1 files changed, 24 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)
+{
+    union { int i; float x; } tmp;
+    float xhalf = 0.5f * x;
+    tmp.x = x;
+    tmp.i = 0x5f375a86 - (tmp.i >> 1);
+    x = tmp.x;
+    x = x * (1.5f - xhalf * x * x);
+    return x;
+}
+
+float fastSqrt(float x)
+{
+    return 1.0f / fastInvSqrt(x);
+}