1 files changed, 23 insertions, 0 deletions
diff --git a/src/utils/fastsqrt.h b/src/utils/fastsqrt.h
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+++ b/src/utils/fastsqrt.h
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+/* A very fast function to calculate the approximate inverse square root of a
+ * floating point value and a helper function that uses it for getting the
+ * normal squareroot. For an explanation of the inverse squareroot function
+ * read:
+ * http://www.math.purdue.edu/~clomont/Math/Papers/2003/InvSqrt.pdf
+ *
+ * Unfortunately the original creator of this function seems to be unknown.
+ */
+
+float fastInvSqrt(float x)
+{
+    float xhalf = 0.5f*x;
+    int i = *(int*)&x;
+    i = 0x5f375a86- (i>>1);
+    x = *(float*)&i;
+    x = x*(1.5f-xhalf*x*x);
+    return x;
+}
+
+float fastSqrt(float x)
+{
+    return 1.0f/fastInvSqrt(x);
+}