package mailmux

import (
	"bufio"
	"errors"
	"fmt"
	"io"
	"math/rand"
	"os"
	"strings"
)

func randomLine(f *os.File) (string, error) {
	fi, err := f.Stat()
	if err != nil {
		return "", err
	}
	offset := rand.Int63n(fi.Size())
	offset, err = f.Seek(offset, io.SeekStart)
	if err != nil {
		return "", err
	}

	br := bufio.NewReader(f)
	for {
		b, err := br.ReadByte()
		if b == '\n' {
			break
		}
		if err != nil {
			return "", err
		}
	}
	line, err := br.ReadString('\n')
	if errors.Is(err, io.EOF) {
		// the file ends without a newline - no problem
	} else if err != nil {
		return "", err
	}

	line = strings.TrimSpace(line)
	if line == "" {
		// empty line. we're either at the end or hit a blank line. try again
		return randomLine(f)
	}
	return line, nil
}

// RandomUsername generates a random username from the dictionary file at dictpath.
// The dictionary file should be a newline delimited text file, one word per line.
// On Unix systems, the file /usr/share/dic/words is a suitable file.
func RandomUsername(dictpath string) (string, error) {
	f, err := os.Open(dictpath)
	if err != nil {
		return "", fmt.Errorf("open dictionary: %w", err)
	}
	defer f.Close()

	first, err := randomLine(f)
	if err != nil {
		return "", fmt.Errorf("first random word: %w", err)
	}
	first = strings.ToLower(first)
	second, err := randomLine(f)
	if err != nil {
		return "", fmt.Errorf("second random word: %w", err)
	}
	second = strings.ToLower(second)

	return fmt.Sprintf("%s_%s%02d", first, second, rand.Intn(99)), nil
}