/* -*- mode: c++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */

/*

 Copyright (C) 2026 Chirag Desai

 This file is part of QuantLib, a free-software/open-source library

 for financial quantitative analysts and developers - http://quantlib.org/

 QuantLib is free software: you can redistribute it and/or modify it

 under the terms of the QuantLib license.  You should have received a

 copy of the license along with this program; if not, please email

 <quantlib-dev@lists.sf.net>. The license is also available online at

 <https://www.quantlib.org/license.shtml>.

 This program is distributed in the hope that it will be useful, but WITHOUT

 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS

 FOR A PARTICULAR PURPOSE.  See the license for more details.

*/

#include <ql/pricingengines/forward/discountingfxforwardengine.hpp>

#include <utility>

namespace QuantLib {

    DiscountingFxForwardEngine::DiscountingFxForwardEngine(

        Handle<YieldTermStructure> sourceCurrencyDiscountCurve,

        Handle<YieldTermStructure> targetCurrencyDiscountCurve,

        Handle<Quote> spotFx)

    : sourceCurrencyDiscountCurve_(std::move(sourceCurrencyDiscountCurve)),

      targetCurrencyDiscountCurve_(std::move(targetCurrencyDiscountCurve)),

      spotFx_(std::move(spotFx)) {

        registerWith(sourceCurrencyDiscountCurve_);

        registerWith(targetCurrencyDiscountCurve_);

        registerWith(spotFx_);

    }

    void DiscountingFxForwardEngine::calculate() const {

        QL_REQUIRE(!sourceCurrencyDiscountCurve_.empty(),

                   "source currency discount curve handle is empty");

        QL_REQUIRE(!targetCurrencyDiscountCurve_.empty(),

                   "target currency discount curve handle is empty");

        QL_REQUIRE(!spotFx_.empty(), "spot FX quote handle is empty");

        results_.value = 0.0;

        results_.errorEstimate = Null<Real>();

        const Date& maturityDate = arguments_.maturityDate;

        const Date& settlementDate = arguments_.settlementDate;

        // Validate that curve reference dates are on or before settlement date

        const Date sourceRefDate = sourceCurrencyDiscountCurve_->referenceDate();

        const Date targetRefDate = targetCurrencyDiscountCurve_->referenceDate();

        QL_REQUIRE(sourceRefDate <= settlementDate,

                   "source currency discount curve reference date (" << sourceRefDate

                   << ") must be on or before settlement date (" << settlementDate << ")");

        QL_REQUIRE(targetRefDate <= settlementDate,

                   "target currency discount curve reference date (" << targetRefDate

                   << ") must be on or before settlement date (" << settlementDate << ")");

        // Get the spot FX rate (targetCurrency/sourceCurrency)

        const Real spotFxRate = spotFx_->value();

        QL_REQUIRE(spotFxRate > 0.0, "spot FX rate must be positive");

        // Get discount factors from settlement to maturity

        const DiscountFactor dfSource =

            sourceCurrencyDiscountCurve_->discount(maturityDate) /

            sourceCurrencyDiscountCurve_->discount(settlementDate);

        const DiscountFactor dfTarget =

            targetCurrencyDiscountCurve_->discount(maturityDate) /

            targetCurrencyDiscountCurve_->discount(settlementDate);

        const DiscountFactor dfSourceSettlement =

            sourceCurrencyDiscountCurve_->discount(settlementDate);

        const DiscountFactor dfTargetSettlement =

            targetCurrencyDiscountCurve_->discount(settlementDate);

        // Calculate fair forward rate: F = S * dfSource / dfTarget

        // This is the forward rate targetCurrency/sourceCurrency

        results_.fairForwardRate = spotFxRate * dfSource / dfTarget;

        // Calculate settlement-date present values of each leg

        // PV of source currency leg (in source currency)

        const Real pvSource = arguments_.sourceNominal * dfSource;

        // PV of target currency leg (in target currency)

        const Real pvTarget = arguments_.targetNominal * dfTarget;

        // Convert target currency PV to source currency using spot FX rate

        const Real pvTargetInSourceCurrency = pvTarget / spotFxRate;

        // Calculate NPV based on direction of trade

        // If paySourceCurrency is true: pay source currency, receive target currency

        //   NPV = -PVSource + PVTarget (in source currency terms)

        // If paySourceCurrency is false: receive source currency, pay target currency

        //   NPV = +PVSource - PVTarget (in source currency terms)

        const Real npvAtSettlementInSourceCurrency =

            arguments_.paySourceCurrency ?

                Real(-pvSource + pvTargetInSourceCurrency) :

                Real(pvSource - pvTargetInSourceCurrency);

        const Real npvInSourceCurrency =

            npvAtSettlementInSourceCurrency * dfSourceSettlement;

        const Real npvInTargetCurrency =

            npvAtSettlementInSourceCurrency * spotFxRate * dfTargetSettlement;

        // Store results - NPV is as of the curve reference date

        results_.value = npvInSourceCurrency;

        results_.npvSourceCurrency = npvInSourceCurrency;

        results_.npvTargetCurrency = npvInTargetCurrency;

        // Store additional results for inspection

        results_.additionalResults["spotFx"] = spotFxRate;

        results_.additionalResults["sourceCurrencyDiscountFactor"] = dfSource;

        results_.additionalResults["targetCurrencyDiscountFactor"] = dfTarget;

        results_.additionalResults["sourceCurrencySettlementDiscountFactor"] =

            dfSourceSettlement;

        results_.additionalResults["targetCurrencySettlementDiscountFactor"] =

            dfTargetSettlement;

        results_.additionalResults["sourceCurrencyPV"] = pvSource;

        results_.additionalResults["targetCurrencyPV"] = pvTarget;

    }

}