package handler

import (
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"log/slog"
	"net/http"
	"os"
	"path/filepath"
	"strings"

	"codeberg.org/VARASYS/ZDDC/zddc/internal/apps"
	"codeberg.org/VARASYS/ZDDC/zddc/internal/config"
	appfs "codeberg.org/VARASYS/ZDDC/zddc/internal/fs"
	"codeberg.org/VARASYS/ZDDC/zddc/internal/policy"
	"codeberg.org/VARASYS/ZDDC/zddc/internal/zddc"
)

// listingETag returns a hex-encoded SHA-256 prefix of the rendered JSON
// listing body. Truncated to 16 chars (64 bits) — collisions on a
// listing of any realistic size are vanishingly unlikely, and the short
// header keeps the wire footprint trim.
func listingETag(body []byte) string {
	h := sha256.Sum256(body)
	return hex.EncodeToString(h[:8])
}

// safeJoin joins fsRoot and relPath, then verifies the result is under fsRoot.
// Returns ("", false) if relPath would escape fsRoot.
func safeJoin(fsRoot, relPath string) (string, bool) {
	abs := filepath.Join(fsRoot, filepath.FromSlash(relPath))
	if !strings.HasPrefix(abs, fsRoot+string(filepath.Separator)) && abs != fsRoot {
		return "", false
	}
	return abs, true
}

// ServeDirectory handles a request for a directory path.
// If Accept: application/json → return Caddy-compatible JSON listing.
// Otherwise → return minimal HTML.
func ServeDirectory(cfg config.Config, w http.ResponseWriter, r *http.Request) {
	urlPath := r.URL.Path
	if !strings.HasSuffix(urlPath, "/") {
		http.Redirect(w, r, urlPath+"/", http.StatusMovedPermanently)
		return
	}

	email := EmailFromContext(r)
	decider := DeciderFromContext(r)
	ctx := r.Context()

	// Compute relative dir path (no leading or trailing slash)
	dirPath := strings.TrimPrefix(urlPath, "/")
	dirPath = strings.TrimSuffix(dirPath, "/")

	// ACL check on this directory itself.
	// Bypassed at the root path: the landing page is a public project
	// picker. Per-project filtering inside fs.ListDirectory still hides
	// directories the caller can't reach.
	absDir, ok := safeJoin(cfg.Root, dirPath)
	if !ok {
		http.Error(w, "Not Found", http.StatusNotFound)
		return
	}
	chain, err := zddc.EffectivePolicy(cfg.Root, absDir)
	if err != nil {
		slog.Warn("ACL policy error", "path", absDir, "err", err)
	}
	isRoot := dirPath == ""
	if !isRoot {
		if allowed, _ := policy.AllowFromChain(ctx, decider, chain, email, urlPath); !allowed {
			http.Error(w, "Forbidden", http.StatusForbidden)
			return
		}
	}

	accept := r.Header.Get("Accept")

	// For HTML requests, serve index.html if present (landing page convention)
	if !strings.Contains(accept, "application/json") {
		indexPath := filepath.Join(absDir, "index.html")
		if info, err := os.Stat(indexPath); err == nil && !info.IsDir() {
			ServeFile(w, r, indexPath)
			return
		}
	}

	// Build base URL for listing entries
	baseURL := urlPath // relative URLs suffice for JSON listings

	entries, err := appfs.ListDirectory(ctx, decider, cfg.Root, dirPath, email, baseURL)
	if err != nil {
		if os.IsNotExist(err) {
			http.Error(w, "Not Found", http.StatusNotFound)
		} else {
			slog.Error("listing directory", "path", dirPath, "err", err)
			http.Error(w, "Internal Server Error", http.StatusInternalServerError)
		}
		return
	}

	// Vary: Accept is critical — the same URL serves either the JSON
	// listing or the embedded browse.html depending on the Accept
	// header. Without Vary, browsers/CDNs cache one response and
	// serve it for the other Accept value, breaking browse.html's
	// auto-detect (which fetches the same URL with Accept: JSON).
	w.Header().Set("Vary", "Accept")

	if strings.Contains(accept, "application/json") {
		// Content-hash ETag on the listing payload. Re-fetched on every
		// request (the cascade is walked, ACL filter applied, JSON
		// rendered, hashed) — that's the same server work the previous
		// no-cache version did. The win is on the *response*: identical
		// listings (e.g. the same vendor refreshing their archive page)
		// short-circuit to 304 with no body.
		//
		// Crucially, this scheme tolerates unreliable filesystem
		// watching (Azure SMB, network shares with delayed inotify):
		// the ETag is the actual response hash, not a watcher-derived
		// invalidation token, so it can never lie about content.
		body, err := json.Marshal(entries)
		if err != nil {
			slog.Error("encoding directory listing", "err", err)
			http.Error(w, "Internal Server Error", http.StatusInternalServerError)
			return
		}
		etag := `"` + listingETag(body) + `"`
		w.Header().Set("Content-Type", "application/json")
		w.Header().Set("ETag", etag)
		w.Header().Set("Cache-Control", "private, max-age=0, must-revalidate")
		if match := r.Header.Get("If-None-Match"); match != "" && match == etag {
			w.WriteHeader(http.StatusNotModified)
			return
		}
		_, _ = w.Write(body)
		return
	}

	// Browser HTML fallback: serve the embedded `browse` tool. It's a
	// single-file SPA whose autoDetectServerMode loads the JSON listing
	// for the current directory and renders it as a sortable, filterable
	// tree. Same bytes that get served at /<dir>/browse.html — but at
	// the bare directory URL too, so any zddc-served folder presents a
	// usable file browser to anyone who navigates to it.
	body := apps.EmbeddedBytes("browse")
	if len(body) == 0 {
		// Bootstrap state: a fresh build hasn't populated browse.html
		// into the embed yet. Fall through to JSON for clients that
		// will still parse it.
		w.Header().Set("Content-Type", "application/json")
		w.Header().Set("Cache-Control", "no-cache")
		if err := json.NewEncoder(w).Encode(entries); err != nil {
			slog.Error("encoding directory listing (no-embed fallback)", "err", err)
		}
		return
	}
	// ETag + max-age=0 + must-revalidate: every request re-validates and
	// gets a 304 unless the binary has been redeployed (the ETag is a
	// content hash, computed once at startup and memoized in apps.embed).
	// Saves re-transmitting ~230 KB of browse.html on every page load
	// while still picking up redeploys immediately.
	etag := `"` + apps.EmbeddedETag("browse") + `"`
	w.Header().Set("ETag", etag)
	w.Header().Set("Cache-Control", "public, max-age=0, must-revalidate")
	w.Header().Set("Content-Type", "text/html; charset=utf-8")
	w.Header().Set("X-ZDDC-Source", "embedded:browse")
	if match := r.Header.Get("If-None-Match"); match != "" && match == etag {
		w.WriteHeader(http.StatusNotModified)
		return
	}
	_, _ = w.Write(body)
}