1 files changed, 48 insertions, 0 deletions
diff --git a/src/math/atan.c b/src/math/atan.c
new file mode 100644
index 00000000..5b4b0330
--- /dev/null
+++ b/src/math/atan.c
@@ -0,0 +1,48 @@
+# define TGSOURCE "atan.c"
+#include <math.h>
+#include "errno.h"
+#include "nonstd/tgmath.h"
+
+/** arc tangent **/
+TYPE TGFN(atan)(TYPE x)
+{
+	int MAXLOOPS = 10;
+	TYPE arctan = 0.0;
+	TYPE power = 1.0;
+	int i;
+
+	if (0) {
+		errno = ERANGE; /* The result cannot be represented */
+		/* RETURN_FAILURE(CONSTANT(HUGE_VAL), A range error occurred); */
+		return TGHUGE;
+	}
+
+	/* TODO: correct input domain */
+	/* TODO: tweak number of loops */
+
+	/* arctan x = x - x^3/3 + x^5/5 - x^7/7 ... */
+	for (i = 1; i < MAXLOOPS; i++) {
+		power *= x;
+
+		if (i % 4 == 1) {
+			arctan += power / i;
+		} else if (i % 4 == 3) {
+			arctan -= power / i;
+		}
+	}
+
+	/* RETURN_SUCCESS(a value in the range `[-PI()/2, +PI()/2]'); */
+	return arctan;
+}
+
+/***
+compute the principal value of the arc tangent of ARGUMENT(x).
+***/
+
+/*
+IMPLEMENTATION(The value returned on a domain error, CONSTANT(HUGE_VAL))
+LINK(m)
+*/
+/*
+STDC(1)
+*/