1 files changed, 58 insertions, 0 deletions
diff --git a/libm/double/acos.c b/libm/double/acos.c
new file mode 100644
index 000000000..60f61dc98
--- /dev/null
+++ b/libm/double/acos.c
@@ -0,0 +1,58 @@
+/*							acos()
+ *
+ *	Inverse circular cosine
+ *
+ *
+ *
+ * SYNOPSIS:
+ *
+ * double x, y, acos();
+ *
+ * y = acos( x );
+ *
+ *
+ *
+ * DESCRIPTION:
+ *
+ * Returns radian angle between 0 and pi whose cosine
+ * is x.
+ *
+ * Analytically, acos(x) = pi/2 - asin(x).  However if |x| is
+ * near 1, there is cancellation error in subtracting asin(x)
+ * from pi/2.  Hence if x < -0.5,
+ *
+ *    acos(x) =	 pi - 2.0 * asin( sqrt((1+x)/2) );
+ *
+ * or if x > +0.5,
+ *
+ *    acos(x) =	 2.0 * asin(  sqrt((1-x)/2) ).
+ *
+ *
+ * ACCURACY:
+ *
+ *                      Relative error:
+ * arithmetic   domain     # trials      peak         rms
+ *    DEC       -1, 1       50000       3.3e-17     8.2e-18
+ *    IEEE      -1, 1       10^6        2.2e-16     6.5e-17
+ *
+ *
+ * ERROR MESSAGES:
+ *
+ *   message         condition      value returned
+ * asin domain        |x| > 1           NAN
+ */
+
+#define __USE_BSD
+#include <math.h>
+
+double acos(double x)
+{
+    if (x < -0.5) {
+	return (M_PI - 2.0 * asin( sqrt((1+x)/2) ));
+    }
+    if (x > 0.5) {
+	return (2.0 * asin(  sqrt((1-x)/2) ));
+    }
+
+    return(M_PI_2 - asin(x));
+}