gam 0.3.18

// Pólya-Gamma PG(1, c) sampler via Devroye's algorithm.
//
// Adapted from the `polya-gamma` crate (v0.5.3) by Daniel Lyng,
// dual-licensed MIT OR Apache-2.0.  We inline only the PG(1, c) path
// because that is all the Gibbs sampler needs.
//
// Reference: Polson, Scott & Windle (2013), "Bayesian Inference for
// Logistic Models Using Pólya-Gamma Latent Variables", JASA 108(504).

use rand::{Rng, RngExt};
use rand_distr::{Distribution, Exp as RandExp, Normal as RandNormal};
use statrs::distribution::{ContinuousCDF, Normal};
use std::f64::consts::{FRAC_2_PI, FRAC_PI_2, PI};

const PI_SQ: f64 = PI * PI;
// sqrt(2 / pi) — the InverseGamma(α=0.5, β=2k²) PDF coefficient is
//   (β^α / Γ(α)) = sqrt(2k²) / sqrt(π) = k · sqrt(2) / sqrt(π)
//                = k · sqrt(2 / π),
// so the n-th series term in the small-x arm is
//   2 · InverseGamma(α=0.5, β=2k²) PDF
//     = 2 · k · sqrt(2 / π) · x^{-3/2} · exp(-2k²/x).
// Multiplied by 2 below so `coeff` is the full `2k · sqrt(2/π)` factor.
const SQRT_2_OVER_SQRT_PI: f64 = 0.797_884_560_802_865_4;

/// Sampler for the Pólya-Gamma PG(1, c) distribution.
#[derive(Debug, Clone)]
pub struct PolyaGamma {
    exp: RandExp<f64>,
    std_norm: Normal,
    std_norm_sampler: RandNormal<f64>,
}

impl PolyaGamma {
    pub fn new() -> Self {
        Self {
            exp: RandExp::new(1.0).expect("Exp(1) valid"),
            std_norm: Normal::standard(),
            std_norm_sampler: RandNormal::new(0.0, 1.0).expect("N(0,1) valid"),
        }
    }

    /// Draw a single PG(1, c) variate using Devroye's exact algorithm.
    pub fn draw<R: Rng + ?Sized>(&self, rng: &mut R, tilt: f64) -> f64 {
        let half_tilt = tilt.abs() * 0.5;
        let half_tilt_sq = half_tilt * half_tilt;
        let scale_factor = 0.125 * PI_SQ + 0.5 * half_tilt_sq;
        let exp_mass = self.exponential_tail_mass(half_tilt);

        loop {
            let u: f64 = rng.random();
            let proposal = if u < exp_mass {
                FRAC_2_PI + self.sample_exp(rng) / scale_factor
            } else {
                self.sample_trunc_inv_gauss(rng, half_tilt, FRAC_2_PI)
            };

            let mut series_sum = self.series_coefficient(0, proposal);
            let threshold = rng.random::<f64>() * series_sum;
            let mut idx = 0;

            'series: loop {
                idx += 1;
                let term = self.series_coefficient(idx, proposal);
                if idx % 2 == 1 {
                    series_sum -= term;
                    if threshold <= series_sum {
                        return 0.25 * proposal;
                    }
                } else {
                    series_sum += term;
                    if threshold >= series_sum {
                        break 'series;
                    }
                }
            }
        }
    }

    // ── internals ──────────────────────────────────────────────────────

    fn exponential_tail_mass(&self, tilt: f64) -> f64 {
        let base = 0.125 * PI_SQ + 0.5 * tilt * tilt;
        let upper = FRAC_PI_2.sqrt() * (FRAC_2_PI * tilt - 1.0);
        let lower = -(FRAC_PI_2.sqrt() * (FRAC_2_PI * tilt + 1.0));
        let log_base = base.ln() + base * FRAC_2_PI;
        let log_p_upper = log_base - tilt + self.std_norm.cdf(upper).ln();
        let log_p_lower = log_base + tilt + self.std_norm.cdf(lower).ln();
        let exp_terms = (4.0 / PI) * (log_p_upper.exp() + log_p_lower.exp());
        1.0 / (1.0 + exp_terms)
    }

    #[inline]
    fn series_coefficient(&self, n: usize, x: f64) -> f64 {
        if x <= 0.0 {
            return 0.0;
        }
        let k0 = n as f64 + 0.5;
        let k_sq = k0 * k0;
        if x <= FRAC_2_PI {
            // 2 · InverseGamma(α=1/2, β=2k²) PDF at x
            //   = 2 · (β^α / Γ(α)) · x^{-α-1} · exp(-β/x)
            //   = 2 · sqrt(2k²/π) · x^{-3/2} · exp(-2k²/x)
            //   = (2 · k · sqrt(2/π)) · x^{-3/2} · exp(-2k²/x).
            let coeff = 2.0 * k0 * SQRT_2_OVER_SQRT_PI;
            let inv_x = 1.0 / x;
            // x^{-3/2} = inv_x · sqrt(inv_x) — avoids a `powf`.
            let x_neg_three_half = inv_x * inv_x.sqrt();
            coeff * x_neg_three_half * (-2.0 * k_sq * inv_x).exp()
        } else {
            // (1 / (π · k²)) · Exp(rate = k² π² / 2) PDF at x
            //   = (1 / (π · k²)) · (k² π² / 2) · exp(-k² π² x / 2)
            //   = (π / 2) · exp(-k² π² x / 2).
            FRAC_PI_2 * (-0.5 * k_sq * PI_SQ * x).exp()
        }
    }

    fn sample_trunc_inv_gauss<R: Rng + ?Sized>(&self, rng: &mut R, z: f64, trunc: f64) -> f64 {
        let z = z.abs();
        if FRAC_2_PI > z {
            self.sample_small_z(rng, z, trunc)
        } else {
            self.sample_large_z(rng, 1.0 / z, trunc)
        }
    }

    fn sample_small_z<R: Rng + ?Sized>(&self, rng: &mut R, z: f64, trunc: f64) -> f64 {
        let mut accept = 0.0;
        let mut sample = 0.0;
        while accept < rng.random::<f64>() {
            let exp_sample = loop {
                let e1 = self.sample_exp(rng);
                let e2 = self.sample_exp(rng);
                if e1 * e1 <= 2.0 * e2 / trunc {
                    break e1;
                }
            };
            sample = 1.0 + exp_sample * trunc;
            sample = trunc / (sample * sample);
            accept = (-0.5 * z * z * sample).exp();
        }
        sample
    }

    fn sample_large_z<R: Rng + ?Sized>(&self, rng: &mut R, mean: f64, trunc: f64) -> f64 {
        let mut sample = f64::INFINITY;
        while sample > trunc {
            let n = self.sample_norm(rng);
            let n_sq = n * n;
            let half_mean = 0.5 * mean;
            let mn_sq = mean * n_sq;
            let disc = (4.0 * mn_sq + mn_sq * mn_sq).sqrt();
            sample = mean + half_mean * mn_sq - half_mean * disc;
            if rng.random::<f64>() > mean / (mean + sample) {
                sample = mean * mean / sample;
            }
        }
        sample
    }

    #[inline(always)]
    fn sample_exp<R: Rng + ?Sized>(&self, rng: &mut R) -> f64 {
        self.exp.sample(rng)
    }

    #[inline(always)]
    fn sample_norm<R: Rng + ?Sized>(&self, rng: &mut R) -> f64 {
        self.std_norm_sampler.sample(rng)
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use rand::{SeedableRng, rngs::StdRng};

    fn empirical_mean(c: f64, n: usize, seed: u64) -> f64 {
        let pg = PolyaGamma::new();
        let mut rng = StdRng::seed_from_u64(seed);
        (0..n).map(|_| pg.draw(&mut rng, c)).sum::<f64>() / n as f64
    }

    /// E[PG(1,c)] = tanh(c/2) / (2c),  or 1/4 when c = 0.
    fn theoretical_mean(c: f64) -> f64 {
        if c.abs() < 1e-12 {
            0.25
        } else {
            (0.5 * c).tanh() / (2.0 * c)
        }
    }

    #[test]
    fn pg1_mean_matches_theory() {
        let n = 25_000;
        for (c, tol) in [(0.0, 0.05), (1.0, 0.10), (3.0, 0.10)] {
            let emp = empirical_mean(c, n, 42);
            let th = theoretical_mean(c);
            assert!(
                (emp - th).abs() / th.max(1e-12) < tol,
                "PG(1,{c}): empirical {emp}, theory {th}",
            );
        }
    }
}