package storage

import (
	"bufio"
	"os"
	"sync"
)

// DedupIndex keeps an in-memory set of seen IDs and persists them to an append-only file.
type DedupIndex struct {
	path string
	mu   sync.Mutex
	set  map[string]struct{}
	f    *os.File
}

// LoadDedup loads existing IDs from path and prepares to append new ones.
func LoadDedup(path string) (*DedupIndex, error) {
	idx := &DedupIndex{path: path, set: make(map[string]struct{})}
	// Ensure file exists
	f, err := os.OpenFile(path, os.O_CREATE|os.O_RDWR|os.O_APPEND, 0o644)
	if err != nil {
		return nil, err
	}
	idx.f = f
	// Read existing lines
	sc := bufio.NewScanner(f)
	for sc.Scan() {
		idx.set[sc.Text()] = struct{}{}
	}
	// ignore scanner error for now
	return idx, nil
}

// Seen reports whether id was seen before.
func (d *DedupIndex) Seen(id string) bool {
	d.mu.Lock()
	defer d.mu.Unlock()
	_, ok := d.set[id]
	return ok
}

// Add marks id as seen and appends to underlying file.
func (d *DedupIndex) Add(id string) error {
	d.mu.Lock()
	defer d.mu.Unlock()
	if _, ok := d.set[id]; ok {
		return nil
	}
	if d.f == nil {
		// should not happen if loaded properly
		f, err := os.OpenFile(d.path, os.O_CREATE|os.O_RDWR|os.O_APPEND, 0o644)
		if err != nil {
			return err
		}
		d.f = f
	}
	if _, err := d.f.WriteString(id + "\n"); err != nil {
		return err
	}
	if err := d.f.Sync(); err != nil {
		// best-effort; ignore
	}
	d.set[id] = struct{}{}
	return nil
}

// Close closes the underlying file handle.
func (d *DedupIndex) Close() error {
	d.mu.Lock()
	defer d.mu.Unlock()
	if d.f != nil {
		return d.f.Close()
	}
	return nil
}