Add newlib exp2f

Author: Schultzer

src/math/exp2f.rs

@@ -1,136 +1,33 @@
-// origin: FreeBSD /usr/src/lib/msun/src/s_exp2f.c
-//-
-// Copyright (c) 2005 David Schultz <[email protected]>
-// All rights reserved.
-//
-// Redistribution and use in source and binary forms, with or without
-// modification, are permitted provided that the following conditions
-// are met:
-// 1. Redistributions of source code must retain the above copyright
-//    notice, this list of conditions and the following disclaimer.
-// 2. Redistributions in binary form must reproduce the above copyright
-//    notice, this list of conditions and the following disclaimer in the
-//    documentation and/or other materials provided with the distribution.
-//
-// THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
-// ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
-// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
-// ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
-// FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
-// DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
-// OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
-// HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
-// LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
-// OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
-// SUCH DAMAGE.
-
-const TBLSIZE: usize = 16;
-
-static EXP2FT: [u64; TBLSIZE] = [
-    0x3fe6a09e667f3bcd,
-    0x3fe7a11473eb0187,
-    0x3fe8ace5422aa0db,
-    0x3fe9c49182a3f090,
-    0x3feae89f995ad3ad,
-    0x3fec199bdd85529c,
-    0x3fed5818dcfba487,
-    0x3feea4afa2a490da,
-    0x3ff0000000000000,
-    0x3ff0b5586cf9890f,
-    0x3ff172b83c7d517b,
-    0x3ff2387a6e756238,
-    0x3ff306fe0a31b715,
-    0x3ff3dea64c123422,
-    0x3ff4bfdad5362a27,
-    0x3ff5ab07dd485429,
-];
-
-// exp2f(x): compute the base 2 exponential of x
-//
-// Accuracy: Peak error < 0.501 ulp; location of peak: -0.030110927.
-//
-// Method: (equally-spaced tables)
-//
-//   Reduce x:
-//     x = k + y, for integer k and |y| <= 1/2.
-//     Thus we have exp2f(x) = 2**k * exp2(y).
-//
-//   Reduce y:
-//     y = i/TBLSIZE + z for integer i near y * TBLSIZE.
-//     Thus we have exp2(y) = exp2(i/TBLSIZE) * exp2(z),
-//     with |z| <= 2**-(TBLSIZE+1).
-//
-//   We compute exp2(i/TBLSIZE) via table lookup and exp2(z) via a
-//   degree-4 minimax polynomial with maximum error under 1.4 * 2**-33.
-//   Using double precision for everything except the reduction makes
-//   roundoff error insignificant and simplifies the scaling step.
-//
-//   This method is due to Tang, but I do not use his suggested parameters:
-//
-//      Tang, P.  Table-driven Implementation of the Exponential Function
-//      in IEEE Floating-Point Arithmetic.  TOMS 15(2), 144-157 (1989).
+/* wf_exp2.c -- float version of w_exp2.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::powf;
 
 /// Exponential, base 2 (f32)
 ///
 /// Calculate `2^x`, that is, 2 raised to the power `x`.
 #[inline]
 #[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
-pub fn exp2f(mut x: f32) -> f32 {
-    let redux = f32::from_bits(0x4b400000) / TBLSIZE as f32;
-    let p1 = f32::from_bits(0x3f317218);
-    let p2 = f32::from_bits(0x3e75fdf0);
-    let p3 = f32::from_bits(0x3d6359a4);
-    let p4 = f32::from_bits(0x3c1d964e);
-
-    // double_t t, r, z;
-    // uint32_t ix, i0, k;
-
-    let x1p127 = f32::from_bits(0x7f000000);
+pub fn exp2f(x: f32) -> f32 {
+    powf(2f32, x)
+}
 
-    /* Filter out exceptional cases. */
-    let ui = f32::to_bits(x);
-    let ix = ui & 0x7fffffff;
-    if ix > 0x42fc0000 {
-        /* |x| > 126 */
-        if ix > 0x7f800000 {
-            /* NaN */
-            return x;
-        }
-        if ui >= 0x43000000 && ui < 0x80000000 {
-            /* x >= 128 */
-            x *= x1p127;
-            return x;
-        }
-        if ui >= 0x80000000 {
-            /* x < -126 */
-            if ui >= 0xc3160000 || (ui & 0x0000ffff != 0) {
-                force_eval!(f32::from_bits(0x80000001) / x);
-            }
-            if ui >= 0xc3160000 {
-                /* x <= -150 */
-                return 0.0;
-            }
-        }
-    } else if ix <= 0x33000000 {
-        /* |x| <= 0x1p-25 */
-        return 1.0 + x;
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn sanity_check() {
+        assert_eq!(super::exp2f(1.1), 2.143547);
     }
-
-    /* Reduce x, computing z, i0, and k. */
-    let ui = f32::to_bits(x + redux);
-    let mut i0 = ui;
-    i0 += TBLSIZE as u32 / 2;
-    let k = i0 / TBLSIZE as u32;
-    let ukf = f64::from_bits(((0x3ff + k) as u64) << 52);
-    i0 &= TBLSIZE as u32 - 1;
-    let mut uf = f32::from_bits(ui);
-    uf -= redux;
-    let z: f64 = (x - uf) as f64;
-    /* Compute r = exp2(y) = exp2ft[i0] * p(z). */
-    let r: f64 = f64::from_bits(EXP2FT[i0 as usize]);
-    let t: f64 = r as f64 * z;
-    let r: f64 = r + t * (p1 as f64 + z * p2 as f64) + t * (z * z) * (p3 as f64 + z * p4 as f64);
-
-    /* Scale by 2**k */
-    (r * ukf) as f32
 }