/* Copyright 2013 Google Inc. All Rights Reserved.

   Distributed under MIT license.

   See file LICENSE for detail or copy at https://opensource.org/licenses/MIT

*/

/* Literal cost model to allow backward reference replacement to be efficient.

*/

#include "./literal_cost.h"

#include <brotli/types.h>

#include "./fast_log.h"

#include "./port.h"

#include "./utf8_util.h"

#if defined(__cplusplus) || defined(c_plusplus)

extern "C" {

#endif

static size_t UTF8Position(size_t last, size_t c, size_t clamp) {

  if (c < 128) {

    return 0;  /* Next one is the 'Byte 1' again. */

  } else if (c >= 192) {  /* Next one is the 'Byte 2' of utf-8 encoding. */

    return BROTLI_MIN(size_t, 1, clamp);

  } else {

    /* Let's decide over the last byte if this ends the sequence. */

    if (last < 0xe0) {

      return 0;  /* Completed two or three byte coding. */

    } else {  /* Next one is the 'Byte 3' of utf-8 encoding. */

      return BROTLI_MIN(size_t, 2, clamp);

    }

  }

}

static size_t DecideMultiByteStatsLevel(size_t pos, size_t len, size_t mask,

                                        const uint8_t *data) {

  size_t counts[3] = { 0 };

  size_t max_utf8 = 1;  /* should be 2, but 1 compresses better. */

  size_t last_c = 0;

  size_t i;

  for (i = 0; i < len; ++i) {

    size_t c = data[(pos + i) & mask];

    ++counts[UTF8Position(last_c, c, 2)];

    last_c = c;

  }

  if (counts[2] < 500) {

    max_utf8 = 1;

  }

  if (counts[1] + counts[2] < 25) {

    max_utf8 = 0;

  }

  return max_utf8;

}

static void EstimateBitCostsForLiteralsUTF8(size_t pos, size_t len, size_t mask,

                                            const uint8_t *data, float *cost) {

  /* max_utf8 is 0 (normal ASCII single byte modeling),

     1 (for 2-byte UTF-8 modeling), or 2 (for 3-byte UTF-8 modeling). */

  const size_t max_utf8 = DecideMultiByteStatsLevel(pos, len, mask, data);

  size_t histogram[3][256] = { { 0 } };

  size_t window_half = 495;

  size_t in_window = BROTLI_MIN(size_t, window_half, len);

  size_t in_window_utf8[3] = { 0 };

  size_t i;

  {  /* Bootstrap histograms. */

    size_t last_c = 0;

    size_t utf8_pos = 0;

    for (i = 0; i < in_window; ++i) {

      size_t c = data[(pos + i) & mask];

      ++histogram[utf8_pos][c];

      ++in_window_utf8[utf8_pos];

      utf8_pos = UTF8Position(last_c, c, max_utf8);

      last_c = c;

    }

  }

  /* Compute bit costs with sliding window. */

  for (i = 0; i < len; ++i) {

    if (i >= window_half) {

      /* Remove a byte in the past. */

      size_t c =

          i < window_half + 1 ? 0 : data[(pos + i - window_half - 1) & mask];

      size_t last_c =

          i < window_half + 2 ? 0 : data[(pos + i - window_half - 2) & mask];

      size_t utf8_pos2 = UTF8Position(last_c, c, max_utf8);

      --histogram[utf8_pos2][data[(pos + i - window_half) & mask]];

      --in_window_utf8[utf8_pos2];

    }

    if (i + window_half < len) {

      /* Add a byte in the future. */

      size_t c = data[(pos + i + window_half - 1) & mask];

      size_t last_c = data[(pos + i + window_half - 2) & mask];

      size_t utf8_pos2 = UTF8Position(last_c, c, max_utf8);

      ++histogram[utf8_pos2][data[(pos + i + window_half) & mask]];

      ++in_window_utf8[utf8_pos2];

    }

    {

      size_t c = i < 1 ? 0 : data[(pos + i - 1) & mask];

      size_t last_c = i < 2 ? 0 : data[(pos + i - 2) & mask];

      size_t utf8_pos = UTF8Position(last_c, c, max_utf8);

      size_t masked_pos = (pos + i) & mask;

      size_t histo = histogram[utf8_pos][data[masked_pos]];

      double lit_cost;

      if (histo == 0) {

        histo = 1;

      }

      lit_cost = FastLog2(in_window_utf8[utf8_pos]) - FastLog2(histo);

      lit_cost += 0.02905;

      if (lit_cost < 1.0) {

        lit_cost *= 0.5;

        lit_cost += 0.5;

      }

      /* Make the first bytes more expensive -- seems to help, not sure why.

         Perhaps because the entropy source is changing its properties

         rapidly in the beginning of the file, perhaps because the beginning

         of the data is a statistical "anomaly". */

      if (i < 2000) {

        lit_cost += 0.7 - ((double)(2000 - i) / 2000.0 * 0.35);

      }

      cost[i] = (float)lit_cost;

    }

  }

}

void BrotliEstimateBitCostsForLiterals(size_t pos, size_t len, size_t mask,

                                       const uint8_t *data, float *cost) {

  if (BrotliIsMostlyUTF8(data, pos, mask, len, kMinUTF8Ratio)) {

    EstimateBitCostsForLiteralsUTF8(pos, len, mask, data, cost);

    return;

  } else {

    size_t histogram[256] = { 0 };

    size_t window_half = 2000;

    size_t in_window = BROTLI_MIN(size_t, window_half, len);

    /* Bootstrap histogram. */

    size_t i;

    for (i = 0; i < in_window; ++i) {

      ++histogram[data[(pos + i) & mask]];

    }

    /* Compute bit costs with sliding window. */

    for (i = 0; i < len; ++i) {

      size_t histo;

      if (i >= window_half) {

        /* Remove a byte in the past. */

        --histogram[data[(pos + i - window_half) & mask]];

        --in_window;

      }

      if (i + window_half < len) {

        /* Add a byte in the future. */

        ++histogram[data[(pos + i + window_half) & mask]];

        ++in_window;

      }

      histo = histogram[data[(pos + i) & mask]];

      if (histo == 0) {

        histo = 1;

      }

      {

        double lit_cost = FastLog2(in_window) - FastLog2(histo);

        lit_cost += 0.029;

        if (lit_cost < 1.0) {

          lit_cost *= 0.5;

          lit_cost += 0.5;

        }

        cost[i] = (float)lit_cost;

      }

    }

  }

}

#if defined(__cplusplus) || defined(c_plusplus)

}  /* extern "C" */

#endif