// Copyright (c) .NET Foundation and contributors. All rights reserved.
// Licensed under the MIT license. See LICENSE file in the project root for full license information.

using System;
using System.Collections.Generic;
using System.Linq;
using Mono.Cecil;
using Mono.Cecil.Cil;

namespace TLens.Analyzers
{
    sealed class UserOperatorCalledForNullCheckAnalyzer : Analyzer
    {
        sealed class Counters
        {
            public int Total;
            public int Redundant;

            public double Ratio => (double)Redundant / Total;
        }

        readonly Dictionary<MethodDefinition, Counters> operators = new Dictionary<MethodDefinition, Counters>();

        protected override void ProcessMethod(MethodDefinition method)
        {
            var instrs = method.Body.Instructions;

            foreach (var instr in instrs)
            {
                switch (instr.OpCode.Code)
                {
                    case Code.Call:
                        if (instr.Operand is not MethodReference mr)
                            throw new NotImplementedException();

                        if (mr.Name != "op_Equality" && mr.Name != "op_Inequality")
                            continue;

                        var md = mr.Resolve();
                        if (md.Parameters.Count != 2)
                            continue;

                        if (!operators.TryGetValue(md, out Counters data))
                        {
                            data = new Counters();
                            operators.Add(md, data);
                        }

                        if (instr.Previous.OpCode.Code == Code.Ldnull)
                        {
                            data.Redundant++;
                        }

                        data.Total++;
                        break;
                }
            }
        }

        public override void PrintResults(int maxCount)
        {
            var entries = operators.Where(l => l.Value.Ratio > 0).OrderByDescending(l => l.Value.Ratio).Take(maxCount);
            if (!entries.Any())
                return;

            PrintHeader("User operators used for null checks");

            foreach (var e in entries)
            {
                Console.WriteLine($"User operator '{e.Key.ToDisplay()}' was called {e.Value.Redundant} [{e.Value.Ratio:0%}] times with null values");
            }
        }
    }
}