using System.Globalization;
using System.Text;
using Encelado.Core.Market;
using Encelado.Core.Risk;
using Encelado.Core.Strategies;

namespace Encelado.Core.Backtest;

/// <summary>One symbol and the strategy instance that will trade it during a replay.</summary>
public sealed record BacktestSymbol(string Symbol, IStrategy Strategy);

/// <summary>
/// Everything that makes a replay realistic. The defaults model Alpaca crypto: a
/// marketable-limit slippage plus a per-fill fee, both charged in both directions.
/// </summary>
public sealed record BacktestSettings
{
    public RiskLimits Risk { get; init; } = new();

    public double StartingEquity { get; init; } = 10_000;

    /// <summary>Price concession paid on every fill, in basis points.</summary>
    public double SlippageBps { get; init; } = 8;

    /// <summary>
    /// Broker fee per fill, in basis points of notional. Alpaca crypto taker fees start
    /// around 25 bps, so a round trip costs roughly 50 bps. Leaving this at zero is the
    /// single easiest way to produce a backtest that cannot be reproduced live.
    /// </summary>
    public double FeeBps { get; init; } = 25;

    public bool AllowFractional { get; init; } = true;
}

public sealed record ClosedTrade(
    string Symbol,
    Side Side,
    DateTime EntryUtc,
    DateTime ExitUtc,
    double Quantity,
    double EntryPrice,
    double ExitPrice,
    double GrossPnl,
    double Fees,
    string ExitReason)
{
    /// <summary>Profit after slippage and broker fees. This is the only number that matters.</summary>
    public double Pnl => GrossPnl - Fees;

    public bool IsWin => Pnl > 0;

    public TimeSpan Holding => ExitUtc - EntryUtc;

    public double ReturnPct => EntryPrice > 0 && Quantity > 0 ? Pnl / (EntryPrice * Quantity) : 0;
}

public sealed record BacktestReport(
    double StartEquity,
    double EndEquity,
    double MaxDrawdownPct,
    IReadOnlyList<ClosedTrade> Trades,
    int BarsProcessed,
    DateTime FromUtc,
    DateTime ToUtc,
    double TotalFees)
{
    public static readonly BacktestReport Empty =
        new(0, 0, 0, [], 0, DateTime.MinValue, DateTime.MinValue, 0);

    public double NetPnl => EndEquity - StartEquity;

    public double ReturnPct => StartEquity > 0 ? NetPnl / StartEquity : 0;

    public int Wins => Trades.Count(t => t.IsWin);

    public int Losses => Trades.Count - Wins;

    public double WinRate => Trades.Count > 0 ? Wins / (double)Trades.Count : 0;

    public double GrossProfit => Trades.Where(t => t.Pnl > 0).Sum(t => t.Pnl);

    public double GrossLoss => -Trades.Where(t => t.Pnl < 0).Sum(t => t.Pnl);

    public double ProfitFactor =>
        GrossLoss > 0 ? GrossProfit / GrossLoss : GrossProfit > 0 ? double.PositiveInfinity : 0;

    public double AverageWin => Wins > 0 ? GrossProfit / Wins : 0;

    public double AverageLoss => Losses > 0 ? GrossLoss / Losses : 0;

    public double Expectancy => Trades.Count > 0 ? NetPnl / Trades.Count : 0;

    public double Years => (ToUtc - FromUtc).TotalDays / 365.25;

    /// <summary>Compound annual growth rate. Meaningless for very short windows.</summary>
    public double Cagr
    {
        get
        {
            if (StartEquity <= 0 || EndEquity <= 0 || Years < 0.08)
            {
                return 0;
            }

            return Math.Pow(EndEquity / StartEquity, 1.0 / Years) - 1;
        }
    }

    /// <summary>Annualised return divided by max drawdown — the ratio that decides if a book is fundable.</summary>
    public double CalmarRatio => MaxDrawdownPct > 0 ? Cagr / MaxDrawdownPct : 0;

    public TimeSpan AverageHolding => Trades.Count == 0
        ? TimeSpan.Zero
        : TimeSpan.FromMinutes(Trades.Average(t => t.Holding.TotalMinutes));

    public string Render()
    {
        StringBuilder sb = new(1400);
        CultureInfo ci = CultureInfo.InvariantCulture;

        sb.AppendLine(ci, $"period          {FromUtc:yyyy-MM-dd} .. {ToUtc:yyyy-MM-dd}   ({Years:F2} years, {BarsProcessed:N0} bars)");
        sb.AppendLine(ci, $"equity          {StartEquity:N2} -> {EndEquity:N2}   ({ReturnPct:P2})");
        sb.AppendLine(ci, $"CAGR            {Cagr:P2}");
        sb.AppendLine(ci, $"max drawdown    {MaxDrawdownPct:P2}      Calmar {CalmarRatio:F2}");
        sb.AppendLine(ci, $"net P&L         {NetPnl:N2}      fees paid {TotalFees:N2}");
        sb.AppendLine(ci, $"trades          {Trades.Count}  (wins {Wins} / losses {Losses}, win rate {WinRate:P1})");
        sb.AppendLine(ci, $"profit factor   {(double.IsInfinity(ProfitFactor) ? "inf" : ProfitFactor.ToString("F2", ci))}");
        sb.AppendLine(ci, $"avg win / loss  {AverageWin:N2} / {AverageLoss:N2}      expectancy {Expectancy:N2}/trade");
        sb.AppendLine(ci, $"avg holding     {AverageHolding.TotalHours:F1} h");

        if (Trades.Count > 0)
        {
            sb.AppendLine();
            sb.AppendLine("per symbol:");
            foreach (IGrouping<string, ClosedTrade> g in Trades.GroupBy(t => t.Symbol).OrderBy(g => g.Key))
            {
                sb.AppendLine(ci,
                    $"  {g.Key,-12} trades={g.Count(),4}  wins={g.Count(t => t.IsWin),4}  pnl={g.Sum(t => t.Pnl),14:N2}");
            }

            sb.AppendLine();
            sb.AppendLine("exit reasons:");
            foreach (IGrouping<string, ClosedTrade> g in Trades.GroupBy(t => Bucket(t.ExitReason)).OrderByDescending(g => g.Count()))
            {
                sb.AppendLine(ci, $"  {g.Key,-24} {g.Count(),4}   pnl={g.Sum(t => t.Pnl),14:N2}");
            }
        }

        return sb.ToString();

        // A trailing stop is how a trend trade normally *takes profit*, so lumping it
        // in with the protective stop would make a winning exit look like a loss.
        static string Bucket(string reason) =>
            reason.Contains("trailing stop", StringComparison.OrdinalIgnoreCase) ? "trailing stop"
            : reason.Contains("take profit", StringComparison.OrdinalIgnoreCase) ? "take profit"
            : reason.Contains("stop", StringComparison.OrdinalIgnoreCase) ? "stop loss"
            : reason.Contains("end of backtest", StringComparison.OrdinalIgnoreCase) ? "end of data"
            : "signal exit";
    }
}