Use a linear interpolator for the reported RSSI values.

RSSIInterpolator.cpp

@@ -0,0 +1,91 @@
+/*
+ *   Copyright (C) 2016 by Jonathan Naylor G4KLX
+ *
+ *   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., 675 Mass Ave, Cambridge, MA 02139, USA.
+ */
+
+#include "RSSIInterpolator.h"
+
+#include "Log.h"
+
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
+#include <cctype>
+
+CRSSIInterpolator::CRSSIInterpolator() :
+m_map()
+{
+}
+
+CRSSIInterpolator::~CRSSIInterpolator()
+{
+	m_map.clear();
+}
+
+bool CRSSIInterpolator::load(const std::string& filename)
+{
+	FILE* fp = ::fopen(filename.c_str(), "rt");
+	if (fp == NULL) {
+		LogWarning("Cannot open the RSSI data file - %s", filename.c_str());
+		return false;
+	}
+	
+	char buffer[100U];
+	while (::fgets(buffer, 100, fp) != NULL) {
+		if (buffer[0U] == '#')
+			continue;
+
+		char* p1 = ::strtok(buffer, " \t\r\n");
+		char* p2 = ::strtok(NULL,   " \t\r\n");
+
+		if (p1 != NULL && p2 != NULL) {
+			uint16_t raw = (unsigned int)::atoi(p1);
+			int rssi = ::atoi(p2);
+			m_map.insert(std::pair<uint16_t, int>(raw, rssi));
+		}
+	}
+
+	::fclose(fp);
+
+	LogInfo("Loaded %u RSSI data mapping points from %s", m_map.size(), filename.c_str());
+	
+	return true;
+}
+
+int CRSSIInterpolator::interpolate(uint16_t val) const
+{
+	if (m_map.empty())
+		return 0;
+
+	std::map<uint16_t, int>::const_iterator it = m_map.lower_bound(val);
+
+	if (it == m_map.end())
+		return m_map.rbegin()->second;
+
+	if (it == m_map.begin())
+		return it->second;
+
+	unsigned int x2 = it->first;
+	int          y2 = it->second;
+
+	--it;
+	unsigned int x1 = it->first;
+	int          y1 = it->second;
+
+	float p = float(val - x1) / float(x2 - x1);
+
+	return int((1.0F - p) * float(y1) + p * float(y2));
+}