sample_rgb impl based on a "min_stack" idea.

Will it perform? Who can say

src/lib/cimb_translator/Cell.h

@@ -1,8 +1,9 @@
 /* This code is subject to the terms of the Mozilla Public License, v.2.0. http://mozilla.org/MPL/2.0/. */
 #pragma once
 
-#include <opencv2/opencv.hpp>
+#include "util/sampling_min_stack.h"
 
+#include <opencv2/opencv.hpp>
 #include <tuple>
 
 class Cell
@@ -26,7 +27,59 @@ class Cell
 		, _rows(rows)
 	{}
 
-	// it would be nice to use a cropped cv::Mat to get the contiguous memory pointer...
+	std::tuple<uchar,uchar,uchar> sample_rgb_continuous(bool skip) const
+	{
+		/* cmovs everywhere?
+		 * sum += foo
+		 * sum_adj = foo < bar? foo : bar
+		 * sum -= sum_adj
+		 * ... etc, no branches, don't trust the compiler here
+		 * */
+
+		sampling_min_stack<uint16_t> blue;
+		sampling_min_stack<uint16_t> green;
+		sampling_min_stack<uint16_t> red;
+		uint16_t count = 0;
+
+		int channels = _img.channels();
+		int index = (_ystart * _img.cols) + _xstart;
+		const uchar* p = _img.ptr<uchar>(0) + (index * channels);
+
+		int increment = 1 + skip;
+		int toNextCol = channels * (_img.rows - _rows);
+		if (skip)
+			toNextCol += channels * _img.rows;
+
+		for (int i = 0; i < _cols; i+=increment)
+		{
+			for (int j = 0; j < _rows; ++j, ++count)
+			{
+				red += p[0];
+				green += p[1];
+				blue += p[2];
+				p += channels;
+			}
+			p += toNextCol;
+		}
+
+		if (count <= 4)
+			return std::tuple<uchar,uchar,uchar>(0, 0, 0);
+
+		count -= 4;
+		return std::tuple<uchar,uchar,uchar>(red.total()/count, green.total()/count, blue.total()/count);
+	}
+
+	std::tuple<uchar,uchar,uchar> sample_rgb(bool skip=false) const
+	{
+		int channels = _img.channels();
+		if (channels < 3)
+			return std::tuple<uchar,uchar,uchar>(0, 0, 0);
+		if (_img.isContinuous() and _cols > 0)
+			return sample_rgb_continuous(skip);
+
+		return mean_rgb(skip);
+	}
+
 	std::tuple<uchar,uchar,uchar> mean_rgb_continuous(bool skip) const
 	{
 		uint16_t blue = 0;

src/lib/util/CMakeLists.txt

@@ -8,3 +8,7 @@ set(SOURCES
 
 add_library(util INTERFACE)
 
+if(NOT DEFINED DISABLE_TESTS)
+	add_subdirectory(test)
+endif()
+

src/lib/util/sampling_min_stack.h

@@ -0,0 +1,63 @@
+#pragma once
+
+#include <array>
+#include <cstdint>
+
+template <typename Integer=uint16_t>
+class sampling_min_stack
+{
+public:
+	sampling_min_stack()
+	    : _vals{255, 255, 255, 255}
+	    , _minSum(255*4)
+	    , _totalSum(-_minSum)
+	{}
+
+	Integer min_sum() const
+	{
+		return _minSum;
+	}
+
+	Integer avg() const
+	{
+		return _minSum >> 2; // /4, the length of _vals
+	}
+
+	Integer total() const
+	{
+		return _totalSum;
+	}
+
+	// replace max in _vals with val. Returns previous max value.
+	// unconditional -- assumes val < avg(), or none of this makes sense
+	Integer push_pop(Integer replacement)
+	{
+		// get max idx
+		int8_t il = _vals[0] > _vals[1]? 0 : 1;
+		int8_t ir = _vals[2] > _vals[3]? 2 : 3;
+		int8_t i = _vals[il] > _vals[ir]? il : ir;
+
+		Integer res = _vals[i];
+		_vals[i] = replacement;
+		_minSum += replacement - res;
+		return res;
+	}
+
+	void add(Integer prospect)
+	{
+		if (prospect < avg())
+			prospect = push_pop(prospect);
+		_totalSum += prospect;
+	}
+
+	sampling_min_stack& operator+=(Integer prospect)
+	{
+		add(prospect);
+		return *this;
+	}
+
+protected:
+	std::array<Integer, 4> _vals;
+	Integer _minSum;
+	Integer _totalSum;
+};

src/lib/util/test/CMakeLists.txt

@@ -0,0 +1,22 @@
+cmake_minimum_required(VERSION 2.6)
+
+project(util_test)
+
+set (SOURCES
+	test.cpp
+	sampling_min_stackTest.cpp
+)
+
+include_directories(
+	${libcimbar_SOURCE_DIR}/test
+	${libcimbar_SOURCE_DIR}/test/lib
+	${CMAKE_CURRENT_SOURCE_DIR}/..
+)
+
+add_executable (
+	util_test
+	${SOURCES}
+)
+
+add_test(util_test util_test)
+

src/lib/util/test/sampling_min_stackTest.cpp

@@ -0,0 +1,29 @@
+/* This code is subject to the terms of the Mozilla Public License, v.2.0. http://mozilla.org/MPL/2.0/. */
+#include "unittest.h"
+
+#include "sampling_min_stack.h"
+
+TEST_CASE( "sample_min_stackTest/testDefault", "[unit]" )
+{
+	sampling_min_stack<uint16_t> blue;
+	assertEquals(255, blue.avg());
+
+	blue += 200;
+	blue += 200;
+	blue += 200;
+	blue += 200;
+	assertEquals(200, blue.avg());
+	assertEquals(0, blue.total());
+
+	blue += 100;
+	blue += 250;
+	blue += 100;
+	blue += 250;
+	assertEquals(900, blue.total());
+
+	blue += 220;
+	assertEquals(1120, blue.total());
+
+	blue += 60;
+	assertEquals(1320, blue.total());
+}

src/lib/util/test/test.cpp

@@ -0,0 +1,4 @@
+/* This code is subject to the terms of the Mozilla Public License, v.2.0. http://mozilla.org/MPL/2.0/. */
+#define CATCH_CONFIG_MAIN
+#include "catch.hpp"
+