package main

import (
	"crypto/rand"
	"crypto/subtle"
	"database/sql"
	"encoding/base64"
	"net/http"
	"time"

	"github.com/go-chi/chi/v5"
)

// genDeletionToken returns a 32-char url-safe random token
func genDeletionToken() string {
	b := make([]byte, 24)
	rand.Read(b)
	return base64.RawURLEncoding.EncodeToString(b)
}

// readWindowMinutes is the per-viewer dedupe window for burn-after-N-reads
// (#49): the same viewer cookie returning within 15 minutes does not count
// as a new read. See the decision comment on issue #49.
const readWindowMinutes = 15

// timeNow is overridable in tests to inject the clock.
var timeNow = time.Now

// registerRead applies the burn-after-read budget for one view (#49).
// For pastes with reads_limit set: the viewer's paste_views row is checked;
// a view within readWindowMinutes of the viewer's last view is deduped
// (count=false). Otherwise reads_used is incremented, and the paste is
// soft-deleted (burned) once reads_used reaches reads_limit. Viewers without
// a cookie (plain API clients) count as their own viewer id "".
// For legacy plain burn_after_read pastes (no reads_limit), any read burns.
// Returns the number of reads remaining (0 when burned), or nil when no
// budget is set. view_count is tracked separately and unaffected.
func (s *Store) registerRead(row *PasteRow, viewerID string) (remaining *int, count bool) {
	if !row.ReadsLimit.Valid {
		if row.BurnAfterRead {
			s.SoftDelete(row.ID)
			r := 0
			return &r, true
		}
		return nil, false
	}
	now := timeNow().Unix()
	var last sql.NullInt64
	s.db.QueryRow(`SELECT last_viewed FROM paste_views WHERE paste_id=? AND viewer_id=?`,
		row.ID, viewerID).Scan(&last)
	if last.Valid && now-last.Int64 < readWindowMinutes*60 {
		r := int(row.ReadsLimit.Int64) - row.ReadsUsed
		if r < 0 {
			r = 0
		}
		return &r, false
	}
	s.db.Exec(`INSERT INTO paste_views (paste_id, viewer_id, last_viewed) VALUES (?,?,?)
		ON CONFLICT(paste_id, viewer_id) DO UPDATE SET last_viewed = excluded.last_viewed`,
		row.ID, viewerID, now)
	used := row.ReadsUsed + 1
	s.db.Exec(`UPDATE pastes SET reads_used=? WHERE id=?`, used, row.ID)
	if int64(used) >= row.ReadsLimit.Int64 {
		s.SoftDelete(row.ID)
	}
	r := int(row.ReadsLimit.Int64) - int(used)
	if r < 0 {
		r = 0
	}
	return &r, true
}

// burned reports whether a read-limited paste has exhausted its budget.
func (row *PasteRow) burned() bool {
	return row.ReadsLimit.Valid && int64(row.ReadsUsed) >= row.ReadsLimit.Int64
}

func deletionTokenEqual(stored, given string) bool {
	return subtle.ConstantTimeCompare([]byte(stored), []byte(given)) == 1
}

// handleRedeemDeletion lets a holder of the deletion token hard-delete immediately.
// DELETE /api/pastes/{id}/redeem?token=...
func (a *apiServer) handleRedeemDeletion(w http.ResponseWriter, r *http.Request) {
	id := chi.URLParam(r, "id")
	token := r.URL.Query().Get("token")
	if token == "" {
		writeErr(w, 400, "token required")
		return
	}
	row, err := a.store.GetPaste(id)
	if err != nil || row == nil {
		writeErr(w, 404, "paste not found")
		return
	}
	if row.DeletionToken.String == "" || !deletionTokenEqual(row.DeletionToken.String, token) {
		writeErr(w, 403, "invalid token")
		return
	}
	// hard delete: pastes table row goes away entirely
	a.store.db.Exec(`DELETE FROM pastes WHERE id = ?`, row.ID)
	writeJSON(w, 200, map[string]string{"status": "deleted"})
}