Add newlib cbrtf

Author: tesuji

src/math/cbrtf.rs

@@ -0,0 +1,62 @@
+/* sf_cbrt.c -- float version of s_cbrt.c.
+ * Conversion to float by Ian Lance Taylor, Cygnus Support, [email protected].
+ */
+
+/*
+ * ====================================================
+ * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
+ *
+ * Developed at SunPro, a Sun Microsystems, Inc. business.
+ * Permission to use, copy, modify, and distribute this
+ * software is freely granted, provided that this notice
+ * is preserved.
+ * ====================================================
+ *
+ */
+
+use super::fdlibm::{FLT_UWORD_IS_FINITE, FLT_UWORD_IS_SUBNORMAL, FLT_UWORD_IS_ZERO};
+
+const B1: u32 = 709_958_130; /* B1 = (84+2/3-0.03306235651)*2**23 */
+const B2: u32 = 642_849_266; /* B2 = (76+2/3-0.03306235651)*2**23 */
+
+const C: f32 = 5.428_571_701_0e-01; /* 19/35        = 0x3f0a_f8b0 */
+const D: f32 = -7.053_061_127_7e-01; /* -864/1225   = 0xbf34_8ef1 */
+const E: f32 = 1.414_285_659_8e+00; /* 99/70        = 0x3fb5_0750 */
+const F: f32 = 1.607_142_806_1e+00; /* 45/28        = 0x3fcd_b6db */
+const G: f32 = 3.571_428_656_6e-01; /* 5/14         = 0x3eb6_db6e */
+
+/// Cube root (f32)
+///
+/// Computes the cube root of the argument.
+#[inline]
+#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
+pub extern "C" fn cbrtf(x: f32) -> f32 {
+    let hx: u32 = x.to_bits() & 0x7fff_ffff; /* |x| */
+    let sign: u32 = x.to_bits() & 0x8000_0000; /* sign = sign(x) */
+
+    if !FLT_UWORD_IS_FINITE(hx) {
+        return x + x; /* cbrt(NaN,INF) is itself */
+    }
+    if FLT_UWORD_IS_ZERO(hx) {
+        return x; /* cbrt(0) is itself */
+    }
+
+    let x = f32::from_bits(hx); /* x <- |x| */
+    /* rough cbrt to 5 bits */
+    let mut t: f32 = if FLT_UWORD_IS_SUBNORMAL(hx) {
+        /* subnormal number */
+        let t: f32 = f32::from_bits(0x4b80_0000);
+        let high: u32 = (x * t).to_bits(); /* x * (2 ** 24)*/
+        f32::from_bits((high / 3).wrapping_add(B2))
+    } else {
+        f32::from_bits((hx / 3).wrapping_add(B1))
+    };
+
+    /* new cbrt to 23 bits */
+    let r: f32 = t * t / x;
+    let s: f32 = C + r * t;
+    t *= G + F / (s + E + D / s);
+
+    /* restore the sign bit */
+    f32::from_bits(t.to_bits() | sign)
+}