using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;

internal class ReadWriter
{
    public static (char, char) GetMostAndLeastCommonLetters(string path)
    {
        using StreamReader streamReader = new(path);
        var text = streamReader.ReadToEnd();
        var letters = CountLetters(string.Join("", new Regex("[a-zA-z]").Matches(text)).ToLower());
        var (most, least) = (letters.Values.Max(), letters.Values.Min());

        char mostLetter = 'a';
        var mostLetters = letters.Where(x => x.Value == most);
        if (mostLetters.Count() == 1)
        {
            mostLetter = mostLetters.First().Key;
        }
        else
        {
            int minPosition = text.Length + 1;
            foreach (var (letter, _) in mostLetters)
            {
                int position = text.ToLower().IndexOf(letter);
                if (position < minPosition)
                {
                    minPosition = position;
                    mostLetter = letter;
                }
            }
        }

        char leastLetter = 'a';
        var leastLetters = letters.Where(x => x.Value == least);
        if (leastLetters.Count() == 1)
        {
            leastLetter = leastLetters.First().Key;
        }
        else
        {
            int minPosition = text.Length + 1;
            foreach (var (letter, _) in leastLetters)
            {
                int position = text.ToLower().IndexOf(letter);
                if (position < minPosition && letter != mostLetter)
                {
                    minPosition = position;
                    leastLetter = letter;
                }
            }
        }

        return (mostLetter, leastLetter);
    }

    private static Dictionary<char, int> CountLetters(string line)
    {
        var letters = new Dictionary<char, int>();
        foreach (var sym in line)
        {
            if (letters.ContainsKey(sym)) letters[sym]++;
            else letters[sym] = 1;
        }

        return letters;
    }

    public static void ReplaceMostRareLetter((char, char) leastAndMostCommon, string inputPath, string outputPath)
    {
        using StreamWriter streamWriter = new(outputPath);
        using StreamReader streamReader = new(inputPath);
        var text = streamReader.ReadToEnd();
        var r = text.Replace(leastAndMostCommon.Item2, leastAndMostCommon.Item1)
            .Replace(leastAndMostCommon.Item2.ToString().ToUpper(), leastAndMostCommon.Item1.ToString().ToUpper());
        streamWriter.Write(r);
    }
}