statrs/function/
evaluate.rs

1//! Provides functions that don't have a numerical solution and must
2//! be solved computationally (e.g. evaluation of a polynomial)
3
4/// evaluates a polynomial at `z` where `coeff` are the coeffecients
5/// to a polynomial of order `k` where `k` is the length of `coeff` and the
6/// coeffecient
7/// to the `k`th power is the `k`th element in coeff. E.g. [3,-1,2] equates to
8/// `2z^2 - z + 3`
9///
10/// # Remarks
11///
12/// Returns 0 for a 0 length coefficient slice
13pub fn polynomial(z: f64, coeff: &[f64]) -> f64 {
14    let n = coeff.len();
15    if n == 0 {
16        return 0.0;
17    }
18
19    let mut sum = *coeff.last().unwrap();
20    for c in coeff[0..n - 1].iter().rev() {
21        sum = *c + z * sum;
22    }
23    sum
24}
25
26#[rustfmt::skip]
27#[cfg(test)]
28mod tests {
29    use std::f64;
30
31    // these tests probably could be more robust
32    #[test]
33    fn test_polynomial() {
34        let empty: [f64; 0] = [];
35        assert_eq!(super::polynomial(2.0, &empty), 0.0);
36
37        let zero = [0.0];
38        assert_eq!(super::polynomial(2.0, &zero), 0.0);
39
40        let mut coeff = [1.0, 0.0, 5.0];
41        assert_eq!(super::polynomial(2.0, &coeff), 21.0);
42
43        coeff = [-5.0, -2.0, 3.0];
44        assert_eq!(super::polynomial(2.0, &coeff), 3.0);
45        assert_eq!(super::polynomial(-2.0, &coeff), 11.0);
46
47        let large_coeff = [-1.35e3, 2.5e2, 8.0, -4.0, 1e2, 3.0];
48        assert_eq!(super::polynomial(5.0, &large_coeff), 71475.0);
49        assert_eq!(super::polynomial(-5.0, &large_coeff), 51225.0);
50
51        coeff = [f64::INFINITY, -2.0, 3.0];
52        assert_eq!(super::polynomial(2.0, &coeff), f64::INFINITY);
53        assert_eq!(super::polynomial(-2.0, &coeff), f64::INFINITY);
54
55        coeff = [f64::NEG_INFINITY, -2.0, 3.0];
56        assert_eq!(super::polynomial(2.0, &coeff), f64::NEG_INFINITY);
57        assert_eq!(super::polynomial(-2.0, &coeff), f64::NEG_INFINITY);
58
59        coeff = [f64::NAN, -2.0, 3.0];
60        assert!(super::polynomial(2.0, &coeff).is_nan());
61        assert!(super::polynomial(-2.0, &coeff).is_nan());
62    }
63}
statrs/function/evaluate.rs

statrs/function/
evaluate.rs
