Version 0.9

Added: Filter type "IIR", which allows to specify a user-defined IIR filter of arbitrary order with custom coefficients.
Improved: For playback devices, the APO is now installed both as GFX (post-mix) and LFX (pre-mix) APO. Normally the filtering happens in GFX, but the command "Stage" allows to select LFX to apply effects based on the number of input channels.
Added: Expression language. The commands "If", "ElseIf", "Else", "EndIf", "Eval" and inline expressions allow to change the filter behaviour based on runtime conditions.
Added: Command "Copy", which copies audio data between channels.
Added: Command "Delay", which delays the audio on the currently selected channels.
Improved: Configuration files are now locked exclusively for writing while reading them.
Improved: There is now a transition (10 ms) from old to new configuration after loading.
Improved: Substantial internal restructuring to improve extensibility and maintainability. Filtering commands now work exactly in the order they were specified.
Fixed: When the microphone input was in stereo but the application requested a mono stream, the audio was heavily distorted.
Fixed: The sine sweep generated by the Benchmark application had floating-point precision issues leading to severe artifacts when certain filters were applied.

filters/IIRFilterFactory.cpp

@@ -0,0 +1,108 @@
+/*
+This file is part of EqualizerAPO, a system-wide equalizer.
+Copyright (C) 2014  Jonas Thedering
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License along
+with this program; if not, write to the Free Software Foundation, Inc.,
+51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+*/
+
+#define _USE_MATH_DEFINES
+#include <cmath>
+#include <regex>
+#include <sstream>
+
+#include "helpers/MemoryHelper.h"
+#include "helpers/StringHelper.h"
+#include "helpers/LogHelper.h"
+#include "IIRFilter.h"
+#include "IIRFilterFactory.h"
+
+using namespace std;
+
+static wregex regexType(L"^\\s*ON\\s+([A-Za-z]+)");
+static wregex regexOrder(L"\\s*Order\\s+([0-9]+)");
+static wregex regexCoefficients(L"\\s+Coefficients((?: [-+0-9.eE]+)+)");
+
+IIRFilterFactory::IIRFilterFactory()
+{
+}
+
+vector<IFilter*> IIRFilterFactory::createFilter(const wstring& configPath, wstring& command, wstring& parameters)
+{
+	IIRFilter* filter = NULL;
+
+	if(command.find(L"Filter") == 0)
+	{
+		wsmatch match;
+		wstring typeString;
+
+		bool found = regex_search(parameters, match, regexType);
+		if(found)
+		{
+			typeString = match.str(1);
+			if(typeString == L"IIR")
+			{
+				bool found = regex_search(parameters, match, regexOrder);
+				if(found)
+				{
+					wstring orderString = match.str(1);
+					unsigned order = wcstol(orderString.c_str(), NULL, 10);
+
+					if(order<1)
+					{
+						LogF(L"Order %d not supported, must at least be 1", order);
+					}
+					else
+					{
+						found = regex_search(parameters, match, regexCoefficients);
+
+						if(found)
+						{
+							wstring coefficientsString = match.str(1);
+							vector<wstring> coefficientStrings = StringHelper::split(coefficientsString, L' ');
+							if(coefficientStrings.size() != (order + 1) * 2)
+							{
+								LogF(L"Invalid number of coefficients. Expected %d coefficients instead of %d", (order + 1) * 2, coefficientStrings.size());
+							}
+							else
+							{
+								vector<double> coefficients;
+								for(auto it = coefficientStrings.begin(); it!=coefficientStrings.end(); it++)
+								{
+									coefficients.push_back(wcstod(it->c_str(), NULL));
+								}
+
+								wstringstream stream;
+								stream << L"Adding IIR filter of order " << order << " with coefficients";
+								for(unsigned i=0; i<=order; i++)
+									stream << L" b" << i << L"=" << coefficients[i];
+								for(unsigned i=0; i<=order; i++)
+									stream << L" a" << i << L"=" << coefficients[i + order + 1];
+
+								TraceF(L"%s", stream.str().c_str());
+
+								void* mem = MemoryHelper::alloc(sizeof(IIRFilter));
+								filter = new(mem) IIRFilter(coefficients);
+							}
+						}
+					}
+				}
+			}
+		}
+	}
+
+	if(filter == NULL)
+		return vector<IFilter*>(0);
+	return vector<IFilter*>(1, filter);
+}