using System.Security.Cryptography;
using System.Text;

namespace QSafe.Core;

public enum PasswordMode { Complex, Digits, Memorable }

public static class PasswordGenerator
{
    private static readonly string[] Words = LoadWords();

    private static string[] LoadWords()
    {
        using var stream = typeof(PasswordGenerator).Assembly.GetManifestResourceStream("QSafe.PasswordWords")
            ?? throw new InvalidOperationException("Missing password word list");
        using var reader = new StreamReader(stream);
        var words = reader.ReadToEnd().Split((char[]?)null, StringSplitOptions.RemoveEmptyEntries);
        if (words.Length != 256 || words.Distinct().Count() != 256) throw new InvalidOperationException("Invalid password word list");
        return words;
    }

    public static string Generate(PasswordMode mode, int length, bool symbols = true, bool digits = true)
    {
        switch (mode)
        {
            case PasswordMode.Complex: return VaultCrypto.Generate(length, symbols);
            case PasswordMode.Digits:
                if (length is < 4 or > 128) throw new ArgumentOutOfRangeException(nameof(length));
                return new string(Enumerable.Range(0, length).Select(_ => (char)('0' + RandomNumberGenerator.GetInt32(10))).ToArray());
            case PasswordMode.Memorable:
                if (length is < 2 or > 12) throw new ArgumentOutOfRangeException(nameof(length));
                const string signs = "!@#$%&*+-=?";
                // Treat each word as a whole: extras only go before, between or after words.
                var boundaries = Enumerable.Range(0, length + 1).Select(_ => new StringBuilder()).ToArray();
                void InsertAtBoundary(char character)
                {
                    var boundary = boundaries[RandomNumberGenerator.GetInt32(boundaries.Length)];
                    boundary.Insert(RandomNumberGenerator.GetInt32(boundary.Length + 1), character);
                }
                if (digits) for (int i = 0; i < 2; i++) InsertAtBoundary((char)('0' + RandomNumberGenerator.GetInt32(10)));
                if (symbols) for (int i = 1; i < length; i++) InsertAtBoundary(signs[RandomNumberGenerator.GetInt32(signs.Length)]);
                var value = new StringBuilder();
                for (int i = 0; i < length; i++)
                {
                    string word = Words[RandomNumberGenerator.GetInt32(Words.Length)];
                    value.Append(boundaries[i]).Append(char.ToUpperInvariant(word[0])).Append(word.AsSpan(1));
                }
                return value.Append(boundaries[length]).ToString();
            default: throw new ArgumentOutOfRangeException(nameof(mode));
        }
    }
}