IndirectContext.hpp

#pragma once

#include "Array.hpp"
#include <cassert>
#include <cstdint>

template<typename T>
class IndirectContext {
private:
  Array<T> data;
  T *ctx;
  const uint32_t ctxMask, inputMask, inputBits, contextBits;

public:
  IndirectContext(const int bitsPerContext, const int inputBits, const int contextBits = sizeof(T)*8) :
    data(UINT64_C(1) << bitsPerContext), ctx(&data[0]),
    ctxMask((UINT32_C(1) << bitsPerContext) - 1), 
    inputMask((UINT32_C(1) << inputBits) - 1), 
    inputBits(inputBits),
    contextBits(contextBits) {
    assert(bitsPerContext > 0 && bitsPerContext <= 20);
    assert(inputBits > 0 && inputBits <= 8);
    assert(contextBits <= sizeof(T)*8);
    if (contextBits < sizeof(T) * 8) // need for leading bit -> include it
      reset(); 
  }

  void reset() {
    for (uint64_t i = 0; i < data.size(); ++i) {
      data[i] = contextBits < sizeof(T) * 8 ? 1 : 0; // 1: leading bit to indicate the number of used bits
    }
  }

  void operator+=(const uint32_t i) {
    assert(i <= inputMask);
    // note: when the context is fully mature, we need to keep the leading bit in front of the contextbits as the MSB
    T leadingBit = (*ctx) & (1 << contextBits); 
    (*ctx) <<= inputBits;
    (*ctx) |= i | leadingBit;
    (*ctx) &= (1 << (contextBits + 1)) - 1;
  };

  void operator=(const uint32_t i) {
    ctx = &data[i & ctxMask];
  }

  T& operator()() {
    return *ctx;
  };
};