package main

import (
	"bufio"
	"context"
	"encoding/json"
	"errors"
	"fmt"
	"os"
	"path/filepath"
)

// journal appends JSON lines to one file for each day in dir, named
// YYYY-MM-DD.jsonl. It keeps the file of the last day open.
type journal struct {
	dir  string
	file *os.File
	day  string
}

// Write appends line to the file of day with one write to the operating system,
// without a buffer. A crash of the process then loses nothing.
func (j *journal) Write(day string, line []byte) error {
	if day != j.day {
		err := j.Close()
		if err != nil {
			return err
		}

		j.file, err = os.OpenFile(filepath.Join(j.dir, day+".jsonl"), os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0o600)
		if err != nil {
			return fmt.Errorf("open day file: %w", err)
		}

		j.day = day
	}

	_, err := j.file.Write(line)
	if err != nil {
		return fmt.Errorf("write line: %w", err)
	}

	return nil
}

// Close closes the open day file.
func (j *journal) Close() error {
	if j.file == nil {
		return nil
	}

	err := j.file.Close()
	j.file = nil
	j.day = ""
	if err != nil {
		return fmt.Errorf("close day file: %w", err)
	}

	return nil
}

// readLines decodes each line of the day files in dir, oldest day first, and
// gives it to add in file order.
func readLines[T any](ctx context.Context, dir string, add func(T) error) error {
	paths, err := filepath.Glob(filepath.Join(dir, "*.jsonl"))
	if err != nil {
		return fmt.Errorf("list day files: %w", err)
	}

	for _, path := range paths {
		err = readFile(ctx, path, add)
		if err != nil {
			return err
		}
	}

	return nil
}

func readFile[T any](ctx context.Context, path string, add func(T) error) (err error) {
	file, err := os.Open(path)
	if err != nil {
		return fmt.Errorf("open day file: %w", err)
	}

	defer func() {
		err = errors.Join(err, file.Close())
	}()

	scanner := bufio.NewScanner(file)
	scanner.Buffer(nil, 1<<30)
	for number := 1; scanner.Scan(); number++ {
		if ctx.Err() != nil {
			return ctx.Err()
		}

		var value T
		err = json.Unmarshal(scanner.Bytes(), &value)
		if err == nil {
			err = add(value)
		}

		if err != nil {
			return fmt.Errorf("%s line %d: %w", filepath.Base(path), number, err)
		}
	}

	err = scanner.Err()
	if err != nil {
		return fmt.Errorf("read %s: %w", filepath.Base(path), err)
	}

	return nil
}