using System;

namespace Utils;

/// <summary>
/// "Nullable" wrapper. Can wrap both value types and reference types.
/// </summary>
/// <typeparam name="T"></typeparam>
public readonly struct Option<T> : IEquatable<Option<T>>
{
    public bool HasValue { get; } = false;
    private readonly T _value = default!;

    public Option() { }
    public Option(T value)
    {
        _value = value;
        HasValue = true;
    }

    public T Value
    {
        get {
            if (!HasValue)
                throw new InvalidOperationException("Cannot access value of None");
            return _value;
        }
    }

    public bool Equals(Option<T> other)
    {
        return (!HasValue && !other.HasValue)
            || (HasValue && other.HasValue && _value!.Equals(other._value));
    }

    public override bool Equals(object obj)
    {
        if (obj is Option<T> other)
            return Equals(other);
        return false;
    }

    public override int GetHashCode()
    {
        return HashCode.Combine(HasValue, _value);
    }

    public static bool operator ==(Option<T> left, Option<T> right)
    {
        return left.Equals(right);
    }

    public static bool operator !=(Option<T> left, Option<T> right)
    {
        return !(left == right);
    }

    public Option<TResult> Map<TResult>(Func<T, TResult> map)
    {
        if (HasValue)
            return Option.Some(map(_value));
        return Option.None<TResult>();
    }

    public T Or(T otherwise)
    {
        if (HasValue)
            return _value;
        return otherwise;
    }

    public Option<T> Or(Option<T> otherwise)
    {
        if (HasValue)
            return this;
        return otherwise;
    }
}

public static class Option
{
    public static Option<T> None<T>()
        => default;

    public static Option<T> Some<T>(T value)
        => new(value);

    public static Option<T> FromNullable<T>(T? value)
        where T : struct
    {
        if (!value.HasValue)
            return default;
        return new(value.Value);
    }

    public static Option<T> FromObject<T>(T? value)
    {
        if (value is null)
            return default;
        return new(value);
    }
}