package main

import (
	"crypto/sha256"
	"encoding/hex"
	"net/http"
	"os"
	"path/filepath"
)

// Root todo.txt rail widget (assets/todo-rail.js). /_todo reads and writes the
// single wiki-root todo.txt — the path is fixed, so there is no traversal
// surface. GET returns the raw text (404 when the file is absent, which the
// widget treats as "render nothing"); POST replaces the whole file atomically.
// Unlike the index.md save path, empty content does NOT delete the file: the
// widget has no in-UI way to recreate it, so an empty save keeps an empty file.
//
// Both GET and POST carry an ETag (a content hash) so the widget can poll for
// external edits: it revalidates GET with If-None-Match and reloads only when
// another device actually changed the bytes. The POST response echoes the new
// ETag so a device's own save comes back 304 rather than reloading itself.

func (h *handler) todoPath() string {
	return filepath.Join(h.root, "todo.txt")
}

// todoETag is a strong validator derived from the file content — not mtime, so
// a save of identical bytes (or a touch) does not look like a change.
func todoETag(data []byte) string {
	sum := sha256.Sum256(data)
	return `"` + hex.EncodeToString(sum[:])[:16] + `"`
}

func (h *handler) handleTodo(w http.ResponseWriter, r *http.Request) {
	if !h.checkAuth(w, r) {
		return
	}
	switch r.Method {
	case http.MethodGet:
		h.handleTodoGet(w, r)
	case http.MethodPost:
		h.handleTodoPost(w, r)
	default:
		http.Error(w, "method not allowed", http.StatusMethodNotAllowed)
	}
}

func (h *handler) handleTodoGet(w http.ResponseWriter, r *http.Request) {
	data, err := os.ReadFile(h.todoPath())
	if err != nil {
		if os.IsNotExist(err) {
			http.NotFound(w, r)
			return
		}
		http.Error(w, "read failed: "+err.Error(), http.StatusInternalServerError)
		return
	}
	// The rail re-fetches on every page load and polls for external edits; a
	// heuristically cached copy would show stale todos. Revalidation is driven
	// by the ETag (If-None-Match) rather than the browser's own HTTP cache.
	etag := todoETag(data)
	w.Header().Set("ETag", etag)
	w.Header().Set("Cache-Control", "no-store")
	if r.Header.Get("If-None-Match") == etag {
		w.WriteHeader(http.StatusNotModified)
		return
	}
	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
	w.Write(data)
}

func (h *handler) handleTodoPost(w http.ResponseWriter, r *http.Request) {
	if err := r.ParseForm(); err != nil {
		http.Error(w, "bad request", http.StatusBadRequest)
		return
	}
	content := r.FormValue("content")
	if err := writeFileAtomic(h.todoPath(), []byte(content), 0644); err != nil {
		http.Error(w, "write failed: "+err.Error(), http.StatusInternalServerError)
		return
	}
	// Echo the new ETag so the saving client can adopt it and skip reloading
	// its own write on the next poll.
	w.Header().Set("ETag", todoETag([]byte(content)))
	w.WriteHeader(http.StatusNoContent)
}