lzfse-4591228/src/lzfse_fse.c

/*
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    in the documentation and/or other materials provided with the distribution.

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    from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
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*/

#include "lzfse_internal.h"

// Initialize encoder table T[NSYMBOLS].
// NSTATES = sum FREQ[i] is the number of states (a power of 2)
// NSYMBOLS is the number of symbols.
// FREQ[NSYMBOLS] is a normalized histogram of symbol frequencies, with FREQ[i]
// >= 0.
// Some symbols may have a 0 frequency.  In that case, they should not be
// present in the data.
void fse_init_encoder_table(int nstates, int nsymbols,
                            const uint16_t *__restrict freq,
                            fse_encoder_entry *__restrict t) {
  int offset = 0; // current offset
  int n_clz = __builtin_clz(nstates);
  for (int i = 0; i < nsymbols; i++) {
    int f = (int)freq[i];
    if (f == 0)
      continue; // skip this symbol, no occurrences
    int k =
        __builtin_clz(f) - n_clz; // shift needed to ensure N <= (F<<K) < 2*N
    t[i].s0 = (int16_t)((f << k) - nstates);
    t[i].k = (int16_t)k;
    t[i].delta0 = (int16_t)(offset - f + (nstates >> k));
    t[i].delta1 = (int16_t)(offset - f + (nstates >> (k - 1)));
    offset += f;
  }
}

// Initialize decoder table T[NSTATES].
// NSTATES = sum FREQ[i] is the number of states (a power of 2)
// NSYMBOLS is the number of symbols.
// FREQ[NSYMBOLS] is a normalized histogram of symbol frequencies, with FREQ[i]
// >= 0.
// Some symbols may have a 0 frequency.  In that case, they should not be
// present in the data.
int fse_init_decoder_table(int nstates, int nsymbols,
                           const uint16_t *__restrict freq,
                           int32_t *__restrict t) {
  assert(nsymbols <= 256);
  assert(fse_check_freq(freq, nsymbols, nstates) == 0);
  int n_clz = __builtin_clz(nstates);
  int sum_of_freq = 0;
  for (int i = 0; i < nsymbols; i++) {
    int f = (int)freq[i];
    if (f == 0)
      continue; // skip this symbol, no occurrences

    sum_of_freq += f;

    if (sum_of_freq > nstates) {
      return -1;
    }

    int k =
        __builtin_clz(f) - n_clz; // shift needed to ensure N <= (F<<K) < 2*N
    int j0 = ((2 * nstates) >> k) - f;

    // Initialize all states S reached by this symbol: OFFSET <= S < OFFSET + F
    for (int j = 0; j < f; j++) {
      fse_decoder_entry e;

      e.symbol = (uint8_t)i;
      if (j < j0) {
        e.k = (int8_t)k;
        e.delta = (int16_t)(((f + j) << k) - nstates);
      } else {
        e.k = (int8_t)(k - 1);
        e.delta = (int16_t)((j - j0) << (k - 1));
      }

      memcpy(t, &e, sizeof(e));
      t++;
    }
  }

  return 0; // OK
}

// Initialize value decoder table T[NSTATES].
// NSTATES = sum FREQ[i] is the number of states (a power of 2)
// NSYMBOLS is the number of symbols.
// FREQ[NSYMBOLS] is a normalized histogram of symbol frequencies, with FREQ[i]
// >= 0.
// SYMBOL_VBITS[NSYMBOLS] and SYMBOLS_VBASE[NSYMBOLS] are the number of value
// bits to read and the base value for each symbol.
// Some symbols may have a 0 frequency.  In that case, they should not be
// present in the data.
void fse_init_value_decoder_table(int nstates, int nsymbols,
                                  const uint16_t *__restrict freq,
                                  const uint8_t *__restrict symbol_vbits,
                                  const int32_t *__restrict symbol_vbase,
                                  fse_value_decoder_entry *__restrict t) {
  assert(nsymbols <= 256);
  assert(fse_check_freq(freq, nsymbols, nstates) == 0);

  int n_clz = __builtin_clz(nstates);
  for (int i = 0; i < nsymbols; i++) {
    int f = (int)freq[i];
    if (f == 0)
      continue; // skip this symbol, no occurrences

    int k =
        __builtin_clz(f) - n_clz; // shift needed to ensure N <= (F<<K) < 2*N
    int j0 = ((2 * nstates) >> k) - f;

    fse_value_decoder_entry ei = {0};
    ei.value_bits = symbol_vbits[i];
    ei.vbase = symbol_vbase[i];

    // Initialize all states S reached by this symbol: OFFSET <= S < OFFSET + F
    for (int j = 0; j < f; j++) {
      fse_value_decoder_entry e = ei;

      if (j < j0) {
        e.total_bits = (uint8_t)k + e.value_bits;
        e.delta = (int16_t)(((f + j) << k) - nstates);
      } else {
        e.total_bits = (uint8_t)(k - 1) + e.value_bits;
        e.delta = (int16_t)((j - j0) << (k - 1));
      }

      memcpy(t, &e, 8);
      t++;
    }
  }
}

// Remove states from symbols until the correct number of states is used.
static void fse_adjust_freqs(uint16_t *freq, int overrun, int nsymbols) {
  for (int shift = 3; overrun != 0; shift--) {
    for (int sym = 0; sym < nsymbols; sym++) {
      if (freq[sym] > 1) {
        int n = (freq[sym] - 1) >> shift;
        if (n > overrun)
          n = overrun;
        freq[sym] -= n;
        overrun -= n;
        if (overrun == 0)
          break;
      }
    }
  }
}

// Normalize a table T[NSYMBOLS] of occurrences to FREQ[NSYMBOLS].
void fse_normalize_freq(int nstates, int nsymbols, const uint32_t *__restrict t,
                        uint16_t *__restrict freq) {
  uint32_t s_count = 0;
  int remaining = nstates; // must be signed; this may become < 0
  int max_freq = 0;
  int max_freq_sym = 0;
  int shift = __builtin_clz(nstates) - 1;
  uint32_t highprec_step;

  // Compute the total number of symbol occurrences
  for (int i = 0; i < nsymbols; i++)
    s_count += t[i];

  if (s_count == 0)
    highprec_step = 0; // no symbols used
  else
    highprec_step = ((uint32_t)1 << 31) / s_count;

  for (int i = 0; i < nsymbols; i++) {

    // Rescale the occurrence count to get the normalized frequency.
    // Round up if the fractional part is >= 0.5; otherwise round down.
    // For efficiency, we do this calculation using integer arithmetic.
    int f = (((t[i] * highprec_step) >> shift) + 1) >> 1;

    // If a symbol was used, it must be given a nonzero normalized frequency.
    if (f == 0 && t[i] != 0)
      f = 1;

    freq[i] = f;
    remaining -= f;

    // Remember the maximum frequency and which symbol had it.
    if (f > max_freq) {
      max_freq = f;
      max_freq_sym = i;
    }
  }

  // If there remain states to be assigned, then just assign them to the most
  // frequent symbol.  Alternatively, if we assigned more states than were
  // actually available, then either remove states from the most frequent symbol
  // (for minor overruns) or use the slower adjustment algorithm (for major
  // overruns).
  if (-remaining < (max_freq >> 2)) {
    freq[max_freq_sym] += remaining;
  } else {
    fse_adjust_freqs(freq, -remaining, nsymbols);
  }
}