// Package github implements a pull-through cache for GitHub and Gitea
// release metadata and assets (the gitea vrepo type shares this handler
// with its own URL prefix and cache namespace).
//
// Paths under /<prefix>/<vrepo>/<owner>/<repo>/ (<prefix> = github|gitea):
//
//	latest, snapshot          resolve alias; 302 to the tag path (default)
//	                          or serve in place (vrepo redirect: false)
//	<tag>                     302 to <tag>.json
//	<tag>.json, latest.json, snapshot.json
//	                          release metadata (GitHub API JSON with asset
//	                          URLs rewritten to this proxy), TTL-cached
//	<tag>/<asset>, latest/<asset>, snapshot/<asset>
//	                          asset download, cached forever
//
// "latest" is the latest stable release (GitHub releases/latest), "snapshot"
// the most recent prerelease. Both names are reserved. Tags containing "/"
// work in the .json and asset forms (asset names never contain "/") but not
// bare. A request ending in ".json" with a "/" before it is ambiguous
// (slash-tag metadata vs .json-named asset): metadata is tried first, and
// an upstream 404 falls back to the asset form.
package github

import (
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"log/slog"
	"net/http"
	"net/url"
	"path"
	"strconv"
	"strings"

	"tiny-proxy/internal/auth"
	"tiny-proxy/internal/config"
	"tiny-proxy/internal/metrics"
	"tiny-proxy/internal/proxycache"
	"tiny-proxy/internal/upstream"
)

type repo struct {
	cfg       config.VRepo
	client    *http.Client
	prefix    string // "github" | "gitea" — URL and cache-key namespace
	webBase   string // e.g. https://github.com
	apiBase   string // e.g. https://api.github.com
	apiAccept string // Accept header for API requests
	// noStore mirrors upstream cache: false — release metadata, resolved
	// aliases and assets are proxied through instead of being stored.
	noStore bool
}

// result tags an upstream response with this repo's cache policy, for the
// proxycache callbacks.
func (rp *repo) result(resp *http.Response, err error) (proxycache.Result, error) {
	return proxycache.Result{Resp: resp, NoStore: rp.noStore}, err
}

type Handler struct {
	repos map[string]*repo
	cache *proxycache.Cache
}

// NewHandler collects github and gitea vrepos. The same handler is mounted
// at both /github/ and /gitea/; rp.prefix disambiguates the two namespaces.
func NewHandler(repos []config.VRepo, clients map[string]*http.Client, cache *proxycache.Cache) *Handler {
	h := &Handler{repos: map[string]*repo{}, cache: cache}
	for _, rc := range repos {
		if rc.Type != config.VRepoGithub && rc.Type != config.VRepoGitea {
			continue
		}
		h.repos[rc.Name] = newRepo(rc, clients[rc.Name])
	}
	return h
}

// newRepo derives the API base and Accept header from the vrepo type: gitea
// serves its API at <web>/api/v1, github.com at api.github.com, and GitHub
// Enterprise at <host>/api/v3.
func newRepo(rc config.VRepo, client *http.Client) *repo {
	web := strings.TrimSuffix(rc.Upstream.URL, "/")
	rp := &repo{
		cfg:     rc,
		client:  client,
		prefix:  string(rc.Type),
		webBase: web,
		noStore: !rc.Upstream.CacheEnabled(),
	}
	if rc.Type == config.VRepoGitea {
		rp.apiBase = web + "/api/v1"
		rp.apiAccept = "application/json"
	} else {
		rp.apiBase = apiBase(web)
		rp.apiAccept = "application/vnd.github+json"
	}
	return rp
}

// apiBase derives the REST API base from the web base: github.com uses
// api.github.com, GitHub Enterprise serves the API at <host>/api/v3.
func apiBase(webBase string) string {
	if u, err := url.Parse(webBase); err == nil && (u.Host == "github.com" || u.Host == "www.github.com") {
		return "https://api.github.com"
	}
	return webBase + "/api/v3"
}

func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	if r.Method != http.MethodGet && r.Method != http.MethodHead {
		http.Error(w, "read-only repository", http.StatusMethodNotAllowed)
		return
	}
	// The handler is mounted at both /github/ and /gitea/; the first path
	// segment is the mount prefix and must match the vrepo's type so the
	// two namespaces never mix.
	rest := strings.TrimPrefix(r.URL.Path, "/")
	mount, rest, ok0 := strings.Cut(rest, "/")
	vrepo, rest, ok := strings.Cut(rest, "/")
	owner, rest, ok2 := strings.Cut(rest, "/")
	ghRepo, rest, _ := strings.Cut(rest, "/")
	if !ok0 || !ok || !ok2 || ghRepo == "" {
		http.Error(w, "bad path", http.StatusBadRequest)
		return
	}
	rp, exists := h.repos[vrepo]
	if !exists || mount != rp.prefix {
		http.Error(w, "unknown repository", http.StatusNotFound)
		return
	}
	if !auth.Check(rp.cfg.Auth, r) {
		auth.Unauthorized(w)
		return
	}
	// Configured and authorized, so it is safe as a metrics label; the cache
	// reads it back from the context to attribute its hits and misses.
	metrics.MarkVRepo(r.Context(), vrepo)
	if !validName(owner) || !validName(ghRepo) || !allowed(rp.cfg.Allow, owner, ghRepo) {
		http.Error(w, "not found", http.StatusNotFound)
		return
	}
	if rest == "" {
		h.redirect(w, r, rp, owner, ghRepo, "latest.json")
		return
	}

	// A ".json" suffix marks metadata, but "<tag>/<asset>.json" is
	// ambiguous with a slash-containing tag: try metadata, fall back to
	// the asset form on an upstream 404.
	if s, found := strings.CutSuffix(rest, ".json"); found {
		h.serveMetadata(w, r, rp, owner, ghRepo, s, strings.Contains(s, "/"))
		return
	}

	// Otherwise a "/" splits at the LAST slash: tag before, asset after.
	selector, asset := rest, ""
	if i := strings.LastIndex(rest, "/"); i >= 0 {
		selector, asset = rest[:i], rest[i+1:]
		if asset == "" {
			http.Error(w, "bad path", http.StatusBadRequest)
			return
		}
	}

	if isAlias(selector) {
		tag, err := h.resolve(r.Context(), rp, owner, ghRepo, selector)
		if err != nil {
			h.resolveError(w, selector, err)
			return
		}
		// A tag literally named "latest"/"snapshot" would redirect onto
		// the same URL forever — serve it in place instead.
		if rp.cfg.AliasRedirect() && tag != selector {
			target := tag
			if asset != "" {
				target += "/" + asset
			}
			h.redirect(w, r, rp, owner, ghRepo, target)
			return
		}
		if asset == "" {
			h.serveTagMetadata(w, r, rp, owner, ghRepo, tag, false)
			return
		}
		h.serveAsset(w, r, rp, owner, ghRepo, tag, asset)
		return
	}
	if asset == "" {
		// Bare tag: canonical metadata URL is <tag>.json.
		h.redirect(w, r, rp, owner, ghRepo, selector+".json")
		return
	}
	h.serveAsset(w, r, rp, owner, ghRepo, selector, asset)
}

// serveMetadata answers <selector>.json, resolving aliases first.
// assetFallback reports that the selector contained a "/", so a 404 from
// the tag endpoint should retry as <tag>/<asset>.json.
func (h *Handler) serveMetadata(w http.ResponseWriter, r *http.Request, rp *repo, owner, ghRepo, selector string, assetFallback bool) {
	if isAlias(selector) {
		tag, err := h.resolve(r.Context(), rp, owner, ghRepo, selector)
		if err != nil {
			h.resolveError(w, selector, err)
			return
		}
		selector = tag
	}
	h.serveTagMetadata(w, r, rp, owner, ghRepo, selector, assetFallback)
}

// serveTagMetadata answers with the cached GitHub release object for tag,
// asset download URLs rewritten to this proxy.
func (h *Handler) serveTagMetadata(w http.ResponseWriter, r *http.Request, rp *repo, owner, ghRepo, tag string, assetFallback bool) {
	// Ambiguous ".json" request: if the asset form is already cached, serve
	// it directly and skip the API probe entirely.
	if assetFallback {
		if i := strings.LastIndex(tag, "/"); i > 0 {
			key := assetKey(rp, owner, ghRepo, tag[:i], tag[i+1:]+".json")
			if _, err := h.cache.Store.Stat(r.Context(), key); err == nil {
				h.serveAsset(w, r, rp, owner, ghRepo, tag[:i], tag[i+1:]+".json")
				return
			}
		}
	}

	key := rp.prefix + "/" + rp.cfg.Name + "/" + owner + "/" + ghRepo + "/" + tag + ".json"
	body, err := h.cache.Fetch(r.Context(), key, rp.cfg.MutableTTL(), func(ctx context.Context) (proxycache.Result, error) {
		return rp.result(h.fetchRelease(ctx, rp, owner, ghRepo, tag))
	})
	if err != nil {
		var ue *proxycache.UpstreamError
		if errors.As(err, &ue) && ue.StatusCode == http.StatusNotFound && assetFallback {
			if i := strings.LastIndex(tag, "/"); i > 0 {
				h.serveAsset(w, r, rp, owner, ghRepo, tag[:i], tag[i+1:]+".json")
				return
			}
		}
		if errors.As(err, &ue) && ue.StatusCode == http.StatusNotFound {
			http.Error(w, "not found", http.StatusNotFound)
			return
		}
		slog.Error("metadata fetch failed", "key", key, "err", err)
		http.Error(w, "upstream error", http.StatusBadGateway)
		return
	}
	w.Header().Set("Content-Type", "application/json")
	w.Header().Set("Content-Length", strconv.Itoa(len(body)))
	w.WriteHeader(http.StatusOK)
	if r.Method == http.MethodGet {
		w.Write(body)
	}
}

// fetchRelease loads the release object for tag from the GitHub API and
// rewrites each asset's browser_download_url to this proxy's path.
func (h *Handler) fetchRelease(ctx context.Context, rp *repo, owner, ghRepo, tag string) (*http.Response, error) {
	apiURL := rp.apiBase + "/repos/" + owner + "/" + ghRepo + "/releases/tags/" + url.PathEscape(tag)
	resp, err := h.get(ctx, rp, apiURL)
	if err != nil {
		return nil, err
	}
	if resp.StatusCode != http.StatusOK {
		return resp, nil // caller treats as uncacheable
	}
	defer resp.Body.Close()
	var rel map[string]any
	if err := json.NewDecoder(resp.Body).Decode(&rel); err != nil {
		return nil, err
	}
	assetBase := escapePath("/" + rp.prefix + "/" + rp.cfg.Name + "/" + owner + "/" + ghRepo + "/" + tag + "/")
	if assets, ok := rel["assets"].([]any); ok {
		for _, a := range assets {
			asset, ok := a.(map[string]any)
			if !ok {
				continue
			}
			if name, ok := asset["name"].(string); ok && name != "" {
				asset["browser_download_url"] = assetBase + url.PathEscape(name)
			}
		}
	}
	body, err := json.Marshal(rel)
	if err != nil {
		return nil, err
	}
	return jsonResponse(body), nil
}

// serveAsset answers <tag>/<asset> by proxying the GitHub download URL
// (which itself 302s to a signed CDN URL; the client follows it and the
// upstream transport drops credentials on redirects). Cached as immutable.
func (h *Handler) serveAsset(w http.ResponseWriter, r *http.Request, rp *repo, owner, ghRepo, tag, asset string) {
	key := assetKey(rp, owner, ghRepo, tag, asset)
	upstreamURL := rp.webBase + escapePath("/"+owner+"/"+ghRepo+"/releases/download/"+tag+"/"+asset)
	h.cache.Serve(w, r, key, 0, func(ctx context.Context) (proxycache.Result, error) {
		req, err := http.NewRequestWithContext(ctx, r.Method, upstreamURL, nil)
		if err != nil {
			return proxycache.Result{}, err
		}
		if rng := r.Header.Get("Range"); rng != "" {
			req.Header.Set("Range", rng)
		}
		res, err := rp.result(rp.client.Do(req))
		if err == nil && res.Resp != nil {
			res.Resume = upstream.Resumer(rp.client, r.Method, upstreamURL)
			res.ResumeAttempts = rp.cfg.Upstream.RetryAttempts()
			res.ResumeMaxWait = rp.cfg.Upstream.RetryMaxWait()
		}
		return res, err
	})
}

func assetKey(rp *repo, owner, ghRepo, tag, asset string) string {
	return rp.prefix + "/" + rp.cfg.Name + "/" + owner + "/" + ghRepo + "/" + tag + "/" + asset
}

// escapePath escapes a URL path while keeping "/" separators, so special
// characters in tags or asset names (#, %, …) survive into links and
// redirects.
func escapePath(p string) string {
	return (&url.URL{Path: p}).String()
}

// resolve maps an alias ("latest" | "snapshot") to a concrete tag, caching
// the answer under <prefix>-refs/<v>/<o>/<r>/<alias> with the metadata TTL —
// a separate namespace from asset keys so no tag/asset pair can collide.
func (h *Handler) resolve(ctx context.Context, rp *repo, owner, ghRepo, alias string) (string, error) {
	key := rp.prefix + "-refs/" + rp.cfg.Name + "/" + owner + "/" + ghRepo + "/" + alias
	body, err := h.cache.Fetch(ctx, key, rp.cfg.MutableTTL(), func(ctx context.Context) (proxycache.Result, error) {
		if alias == "latest" {
			return rp.result(h.fetchLatest(ctx, rp, owner, ghRepo))
		}
		return rp.result(h.fetchSnapshot(ctx, rp, owner, ghRepo))
	})
	if err != nil {
		return "", err
	}
	tag := string(body)
	if !validTag(tag) {
		return "", fmt.Errorf("upstream returned invalid tag %q", tag)
	}
	return tag, nil
}

// fetchLatest asks GitHub for the latest stable release (releases/latest
// excludes prereleases and drafts) and returns the tag as a 200 body.
func (h *Handler) fetchLatest(ctx context.Context, rp *repo, owner, ghRepo string) (*http.Response, error) {
	apiURL := rp.apiBase + "/repos/" + owner + "/" + ghRepo + "/releases/latest"
	resp, err := h.get(ctx, rp, apiURL)
	if err != nil {
		return nil, err
	}
	if resp.StatusCode != http.StatusOK {
		return resp, nil
	}
	defer resp.Body.Close()
	tag, err := decodeTag(resp.Body)
	if err != nil {
		return nil, err
	}
	return textResponse([]byte(tag)), nil
}

// fetchSnapshot lists releases (paging until a prerelease turns up, capped
// at 5 pages) and returns the tag of the most recent non-draft prerelease;
// a 404 body when the repo has none.
func (h *Handler) fetchSnapshot(ctx context.Context, rp *repo, owner, ghRepo string) (*http.Response, error) {
	for page := 1; page <= 5; page++ {
		apiURL := fmt.Sprintf("%s/repos/%s/%s/releases?per_page=100&page=%d", rp.apiBase, owner, ghRepo, page)
		resp, err := h.get(ctx, rp, apiURL)
		if err != nil {
			return nil, err
		}
		if resp.StatusCode != http.StatusOK {
			return resp, nil
		}
		var rels []struct {
			TagName    string `json:"tag_name"`
			Prerelease bool   `json:"prerelease"`
			Draft      bool   `json:"draft"`
		}
		err = json.NewDecoder(resp.Body).Decode(&rels)
		resp.Body.Close()
		if err != nil {
			return nil, err
		}
		for _, rel := range rels {
			if rel.Prerelease && !rel.Draft && rel.TagName != "" {
				return textResponse([]byte(rel.TagName)), nil
			}
		}
		if len(rels) < 100 {
			break
		}
	}
	return &http.Response{
		StatusCode: http.StatusNotFound,
		Header:     http.Header{"Content-Type": {"text/plain"}},
		Body:       io.NopCloser(strings.NewReader("no prerelease")),
	}, nil
}

func (h *Handler) get(ctx context.Context, rp *repo, apiURL string) (*http.Response, error) {
	req, err := http.NewRequestWithContext(ctx, http.MethodGet, apiURL, nil)
	if err != nil {
		return nil, err
	}
	req.Header.Set("Accept", rp.apiAccept)
	return rp.client.Do(req)
}

// redirect sends a 302 to an absolute-path URL under this vrepo.
func (h *Handler) redirect(w http.ResponseWriter, r *http.Request, rp *repo, owner, ghRepo, target string) {
	p := "/" + rp.prefix + "/" + rp.cfg.Name + "/" + owner + "/" + ghRepo + "/" + target
	http.Redirect(w, r, escapePath(p), http.StatusFound)
}

// resolveError maps an alias resolution failure to the client: upstream 404
// (no such release) stays a 404, anything else is a bad gateway.
func (h *Handler) resolveError(w http.ResponseWriter, alias string, err error) {
	var ue *proxycache.UpstreamError
	if errors.As(err, &ue) && ue.StatusCode == http.StatusNotFound {
		http.Error(w, "not found", http.StatusNotFound)
		return
	}
	slog.Error("alias resolution failed", "alias", alias, "err", err)
	http.Error(w, "upstream error", http.StatusBadGateway)
}

func isAlias(selector string) bool {
	return selector == "latest" || selector == "snapshot"
}

// allowed reports whether the allow list permits owner/repo. An empty list
// disables filtering entirely.
func allowed(patterns []string, owner, repo string) bool {
	if len(patterns) == 0 {
		return true
	}
	full := owner + "/" + repo
	for _, p := range patterns {
		if ok, err := path.Match(p, full); err == nil && ok {
			return true
		}
	}
	return false
}

// validName restricts owner/repo to GitHub's character set, rejecting
// path traversal and empty segments.
func validName(s string) bool {
	if s == "" || s == "." || s == ".." {
		return false
	}
	for _, c := range s {
		if c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z' || c >= '0' && c <= '9' ||
			c == '-' || c == '_' || c == '.' {
			continue
		}
		return false
	}
	return true
}

// validTag rejects tags that could escape the vrepo's cache-key namespace
// or produce broken URLs (defense-in-depth; git refnames already forbid
// most of this).
func validTag(tag string) bool {
	if tag == "" || strings.Contains(tag, "..") ||
		strings.HasPrefix(tag, "/") || strings.HasSuffix(tag, "/") {
		return false
	}
	for _, c := range tag {
		if c < 0x20 || c == 0x7f {
			return false
		}
	}
	return true
}

func decodeTag(r io.Reader) (string, error) {
	var rel struct {
		TagName string `json:"tag_name"`
	}
	if err := json.NewDecoder(r).Decode(&rel); err != nil {
		return "", err
	}
	if rel.TagName == "" {
		return "", errors.New("release has no tag_name")
	}
	return rel.TagName, nil
}

func jsonResponse(body []byte) *http.Response {
	return &http.Response{
		StatusCode:    http.StatusOK,
		Header:        http.Header{"Content-Type": {"application/json"}},
		Body:          io.NopCloser(bytes.NewReader(body)),
		ContentLength: int64(len(body)),
	}
}

func textResponse(body []byte) *http.Response {
	return &http.Response{
		StatusCode:    http.StatusOK,
		Header:        http.Header{"Content-Type": {"text/plain"}},
		Body:          io.NopCloser(bytes.NewReader(body)),
		ContentLength: int64(len(body)),
	}
}