using System;
using System.Collections.Generic;
using System.Linq;

namespace MaybeMonad
{
    /// <summary>
    /// Contains Extensions Methods to work with possible null values.
    /// </summary>
    public static class MonadExtensions
    {
        /// <summary>
        /// Returns the specified propertyvalue or null if the calling object is null.
        /// </summary>
        /// <typeparam name="TInput">The type of the input.</typeparam>
        /// <typeparam name="TResult">The type of the result.</typeparam>
        /// <param name="o">The object which may be null.</param>
        /// <param name="evaluator">A Function to access the property.</param>
        /// <returns>The specified propertyvalue or null if the calling object is null</returns>
        public static TResult With<TInput, TResult>(this TInput o, Func<TInput, TResult> evaluator)
            where TResult : class
            where TInput : class
        {
            if (o == null)
                return null;
            return evaluator(o);
        }

        /// <summary>
        /// Returns the given collection or an empty collection if it is null.
        /// </summary>
        /// <typeparam name="TSource">The type of the source.</typeparam>
        /// <param name="source">The source collection.</param>
        /// <returns>The given collection or an empty collection if it is null.</returns>
        public static IList<TSource> NullToEmpty<TSource>(this IList<TSource> source)
        {
            return source ?? new List<TSource>();
        }

        /// <summary>
        /// Returns the given collection or an empty collection if it is null.
        /// </summary>
        /// <typeparam name="TSource">The type of the source.</typeparam>
        /// <param name="source">The source collection.</param>
        /// <returns>The given collection or an empty collection if it is null.</returns>
        public static IEnumerable<TSource> NullToEmpty<TSource>(this IEnumerable<TSource> source)
        {
            return source ?? Enumerable.Empty<TSource>();
        }

        /// <summary>
        /// Returns the specified propertyvalue or null if the calling object is null.
        /// </summary>
        /// <typeparam name="TInput">The type of the input.</typeparam>
        /// <typeparam name="TResult">The type of the result.</typeparam>
        /// <param name="o">The object which may be null.</param>
        /// <param name="evaluator">A Function to access the property.</param>
        /// <returns>The specified propertyvalue or null if the calling object is null</returns>
        public static Nullable<TResult> WithNullable<TInput, TResult>(this TInput o, Func<TInput, TResult> evaluator)
            where TResult : struct
            where TInput : class
        {
            if (o == null)
                return new Nullable<TResult>();
            return evaluator(o);
        }

        /// <summary>
        /// Returns the specified propertyvalue or null if the calling object is null.
        /// </summary>
        /// <typeparam name="TInput">The type of the input.</typeparam>
        /// <typeparam name="TResult">The type of the result.</typeparam>
        /// <param name="o">The object which may be null.</param>
        /// <param name="evaluator">A Function to access the property.</param>
        /// <returns>
        /// The specified propertyvalue or null if the calling object is null
        /// </returns>
        public static Nullable<TResult> WithNullable<TInput, TResult>(this TInput o, Func<TInput, Nullable<TResult>> evaluator)
            where TResult : struct
            where TInput : class
        {
            if (o == null)
                return new Nullable<TResult>();
            return evaluator(o);
        }

        /// <summary>
        /// Returns the specified propertyvalue or null if the calling object is null.
        /// </summary>
        /// <typeparam name="TInput">The type of the input.</typeparam>
        /// <typeparam name="TResult">The type of the result.</typeparam>
        /// <param name="o">The object which may be null.</param>
        /// <param name="evaluator">A Function to access the property.</param>
        /// <returns>
        /// The specified propertyvalue or null if the calling object is null
        /// </returns>
        public static TResult WithValue<TInput, TResult>(this TInput o, Func<TInput, TResult> evaluator)
            where TResult : struct
            where TInput : class
        {
            if (o == null)
                return default(TResult);
            return evaluator(o);
        }

        /// <summary>
        /// Returns the specified propertyvalue or the specified failureValue if the calling object is null.
        /// </summary>
        /// <typeparam name="TInput">The type of the input.</typeparam>
        /// <typeparam name="TResult">The type of the result.</typeparam>
        /// <param name="o">The object which may be null.</param>
        /// <param name="evaluator">The evaluator.</param>
        /// <param name="failureValue">The failure value.</param>
        /// <returns>
        /// The specified propertyvalue or the specified failureValue if the calling object is null.
        /// </returns>
        public static TResult Return<TInput, TResult>(this TInput o, Func<TInput, TResult> evaluator, TResult failureValue)
            where TInput : class
        {
            if (o == null)
                return failureValue;
            return evaluator(o);
        }

        /// <summary>
        /// Returns the empty string if called on null, otherwise the same as ToString.
        /// </summary>
        /// <typeparam name="TInput">The type of the input.</typeparam>
        /// <typeparam name="TResult">The type of the result.</typeparam>
        /// <param name="o">The object which may be null.</param>
        /// <param name="evaluator">The evaluator.</param>
        /// <param name="formatString">The format string.</param>
        /// <returns>
        /// The empty string if called on null, otherwise the same as ToString.
        /// </returns>
        public static string ToStringSave<TInput, TResult>(this TInput o, Func<TInput, TResult> evaluator, string formatString = null)
            where TInput : class
        {
            if (o == null)
                return "";
            var val = evaluator(o);
            if (val == null)
                return "";
            if (formatString == null)
                return val.ToString();
            return string.Format("{0:" + formatString + "}", val);
        }

        /// <summary>
        /// Returns the specified object if the expression is true or null if it is not.
        /// </summary>
        /// <typeparam name="TInput">The type of the input.</typeparam>
        /// <param name="o">The o.</param>
        /// <param name="evaluator">The evaluator.</param>
        /// <returns>The specified object if the expression is true or null if it is not.</returns>
        public static TInput If<TInput>(this TInput o, Func<TInput, bool> evaluator)
            where TInput : class
        {
            if (o == null)
                return null;
            return evaluator(o) ? o : null;
        }

        /// <summary>
        /// Executes the specified Action if the object is not null.
        /// </summary>
        /// <typeparam name="TInput">The type of the input.</typeparam>
        /// <param name="o">The o.</param>
        /// <param name="action">The action.</param>
        /// <returns>The calling object.</returns>
        public static TInput Do<TInput>(this TInput o, Action<TInput> action)
            where TInput : class
        {
            if (o == null)
                return null;
            action(o);
            return o;
        }

        /// <summary>
        /// Returns the Value or null if the key is not found.
        /// </summary>
        /// <typeparam name="TKey">The type of the key.</typeparam>
        /// <typeparam name="TValue">The type of the value.</typeparam>
        /// <param name="dictionary">The dictionary.</param>
        /// <param name="key">The key.</param>
        /// <returns></returns>
        public static TValue With<TKey, TValue> (this IDictionary<TKey, TValue> dictionary, TKey key)
        {
            TValue value;
            if (dictionary.TryGetValue(key, out value))
                return value;

            return default(TValue);
        }
    }
}