// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: MIT

//go:build linux || freebsd

// Package storage provides file-based persistence for nuntius.
//
// Today it contains only the newsletter subscriber log. It is designed
// to be zero-dependency (stdlib only) and crash-safe: each Append writes
// a single JSON line to an append-only file, so partial writes do not
// corrupt earlier records.
package storage

import (
	"bufio"
	"encoding/json"
	"fmt"
	"os"
	"path/filepath"
	"sync"
	"time"
)

// Subscriber is a single newsletter signup, one JSON object per line.
type Subscriber struct {
	Email     string    `json:"email"`
	IP        string    `json:"ip,omitempty"`
	Form      string    `json:"form"`
	CreatedAt time.Time `json:"created_at"`
}

// NewsletterStore is a file-based append-only log of subscribers.
// All methods are safe for concurrent use.
type NewsletterStore struct {
	path string
	mu   sync.Mutex
}

// NewNewsletterStore returns a store that appends to path.
// The file and its parent directory are created lazily on first Append.
func NewNewsletterStore(path string) *NewsletterStore {
	return &NewsletterStore{path: path}
}

// Path returns the file path this store writes to.
func (s *NewsletterStore) Path() string {
	return s.path
}

// Append writes sub as a single JSON line to the log.
// The file and parent directory are created on first call.
func (s *NewsletterStore) Append(sub Subscriber) error {
	if sub.CreatedAt.IsZero() {
		sub.CreatedAt = time.Now().UTC()
	}
	line, err := json.Marshal(sub)
	if err != nil {
		return fmt.Errorf("marshal subscriber: %w", err)
	}
	line = append(line, '\n')

	s.mu.Lock()
	defer s.mu.Unlock()

	if err := os.MkdirAll(filepath.Dir(s.path), 0755); err != nil {
		return fmt.Errorf("mkdir %s: %w", filepath.Dir(s.path), err)
	}
	f, err := os.OpenFile(s.path, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
	if err != nil {
		return fmt.Errorf("open %s: %w", s.path, err)
	}
	defer f.Close()
	if _, err := f.Write(line); err != nil {
		return fmt.Errorf("write %s: %w", s.path, err)
	}
	return nil
}

// Count returns the number of valid subscriber lines in the log.
// Malformed lines are silently skipped so a partial write does not
// brick the entire file.
func (s *NewsletterStore) Count() (int, error) {
	s.mu.Lock()
	defer s.mu.Unlock()
	return s.countLocked()
}

func (s *NewsletterStore) countLocked() (int, error) {
	f, err := os.Open(s.path)
	if os.IsNotExist(err) {
		return 0, nil
	}
	if err != nil {
		return 0, fmt.Errorf("open %s: %w", s.path, err)
	}
	defer f.Close()

	n := 0
	scanner := bufio.NewScanner(f)
	// Allow up to 1 MB per line in case a single record balloons.
	scanner.Buffer(make([]byte, 64*1024), 1024*1024)
	for scanner.Scan() {
		line := scanner.Bytes()
		if len(line) == 0 {
			continue
		}
		var sub Subscriber
		if err := json.Unmarshal(line, &sub); err != nil {
			// Skip malformed lines rather than failing the whole count.
			continue
		}
		if sub.Email != "" {
			n++
		}
	}
	if err := scanner.Err(); err != nil {
		return n, fmt.Errorf("scan %s: %w", s.path, err)
	}
	return n, nil
}

// List returns all valid subscribers in insertion order.
// Use with care on large files; it reads the whole log into memory.
func (s *NewsletterStore) List() ([]Subscriber, error) {
	s.mu.Lock()
	defer s.mu.Unlock()

	f, err := os.Open(s.path)
	if os.IsNotExist(err) {
		return nil, nil
	}
	if err != nil {
		return nil, fmt.Errorf("open %s: %w", s.path, err)
	}
	defer f.Close()

	var out []Subscriber
	scanner := bufio.NewScanner(f)
	scanner.Buffer(make([]byte, 64*1024), 1024*1024)
	for scanner.Scan() {
		line := scanner.Bytes()
		if len(line) == 0 {
			continue
		}
		var sub Subscriber
		if err := json.Unmarshal(line, &sub); err != nil {
			continue
		}
		if sub.Email != "" {
			out = append(out, sub)
		}
	}
	if err := scanner.Err(); err != nil {
		return nil, fmt.Errorf("scan %s: %w", s.path, err)
	}
	return out, nil
}