diff --git a/lib/decoding/Viterbi.h b/lib/decoding/Viterbi.h
new file mode 100644
index 0000000..8635bf5
--- /dev/null
+++ b/lib/decoding/Viterbi.h
@@ -0,0 +1,35 @@
+/*
+* Copyright 2013, 2014 Range Networks, Inc.
+*
+* This software is distributed under multiple licenses;
+* see the COPYING file in the main directory for licensing
+* information for this specific distribution.
+*
+* This use of this software may be subject to additional restrictions.
+* See the LEGAL file in the main directory for details.
+
+    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.
+
+*/
+
+
+#ifndef _VITERBI_H_
+#define _VITERBI_H_ 1
+
+// (pat) Virtual base class for Viterbi and Turbo coder/decoders.
+class ViterbiBase {
+	public:
+	virtual void encode(const BitVector &in, BitVector& target) const = 0;
+	virtual void decode(const SoftVector &in, BitVector& target) = 0;
+	// (pat) Return error count from most recent decoder run.
+	// If you get -1 from this, the method is not defined in the Viterbi class.
+	virtual int getBEC() { return -1; }
+	//virtual ~ViterbiBase();   Currently None of these have destructors.
+
+	// These functions are logically part of the Viterbi functionality, even though they do not use any class variables.
+	unsigned applyPoly(uint64_t val, uint64_t poly);
+	unsigned applyPoly(uint64_t val, uint64_t poly, unsigned order);
+};
+#endif
diff --git a/lib/decoding/ViterbiR204.cpp b/lib/decoding/ViterbiR204.cpp
new file mode 100644
index 0000000..999b31b
--- /dev/null
+++ b/lib/decoding/ViterbiR204.cpp
@@ -0,0 +1,306 @@
+/*
+* Copyright 2008, 2009, 2014 Free Software Foundation, Inc.
+* Copyright 2014 Range Networks, Inc.
+*
+*
+* This software is distributed under the terms of the GNU Affero Public License.
+* See the COPYING file in the main directory for details.
+*
+* This use of this software may be subject to additional restrictions.
+* See the LEGAL file in the main directory for details.
+
+	This program is free software: you can redistribute it and/or modify
+	it under the terms of the GNU Affero General Public License as published by
+	the Free Software Foundation, either version 3 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 Affero General Public License for more details.
+
+	You should have received a copy of the GNU Affero General Public License
+	along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+*/
+
+
+
+
+#include "BitVector.h"
+#include "ViterbiR204.h"
+#include <iostream>
+#include <stdio.h>
+#include <sstream>
+#include <string.h>
+
+using namespace std;
+
+
+/**
+  Apply a Galois polymonial to a binary seqeunce.
+  @param val The input sequence.
+  @param poly The polynomial.
+  @param order The order of the polynomial.
+  @return Single-bit result.
+*/
+unsigned ViterbiBase::applyPoly(uint64_t val, uint64_t poly, unsigned order)
+{
+	uint64_t prod = val & poly;
+	unsigned sum = prod;
+	for (unsigned i=1; i<order; i++) sum ^= prod>>i;
+	return sum & 0x01;
+}
+
+unsigned ViterbiBase::applyPoly(uint64_t val, uint64_t poly)
+{
+	uint64_t prod = val & poly;
+	prod = (prod ^ (prod >> 32));
+	prod = (prod ^ (prod >> 16));
+	prod = (prod ^ (prod >> 8));
+	prod = (prod ^ (prod >> 4));
+	prod = (prod ^ (prod >> 2));
+	prod = (prod ^ (prod >> 1));
+	return prod & 0x01;
+}
+
+
+
+//void BitVector::encode(const ViterbiR2O4& coder, BitVector& target)
+void ViterbiR2O4::encode(const BitVector& in, BitVector& target) const
+{
+	const ViterbiR2O4& coder = *this;
+	size_t sz = in.size();
+
+	assert(sz*coder.iRate() == target.size());
+
+	// Build a "history" array where each element contains the full history.
+	uint32_t history[sz];
+	uint32_t accum = 0;
+	for (size_t i=0; i<sz; i++) {
+		accum = (accum<<1) | in.bit(i);
+		history[i] = accum;
+	}
+
+	// Look up histories in the pre-generated state table.
+	char *op = target.begin();
+	for (size_t i=0; i<sz; i++) {
+		unsigned index = coder.cMask() & history[i];
+		for (unsigned g=0; g<coder.iRate(); g++) {
+			*op++ = coder.stateTable(g,index);
+		}
+	}
+}
+
+
+ViterbiR2O4::ViterbiR2O4()
+{
+	assert(mDeferral < 32);
+	// (pat) The generator polynomials are: G0 = 1 + D**3 + D**4; and G1 = 1 + D + D**3 + D**4
+	mCoeffs[0] = 0x019;     // G0 = D**4 + D**3 + 1; represented as binary 11001,
+	mCoeffs[1] = 0x01b;     // G1 = + D**4 + D**3 + D + 1; represented as binary 11011
+	computeStateTables(0);
+	computeStateTables(1);
+	computeGeneratorTable();
+}
+
+
+void ViterbiR2O4::initializeStates()
+{
+	for (unsigned i=0; i<mIStates; i++) vitClear(mSurvivors[i]);
+	for (unsigned i=0; i<mNumCands; i++) vitClear(mCandidates[i]);
+}
+
+
+
+// (pat) The state machine has 16 states.
+// Each state has two possible next states corresponding to 0 or 1 inputs to original encoder.
+// which are saved in mStateTable in consecutive locations.
+// In other words the mStateTable second index is ((current_state <<1) + encoder_bit)
+// g is 0 or 1 for the first or second bit of the encoded stream, ie, the one we are decoding.
+void ViterbiR2O4::computeStateTables(unsigned g)
+{
+	assert(g<mIRate);
+	for (unsigned state=0; state<mIStates; state++) {
+		// 0 input
+		uint32_t inputVal = state<<1;
+		mStateTable[g][inputVal] = applyPoly(inputVal, mCoeffs[g], mOrder+1);
+		// 1 input
+		inputVal |= 1;
+		mStateTable[g][inputVal] = applyPoly(inputVal, mCoeffs[g], mOrder+1);
+	}
+}
+
+void ViterbiR2O4::computeGeneratorTable()
+{
+	for (unsigned index=0; index<mIStates*2; index++) {
+		mGeneratorTable[index] = (mStateTable[0][index]<<1) | mStateTable[1][index];
+	}
+}
+
+
+void ViterbiR2O4::branchCandidates()
+{
+	// Branch to generate new input states.
+	const vCand *sp = mSurvivors;
+	for (unsigned i=0; i<mNumCands; i+=2) {
+		// extend and suffix
+		const uint32_t iState0 = (sp->iState) << 1;				// input state for 0
+		const uint32_t iState1 = iState0 | 0x01;				// input state for 1
+		const uint32_t oStateShifted = (sp->oState) << mIRate;	// shifted output (by 2)
+		const float cost = sp->cost;
+		int bec = sp->bitErrorCnt;
+		sp++;
+		// 0 input extension
+		mCandidates[i].cost = cost;
+		// mCMask is the low 5 bits, ie, full width of mGeneratorTable.
+		mCandidates[i].oState = oStateShifted | mGeneratorTable[iState0 & mCMask];
+		mCandidates[i].iState = iState0;
+		mCandidates[i].bitErrorCnt = bec;
+		// 1 input extension
+		mCandidates[i+1].cost = cost;
+		mCandidates[i+1].oState = oStateShifted | mGeneratorTable[iState1 & mCMask];
+		mCandidates[i+1].iState = iState1;
+		mCandidates[i+1].bitErrorCnt = bec;
+	}
+}
+
+
+void ViterbiR2O4::getSoftCostMetrics(const uint32_t inSample, const float *matchCost, const float *mismatchCost)
+{
+	const float *cTab[2] = {matchCost,mismatchCost};
+	for (unsigned i=0; i<mNumCands; i++) {
+		vCand& thisCand = mCandidates[i];
+		// We examine input bits 2 at a time for a rate 1/2 coder.
+		// (pat) mismatched will end up with bits in it for previous transitions,
+		// but we only use the bottom two bits of mismatched so it is ok.
+		const unsigned mismatched = inSample ^ (thisCand.oState);
+		// (pat) TODO: Are these two tests swapped?
+		thisCand.cost += cTab[mismatched&0x01][1] + cTab[(mismatched>>1)&0x01][0];
+		if (mismatched & 1) { thisCand.bitErrorCnt++; }
+		if (mismatched & 2) { thisCand.bitErrorCnt++; }
+	}
+}
+
+
+void ViterbiR2O4::pruneCandidates()
+{
+	const vCand* c1 = mCandidates;					// 0-prefix
+	const vCand* c2 = mCandidates + mIStates;		// 1-prefix
+	for (unsigned i=0; i<mIStates; i++) {
+		if (c1[i].cost < c2[i].cost) mSurvivors[i] = c1[i];
+		else mSurvivors[i] = c2[i];
+	}
+}
+
+
+const ViterbiR2O4::vCand& ViterbiR2O4::minCost() const
+{
+	int minIndex = 0;
+	float minCost = mSurvivors[0].cost;
+	for (unsigned i=1; i<mIStates; i++) {
+		const float thisCost = mSurvivors[i].cost;
+		if (thisCost>=minCost) continue;
+		minCost = thisCost;
+		minIndex=i;
+	}
+	return mSurvivors[minIndex];
+}
+
+
+const ViterbiR2O4::vCand* ViterbiR2O4::vstep(uint32_t inSample, const float *probs, const float *iprobs, bool isNotTailBits)
+{
+	branchCandidates();
+	// (pat) tail bits do not affect cost or error bit count of any branch.
+	if (isNotTailBits) getSoftCostMetrics(inSample,probs,iprobs);
+	pruneCandidates();
+	return &minCost();
+}
+
+
+void ViterbiR2O4::decode(const SoftVector &in, BitVector& target)
+{
+	ViterbiR2O4& decoder = *this;
+	const size_t sz = in.size();
+	const unsigned oSize = in.size() / decoder.iRate();
+	const unsigned deferral = decoder.deferral();
+	const size_t ctsz = sz + deferral*decoder.iRate();
+	assert(sz <= decoder.iRate()*target.size());
+
+	// Build a "history" array where each element contains the full history.
+	// (pat) We only use every other history element, so why are we setting them?
+	uint32_t history[ctsz];
+	{
+		BitVector bits = in.sliced();
+		uint32_t accum = 0;
+		for (size_t i=0; i<sz; i++) {
+			accum = (accum<<1) | bits.bit(i);
+			history[i] = accum;
+		}
+		// Repeat last bit at the end.
+		// (pat) TODO: really?  Does this matter?
+		for (size_t i=sz; i<ctsz; i++) {
+			accum = (accum<<1) | (accum & 0x01);
+			history[i] = accum;
+		}
+	}
+
+	// Precompute metric tables.
+	float matchCostTable[ctsz];
+	float mismatchCostTable[ctsz];
+	{
+		const float *dp = in.begin();
+		for (size_t i=0; i<sz; i++) {
+			// pVal is the probability that a bit is correct.
+			// ipVal is the probability that a bit is incorrect.
+			float pVal = dp[i];
+			if (pVal>0.5F) pVal = 1.0F-pVal;
+			float ipVal = 1.0F-pVal;
+			// This is a cheap approximation to an ideal cost function.
+			if (pVal<0.01F) pVal = 0.01;
+			if (ipVal<0.01F) ipVal = 0.01;
+			matchCostTable[i] = 0.25F/ipVal;
+			mismatchCostTable[i] = 0.25F/pVal;
+		}
+	
+		// pad end of table with unknowns
+		// Note that these bits should not contribute to Bit Error Count.
+		for (size_t i=sz; i<ctsz; i++) {
+			matchCostTable[i] = 0.5F;
+			mismatchCostTable[i] = 0.5F;
+		}
+	}
+
+	{
+		decoder.initializeStates();
+		// Each sample of history[] carries its history.
+		// So we only have to process every iRate-th sample.
+		const unsigned step = decoder.iRate();
+		// input pointer
+		const uint32_t *ip = history + step - 1;
+		// output pointers
+		char *op = target.begin();
+		const char *const opt = target.end();	// (pat) Not right if target is larger than needed; should be: op + sz/2;
+		// table pointers
+		const float* match = matchCostTable;
+		const float* mismatch = mismatchCostTable;
+		size_t oCount = 0;
+		const ViterbiR2O4::vCand *minCost = NULL;
+		while (op<opt) {
+			// Viterbi algorithm
+			assert(match-matchCostTable<(float)sizeof(matchCostTable)/sizeof(matchCostTable[0])-1);
+			assert(mismatch-mismatchCostTable<(float)sizeof(mismatchCostTable)/sizeof(mismatchCostTable[0])-1);
+			minCost = decoder.vstep(*ip, match, mismatch, oCount < oSize);
+			ip += step;
+			match += step;
+			mismatch += step;
+			// output
+			if (oCount>=deferral) *op++ = (minCost->iState >> deferral)&0x01;
+			oCount++;
+		}
+		// Dont think minCost == NULL can happen.
+		mBitErrorCnt = minCost ? minCost->bitErrorCnt : 0;
+	}
+}
+
+// vim: ts=4 sw=4
diff --git a/lib/decoding/ViterbiR204.h b/lib/decoding/ViterbiR204.h
new file mode 100644
index 0000000..4fd053f
--- /dev/null
+++ b/lib/decoding/ViterbiR204.h
@@ -0,0 +1,149 @@
+/*
+* Copyright 2008, 2009, 2014 Free Software Foundation, Inc.
+* Copyright 2014 Range Networks, Inc.
+*
+* This software is distributed under the terms of the GNU Affero Public License.
+* See the COPYING file in the main directory for details.
+*
+* This use of this software may be subject to additional restrictions.
+* See the LEGAL file in the main directory for details.
+
+	This program is free software: you can redistribute it and/or modify
+	it under the terms of the GNU Affero General Public License as published by
+	the Free Software Foundation, either version 3 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 Affero General Public License for more details.
+
+	You should have received a copy of the GNU Affero General Public License
+	along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+*/
+
+#ifndef _VITERBIR204_H_
+#define _VITERBIR204_H_ 1
+
+#include "Viterbi.h"
+
+
+/**
+	Class to represent convolutional coders/decoders of rate 1/2, memory length 4.
+	This is the "workhorse" coder for most GSM channels.
+*/
+class ViterbiR2O4 : public ViterbiBase {
+
+	private:
+		/**name Lots of precomputed elements so the compiler can optimize like hell. */
+		//@{
+		/**@name Core values. */
+		//@{
+		static const unsigned mIRate = 2;	///< reciprocal of rate
+		static const unsigned mOrder = 4;	///< memory length of generators
+		//@}
+		/**@name Derived values. */
+		//@{
+		static const unsigned mIStates = 0x01 << mOrder;	///< (16) number of states, number of survivors
+		static const uint32_t mSMask = mIStates-1;			///< survivor mask
+		static const uint32_t mCMask = (mSMask<<1) | 0x01;	///< candidate mask
+		static const uint32_t mOMask = (0x01<<mIRate)-1;	///< ouput mask, all iRate low bits set
+		static const unsigned mNumCands = mIStates*2;		///< number of candidates to generate during branching
+		static const unsigned mDeferral = 6*mOrder;			///< deferral to be used
+		//@}
+		//@}
+
+		/** Precomputed tables. */
+		//@{
+		uint32_t mCoeffs[mIRate];					///< polynomial for each generator
+		// (pat) There are 16 states, each of which has two possible output states.
+		// These are stored in these two tables in consecutive locations.
+		uint32_t mStateTable[mIRate][2*mIStates];	///< precomputed generator output tables
+		// mGeneratorTable is the encoder output state for a given input state and encoder input bit.
+		uint32_t mGeneratorTable[2*mIStates];		///< precomputed coder output table
+		//@}
+		int mBitErrorCnt;
+	
+	public:
+
+		/**
+		  A candidate sequence in a Viterbi decoder.
+		  The 32-bit state register can support a deferral of 6 with a 4th-order coder.
+		 */
+		typedef struct candStruct {
+			uint32_t iState;	///< encoder input associated with this candidate
+			uint32_t oState;	///< encoder output associated with this candidate
+			float cost;			///< cost (metric value), float to support soft inputs
+			int bitErrorCnt;	///< number of bit errors in the encoded vector being decoded.
+		} vCand;
+
+		/** Clear a structure. */
+		void vitClear(vCand& v)
+		{
+			v.iState=0;
+			v.oState=0;
+			v.cost=0;
+			v.bitErrorCnt = 0;
+		}
+		
+
+	private:
+
+		/**@name Survivors and candidates. */
+		//@{
+		vCand mSurvivors[mIStates];			///< current survivor pool
+		vCand mCandidates[2*mIStates];		///< current candidate pool
+		//@}
+
+	public:
+
+		unsigned iRate() const { return mIRate; }
+		uint32_t cMask() const { return mCMask; }
+		uint32_t stateTable(unsigned g, unsigned i) const { return mStateTable[g][i]; }
+		unsigned deferral() const { return mDeferral; }
+		
+
+		ViterbiR2O4();
+
+		/** Set all cost metrics to zero. */
+		void initializeStates();
+
+		/**
+			Full cycle of the Viterbi algorithm: branch, metrics, prune, select.
+			@return reference to minimum-cost candidate.
+		*/
+		const vCand* vstep(uint32_t inSample, const float *probs, const float *iprobs, bool isNotTailBits);
+
+	private:
+
+		/** Branch survivors into new candidates. */
+		void branchCandidates();
+
+		/** Compute cost metrics for soft-inputs. */
+		void getSoftCostMetrics(uint32_t inSample, const float *probs, const float *iprobs);
+
+		/** Select survivors from the candidate set. */
+		void pruneCandidates();
+
+		/** Find the minimum cost survivor. */
+		const vCand& minCost() const;
+
+		/**
+			Precompute the state tables.
+			@param g Generator index 0..((1/rate)-1)
+		*/
+		void computeStateTables(unsigned g);
+
+		/**
+			Precompute the generator outputs.
+			mCoeffs must be defined first.
+		*/
+		void computeGeneratorTable();
+
+	public:
+		void encode(const BitVector &in, BitVector& target) const;
+		void decode(const SoftVector &in, BitVector& target);
+		int getBEC() { return mBitErrorCnt; }
+};
+#endif