using System;

namespace Umbraco.Cms.Core
{
    /// <summary>
    /// Represents a value that can be assigned a value.
    /// </summary>
    /// <typeparam name="T">The type of the value</typeparam>
    public class Settable<T>
    {
        private T? _value;

        /// <summary>
        /// Assigns a value to this <see cref="Settable{T}"/> instance.
        /// </summary>
        /// <param name="value">The value.</param>
        public void Set(T? value)
        {
            if (value is not null)
            {
                HasValue = true;
            }
            _value = value;
        }

        /// <summary>
        /// Assigns a value to this <see cref="Settable{T}"/> instance by copying the value
        /// of another instance, if the other instance has a value.
        /// </summary>
        /// <param name="other">The other instance.</param>
        public void Set(Settable<T> other)
        {
            // set only if has value else don't change anything
            if (other.HasValue) Set(other.Value);
        }

        /// <summary>
        /// Clears the value.
        /// </summary>
        public void Clear()
        {
            HasValue = false;
            _value = default (T);
        }

        /// <summary>
        /// Gets a value indicating whether a value has been assigned to this <see cref="Settable{T}"/> instance.
        /// </summary>
        public bool HasValue { get; private set; }

        /// <summary>
        /// Gets the value assigned to this <see cref="Settable{T}"/> instance.
        /// </summary>
        /// <remarks>An exception is thrown if the HasValue property is false.</remarks>
        /// <exception cref="InvalidOperationException">No value has been assigned to this instance.</exception>
        public T? Value
        {
            get
            {
                if (HasValue == false)
                    throw new InvalidOperationException("The HasValue property is false.");
                return _value;
            }
        }

        /// <summary>
        /// Gets the value assigned to this <see cref="Settable{T}"/> instance, if a value has been assigned,
        /// otherwise the default value of <typeparamref name="T"/>.
        /// </summary>
        /// <returns>The value assigned to this <see cref="Settable{T}"/> instance, if a value has been assigned,
        /// else the default value of <typeparamref name="T"/>.</returns>
        public T? ValueOrDefault()
        {
            return HasValue ? _value : default(T);
        }

        /// <summary>
        /// Gets the value assigned to this <see cref="Settable{T}"/> instance, if a value has been assigned,
        /// otherwise a specified default value.
        /// </summary>
        /// <param name="defaultValue">The default value.</param>
        /// <returns>The value assigned to this <see cref="Settable{T}"/> instance, if a value has been assigned,
        /// else <paramref name="defaultValue" />.</returns>
        public T? ValueOrDefault(T defaultValue)
        {
            return HasValue ? _value : defaultValue;
        }

        /// <inheritdoc />
        public override string? ToString()
        {
            return HasValue ? _value?.ToString() : "void";
        }
    }
}