Implement p_exp aproximation function

src/math/p_exp.c

@@ -1,10 +1,12 @@
 #include <pal.h>
-
 /**
  *
  * Calculate exponent (e^a), where e is the base of the natural logarithm
  * (2.71828.)
  *
+ * Based of the algorithm in this paper: http://www.schraudolph.org/pubs/Schraudolph99.pdf,
+ * It calculates a approximation of e^a very efficiently, but at the cost of accuracy.
+ *
  * @param a     Pointer to input vector
  *
  * @param c     Pointer to output vector
@@ -14,12 +16,19 @@
  * @return      None
  *
  */
-#include <math.h>
+
 void p_exp_f32(const float *a, float *c, int n)
 {
-
     int i;
     for (i = 0; i < n; i++) {
-        *(c + i) = expf(*(a + i));
+        union
+        {
+            float f;
+            uint32_t i;
+        } u;
+
+        // = 2^23 / M_LN2 * (*(a+i)) + (127 * 2^23 - 100000)
+        u.i = 12102203.16156 * ( *( a + i )) + (1065353216 - 100000);
+        *(c + i) = u.f;
     }
 }