package store

import (
	"crypto/subtle"
	"database/sql"
	"encoding/base64"
	"time"
)

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

// 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 the burn viewer window 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.
// #58: admission is atomic. Returns (remaining, admitted). admitted is true
// only when this caller may serve the content: for legacy burn pastes the
// caller wins exactly when its conditional soft delete flipped deleted_at
// (RowsAffected), and for read-budget pastes the caller wins exactly when its
// conditional UPDATE (reads_used < reads_limit) incremented the counter - so
// concurrent readers can never both consume the last read. Losing callers
// must treat the paste as gone. view_count is tracked separately and
// unaffected.
func (s *Store) RegisterRead(row *PasteRow, viewerID string, burnWindowMinutes int) (remaining *int, admitted bool) {
	if !row.ReadsLimit.Valid {
		if row.BurnAfterRead {
			// #58: atomic claim - only the caller whose UPDATE actually
			// flips deleted_at from NULL may serve the content.
			ok, err := s.SoftDelete(row.ID)
			r := 0
			if err != nil || !ok {
				return &r, false
			}
			return &r, true
		}
		return nil, true
	}
	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 < int64(burnWindowMinutes)*60 {
		r := int(row.ReadsLimit.Int64) - row.ReadsUsed
		if r < 0 {
			r = 0
		}
		return &r, true
	}
	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)
	// #58: conditional increment - only succeeds while budget remains, so
	// concurrent readers cannot both consume the final read.
	res, err := s.db.Exec(`UPDATE pastes SET reads_used = reads_used + 1
		WHERE id = ? AND deleted_at IS NULL AND reads_used < ?`, row.ID, row.ReadsLimit.Int64)
	if err != nil {
		r := 0
		return &r, false
	}
	if n, _ := res.RowsAffected(); n == 0 {
		// Lost the race: budget exhausted (or paste already burned).
		r := 0
		return &r, false
	}
	var used int64
	s.db.QueryRow(`SELECT reads_used FROM pastes WHERE id = ?`, row.ID).Scan(&used)
	if used >= row.ReadsLimit.Int64 {
		// Atomic burn; either way the paste is gone for future readers.
		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
}