc.Digest.String(), nil
}

// Tags returns every tag of repo ("host/path", or a Docker Hub name), in the
// order the registry lists them. It sends GET /v2/<repo>/tags/list, following
// the registry's pagination links. Errors are sanitised like Digest's.
func (c *Client) Tags(ctx context.Context, repo string) ([]string, error) {
	r, err := name.NewRepository(repo)
	if err != nil {
		return nil, fmt.Errorf("parse %s: %w", repo, err)
	}
	auth, err := c.keychain.Resolve(r)
	if err != nil {
		return nil, fmt.Errorf("list %s: %w", r, err)
	}
	tags, err := remote.List(r,
		remote.WithContext(ctx),
		remote.WithAuth(auth),
		remote.WithUserAgent(c.userAgent))
	if err != nil {
		return nil, c.lookupError("list", r.String(), repo, err)
	}
	return tags, nil
}

// lookupError turns a failed registry request into an error that is safe to
// show: a registry answer is described by its status and codes, anything
// else by a fixed text. The full error goes to the log. verb and target
// start the message; ref is what the log line names.
func (c *Client) lookupError(verb, target, ref string, err error) error {
	var terr *transport.Error
	if errors.As(err, &terr) {
		return fmt.Errorf("%s %s: %s", verb, target, describe(terr))
	}
	c.log.Warn("registry_request_failed", "ref", ref, "error", err)
	return fmt.Errorf("%s %s: %s", verb, target, classify(err))
}

// classify returns a fixed text for an error that is not a registry answer.
// ggcr copies registry-controlled text (token responses, authentication
// challenges, redirect URLs, certificate names) into such errors, so none of
// it is passed on.
func classify(err error) string {
	var netErr net.Error
	var urlErr *url.Error
	switch {
	case errors.Is(err, context.DeadlineExceeded), errors.Is(err, context.Canceled):
		return "timeout"
	case errors.As(err, &netErr), errors.As(err, &urlErr):
		return "cannot reach registry"
	}
	return "lookup failed"
}

// errorCodePattern matches the registry error codes of the distribution
// spec ("MANIFEST_UNKNOWN"). Anything else is dropped from error text.
var errorCodePattern = regexp.MustCompile(`^[A-Z_]{1,40}$`)

// describe returns the HTTP status and the structured error codes of a
// registry error, without the messages, details or raw body that ggcr copies
// into its error text. The registry host comes from an image name, so
// whatever it answers must not end up in the UI.
func describe(e *transport.Error) string {
	msg := fmt.Sprintf("registry answered %d %s", e.StatusCode, http.StatusText(e.StatusCode))
	var codes []string
	for _, d := range e.Errors {
		if errorCodePattern.MatchString(string(d.Code)) {
			codes = append(codes, string(d.Code))
		}
	}
	if len(codes) > 0 {
		msg += " (" + strings.Join(codes, ", ") + ")"
	}
	return msg
}