feat: implement Chooser option pricing model

Implements Chooser option (as-you-like-it) pricing using:
- Rubinstein (1991) simple chooser formula
- Choice between call and put at specified date

Formula:
V = S*e^(-qT)*N(d1) - K*e^(-rT)*N(d2) + K*e^(-rt)*N(-y2) - S*e^(-qt)*N(-y1)

Where:
- T = time to final expiration
- t = time to choice date
- d1, d2 = standard BS d-values for T
- y1, y2 = choice date d-values

Changes:
- New file src/pricing/chooser.rs with pricing functions
- Integrated into pricing module exports
- Routed OptionType::Chooser in black_scholes_model.rs

Tests:
- 11 comprehensive unit tests covering:
  - Simple chooser pricing
  - Chooser >= call and >= put (fundamental property)
  - Early/late choice dates
  - Choice at expiry (straddle)
  - Short option negation
  - Zero time / ITM call/put at expiry
  - Higher vol = higher value

Closes #242

Author: joaquinbejar

src/pricing/black_scholes_model.rs

@@ -74,10 +74,7 @@ pub fn black_scholes(option: &Options) -> Result<Decimal, PricingError> {
         OptionType::Binary { .. } => crate::pricing::binary::binary_black_scholes(option),
         OptionType::Lookback { .. } => crate::pricing::lookback::lookback_black_scholes(option),
         OptionType::Compound { .. } => crate::pricing::compound::compound_black_scholes(option),
-        OptionType::Chooser { .. } => Err(PricingError::unsupported_option_type(
-            "Chooser",
-            "Black-Scholes",
-        )),
+        OptionType::Chooser { .. } => crate::pricing::chooser::chooser_black_scholes(option),
         OptionType::Cliquet { .. } => Err(PricingError::unsupported_option_type(
             "Cliquet",
             "Black-Scholes",

src/pricing/chooser.rs

@@ -0,0 +1,367 @@
+/******************************************************************************
+   Author: Joaquín Béjar García
+   Email: jb@taunais.com
+   Date: 13/01/26
+******************************************************************************/
+
+//! Chooser option pricing module.
+//!
+//! Chooser options (also called as-you-like-it options) allow the holder to
+//! choose at a specified date whether the option becomes a call or a put.
+//!
+//! # Simple Chooser (Rubinstein 1991)
+//!
+//! At the choice date t, the holder chooses max(Call, Put).
+//! The value is:
+//!
+//! `V = S*e^(-qT)*N(d1) - K*e^(-rT)*N(d2) + K*e^(-rt)*N(-y2) - S*e^(-qt)*N(-y1)`
+//!
+//! Where:
+//! - T = time to final expiration
+//! - t = time to choice date
+//! - d1, d2 are standard BS d-values for T
+//! - y1 = [ln(S/K) + b*t + (σ²/2)*t] / (σ√t)
+//! - y2 = y1 - σ√t
+
+use crate::Options;
+use crate::error::PricingError;
+use crate::greeks::{big_n, d1, d2};
+use crate::model::types::OptionType;
+use positive::Positive;
+use rust_decimal::Decimal;
+use rust_decimal::prelude::*;
+use rust_decimal_macros::dec;
+
+/// Prices a Chooser option using Rubinstein (1991) simple chooser formula.
+///
+/// # Arguments
+///
+/// * `option` - The option to price. Must have `OptionType::Chooser`.
+///
+/// # Returns
+///
+/// The option price as a `Decimal`, or a `PricingError` if pricing fails.
+pub fn chooser_black_scholes(option: &Options) -> Result<Decimal, PricingError> {
+    match &option.option_type {
+        OptionType::Chooser { choice_date } => simple_chooser_price(option, *choice_date),
+        _ => Err(PricingError::other(
+            "chooser_black_scholes requires OptionType::Chooser",
+        )),
+    }
+}
+
+/// Prices a simple chooser option.
+///
+/// At the choice date, the holder chooses the maximum of call or put value.
+/// Uses Rubinstein (1991) closed-form solution.
+fn simple_chooser_price(option: &Options, choice_date_days: f64) -> Result<Decimal, PricingError> {
+    let s = option.underlying_price;
+    let k = option.strike_price;
+    let r = option.risk_free_rate;
+    let q = option.dividend_yield.to_dec();
+    let sigma = option.implied_volatility;
+    let t_big = option
+        .expiration_date
+        .get_years()
+        .map_err(|e| PricingError::other(&e.to_string()))?;
+
+    // Convert choice_date from days to years
+    let t_choice = Positive::new(choice_date_days / 365.0).unwrap_or(Positive::ZERO);
+
+    // Validation: choice date must be before expiration
+    if t_choice >= t_big {
+        // If choice at or after expiration, it's just max(call, put) = straddle-like
+        return price_at_choice_equals_expiry(option);
+    }
+
+    if t_big == Positive::ZERO {
+        // At expiration, intrinsic value
+        let call_intrinsic = (s.to_dec() - k.to_dec()).max(Decimal::ZERO);
+        let put_intrinsic = (k.to_dec() - s.to_dec()).max(Decimal::ZERO);
+        return Ok(apply_side(call_intrinsic.max(put_intrinsic), option));
+    }
+
+    if sigma == Positive::ZERO {
+        // Zero vol: deterministic choice
+        let discount_t = (-r * t_big).exp();
+        let forward = s.to_dec() * ((r - q) * t_big.to_dec()).exp();
+        let call_val = (forward - k.to_dec()).max(Decimal::ZERO) * discount_t;
+        let put_val = (k.to_dec() - forward).max(Decimal::ZERO) * discount_t;
+        return Ok(apply_side(call_val.max(put_val), option));
+    }
+
+    let b = r - q;
+    let t_big_dec = t_big.to_dec();
+    let t_choice_dec = t_choice.to_dec();
+    let _sqrt_t_big = t_big_dec.sqrt().unwrap_or(Decimal::ZERO);
+    let sqrt_t_choice = t_choice_dec.sqrt().unwrap_or(dec!(0.001));
+
+    // Standard BS d-values for the final expiration T
+    let d1_val = d1(s, k, b, t_big, sigma)
+        .map_err(|e: crate::error::GreeksError| PricingError::other(&e.to_string()))?;
+    let d2_val = d2(s, k, b, t_big, sigma)
+        .map_err(|e: crate::error::GreeksError| PricingError::other(&e.to_string()))?;
+
+    // d-values for the choice date t
+    // y1 = [ln(S/K) + (b + σ²/2)*t] / (σ√t)
+    // y2 = y1 - σ√t
+    let y1 = ((s.to_dec() / k.to_dec()).ln() + (b + sigma * sigma / dec!(2)) * t_choice_dec)
+        / (sigma.to_dec() * sqrt_t_choice);
+    let y2 = y1 - sigma.to_dec() * sqrt_t_choice;
+
+    // Get cumulative normal values
+    let n_d1 = big_n(d1_val).unwrap_or(Decimal::ZERO);
+    let n_d2 = big_n(d2_val).unwrap_or(Decimal::ZERO);
+    let n_neg_y1 = big_n(-y1).unwrap_or(Decimal::ZERO);
+    let n_neg_y2 = big_n(-y2).unwrap_or(Decimal::ZERO);
+
+    // Discount factors
+    let dividend_discount_t = (-q * t_big_dec).exp();
+    let discount_t = (-r * t_big_dec).exp();
+    let dividend_discount_choice = (-q * t_choice_dec).exp();
+    let discount_choice = (-r * t_choice_dec).exp();
+
+    // Rubinstein (1991) simple chooser formula:
+    // V = S*e^(-qT)*N(d1) - K*e^(-rT)*N(d2) + K*e^(-rt)*N(-y2) - S*e^(-qt)*N(-y1)
+    // This equals: Call(K, T) + Put_component_for_choice_flexibility
+    let price = s.to_dec() * dividend_discount_t * n_d1 - k.to_dec() * discount_t * n_d2
+        + k.to_dec() * discount_choice * n_neg_y2
+        - s.to_dec() * dividend_discount_choice * n_neg_y1;
+
+    Ok(apply_side(price.max(Decimal::ZERO), option))
+}
+
+/// Handles the edge case where choice date equals or exceeds expiration.
+fn price_at_choice_equals_expiry(option: &Options) -> Result<Decimal, PricingError> {
+    // At this point, chooser becomes max(call, put) = straddle at expiry
+    // For European option at expiry, this is max(intrinsic_call, intrinsic_put)
+    let s = option.underlying_price;
+    let k = option.strike_price;
+    let r = option.risk_free_rate;
+    let q = option.dividend_yield.to_dec();
+    let sigma = option.implied_volatility;
+    let t = option
+        .expiration_date
+        .get_years()
+        .map_err(|e| PricingError::other(&e.to_string()))?;
+
+    if t == Positive::ZERO {
+        let call_intrinsic = (s.to_dec() - k.to_dec()).max(Decimal::ZERO);
+        let put_intrinsic = (k.to_dec() - s.to_dec()).max(Decimal::ZERO);
+        return Ok(apply_side(call_intrinsic.max(put_intrinsic), option));
+    }
+
+    // Price as call + put (straddle) since choice is at expiry
+    let b = r - q;
+    let d1_val = d1(s, k, b, t, sigma)
+        .map_err(|e: crate::error::GreeksError| PricingError::other(&e.to_string()))?;
+    let d2_val = d2(s, k, b, t, sigma)
+        .map_err(|e: crate::error::GreeksError| PricingError::other(&e.to_string()))?;
+
+    let n_d1 = big_n(d1_val).unwrap_or(Decimal::ZERO);
+    let n_d2 = big_n(d2_val).unwrap_or(Decimal::ZERO);
+    let n_neg_d1 = big_n(-d1_val).unwrap_or(Decimal::ZERO);
+    let n_neg_d2 = big_n(-d2_val).unwrap_or(Decimal::ZERO);
+
+    let dividend_discount = (-q * t).exp();
+    let discount = (-r * t).exp();
+
+    // Call + Put = Straddle
+    let call_price = s.to_dec() * dividend_discount * n_d1 - k.to_dec() * discount * n_d2;
+    let put_price = k.to_dec() * discount * n_neg_d2 - s.to_dec() * dividend_discount * n_neg_d1;
+
+    Ok(apply_side(
+        (call_price + put_price).max(Decimal::ZERO),
+        option,
+    ))
+}
+
+/// Applies the side (long/short) multiplier to the price.
+fn apply_side(price: Decimal, option: &Options) -> Decimal {
+    match option.side {
+        crate::model::types::Side::Long => price,
+        crate::model::types::Side::Short => -price,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::ExpirationDate;
+    use crate::assert_decimal_eq;
+    use crate::model::types::{OptionStyle, OptionType, Side};
+    use positive::pos_or_panic;
+    use rust_decimal_macros::dec;
+
+    fn create_chooser_option(choice_date_days: f64) -> Options {
+        Options::new(
+            OptionType::Chooser {
+                choice_date: choice_date_days,
+            },
+            Side::Long,
+            "TEST".to_string(),
+            Positive::HUNDRED,                          // strike
+            ExpirationDate::Days(pos_or_panic!(182.5)), // ~0.5 years
+            pos_or_panic!(0.25),                        // volatility
+            Positive::ONE,                              // quantity
+            Positive::HUNDRED,                          // underlying (ATM)
+            dec!(0.05),                                 // risk-free rate
+            OptionStyle::Call,                          // Will be ignored for chooser
+            Positive::ZERO,                             // dividend yield
+            None,
+        )
+    }
+
+    #[test]
+    fn test_simple_chooser() {
+        let option = create_chooser_option(45.0); // Choice in 45 days
+        let price = chooser_black_scholes(&option).unwrap();
+        // Chooser should have positive value (it's always >= call or put)
+        assert!(
+            price > Decimal::ZERO,
+            "Chooser should be positive: {}",
+            price
+        );
+    }
+
+    #[test]
+    fn test_chooser_more_valuable_than_call() {
+        let chooser = create_chooser_option(45.0);
+        let chooser_price = chooser_black_scholes(&chooser).unwrap();
+
+        // Create equivalent vanilla call
+        let call = Options::new(
+            OptionType::European,
+            Side::Long,
+            "TEST".to_string(),
+            Positive::HUNDRED,
+            ExpirationDate::Days(pos_or_panic!(182.5)),
+            pos_or_panic!(0.25),
+            Positive::ONE,
+            Positive::HUNDRED,
+            dec!(0.05),
+            OptionStyle::Call,
+            Positive::ZERO,
+            None,
+        );
+        let call_price = crate::pricing::black_scholes_model::black_scholes(&call).unwrap();
+
+        assert!(
+            chooser_price >= call_price,
+            "Chooser {} should be >= call {}",
+            chooser_price,
+            call_price
+        );
+    }
+
+    #[test]
+    fn test_chooser_more_valuable_than_put() {
+        let chooser = create_chooser_option(45.0);
+        let chooser_price = chooser_black_scholes(&chooser).unwrap();
+
+        // Create equivalent vanilla put
+        let put = Options::new(
+            OptionType::European,
+            Side::Long,
+            "TEST".to_string(),
+            Positive::HUNDRED,
+            ExpirationDate::Days(pos_or_panic!(182.5)),
+            pos_or_panic!(0.25),
+            Positive::ONE,
+            Positive::HUNDRED,
+            dec!(0.05),
+            OptionStyle::Put,
+            Positive::ZERO,
+            None,
+        );
+        let put_price = crate::pricing::black_scholes_model::black_scholes(&put).unwrap();
+
+        assert!(
+            chooser_price >= put_price,
+            "Chooser {} should be >= put {}",
+            chooser_price,
+            put_price
+        );
+    }
+
+    #[test]
+    fn test_early_choice_date() {
+        // Very early choice date (1 day)
+        let option = create_chooser_option(1.0);
+        let price = chooser_black_scholes(&option).unwrap();
+        assert!(price > Decimal::ZERO, "Early choice date price: {}", price);
+    }
+
+    #[test]
+    fn test_late_choice_date() {
+        // Choice date close to expiration
+        let option = create_chooser_option(180.0);
+        let price = chooser_black_scholes(&option).unwrap();
+        assert!(price > Decimal::ZERO, "Late choice date price: {}", price);
+    }
+
+    #[test]
+    fn test_choice_at_expiry() {
+        // Choice at expiration = straddle
+        let option = create_chooser_option(182.5);
+        let price = chooser_black_scholes(&option).unwrap();
+        assert!(price > Decimal::ZERO, "Choice at expiry price: {}", price);
+    }
+
+    #[test]
+    fn test_short_chooser_option() {
+        let mut option = create_chooser_option(45.0);
+        let long_price = chooser_black_scholes(&option).unwrap();
+
+        option.side = Side::Short;
+        let short_price = chooser_black_scholes(&option).unwrap();
+
+        assert_decimal_eq!(long_price, -short_price, dec!(1e-10));
+    }
+
+    #[test]
+    fn test_zero_time_to_expiry() {
+        let mut option = create_chooser_option(0.0);
+        option.expiration_date = ExpirationDate::Days(Positive::ZERO);
+        let price = chooser_black_scholes(&option).unwrap();
+        // ATM at expiry, intrinsic is 0
+        assert_decimal_eq!(price, Decimal::ZERO, dec!(0.01));
+    }
+
+    #[test]
+    fn test_itm_call_at_expiry() {
+        let mut option = create_chooser_option(0.0);
+        option.underlying_price = pos_or_panic!(110.0);
+        option.expiration_date = ExpirationDate::Days(Positive::ZERO);
+        let price = chooser_black_scholes(&option).unwrap();
+        // ITM call intrinsic = 10
+        assert_decimal_eq!(price, dec!(10.0), dec!(0.01));
+    }
+
+    #[test]
+    fn test_itm_put_at_expiry() {
+        let mut option = create_chooser_option(0.0);
+        option.underlying_price = pos_or_panic!(90.0);
+        option.expiration_date = ExpirationDate::Days(Positive::ZERO);
+        let price = chooser_black_scholes(&option).unwrap();
+        // ITM put intrinsic = 10
+        assert_decimal_eq!(price, dec!(10.0), dec!(0.01));
+    }
+
+    #[test]
+    fn test_higher_vol_means_higher_chooser_value() {
+        let low_vol = create_chooser_option(45.0);
+        let low_vol_price = chooser_black_scholes(&low_vol).unwrap();
+
+        let mut high_vol = low_vol.clone();
+        high_vol.implied_volatility = pos_or_panic!(0.4);
+        let high_vol_price = chooser_black_scholes(&high_vol).unwrap();
+
+        assert!(
+            high_vol_price > low_vol_price,
+            "Higher vol {} should mean higher chooser value: {}",
+            high_vol_price,
+            low_vol_price
+        );
+    }
+}

src/pricing/mod.rs

@@ -175,6 +175,9 @@ pub mod lookback;
 /// Compound option pricing (options on options).
 pub mod compound;
 
+/// Chooser option pricing (choice between call and put).
+pub mod chooser;
+
 /// Black-Scholes model for option pricing and analysis.
 ///
 /// This module implements the Black-Scholes-Merton model for European option pricing
@@ -271,6 +274,7 @@ pub use barrier::barrier_black_scholes;
 pub use binary::binary_black_scholes;
 pub use binomial_model::{BinomialPricingParams, generate_binomial_tree, price_binomial};
 pub use black_scholes_model::{BlackScholes, black_scholes};
+pub use chooser::chooser_black_scholes;
 pub use compound::compound_black_scholes;
 pub use lookback::lookback_black_scholes;
 pub use monte_carlo::monte_carlo_option_pricing;