Repo created

libs/coding/simple_dense_coding.hpp

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+#pragma once
+
+#include <cstdint>
+#include <vector>
+
+#if defined(__clang__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wunused-private-field"
+#endif
+
+#include "3party/succinct/elias_fano_compressed_list.hpp"
+
+#if defined(__clang__)
+#pragma clang diagnostic pop
+#endif
+
+namespace coding
+{
+// This class represents a variant of a so-called simple dense coding
+// scheme for byte strings.  It can be used when it's necessary to
+// compress strings with skewed entropy and nevertheless efficient
+// access to the string's elements is needed.
+//
+// The main idea is to assign codewords from the set { 0, 1, 00, 01,
+// 10, 11, 000, ... } to string's symbols in accordance with their
+// frequencies and to create a helper bit-vector for starting
+// positions of the codewords in compressed string.
+//
+// Memory complexity: 2 * n * (H_0(T) + 1) bits for a text T, but note
+// that this is an upper bound and too pessimistic.
+
+// Time complexity: O(log(n * H_0(T))) to access i-th element of the
+// string, because of logarithmic complexity of
+// rs_bit_vector::select. This will be fixed when RRR will be
+// implemented.
+//
+// For details, see Kimmo Fredriksson, Fedor Nikitin, "Simple Random
+// Access Compression", Fundamenta Informaticae 2009,
+// http://www.cs.uku.fi/~fredriks/pub/papers/fi09.pdf.
+class SimpleDenseCoding
+{
+public:
+  SimpleDenseCoding() = default;
+
+  SimpleDenseCoding(std::vector<uint8_t> const & data);
+
+  SimpleDenseCoding(SimpleDenseCoding && rhs);
+
+  uint8_t Get(uint64_t i) const;
+
+  inline uint64_t Size() const { return m_ranks.size(); }
+
+  // map is used here (instead of Map) for compatibility with succinct
+  // structures.
+  template <typename TVisitor>
+  void map(TVisitor & visitor)
+  {
+    visitor(m_ranks, "m_ranks");
+    visitor(m_symbols, "m_symbols");
+  }
+
+private:
+  succinct::elias_fano_compressed_list m_ranks;
+  succinct::mapper::mappable_vector<uint8_t> m_symbols;
+};
+}  // namespace coding