Fix pert for mode approx eq mean; use builder pattern (#1452)

- Fix #1311 (mode close to mean)
- Use a builder pattern, allowing specification via mode OR mean

rand_distr/CHANGELOG.md

@@ -5,9 +5,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## Unreleased
-
 ### Added
 - Add plots for `rand_distr` distributions to documentation (#1434)
+- Add `PertBuilder`, fix case where mode ≅ mean (#1452)
 
 ## [0.5.0-alpha.1] - 2024-03-18
 - Target `rand` version `0.9.0-alpha.1`

rand_distr/src/lib.rs

@@ -117,7 +117,7 @@ pub use self::normal_inverse_gaussian::{
     Error as NormalInverseGaussianError, NormalInverseGaussian,
 };
 pub use self::pareto::{Error as ParetoError, Pareto};
-pub use self::pert::{Pert, PertError};
+pub use self::pert::{Pert, PertBuilder, PertError};
 pub use self::poisson::{Error as PoissonError, Poisson};
 pub use self::skew_normal::{Error as SkewNormalError, SkewNormal};
 pub use self::triangular::{Triangular, TriangularError};

rand_distr/src/pert.rs

@@ -31,7 +31,7 @@ use rand::Rng;
 /// ```rust
 /// use rand_distr::{Pert, Distribution};
 ///
-/// let d = Pert::new(0., 5., 2.5).unwrap();
+/// let d = Pert::new(0., 5.).with_mode(2.5).unwrap();
 /// let v = d.sample(&mut rand::thread_rng());
 /// println!("{} is from a PERT distribution", v);
 /// ```
@@ -82,35 +82,75 @@ where
     Exp1: Distribution<F>,
     Open01: Distribution<F>,
 {
-    /// Set up the PERT distribution with defined `min`, `max` and `mode`.
+    /// Construct a PERT distribution with defined `min`, `max`
     ///
-    /// This is equivalent to calling `Pert::new_with_shape` with `shape == 4.0`.
+    /// # Example
+    ///
+    /// ```
+    /// use rand_distr::Pert;
+    /// let pert_dist = Pert::new(0.0, 10.0)
+    ///     .with_shape(3.5)
+    ///     .with_mean(3.0)
+    ///     .unwrap();
+    /// # let _unused: Pert<f64> = pert_dist;
+    /// ```
+    #[allow(clippy::new_ret_no_self)]
     #[inline]
-    pub fn new(min: F, max: F, mode: F) -> Result<Pert<F>, PertError> {
-        Pert::new_with_shape(min, max, mode, F::from(4.).unwrap())
+    pub fn new(min: F, max: F) -> PertBuilder<F> {
+        let shape = F::from(4.0).unwrap();
+        PertBuilder { min, max, shape }
+    }
+}
+
+/// Struct used to build a [`Pert`]
+#[derive(Debug)]
+pub struct PertBuilder<F> {
+    min: F,
+    max: F,
+    shape: F,
+}
+
+impl<F> PertBuilder<F>
+where
+    F: Float,
+    StandardNormal: Distribution<F>,
+    Exp1: Distribution<F>,
+    Open01: Distribution<F>,
+{
+    /// Set the shape parameter
+    ///
+    /// If not specified, this defaults to 4.
+    #[inline]
+    pub fn with_shape(mut self, shape: F) -> PertBuilder<F> {
+        self.shape = shape;
+        self
     }
 
-    /// Set up the PERT distribution with defined `min`, `max`, `mode` and
-    /// `shape`.
-    pub fn new_with_shape(min: F, max: F, mode: F, shape: F) -> Result<Pert<F>, PertError> {
-        if !(max > min) {
+    /// Specify the mean
+    #[inline]
+    pub fn with_mean(self, mean: F) -> Result<Pert<F>, PertError> {
+        let two = F::from(2.0).unwrap();
+        let mode = ((self.shape + two) * mean - self.min - self.max) / self.shape;
+        self.with_mode(mode)
+    }
+
+    /// Specify the mode
+    #[inline]
+    pub fn with_mode(self, mode: F) -> Result<Pert<F>, PertError> {
+        if !(self.max > self.min) {
             return Err(PertError::RangeTooSmall);
         }
-        if !(mode >= min && max >= mode) {
+        if !(mode >= self.min && self.max >= mode) {
             return Err(PertError::ModeRange);
         }
-        if !(shape >= F::from(0.).unwrap()) {
+        if !(self.shape >= F::from(0.).unwrap()) {
             return Err(PertError::ShapeTooSmall);
         }
 
+        let (min, max, shape) = (self.min, self.max, self.shape);
         let range = max - min;
-        let mu = (min + max + shape * mode) / (shape + F::from(2.).unwrap());
-        let v = if mu == mode {
-            shape * F::from(0.5).unwrap() + F::from(1.).unwrap()
-        } else {
-            (mu - min) * (F::from(2.).unwrap() * mode - min - max) / ((mode - mu) * (max - min))
-        };
-        let w = v * (max - mu) / (mu - min);
+        let v = F::from(1.0).unwrap() + shape * (mode - min) / range;
+        let w = F::from(1.0).unwrap() + shape * (max - mode) / range;
         let beta = Beta::new(v, w).map_err(|_| PertError::RangeTooSmall)?;
         Ok(Pert { min, range, beta })
     }
@@ -136,17 +176,38 @@ mod test {
     #[test]
     fn test_pert() {
         for &(min, max, mode) in &[(-1., 1., 0.), (1., 2., 1.), (5., 25., 25.)] {
-            let _distr = Pert::new(min, max, mode).unwrap();
+            let _distr = Pert::new(min, max).with_mode(mode).unwrap();
             // TODO: test correctness
         }
 
         for &(min, max, mode) in &[(-1., 1., 2.), (-1., 1., -2.), (2., 1., 1.)] {
-            assert!(Pert::new(min, max, mode).is_err());
+            assert!(Pert::new(min, max).with_mode(mode).is_err());
         }
     }
 
     #[test]
-    fn pert_distributions_can_be_compared() {
-        assert_eq!(Pert::new(1.0, 3.0, 2.0), Pert::new(1.0, 3.0, 2.0));
+    fn distributions_can_be_compared() {
+        let (min, mode, max, shape) = (1.0, 2.0, 3.0, 4.0);
+        let p1 = Pert::new(min, max).with_mode(mode).unwrap();
+        let mean = (min + shape * mode + max) / (shape + 2.0);
+        let p2 = Pert::new(min, max).with_mean(mean).unwrap();
+        assert_eq!(p1, p2);
+    }
+
+    #[test]
+    fn mode_almost_half_range() {
+        assert!(Pert::new(0.0f32, 0.48258883).with_mode(0.24129441).is_ok());
+    }
+
+    #[test]
+    fn almost_symmetric_about_zero() {
+        let distr = Pert::new(-10f32, 10f32).with_mode(f32::EPSILON);
+        assert!(distr.is_ok());
+    }
+
+    #[test]
+    fn almost_symmetric() {
+        let distr = Pert::new(0f32, 2f32).with_mode(1f32 + f32::EPSILON);
+        assert!(distr.is_ok());
     }
 }

rand_distr/tests/value_stability.rs

@@ -250,7 +250,7 @@ fn pert_stability() {
     // mean = 4, var = 12/7
     test_samples(
         860,
-        Pert::new(2., 10., 3.).unwrap(),
+        Pert::new(2., 10.).with_mode(3.).unwrap(),
         &[
             4.908681667460367,
             4.014196196158352,