polar_sequence.hh

/*
Construct reliability sequences for polar codes

Copyright 2023 Ahmet Inan <inan@aicodix.de>
*/

#pragma once

#include <algorithm>

namespace CODE {

template <int MAX_M>
class PolarSeqConst0
{
	void compute(double pe, int i, int h)
	{
		if (h) {
			compute(pe * (2-pe), i, h/2);
			compute(pe * pe, i+h, h/2);
		} else {
			prob[i] = pe;
		}
	}
	double prob[1<<MAX_M];
	int index[1<<MAX_M];
public:
	void operator()(int *sequence, int level, double erasure_probability = std::exp(-1.))
	{
		assert(level <= MAX_M);
		int length = 1 << level;
		compute(erasure_probability, 0, length / 2);
		for (int i = 0; i < length; ++i)
			sequence[i] = i;
		std::sort(sequence, sequence+length, [this](int a, int b){ return prob[a] > prob[b]; });
	}
};

}