// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT License.
// See the LICENSE file in the project root for more information. 

using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Reactive.Concurrency;
using System.Reflection;

namespace System.Reactive.Linq
{
    /// <summary>
    /// Provides a set of static methods for writing queries over observable sequences, allowing translation to a target query language.
    /// </summary>
    public static partial class Qbservable
    {
        /// <summary>
        /// Returns the input typed as an <see cref="IObservable{TSource}"/>.
        /// This operator is used to separate the part of the query that's captured as an expression tree from the part that's executed locally.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <param name="source">An <see cref="IQbservable{TSource}"/> sequence to convert to an <see cref="IObservable{TSource}"/> sequence.</param>
        /// <returns>The original source object, but typed as an <see cref="IObservable{TSource}"/>.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> is null.</exception>
        public static IObservable<TSource> AsObservable<TSource>(this IQbservable<TSource> source)
        {
            if (source == null)
            {
                throw new ArgumentNullException(nameof(source));
            }

            return source;
        }

        /// <summary>
        /// Converts an enumerable sequence to an observable sequence.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <param name="source">Enumerable sequence to convert to an observable sequence.</param>
        /// <returns>The observable sequence whose elements are pulled from the given enumerable sequence.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> is null.</exception>
        /// <remarks>This operator requires the source's <see cref="IQueryProvider"/> object (see <see cref="IQueryable.Provider"/>) to implement <see cref="IQbservableProvider"/>.</remarks>
        public static IQbservable<TSource> ToQbservable<TSource>(this IQueryable<TSource> source)
        {
            if (source == null)
            {
                throw new ArgumentNullException(nameof(source));
            }

            return ((IQbservableProvider)source.Provider).CreateQuery<TSource>(
                Expression.Call(
                    null,
#pragma warning disable IL2060 // ('System.Reflection.MethodInfo.MakeGenericMethod' can not be statically analyzed.) This gets the MethodInfo for the method running right now, so it can't have been trimmed
                    ((MethodInfo)MethodBase.GetCurrentMethod()!).MakeGenericMethod(typeof(TSource)),
#pragma warning restore IL2060
                    source.Expression
                )
            );
        }

        /// <summary>
        /// Converts an enumerable sequence to an observable sequence, using the specified scheduler to run the enumeration loop.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <param name="source">Enumerable sequence to convert to an observable sequence.</param>
        /// <param name="scheduler">Scheduler to run the enumeration of the input sequence on.</param>
        /// <returns>The observable sequence whose elements are pulled from the given enumerable sequence.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="scheduler"/> is null.</exception>
        /// <remarks>This operator requires the source's <see cref="IQueryProvider"/> object (see <see cref="IQueryable.Provider"/>) to implement <see cref="IQbservableProvider"/>.</remarks>
        public static IQbservable<TSource> ToQbservable<TSource>(this IQueryable<TSource> source, IScheduler scheduler)
        {
            if (source == null)
            {
                throw new ArgumentNullException(nameof(source));
            }

            if (scheduler == null)
            {
                throw new ArgumentNullException(nameof(scheduler));
            }

            return ((IQbservableProvider)source.Provider).CreateQuery<TSource>(
                Expression.Call(
                    null,
#pragma warning disable IL2060 // ('System.Reflection.MethodInfo.MakeGenericMethod' can not be statically analyzed.) This gets the MethodInfo for the method running right now, so it can't have been trimmed
                    ((MethodInfo)MethodBase.GetCurrentMethod()!).MakeGenericMethod(typeof(TSource)),
#pragma warning restore IL2060
                    source.Expression,
                    Expression.Constant(scheduler)
                )
            );
        }

        internal static Expression GetSourceExpression<TSource>(IObservable<TSource> source)
        {
            if (source is IQbservable<TSource> q)
            {
                return q.Expression;
            }

            return Expression.Constant(source, typeof(IObservable<TSource>));
        }

        internal static Expression GetSourceExpression<TSource>(IEnumerable<TSource> source)
        {
            if (source is IQueryable<TSource> q)
            {
                return q.Expression;
            }

            return Expression.Constant(source, typeof(IEnumerable<TSource>));
        }

        internal static Expression GetSourceExpression<TSource>(IObservable<TSource>[] sources)
        {
            return Expression.NewArrayInit(
                typeof(IObservable<TSource>),
                sources.Select(source => GetSourceExpression(source))
            );
        }

        internal static Expression GetSourceExpression<TSource>(IEnumerable<TSource>[] sources)
        {
            return Expression.NewArrayInit(
                typeof(IEnumerable<TSource>),
                sources.Select(source => GetSourceExpression(source))
            );
        }

        internal static MethodInfo InfoOf<R>(Expression<Func<R>> f)
        {
            return ((MethodCallExpression)f.Body).Method;
        }
    }
}