Add some external dependencies.

extern/EtcLib/Etc/EtcMath.cpp

@@ -0,0 +1,64 @@
+/*
+ * Copyright 2015 The Etc2Comp Authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "EtcConfig.h"
+#include "EtcMath.h"
+
+namespace Etc
+{
+
+	// ----------------------------------------------------------------------------------------------------
+	// calculate the line that best fits the set of XY points contained in a_afX[] and a_afY[]
+	// use a_fSlope and a_fOffset to define that line
+	//
+	bool Regression(float a_afX[], float a_afY[], unsigned int a_Points,
+					float *a_fSlope, float *a_fOffset)
+	{
+		float fPoints = (float)a_Points;
+
+		float fSumX = 0.0f;
+		float fSumY = 0.0f;
+		float fSumXY = 0.0f;
+		float fSumX2 = 0.0f;
+
+		for (unsigned int uiPoint = 0; uiPoint < a_Points; uiPoint++)
+		{
+			fSumX += a_afX[uiPoint];
+			fSumY += a_afY[uiPoint];
+			fSumXY += a_afX[uiPoint] * a_afY[uiPoint];
+			fSumX2 += a_afX[uiPoint] * a_afX[uiPoint];
+		}
+
+		float fDivisor = fPoints*fSumX2 - fSumX*fSumX;
+
+		// if vertical line
+		if (fDivisor == 0.0f)
+		{
+			*a_fSlope = 0.0f;
+			*a_fOffset = 0.0f;
+			return true;
+		}
+
+		*a_fSlope = (fPoints*fSumXY - fSumX*fSumY) / fDivisor;
+		*a_fOffset = (fSumY - (*a_fSlope)*fSumX) / fPoints;
+
+		return false;
+	}
+
+	// ----------------------------------------------------------------------------------------------------
+	//
+
+} // namespace Etc