package main

import (
	"bufio"
	"fmt"
	"os"
	"strings"
	"syscall"
)

// LineStore gives locked read-modify-write access to a flat text file with
// one record per line. Lines starting with '#' and blank lines are ignored.
// parse/format convert between a line and a record of type T.
type LineStore[T any] struct {
	path   string
	file   *os.File
	header string
	parse  func(line string) (T, error)
	format func(v T) string
}

func openLineStore[T any](path, header string, parse func(string) (T, error), format func(T) string) (*LineStore[T], error) {
	if err := os.MkdirAll(dirOf(path), 0755); err != nil {
		return nil, fmt.Errorf("cannot create directory for %s: %w", path, err)
	}
	f, err := os.OpenFile(path, os.O_RDWR|os.O_CREATE, 0644)
	if err != nil {
		return nil, fmt.Errorf("cannot open %s: %w", path, err)
	}
	if err := syscall.Flock(int(f.Fd()), syscall.LOCK_EX); err != nil {
		f.Close()
		return nil, fmt.Errorf("cannot lock %s: %w", path, err)
	}
	return &LineStore[T]{path: path, file: f, header: header, parse: parse, format: format}, nil
}

func (s *LineStore[T]) Close() error {
	syscall.Flock(int(s.file.Fd()), syscall.LOCK_UN)
	return s.file.Close()
}

func (s *LineStore[T]) Load() ([]T, error) {
	if _, err := s.file.Seek(0, 0); err != nil {
		return nil, err
	}
	var items []T
	sc := bufio.NewScanner(s.file)
	lineNo := 0
	for sc.Scan() {
		lineNo++
		line := strings.TrimRight(sc.Text(), "\r\n")
		if line == "" || strings.HasPrefix(line, "#") {
			continue
		}
		v, err := s.parse(line)
		if err != nil {
			return nil, fmt.Errorf("%s:%d: %w", s.path, lineNo, err)
		}
		items = append(items, v)
	}
	if err := sc.Err(); err != nil {
		return nil, err
	}
	return items, nil
}

func (s *LineStore[T]) Save(items []T) error {
	var b strings.Builder
	b.WriteString(s.header)
	for _, v := range items {
		b.WriteString(s.format(v))
		b.WriteString("\n")
	}
	if err := s.file.Truncate(0); err != nil {
		return err
	}
	if _, err := s.file.Seek(0, 0); err != nil {
		return err
	}
	if _, err := s.file.WriteString(b.String()); err != nil {
		return err
	}
	return s.file.Sync()
}

func dirOf(path string) string {
	i := strings.LastIndexByte(path, '/')
	if i <= 0 {
		return "."
	}
	return path[:i]
}