Rollup merge of rust-lang#99747 - ankane:float_gamma, r=joshtriplett

Add gamma function to f32 and f64

Adds the [gamma function](https://en.wikipedia.org/wiki/Gamma_function) to `f32` and `f64` (`tgamma` and `tgammaf` from C).

Refs:
- rust-lang/rfcs#864
- rust-lang#18271

Author: Dylan-DPC

Diff for: library/std/src/f32.rs

@@ -920,4 +920,24 @@ impl f32 {
     pub fn atanh(self) -> f32 {
         0.5 * ((2.0 * self) / (1.0 - self)).ln_1p()
     }
+
+    /// Gamma function.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(float_gamma)]
+    /// let x = 5.0f32;
+    ///
+    /// let abs_difference = (x.gamma() - 24.0).abs();
+    ///
+    /// assert!(abs_difference <= f32::EPSILON);
+    /// ```
+    #[rustc_allow_incoherent_impl]
+    #[must_use = "method returns a new number and does not mutate the original value"]
+    #[unstable(feature = "float_gamma", issue = "99842")]
+    #[inline]
+    pub fn gamma(self) -> f32 {
+        unsafe { cmath::tgammaf(self) }
+    }
 }

Diff for: library/std/src/f32/tests.rs

@@ -550,6 +550,21 @@ fn test_atanh() {
     assert_approx_eq!((-0.5f32).atanh(), -0.54930614433405484569762261846126285f32);
 }
 
+#[test]
+fn test_gamma() {
+    assert_eq!(0.0f32.gamma(), f32::INFINITY);
+    assert_approx_eq!(1.0f32.gamma(), 1.0f32);
+    assert_approx_eq!(2.0f32.gamma(), 1.0f32);
+    assert_approx_eq!(3.0f32.gamma(), 2.0f32);
+    assert_approx_eq!(4.0f32.gamma(), 6.0f32);
+    assert_approx_eq!(5.0f32.gamma(), 24.0f32);
+
+    assert_approx_eq!(0.5f32.gamma(), consts::PI.sqrt());
+    assert_approx_eq!((-0.5f32).gamma(), -2.0 * consts::PI.sqrt());
+
+    assert!((-1.0f32).gamma().is_nan());
+}
+
 #[test]
 fn test_real_consts() {
     use super::consts;

Diff for: library/std/src/f64.rs

@@ -946,4 +946,24 @@ impl f64 {
             Self::NAN // log(-Inf) = NaN
         }
     }
+
+    /// Gamma function.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(float_gamma)]
+    /// let x = 5.0f64;
+    ///
+    /// let abs_difference = (x.gamma() - 24.0).abs();
+    ///
+    /// assert!(abs_difference <= f64::EPSILON);
+    /// ```
+    #[rustc_allow_incoherent_impl]
+    #[must_use = "method returns a new number and does not mutate the original value"]
+    #[unstable(feature = "float_gamma", issue = "99842")]
+    #[inline]
+    pub fn gamma(self) -> f64 {
+        unsafe { cmath::tgamma(self) }
+    }
 }

Diff for: library/std/src/f64/tests.rs

@@ -535,6 +535,21 @@ fn test_atanh() {
     assert_approx_eq!((-0.5f64).atanh(), -0.54930614433405484569762261846126285f64);
 }
 
+#[test]
+fn test_gamma() {
+    assert_eq!(0.0f64.gamma(), f64::INFINITY);
+    assert_approx_eq!(1.0f64.gamma(), 1.0f64);
+    assert_approx_eq!(2.0f64.gamma(), 1.0f64);
+    assert_approx_eq!(3.0f64.gamma(), 2.0f64);
+    assert_approx_eq!(4.0f64.gamma(), 6.0f64);
+    assert_approx_eq!(5.0f64.gamma(), 24.0f64);
+
+    assert_approx_eq!(0.5f64.gamma(), consts::PI.sqrt());
+    assert_approx_eq!((-0.5f64).gamma(), -2.0 * consts::PI.sqrt());
+
+    assert!((-1.0f64).gamma().is_nan());
+}
+
 #[test]
 fn test_real_consts() {
     use super::consts;

Diff for: library/std/src/lib.rs

@@ -272,6 +272,7 @@
 #![feature(exact_size_is_empty)]
 #![feature(exclusive_wrapper)]
 #![feature(extend_one)]
+#![feature(float_gamma)]
 #![feature(float_minimum_maximum)]
 #![feature(hasher_prefixfree_extras)]
 #![feature(hashmap_internals)]

Diff for: library/std/src/sys/unix/cmath.rs

@@ -30,4 +30,6 @@ extern "C" {
     pub fn tanf(n: f32) -> f32;
     pub fn tanh(n: f64) -> f64;
     pub fn tanhf(n: f32) -> f32;
+    pub fn tgamma(n: f64) -> f64;
+    pub fn tgammaf(n: f32) -> f32;
 }

Diff for: library/std/src/sys/windows/cmath.rs

@@ -23,6 +23,8 @@ extern "C" {
     pub fn sinh(n: c_double) -> c_double;
     pub fn tan(n: c_double) -> c_double;
     pub fn tanh(n: c_double) -> c_double;
+    pub fn tgamma(n: c_double) -> c_double;
+    pub fn tgammaf(n: c_float) -> c_float;
 }
 
 pub use self::shims::*;