RustyQLib 0.0.3

//! Cliquet (ratchet) options: a strip of forward-start performance
//! periods with local and global caps/floors.
//!
//! The payoff observes period returns `R_i = S_{t_i}/S_{t_{i-1}} - 1`
//! over an equally-spaced reset schedule and pays at maturity
//!
//! ```text
//! N * clamp( sum_i clamp(R_i, local_floor, local_cap),
//!            global_floor, global_cap )
//! ```
//!
//! A **ratchet** is the `local_floor = 0` special case: each period
//! locks in its gain and losses are forgiven. Because the payoff is
//! built from returns it is spot-homogeneous — the classic product
//! whose value is all **forward smile**: under Black-Scholes each
//! period is an independent lognormal and the price collapses to a
//! closed form (a strip of forward-start call spreads); under Heston
//! the forward smile is model-generated and the price genuinely
//! differs, which is the reason desks price cliquets on stochastic-vol
//! models.
//!
//! Engines: `Analytical` (Black-Scholes closed form; requires no
//! global cap/floor, which break the per-period independence) and
//! `MonteCarlo` (GBM per-period sampling, or full Heston paths when
//! parameters are supplied). Under homogeneous dynamics the pure
//! cliquet has zero spot delta; the output reports Monte Carlo
//! standard errors instead of spot Greeks.

use chrono::NaiveDate;
use serde::{Deserialize, Serialize};

use crate::core::montecarlo::{mean_std_err, path_rng};
use crate::core::traits::Instrument;
use crate::core::utils::norm_cdf;
use crate::equity::heston::HestonParams;
use rand::Rng;
use rand_distr::StandardNormal;
use crate::core::errors::RustyQLibError;

/// Pricing engine choice for a cliquet.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum CliquetPricer {
    Analytical,
    MonteCarlo,
}

/// Payoff family on the reset schedule.
#[derive(Debug, Clone, Copy, PartialEq)]
pub enum CliquetStyle {
    /// Sum of locally clamped returns (the classic cliquet; local
    /// floor 0 = ratchet).
    Standard,
    /// Reverse cliquet: a headline `coupon` eroded by the negative
    /// period returns, `coupon + sum_i min(R_i, 0)` — conventionally
    /// sold with `global_floor = 0`. Ignores the local clamp fields.
    Reverse { coupon: f64 },
    /// Napoleon: a `coupon` plus the **worst** period return,
    /// `coupon + min_i R_i` — conventionally `global_floor = 0`.
    /// Ignores the local clamp fields.
    Napoleon { coupon: f64 },
}

/// JSON contract data (`"product_type": "cliquet_option"`).
#[derive(Clone, Debug, Deserialize, Serialize)]
pub struct CliquetOptionData {
    pub symbol: String,
    /// Number of equally-spaced reset periods.
    pub resets: usize,
    /// Maturity date, `YYYY-MM-DD`.
    pub maturity: String,
    /// Per-period floor on the return (e.g. `0.0` for a ratchet).
    pub local_floor: f64,
    /// Per-period cap on the return.
    pub local_cap: Option<f64>,
    pub global_floor: Option<f64>,
    pub global_cap: Option<f64>,
    pub notional: Option<f64>,
    pub risk_free_rate: f64,
    pub dividend: Option<f64>,
    /// Flat Black-Scholes volatility (GBM engine and the closed form).
    pub volatility: f64,
    /// Optional Heston parameters: when present the Monte Carlo engine
    /// simulates Heston dynamics instead of GBM.
    pub heston: Option<HestonParams>,
    pub pricer: Option<String>,
    pub simulation: Option<u64>,
    pub mc_seed: Option<u64>,
    /// "standard" (default) | "reverse" | "napoleon".
    pub style: Option<String>,
    /// Headline coupon for reverse / napoleon styles.
    pub coupon: Option<f64>,
    /// Pricing as-of date (`YYYY-MM-DD`); defaults to today.
    pub valuation_date: Option<String>,
}

/// A cliquet/ratchet option on equally-spaced resets.
#[derive(Debug, Clone)]
pub struct Cliquet {
    pub resets: usize,
    /// Year fraction to maturity.
    pub t: f64,
    pub r: f64,
    pub q: f64,
    pub sigma: f64,
    pub local_floor: f64,
    pub local_cap: Option<f64>,
    pub global_floor: Option<f64>,
    pub global_cap: Option<f64>,
    pub notional: f64,
    pub heston: Option<HestonParams>,
    pub style: CliquetStyle,
    pub pricer: CliquetPricer,
    pub paths: usize,
    pub seed: u64,
}

/// Euler substeps per reset period for the Heston path engine.
const HESTON_SUBSTEPS: usize = 32;

impl Cliquet {
    fn clamp_local(&self, ret: f64) -> f64 {
        let mut x = ret.max(self.local_floor);
        if let Some(cap) = self.local_cap {
            x = x.min(cap);
        }
        x
    }

    fn accumulate(&self, ret: f64, total: &mut f64, worst: &mut f64) {
        match self.style {
            CliquetStyle::Standard => *total += self.clamp_local(ret),
            CliquetStyle::Reverse { .. } => *total += ret.min(0.0),
            CliquetStyle::Napoleon { .. } => *worst = worst.min(ret),
        }
    }

    fn clamp_global(&self, sum: f64) -> f64 {
        let mut x = sum;
        if let Some(floor) = self.global_floor {
            x = x.max(floor);
        }
        if let Some(cap) = self.global_cap {
            x = x.min(cap);
        }
        x
    }

    /// Black-Scholes closed form: each period's clamped return is a
    /// forward-start call spread on the lognormal period ratio, and the
    /// periods are independent, so the sum prices term by term. Errs
    /// when a global cap/floor is present (it couples the periods) or
    /// Heston dynamics are requested.
    pub fn analytic_npv(&self) -> Result<f64, RustyQLibError> {
        if self.global_floor.is_some() || self.global_cap.is_some() {
            return Err(RustyQLibError::UnsupportedEngine("global cap/floor couples the periods: use Monte Carlo".to_string()));
        }
        if self.heston.is_some() {
            return Err(RustyQLibError::UnsupportedEngine("Heston cliquets price by Monte Carlo".to_string()));
        }
        let dt = self.t / self.resets as f64;
        let fwd = ((self.r - self.q) * dt).exp();
        let sd = self.sigma * dt.sqrt();
        // undiscounted E[(X - k)+] for the lognormal period ratio X
        let ratio_call = |k: f64| -> f64 {
            let d1 = ((fwd / k).ln() + 0.5 * sd * sd) / sd;
            fwd * norm_cdf(d1) - k * norm_cdf(d1 - sd)
        };
        let df_n = self.notional * (-self.r * self.t).exp();
        match self.style {
            CliquetStyle::Standard => {
                // E[clamp(R, lf, lc)] = lf + call(1 + lf) - call(1 + lc)
                let mut period = self.local_floor + ratio_call(1.0 + self.local_floor);
                if let Some(cap) = self.local_cap {
                    period -= ratio_call(1.0 + cap);
                }
                Ok(df_n * self.resets as f64 * period)
            }
            CliquetStyle::Reverse { coupon } => {
                // E[min(R, 0)] = -E[(1 - X)+], a put on the period ratio
                // struck at 1, via parity: put(1) = call(1) - (F - 1)
                let ratio_put_at_one = ratio_call(1.0) - (fwd - 1.0);
                Ok(df_n * (coupon - self.resets as f64 * ratio_put_at_one))
            }
            CliquetStyle::Napoleon { .. } => {
                Err(RustyQLibError::UnsupportedEngine("the Napoleon's worst-of statistic prices by Monte Carlo".to_string()))
            }
        }
    }

    /// Monte Carlo price with standard error: per-period lognormal
    /// sampling under GBM, full Euler paths under Heston. Deterministic
    /// per seed.
    pub fn mc_npv(&self) -> (f64, f64) {
        let dt = self.t / self.resets as f64;
        let mut sum = 0.0;
        let mut sum_sq = 0.0;
        for i in 0..self.paths {
            let mut rng = path_rng(self.seed, i as u64);
            let mut total = 0.0;
            let mut worst = f64::INFINITY;
            match &self.heston {
                None => {
                    let drift = (self.r - self.q - 0.5 * self.sigma * self.sigma) * dt;
                    let sd = self.sigma * dt.sqrt();
                    for _ in 0..self.resets {
                        let z: f64 = rng.sample(StandardNormal);
                        let ret = (drift + sd * z).exp() - 1.0;
                        self.accumulate(ret, &mut total, &mut worst);
                    }
                }
                Some(hp) => {
                    let sub = dt / HESTON_SUBSTEPS as f64;
                    let rho_bar = (1.0 - hp.rho * hp.rho).sqrt();
                    let mut v: f64 = hp.v0;
                    for _ in 0..self.resets {
                        let mut log_ret = 0.0;
                        for _ in 0..HESTON_SUBSTEPS {
                            let z1: f64 = rng.sample(StandardNormal);
                            let z2: f64 = rng.sample(StandardNormal);
                            let zv = hp.rho * z1 + rho_bar * z2;
                            let vp = v.max(0.0);
                            log_ret += (self.r - self.q - 0.5 * vp) * sub
                                + (vp * sub).sqrt() * z1;
                            v += hp.kappa * (hp.theta - vp) * sub
                                + hp.vol_of_vol * (vp * sub).sqrt() * zv;
                        }
                        self.accumulate(log_ret.exp() - 1.0, &mut total, &mut worst);
                    }
                }
            }
            let units = match self.style {
                CliquetStyle::Standard => self.clamp_global(total),
                CliquetStyle::Reverse { coupon } => self.clamp_global(coupon + total),
                CliquetStyle::Napoleon { coupon } => self.clamp_global(coupon + worst),
            };
            let payoff = self.notional * (-self.r * self.t).exp() * units;
            sum += payoff;
            sum_sq += payoff * payoff;
        }
        mean_std_err(sum, sum_sq, self.paths)
    }

    /// Price with the configured engine (analytic falls back to Monte
    /// Carlo when global constraints or Heston dynamics require it).
    /// Build from contract data, panicking on any invalid field. Fallible
    /// callers should use [`Cliquet::try_from_json`].
    pub fn from_json(data: &CliquetOptionData) -> Box<Cliquet> {
        Self::try_from_json(data).unwrap_or_else(|e| panic!("{e}"))
    }

    pub fn try_from_json(data: &CliquetOptionData) -> Result<Box<Cliquet>, RustyQLibError> {
        let today =
            crate::core::data_models::parse_valuation_date(data.valuation_date.as_deref())?;
        let maturity = NaiveDate::parse_from_str(&data.maturity, "%Y-%m-%d")
            .map_err(|_| RustyQLibError::invalid_input(
                "maturity",
                format!("invalid date '{}' (expected YYYY-MM-DD)", data.maturity),
            ))?;
        let t = (maturity - today).num_days() as f64 / 365.0;
        if t <= 0.0 {
            return Err(RustyQLibError::invalid_input("maturity", "cliquet is expired"));
        }
        if data.resets < 1 {
            return Err(RustyQLibError::invalid_input("resets", "need at least one reset period"));
        }
        if let Some(hp) = &data.heston {
            hp.validate()?;
        }
        let pricer = match data.pricer.as_deref().map(str::trim) {
            None | Some("Analytical") | Some("analytical") | Some("bs") => {
                CliquetPricer::Analytical
            }
            Some("MonteCarlo") | Some("montecarlo") | Some("MC") | Some("mc") => {
                CliquetPricer::MonteCarlo
            }
            Some(other) => return Err(RustyQLibError::invalid_input(
                "pricer",
                format!("invalid cliquet pricer '{other}' (use Analytical or MonteCarlo)"),
            )),
        };
        let need_coupon = |style: &str| {
            data.coupon.ok_or_else(|| RustyQLibError::invalid_input(
                "coupon",
                format!("{style} cliquet needs a coupon"),
            ))
        };
        let style = match data.style.as_deref().map(str::trim) {
            None | Some("standard") | Some("Standard") => CliquetStyle::Standard,
            Some("reverse") | Some("Reverse") => CliquetStyle::Reverse {
                coupon: need_coupon("reverse")?,
            },
            Some("napoleon") | Some("Napoleon") => CliquetStyle::Napoleon {
                coupon: need_coupon("napoleon")?,
            },
            Some(other) => return Err(RustyQLibError::invalid_input(
                "style",
                format!("invalid cliquet style '{other}' (use standard, reverse or napoleon)"),
            )),
        };
        Ok(Box::new(Cliquet {
            resets: data.resets,
            t,
            r: data.risk_free_rate,
            q: data.dividend.unwrap_or(0.0),
            sigma: data.volatility,
            local_floor: data.local_floor,
            local_cap: data.local_cap,
            global_floor: data.global_floor,
            global_cap: data.global_cap,
            notional: data.notional.unwrap_or(1.0),
            heston: data.heston,
            style,
            pricer,
            paths: data.simulation.unwrap_or(100_000) as usize,
            seed: data.mc_seed.unwrap_or(42),
        }))
    }
}

impl Instrument for Cliquet {
    fn try_npv(&self) -> Result<f64, RustyQLibError> {
        Ok(self.price()?.pv)
    }

    /// Analytic where the payoff permits (falling back to Monte Carlo
    /// otherwise, mirroring the documented pricing policy); the standard
    /// error is reported whenever a simulation produced the value.
    fn price(&self) -> Result<crate::core::results::PricingResult, RustyQLibError> {
        let (pv, std_err) = match self.pricer {
            CliquetPricer::Analytical => match self.analytic_npv() {
                Ok(v) => (v, None),
                Err(_) => {
                    let (pv, se) = self.mc_npv();
                    (pv, Some(se))
                }
            },
            CliquetPricer::MonteCarlo => {
                let (pv, se) = self.mc_npv();
                (pv, Some(se))
            }
        };
        // the return-based payoff is spot-homogeneous: no spot Greeks
        Ok(crate::core::results::PricingResult { pv, greeks: Default::default(), std_err })
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::equity::blackscholes::bs_price;
    use crate::core::trade::PutOrCall;

    fn base() -> Cliquet {
        Cliquet {
            resets: 12,
            t: 1.0,
            r: 0.03,
            q: 0.01,
            sigma: 0.2,
            local_floor: 0.0,
            local_cap: Some(0.03),
            global_floor: None,
            global_cap: None,
            notional: 1.0,
            heston: None,
            style: CliquetStyle::Standard,
            pricer: CliquetPricer::Analytical,
            paths: 60_000,
            seed: 42,
        }
    }

    #[test]
    fn single_period_ratchet_is_a_scaled_atm_call() {
        // one reset, floor 0, no cap: pays (S_T/S_0 - 1)+ = ATM call / S0
        let mut c = base();
        c.resets = 1;
        c.local_cap = None;
        let analytic = c.analytic_npv().unwrap();
        let atm = bs_price(100.0, 100.0, c.r, c.q, c.sigma, c.t, PutOrCall::Call) / 100.0;
        assert!((analytic - atm).abs() < 1e-12, "{analytic} vs {atm}");
    }

    #[test]
    fn monte_carlo_agrees_with_the_closed_form() {
        let c = base();
        let analytic = c.analytic_npv().unwrap();
        let (mc, se) = c.mc_npv();
        assert!(
            (mc - analytic).abs() < 3.0 * se + 1e-4,
            "mc {mc} +/- {se} vs analytic {analytic}"
        );
        // deterministic per seed
        assert_eq!(c.mc_npv().0, mc);
    }

    #[test]
    fn caps_and_floors_move_the_price_the_right_way() {
        let c = base();
        let baseline = c.analytic_npv().unwrap();
        // a tighter local cap must cheapen the strip
        let mut tight = base();
        tight.local_cap = Some(0.01);
        assert!(tight.analytic_npv().unwrap() < baseline);
        // a global floor adds value, a global cap removes it (MC only)
        let mut floored = base();
        floored.global_floor = Some(0.06);
        assert!(floored.analytic_npv().is_err());
        assert!(floored.mc_npv().0 > c.mc_npv().0 - 1e-12);
        let mut capped = base();
        capped.global_cap = Some(0.10);
        assert!(capped.mc_npv().0 < c.mc_npv().0);
        // local floor above the cap is degenerate but bounded
        let mut sunk = base();
        sunk.local_floor = -0.02;
        assert!(sunk.analytic_npv().unwrap() < baseline);
    }

    #[test]
    fn heston_with_tiny_vol_of_vol_matches_the_gbm_closed_form() {
        let mut c = base();
        c.heston = Some(HestonParams {
            v0: c.sigma * c.sigma,
            kappa: 1.0,
            theta: c.sigma * c.sigma,
            vol_of_vol: 1e-4,
            rho: 0.0,
        });
        c.paths = 40_000;
        let (mc, se) = c.mc_npv();
        let analytic = base().analytic_npv().unwrap();
        assert!(
            (mc - analytic).abs() < 4.0 * se + 5e-4,
            "heston-degenerate {mc} +/- {se} vs analytic {analytic}"
        );
    }

    #[test]
    fn heston_forward_smile_moves_the_cliquet_off_black_scholes() {
        // same total variance, real vol-of-vol and negative rho: the
        // capped/floored strip prices differently from flat-vol GBM —
        // the whole reason cliquets are priced on stochastic vol
        let mut c = base();
        c.heston = Some(HestonParams {
            v0: 0.04,
            kappa: 1.5,
            theta: 0.04,
            vol_of_vol: 0.7,
            rho: -0.7,
        });
        c.paths = 60_000;
        let (mc, se) = c.mc_npv();
        let flat = base().analytic_npv().unwrap();
        assert!(
            (mc - flat).abs() > 3.0 * se,
            "expected a forward-smile effect: heston {mc} +/- {se} vs bs {flat}"
        );
    }

    #[test]
    fn reverse_cliquet_closed_form_matches_monte_carlo() {
        // unfloored reverse: coupon minus a strip of forward-start puts
        let mut c = base();
        c.style = CliquetStyle::Reverse { coupon: 0.20 };
        c.local_cap = None;
        let analytic = c.analytic_npv().unwrap();
        let (mc, se) = c.mc_npv();
        assert!(
            (mc - analytic).abs() < 3.0 * se + 1e-4,
            "mc {mc} +/- {se} vs analytic {analytic}"
        );
        // the conventional 0% floor only adds value
        let mut floored = c.clone();
        floored.global_floor = Some(0.0);
        assert!(floored.analytic_npv().is_err());
        assert!(floored.mc_npv().0 >= mc - 1e-12);
    }

    #[test]
    fn single_period_napoleon_is_a_forward_start_call() {
        // one reset, floor 0: max(0, C + R) = (X - (1 - C))+ on the ratio
        let coupon = 0.10;
        let mut c = base();
        c.resets = 1;
        c.local_cap = None;
        c.style = CliquetStyle::Napoleon { coupon };
        c.global_floor = Some(0.0);
        c.paths = 200_000;
        let (mc, se) = c.mc_npv();
        // a call on the unit-spot period ratio struck at 1 - C, already
        // discounted by bs_price
        let analytic = bs_price(1.0, 1.0 - coupon, c.r, c.q, c.sigma, c.t, PutOrCall::Call);
        assert!(
            (mc - analytic).abs() < 3.0 * se + 1e-4,
            "mc {mc} +/- {se} vs analytic {analytic}"
        );
    }

    #[test]
    fn napoleon_worsens_with_more_resets_and_higher_vol() {
        let napoleon = |resets: usize, sigma: f64| -> f64 {
            let mut c = base();
            c.resets = resets;
            c.t = 1.0;
            c.sigma = sigma;
            c.local_cap = None;
            c.style = CliquetStyle::Napoleon { coupon: 0.10 };
            c.global_floor = Some(0.0);
            c.paths = 30_000;
            c.mc_npv().0
        };
        // the minimum of more period returns is worse
        assert!(napoleon(1, 0.2) > napoleon(4, 0.2));
        assert!(napoleon(4, 0.2) > napoleon(12, 0.2));
        // and the structure is short volatility
        assert!(napoleon(12, 0.15) > napoleon(12, 0.30));
    }

    #[test]
    fn napoleon_vol_of_vol_exposure_is_large_and_directionally_convex() {
        // Same total variance, real vol-of-vol. Note the direction: the
        // FLOORED Napoleon is convex in the worst-month distribution (the
        // 0% floor truncates the fat left tail, while calm-vol regimes
        // improve the worst month), so vol-of-vol RAISES the buyer's
        // value here — the famous Napoleon blowups were the sellers'
        // short position in exactly this convexity. The unfloored
        // structure, by contrast, is hurt by vol clustering.
        let mut gbm = base();
        gbm.style = CliquetStyle::Napoleon { coupon: 0.10 };
        gbm.local_cap = None;
        gbm.global_floor = Some(0.0);
        gbm.paths = 60_000;
        let (flat, flat_se) = gbm.mc_npv();
        let hp = HestonParams { v0: 0.04, kappa: 1.5, theta: 0.04, vol_of_vol: 0.7, rho: -0.7 };
        let mut heston = gbm.clone();
        heston.heston = Some(hp);
        let (stoch, stoch_se) = heston.mc_npv();
        let noise = (flat_se * flat_se + stoch_se * stoch_se).sqrt();
        assert!(stoch > flat + 5.0 * noise, "floored: heston {stoch} vs gbm {flat}");

        // without the floor two effects compete (worst-month concavity
        // vs the right-skewed CIR variance making the typical month
        // calmer), so no sign is asserted — only that the model choice
        // moves the price by far more than the Monte Carlo noise
        let mut gbm_unfloored = gbm.clone();
        gbm_unfloored.global_floor = None;
        let (flat_u, se_u) = gbm_unfloored.mc_npv();
        let mut heston_unfloored = gbm_unfloored.clone();
        heston_unfloored.heston = Some(hp);
        let (stoch_u, se_u2) = heston_unfloored.mc_npv();
        let noise_u = (se_u * se_u + se_u2 * se_u2).sqrt();
        assert!(
            (stoch_u - flat_u).abs() > 5.0 * noise_u,
            "unfloored: heston {stoch_u} vs gbm {flat_u} (noise {noise_u})"
        );
    }

    #[test]
    fn styled_json_contracts_parse() {
        let json = r#"{
            "symbol": "NAP", "resets": 12, "maturity": "2030-01-01",
            "local_floor": 0.0, "global_floor": 0.0,
            "risk_free_rate": 0.03, "volatility": 0.2,
            "style": "napoleon", "coupon": 0.08,
            "pricer": "MC", "simulation": 5000
        }"#;
        let data: CliquetOptionData = serde_json::from_str(json).unwrap();
        let napoleon = Cliquet::from_json(&data);
        assert_eq!(napoleon.style, CliquetStyle::Napoleon { coupon: 0.08 });
        let pv = napoleon.npv();
        assert!(pv > 0.0 && pv < 0.08, "{pv}"); // bounded by the coupon
    }

    #[test]
    fn json_contract_round_trip() {
        let json = r#"{
            "symbol": "CLIQ", "resets": 4, "maturity": "2030-01-01",
            "local_floor": 0.0, "local_cap": 0.05, "global_floor": 0.02,
            "notional": 1000000.0, "risk_free_rate": 0.03, "dividend": 0.01,
            "volatility": 0.25, "pricer": "MC", "simulation": 20000, "mc_seed": 7
        }"#;
        let data: CliquetOptionData = serde_json::from_str(json).unwrap();
        let cliquet = Cliquet::from_json(&data);
        assert_eq!(cliquet.resets, 4);
        assert_eq!(cliquet.pricer, CliquetPricer::MonteCarlo);
        let pv = cliquet.npv();
        // bounded by the discounted global-capped maximum
        assert!(pv > 0.0 && pv < 1_000_000.0 * 4.0 * 0.05, "{pv}");
    }
}