1 files changed, 27 insertions, 0 deletions
diff --git a/library/compiler-builtins/libm/src/math/acosh.rs b/library/compiler-builtins/libm/src/math/acosh.rs
new file mode 100644
index 00000000000..d1f5b9fa937
--- /dev/null
+++ b/library/compiler-builtins/libm/src/math/acosh.rs
@@ -0,0 +1,27 @@
+use super::{log, log1p, sqrt};
+
+const LN2: f64 = 0.693147180559945309417232121458176568; /* 0x3fe62e42,  0xfefa39ef*/
+
+/// Inverse hyperbolic cosine (f64)
+///
+/// Calculates the inverse hyperbolic cosine of `x`.
+/// Is defined as `log(x + sqrt(x*x-1))`.
+/// `x` must be a number greater than or equal to 1.
+#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
+pub fn acosh(x: f64) -> f64 {
+    let u = x.to_bits();
+    let e = ((u >> 52) as usize) & 0x7ff;
+
+    /* x < 1 domain error is handled in the called functions */
+
+    if e < 0x3ff + 1 {
+        /* |x| < 2, up to 2ulp error in [1,1.125] */
+        return log1p(x - 1.0 + sqrt((x - 1.0) * (x - 1.0) + 2.0 * (x - 1.0)));
+    }
+    if e < 0x3ff + 26 {
+        /* |x| < 0x1p26 */
+        return log(2.0 * x - 1.0 / (x + sqrt(x * x - 1.0)));
+    }
+    /* |x| >= 0x1p26 or nan */
+    return log(x) + LN2;
+}