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/*
* Copyright 2012, The Android Open Source Project
*
* 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.
*/
//#define LOG_NDEBUG 0
#define LOG_TAG "LinearRegression"
#include <utils/Log.h>
#include "LinearRegression.h"
#include <math.h>
#include <string.h>
namespace android {
LinearRegression::LinearRegression(size_t historySize)
: mHistorySize(historySize),
mCount(0),
mHistory(new Point[mHistorySize]),
mSumX(0.0),
mSumY(0.0) {
}
LinearRegression::~LinearRegression() {
delete[] mHistory;
mHistory = NULL;
}
void LinearRegression::addPoint(float x, float y) {
if (mCount == mHistorySize) {
const Point &oldest = mHistory[0];
mSumX -= oldest.mX;
mSumY -= oldest.mY;
memmove(&mHistory[0], &mHistory[1], (mHistorySize - 1) * sizeof(Point));
--mCount;
}
Point *newest = &mHistory[mCount++];
newest->mX = x;
newest->mY = y;
mSumX += x;
mSumY += y;
}
bool LinearRegression::approxLine(float *n1, float *n2, float *b) const {
static const float kEpsilon = 1.0E-4;
if (mCount < 2) {
return false;
}
float sumX2 = 0.0f;
float sumY2 = 0.0f;
float sumXY = 0.0f;
float meanX = mSumX / (float)mCount;
float meanY = mSumY / (float)mCount;
for (size_t i = 0; i < mCount; ++i) {
const Point &p = mHistory[i];
float x = p.mX - meanX;
float y = p.mY - meanY;
sumX2 += x * x;
sumY2 += y * y;
sumXY += x * y;
}
float T = sumX2 + sumY2;
float D = sumX2 * sumY2 - sumXY * sumXY;
float root = sqrt(T * T * 0.25 - D);
float L1 = T * 0.5 - root;
if (fabs(sumXY) > kEpsilon) {
*n1 = 1.0;
*n2 = (2.0 * L1 - sumX2) / sumXY;
float mag = sqrt((*n1) * (*n1) + (*n2) * (*n2));
*n1 /= mag;
*n2 /= mag;
} else {
*n1 = 0.0;
*n2 = 1.0;
}
*b = (*n1) * meanX + (*n2) * meanY;
return true;
}
} // namespace android
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