src/libm/k_cos.c
changeset 2756 a98604b691c8
child 3162 dc1eb82ffdaa
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/libm/k_cos.c	Mon Sep 15 06:33:23 2008 +0000
@@ -0,0 +1,99 @@
+/* @(#)k_cos.c 5.1 93/09/24 */
+/*
+ * ====================================================
+ * 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.
+ * ====================================================
+ */
+
+#if defined(LIBM_SCCS) && !defined(lint)
+static char rcsid[] = "$NetBSD: k_cos.c,v 1.8 1995/05/10 20:46:22 jtc Exp $";
+#endif
+
+/*
+ * __kernel_cos( x,  y )
+ * kernel cos function on [-pi/4, pi/4], pi/4 ~ 0.785398164
+ * Input x is assumed to be bounded by ~pi/4 in magnitude.
+ * Input y is the tail of x.
+ *
+ * Algorithm
+ *	1. Since cos(-x) = cos(x), we need only to consider positive x.
+ *	2. if x < 2^-27 (hx<0x3e400000 0), return 1 with inexact if x!=0.
+ *	3. cos(x) is approximated by a polynomial of degree 14 on
+ *	   [0,pi/4]
+ *		  	                 4            14
+ *	   	cos(x) ~ 1 - x*x/2 + C1*x + ... + C6*x
+ *	   where the remez error is
+ *
+ * 	|              2     4     6     8     10    12     14 |     -58
+ * 	|cos(x)-(1-.5*x +C1*x +C2*x +C3*x +C4*x +C5*x  +C6*x  )| <= 2
+ * 	|    					               |
+ *
+ * 	               4     6     8     10    12     14
+ *	4. let r = C1*x +C2*x +C3*x +C4*x +C5*x  +C6*x  , then
+ *	       cos(x) = 1 - x*x/2 + r
+ *	   since cos(x+y) ~ cos(x) - sin(x)*y
+ *			  ~ cos(x) - x*y,
+ *	   a correction term is necessary in cos(x) and hence
+ *		cos(x+y) = 1 - (x*x/2 - (r - x*y))
+ *	   For better accuracy when x > 0.3, let qx = |x|/4 with
+ *	   the last 32 bits mask off, and if x > 0.78125, let qx = 0.28125.
+ *	   Then
+ *		cos(x+y) = (1-qx) - ((x*x/2-qx) - (r-x*y)).
+ *	   Note that 1-qx and (x*x/2-qx) is EXACT here, and the
+ *	   magnitude of the latter is at least a quarter of x*x/2,
+ *	   thus, reducing the rounding error in the subtraction.
+ */
+
+#include "math.h"
+#include "math_private.h"
+
+#ifdef __STDC__
+static const double
+#else
+static double
+#endif
+  one = 1.00000000000000000000e+00,     /* 0x3FF00000, 0x00000000 */
+    C1 = 4.16666666666666019037e-02,    /* 0x3FA55555, 0x5555554C */
+    C2 = -1.38888888888741095749e-03,   /* 0xBF56C16C, 0x16C15177 */
+    C3 = 2.48015872894767294178e-05,    /* 0x3EFA01A0, 0x19CB1590 */
+    C4 = -2.75573143513906633035e-07,   /* 0xBE927E4F, 0x809C52AD */
+    C5 = 2.08757232129817482790e-09,    /* 0x3E21EE9E, 0xBDB4B1C4 */
+    C6 = -1.13596475577881948265e-11;   /* 0xBDA8FAE9, 0xBE8838D4 */
+
+#ifdef __STDC__
+double attribute_hidden
+__kernel_cos(double x, double y)
+#else
+double attribute_hidden
+__kernel_cos(x, y)
+     double x, y;
+#endif
+{
+    double a, hz, z, r, qx;
+    int32_t ix;
+    GET_HIGH_WORD(ix, x);
+    ix &= 0x7fffffff;           /* ix = |x|'s high word */
+    if (ix < 0x3e400000) {      /* if x < 2**27 */
+        if (((int) x) == 0)
+            return one;         /* generate inexact */
+    }
+    z = x * x;
+    r = z * (C1 + z * (C2 + z * (C3 + z * (C4 + z * (C5 + z * C6)))));
+    if (ix < 0x3FD33333)        /* if |x| < 0.3 */
+        return one - (0.5 * z - (z * r - x * y));
+    else {
+        if (ix > 0x3fe90000) {  /* x > 0.78125 */
+            qx = 0.28125;
+        } else {
+            INSERT_WORDS(qx, ix - 0x00200000, 0);       /* x/4 */
+        }
+        hz = 0.5 * z - qx;
+        a = one - qx;
+        return a - (hz - (z * r - x * y));
+    }
+}