trig_const/

lib.rs

//! ## trig-const
//!
//! Rust implementation of const trig functions.
//!
//! This is implemented using a 16-term Taylor series approximation of cosine.
//! Correctness is favored over speed, especially considering the main use case for
//! this crate is to expose trigonometric functions for compile time.
//!
//! The implemntation was largely inspired by the work of Dr. Austin Henley and Dr. Stephen Marz:
//!   - GitHub Repo: <https://github.com/AZHenley/cosine>
//!   - Article: <https://austinhenley.com/blog/cosine.html>
//!
//! The implementation carries forward the original MIT license contained in the GitHub repo above.
//!
//! ## Requirements
//!
//! This crate supports any compiler version back to rustc 1.85
//!
//! ```toml
//! [dependencies]
//! trig-const = "0"
//! ```
//!
//! ## Example
//!
//! ```
//! # use trig_const::cos;
//! # use core::f64::consts::PI;
//! # fn float_eq(lhs: f64, rhs: f64) { assert!((lhs - rhs).abs() < 0.0001, "lhs: {}, rhs: {}", lhs, rhs); }
//! const COS_PI: f64 = cos(PI);
//! float_eq(COS_PI, -1.0);
//! ```

#![no_std]
#![forbid(unsafe_code)]

use core::f64::{self, consts::PI};

/// Number of sum iterations for Taylor series
const TAYLOR_SERIES_SUMS: usize = 16;
/// Number of sum iterations for ln
const LN_SUM_TERMS: f64 = 101.0;

/// Cosine
///
/// ```
/// # use trig_const::cos;
/// # use core::f64::consts::PI;
/// # fn float_eq(lhs: f64, rhs: f64) { assert!((lhs - rhs).abs() < 0.0001, "lhs: {}, rhs: {}", lhs, rhs); }
/// const COS_PI: f64 = cos(PI);
/// float_eq(COS_PI, -1.0);
/// ```
pub const fn cos(mut x: f64) -> f64 {
    // If value is large, fold into smaller value
    while x < -0.1 {
        x += 2.0 * PI;
    }
    while x > 2.0 * PI + 0.1 {
        x -= 2.0 * PI;
    }
    let div = (x / PI) as u32;
    x -= div as f64 * PI;
    let sign = if div % 2 != 0 { -1.0 } else { 1.0 };

    let mut result = 1.0;
    let mut inter = 1.0;
    let num = x * x;

    let mut i = 1;
    while i <= TAYLOR_SERIES_SUMS {
        let comp = 2.0 * i as f64;
        let den = comp * (comp - 1.0);
        inter *= num / den;
        if i % 2 == 0 {
            result += inter;
        } else {
            result -= inter;
        }
        i += 1;
    }

    sign * result
}

/// Sine
///
/// ```
/// # use trig_const::sin;
/// # use core::f64::consts::PI;
/// # fn float_eq(lhs: f64, rhs: f64) { assert!((lhs - rhs).abs() < 0.0001, "lhs: {}, rhs: {}", lhs, rhs); }
/// const SIN_PI: f64 = sin(PI);
/// float_eq(SIN_PI, 0.0);
/// ```
pub const fn sin(x: f64) -> f64 {
    cos(x - PI / 2.0)
}

/// Tangent
///
/// ```
/// # use trig_const::tan;
/// # use core::f64::consts::PI;
/// # fn float_eq(lhs: f64, rhs: f64) { assert!((lhs - rhs).abs() < 0.0001, "lhs: {}, rhs: {}", lhs, rhs); }
/// const TAN_PI_4: f64 = tan(PI / 4.0);
/// float_eq(TAN_PI_4, 1.0);
/// ```
pub const fn tan(x: f64) -> f64 {
    sin(x) / cos(x)
}

/// Cotangent
///
/// ```
/// # use trig_const::cot;
/// # use core::f64::consts::PI;
/// # fn float_eq(lhs: f64, rhs: f64) { assert!((lhs - rhs).abs() < 0.0001, "lhs: {}, rhs: {}", lhs, rhs); }
/// const COT_PI_4: f64 = cot(PI / 4.0);
/// float_eq(COT_PI_4, 1.0);
/// ```
pub const fn cot(x: f64) -> f64 {
    let sin_calc = sin(x);
    if sin_calc == 0.0 {
        f64::INFINITY
    } else {
        cos(x) / sin_calc
    }
}

/// Secant
///
/// ```
/// # use trig_const::sec;
/// # use core::f64::consts::PI;
/// # fn float_eq(lhs: f64, rhs: f64) { assert!((lhs - rhs).abs() < 0.0001, "lhs: {}, rhs: {}", lhs, rhs); }
/// const SEC_PI: f64 = sec(PI);
/// float_eq(SEC_PI, -1.0);
/// ```
pub const fn sec(x: f64) -> f64 {
    let cos_calc = cos(x);
    if cos_calc == 0.0 {
        f64::INFINITY
    } else {
        1.0 / cos_calc
    }
}

/// Cosecant
///
/// ```
/// # use trig_const::csc;
/// # use core::f64::consts::PI;
/// # fn float_eq(lhs: f64, rhs: f64) { assert!((lhs - rhs).abs() < 0.0001, "lhs: {}, rhs: {}", lhs, rhs); }
/// const CSC_PI_2: f64 = csc(PI / 2.0);
/// float_eq(CSC_PI_2, 1.0);
/// ```
pub const fn csc(x: f64) -> f64 {
    let sin_calc = sin(x);
    if sin_calc == 0.0 {
        f64::INFINITY
    } else {
        1.0 / sin_calc
    }
}

/// Hyperbolic Sine
///
/// ```
/// # use trig_const::sinh;
/// const SINH_0: f64 = sinh(0.0);
/// assert_eq!(SINH_0, 0.0);
/// ```
pub const fn sinh(x: f64) -> f64 {
    (exp(x) - exp(-x)) / 2.0
}

/// Hyperbolic Cosine
///
/// ```
/// # use trig_const::cosh;
/// const COSH_0: f64 = cosh(0.0);
/// assert_eq!(COSH_0, 1.0);
/// ```
pub const fn cosh(x: f64) -> f64 {
    (exp(x) + exp(-x)) / 2.0
}

/// Arcsine
///
/// ```
/// # use trig_const::asin;
/// const ASIN_PI: f64 = asin(0.0);
/// assert_eq!(ASIN_PI, 0.0);
/// ```
pub const fn asin(x: f64) -> f64 {
    if x.is_infinite() || x.abs() > 1.0 {
        return f64::NAN;
    } else if x == 1.0 {
        return f64::consts::FRAC_PI_2;
    } else if x == -1.0 {
        return -f64::consts::FRAC_PI_2;
    }

    // As we start to get past 0.8, the number of summations needed for an accurate
    // Taylor series approximation starts to get unweidy. We can use the property
    // that arcsin(x) = pi/2 - 2*arcsin(sqrt((1 - x) / 2)) to reduce
    const RANGE_REDUCTION_THRESHOLD: f64 = 0.5;
    if x.abs() > RANGE_REDUCTION_THRESHOLD {
        let sign = x.signum();
        let abs_x = x.abs();

        let y = sqrt((1.0 - abs_x) / 2.0);
        return sign * (f64::consts::FRAC_PI_2 - 2.0 * asin(y));
    }

    let mut n = 1;
    let mut s = x;

    while n < TAYLOR_SERIES_SUMS {
        let numer1 = factorial(2.0 * n as f64);
        let numer2 = expi(x, 2 * n + 1);

        // Calculate all denom terms;
        let denom1 = expi(4.0, n);
        let denom2 = factorial(n as f64) * factorial(n as f64);
        let denom3 = 2.0 * n as f64 + 1.0;

        // Try to match terms to divide to stop number getting too large
        let f1 = numer1 / denom2;
        let f2 = numer2 / denom1;

        s += f1 * f2 / denom3;

        n += 1;
    }

    s
}

/// Arccosine
///
/// ```
/// # use trig_const::acos;
/// # use core::f64::consts::PI;
/// const ACOS_1: f64 = acos(1.0);
/// assert_eq!(ACOS_1, 0.0);
/// ```
pub const fn acos(x: f64) -> f64 {
    if x.is_infinite() || x.abs() > 1.0 {
        f64::NAN
    } else {
        f64::consts::FRAC_PI_2 - asin(x)
    }
}

/// Inverse hyperbolic sine
///
/// ```
/// # use trig_const::asinh;
/// const ASINH_1: f64 = asinh(0.0);
/// assert_eq!(ASINH_1, 0.0);
/// ```
pub const fn asinh(x: f64) -> f64 {
    if x.is_nan() {
        f64::NAN
    } else if x.is_infinite() {
        x
    } else {
        ln(x + sqrt(x * x + 1.0))
    }
}

/// Inverse hyperbolic cosine
///
/// ```
/// # use trig_const::acosh;
/// const ACOSH_1: f64 = acosh(1.0);
/// assert_eq!(ACOSH_1, 0.0);
/// ```
pub const fn acosh(x: f64) -> f64 {
    if x.is_nan() {
        f64::NAN
    } else if x.is_infinite() {
        x
    } else if x < 1.0 {
        f64::NAN
    } else {
        ln(x + sqrt(x * x - 1.0))
    }
}

/// e^x
const fn exp(x: f64) -> f64 {
    let mut i = 1;
    let mut s = 1.0;

    while i < 16 {
        s += expi(x, i) / factorial(i as f64);
        i += 1;
    }

    s
}

/// x^pow
const fn expi(x: f64, mut pow: usize) -> f64 {
    let mut o = 1.0;

    while pow > 0 {
        o *= x;
        pow -= 1;
    }

    o
}

/// Factorial (x!)
const fn factorial(mut x: f64) -> f64 {
    if x == 0.0 || x == 1.0 {
        1.0
    } else {
        let mut s = 1.0;
        while x > 1.0 {
            s *= x;
            x -= 1.0;
        }
        s
    }
}

/// Const sqrt function using Newton's method
const fn sqrt(x: f64) -> f64 {
    if x.is_nan() || x < 0.0 {
        return f64::NAN;
    } else if x.is_infinite() || x == 0.0 {
        return x;
    }

    // Use Newton's method for sqrt calculation
    let mut current_guess = 1.0;

    let mut i = 0;
    while i < TAYLOR_SERIES_SUMS {
        current_guess = 0.5 * (current_guess + x / current_guess);
        i += 1;
    }

    current_guess
}

/// Computes natural log using Taylor series approximation
const fn ln(x: f64) -> f64 {
    if x.is_nan() || x < 0.0 {
        return f64::NAN;
    } else if x == 0.0 {
        return f64::NEG_INFINITY;
    } else if x == 1.0 {
        return 0.0;
    } else if x.is_infinite() {
        return f64::INFINITY;
    }

    // Put into form ln(x) = ln(a * 2^k) = ln(a) + k * ln(2)

    let mut a = x;
    let mut k = 0;

    // Normalize `a` to [1.0, 2.0)
    while a >= 2.0 {
        a /= 2.0;
        k += 1;
    }
    while a < 1.0 {
        a *= 2.0;
        k -= 1;
    }

    let x = a - 1.0;

    let mut s = 0.0;
    let mut term = x;
    let mut n = 1.0;

    while n < LN_SUM_TERMS {
        s += term;
        n += 1.0;
        term = -term * x * (n - 1.0) / n;
    }

    s + (k as f64) * f64::consts::LN_2
}

#[cfg(test)]
mod tests {
    use core::f64::consts::{E, PI};

    use crate::{cos, cosh, exp, expi, factorial, ln, sin, sinh, sqrt};

    macro_rules! float_eq {
        ($lhs:expr, $rhs:expr) => {
            assert!(($lhs - $rhs).abs() < 0.0001, "lhs: {}, rhs: {}", $lhs, $rhs);
        };
    }

    #[test]
    fn test_factorial() {
        assert_eq!(factorial(0.0), 1.0);
        assert_eq!(factorial(1.0), 1.0);
        assert_eq!(factorial(2.0), 2.0);
        assert_eq!(factorial(3.0), 6.0);
        assert_eq!(factorial(4.0), 24.0);
        assert_eq!(factorial(5.0), 120.0);
    }

    #[test]
    fn test_expi() {
        assert_eq!(expi(2.0, 0), 1.0);
        assert_eq!(expi(2.0, 4), 16.0);
        assert_eq!(expi(2.0, 5), 32.0);
        assert_eq!(expi(3.0, 3), 27.0);
    }

    #[test]
    fn test_exp() {
        float_eq!(exp(0.0), 1.0);
        float_eq!(exp(1.0), E);
    }

    #[test]
    fn test_sqrt() {
        float_eq!(sqrt(0.0), 0.0);
        float_eq!(sqrt(1.0), 1.0);
        float_eq!(sqrt(4.0), 2.0);
        float_eq!(sqrt(9.0), 3.0);
        float_eq!(sqrt(16.0), 4.0);
        float_eq!(sqrt(25.0), 5.0);
    }

    #[test]
    fn test_cos() {
        float_eq!(cos(0.0), 0.0_f64.cos());
        float_eq!(cos(1.0), 1.0_f64.cos());
        float_eq!(cos(PI), PI.cos());
        float_eq!(cos(PI * 8.0), (PI * 8.0).cos());
    }

    #[test]
    fn test_sin() {
        float_eq!(sin(0.0), 0.0_f64.sin());
        float_eq!(sin(1.0), 1.0_f64.sin());
        float_eq!(sin(PI), PI.sin());
        float_eq!(sin(PI * 8.0), (PI * 8.0).sin());
    }

    #[test]
    fn test_sinh() {
        for x in [0.0, 0.5, 1.0, 1.5, 2.0, 2.5] {
            float_eq!(sinh(x), x.sinh());
        }
    }

    #[test]
    fn test_cosh() {
        for x in [0.0, 0.5, 1.0, 1.5, 2.0, 2.5] {
            float_eq!(cosh(x), x.cosh());
        }
    }

    #[test]
    fn test_ln() {
        float_eq!(ln(0.01), 0.01_f64.ln());
        float_eq!(ln(0.5), 0.5_f64.ln());
        float_eq!(ln(1.0), 1.0_f64.ln());
        float_eq!(ln(2.0), 2.0_f64.ln());
        float_eq!(ln(10.0), 10.0_f64.ln());
        float_eq!(ln(1_000.0), 1_000.0_f64.ln());
    }
}