// Copyright (c) Microsoft Corporation. All rights reserved.
// Licensed under the MIT License.

namespace Schemy
{
    using System;
    using System.Collections.Generic;
    using System.IO;
    using System.Linq;

    /// <summary>
    /// Extend the interpreter with essential builtin functionalities
    /// </summary>
    public class Builtins
    {
        public static IDictionary<Symbol, object> CreateBuiltins(Interpreter interpreter)
        {
            var builtins = new Dictionary<Symbol, object>();

            builtins[Symbol.FromString("+")] = new NativeProcedure(Utils.MakeVariadic(Add), "+");
            builtins[Symbol.FromString("-")] = new NativeProcedure(Utils.MakeVariadic(Minus), "-");
            builtins[Symbol.FromString("*")] = new NativeProcedure(Utils.MakeVariadic(Multiply), "*");
            builtins[Symbol.FromString("/")] = new NativeProcedure(Utils.MakeVariadic(Divide), "/");
            builtins[Symbol.FromString("=")] = NativeProcedure.Create<double, double, bool>((x, y) => x == y, "=");
            builtins[Symbol.FromString("<")] = NativeProcedure.Create<double, double, bool>((x, y) => x < y, "<");
            builtins[Symbol.FromString("<=")] = NativeProcedure.Create<double, double, bool>((x, y) => x <= y, "<=");
            builtins[Symbol.FromString(">")] = NativeProcedure.Create<double, double, bool>((x, y) => x > y, ">");
            builtins[Symbol.FromString(">=")] = NativeProcedure.Create<double, double, bool>((x, y) => x >= y, ">=");
            builtins[Symbol.FromString("eq?")] = NativeProcedure.Create<object, object, bool>((x, y) => object.ReferenceEquals(x, y), "eq?");
            builtins[Symbol.FromString("equal?")] = NativeProcedure.Create<object, object, bool>(EqualImpl, "equal?");
            builtins[Symbol.FromString("boolean?")] = NativeProcedure.Create<object, bool>(x => x is bool, "boolean?");
            builtins[Symbol.FromString("num?")] = NativeProcedure.Create<object, bool>(x => x is int || x is double, "num?");
            builtins[Symbol.FromString("string?")] = NativeProcedure.Create<object, bool>(x => x is string, "string?");
            builtins[Symbol.FromString("symbol?")] = NativeProcedure.Create<object, bool>(x => x is Symbol, "symbol?");
            builtins[Symbol.FromString("list?")] = NativeProcedure.Create<object, bool>(x => x is List<object>, "list?");
            builtins[Symbol.FromString("map")] = NativeProcedure.Create<ICallable, List<object>, List<object>>((func, ls) => ls.Select(x => func.Call(new List<object> { x })).ToList());
            builtins[Symbol.FromString("reverse")] = NativeProcedure.Create<List<object>, List<object>>(ls => ls.Reverse<object>().ToList());
            builtins[Symbol.FromString("range")] = new NativeProcedure(RangeImpl, "range");
            builtins[Symbol.FromString("apply")] = NativeProcedure.Create<ICallable, List<object>, object>((proc, args) => proc.Call(args), "apply");
            builtins[Symbol.FromString("list")] = new NativeProcedure(args => args, "list");
            builtins[Symbol.FromString("list-ref")] = NativeProcedure.Create<List<object>, int, object>((ls, idx) => ls[idx]);
            builtins[Symbol.FromString("length")] = NativeProcedure.Create<List<object>, int>(list => list.Count, "length");
            builtins[Symbol.FromString("car")] = NativeProcedure.Create<List<object>, object>(args => args[0], "car");
            builtins[Symbol.FromString("cdr")] = NativeProcedure.Create<List<object>, List<object>>(args => args.Skip(1).ToList(), "cdr");
            builtins[Symbol.CONS] = NativeProcedure.Create<object, List<object>, List<object>>((x, ys) => Enumerable.Concat(new[] { x }, ys).ToList(), "cons");
            builtins[Symbol.FromString("not")] = NativeProcedure.Create<bool, bool>(x => !x, "not");
            builtins[Symbol.APPEND] = NativeProcedure.Create<List<object>, List<object>, List<object>>((l1, l2) => Enumerable.Concat(l1, l2).ToList(), "append");
            builtins[Symbol.FromString("null")] = NativeProcedure.Create<object>(() => (object)null, "null");
            builtins[Symbol.FromString("null?")] = NativeProcedure.Create<object, bool>(x => x is List<object> && ((List<object>)x).Count == 0, "null?");
            builtins[Symbol.FromString("assert")] = new NativeProcedure(AssertImpl, "assert");
            builtins[Symbol.FromString("load")] = NativeProcedure.Create<string, None>(filename => LoadImpl(interpreter, filename), "load");

            return builtins;
        }

        #region Builtin Implementations

        private static List<object> RangeImpl(List<object> args)
        {
            Utils.CheckSyntax(args, args.Count >= 1 && args.Count <= 3);
            foreach (var item in args)
            {
                Utils.CheckSyntax(args, item is int, "items must be integers");
            }

            int start, end, step;
            if (args.Count == 1)
            {
                start = 0;
                end = (int)args[0];
                step = 1;
            }
            else if (args.Count == 2)
            {
                start = (int)args[0];
                end = (int)args[1];
                step = 1;
            }
            else
            {
                start = (int)args[0];
                end = (int)args[1];
                step = (int)args[2];
            }

            if (start < end) Utils.CheckSyntax(args, step > 0, "step must make the sequence end");
            if (start > end) Utils.CheckSyntax(args, step < 0, "step must make the sequence end");

            var res = new List<object>();

            if (start <= end) for (int i = start; i < end; i += step) res.Add(i);
            else for (int i = start; i > end; i += step) res.Add(i);

            res.TrimExcess();
            return res;
        }

        private static None AssertImpl(List<object> args)
        {
            Utils.CheckArity(args, 1, 2);
            string msg = "Assertion failed";
            msg += args.Count > 1 ? ": " + Utils.ConvertType<string>(args[1]) : string.Empty;
            bool pred = Utils.ConvertType<bool>(args[0]);
            if (!pred) throw new AssertionFailedError(msg);
            return None.Instance;
        }

        private static None LoadImpl(Interpreter interpreter, string filename)
        {
            using (TextReader reader = new StreamReader(interpreter.FileSystemAccessor.OpenRead(filename)))
            {
                interpreter.Evaluate(reader);
            }

            return None.Instance;
        }

        public static bool EqualImpl(object x, object y)
        {
            if (object.Equals(x, y)) return true;
            if (x == null || y == null) return false;

            if (x is IList<object> && y is IList<object>)
            {
                var x2 = (IList<object>)x;
                var y2 = (IList<object>)y;
                if (x2.Count != y2.Count) return false;
                return Enumerable.Zip(x2, y2, (a, b) => Tuple.Create(a, b))
                                 .All(pair => EqualImpl(pair.Item1, pair.Item2));
            }

            return false;
        }

        private static object Add(object x, object y)
        {
            if (x is int && y is int) return (int)x + (int)y;
            return (double)System.Convert.ChangeType(x, typeof(double)) + (double)System.Convert.ChangeType(y, typeof(double));
        }

        private static object Minus(object x, object y)
        {
            if (x is int && y is int) return (int)x - (int)y;
            return (double)System.Convert.ChangeType(x, typeof(double)) - (double)System.Convert.ChangeType(y, typeof(double));
        }

        private static object Multiply(object x, object y)
        {
            if (x is int && y is int) return (int)x * (int)y;
            return (double)System.Convert.ChangeType(x, typeof(double)) * (double)System.Convert.ChangeType(y, typeof(double));
        }

        private static object Divide(object x, object y)
        {
            if (x is int && y is int) return (int)x / (int)y;
            return (double)System.Convert.ChangeType(x, typeof(double)) / (double)System.Convert.ChangeType(y, typeof(double));
        }
        
        #endregion Builtin Implementations
    }
}