// Copyright (c) the JPEG XL Project Authors. All rights reserved.

//

// Use of this source code is governed by a BSD-style

// license that can be found in the LICENSE file.

#ifndef LIB_JXL_LEHMER_CODE_H_

#define LIB_JXL_LEHMER_CODE_H_

#include <stddef.h>

#include <stdint.h>

#include "lib/jxl/base/bits.h"

#include "lib/jxl/base/compiler_specific.h"

#include "lib/jxl/base/status.h"

namespace jxl {

// Permutation <=> factorial base representation (Lehmer code).

using LehmerT = uint32_t;

template <typename T>

constexpr T ValueOfLowest1Bit(T t) {

  return t & -t;

}

unsigned int jxl::ValueOfLowest1Bit<unsigned int>(unsigned int)

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constexpr T ValueOfLowest1Bit(T t) {

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  return t & -t;

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}

int jxl::ValueOfLowest1Bit<int>(int)

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constexpr T ValueOfLowest1Bit(T t) {

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  return t & -t;

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}

unsigned long jxl::ValueOfLowest1Bit<unsigned long>(unsigned long)

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constexpr T ValueOfLowest1Bit(T t) {

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  return t & -t;

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}

// Computes the Lehmer (factorial basis) code of permutation, an array of n

// unique indices in [0..n), and stores it in code[0..len). N*logN time.

// temp must have n + 1 elements but need not be initialized.

template <typename PermutationT>

Status ComputeLehmerCode(const PermutationT* JXL_RESTRICT permutation,

                         uint32_t* JXL_RESTRICT temp, const size_t n,

                         LehmerT* JXL_RESTRICT code) {

  for (size_t idx = 0; idx < n + 1; ++idx) temp[idx] = 0;

  for (size_t idx = 0; idx < n; ++idx) {

    const PermutationT s = permutation[idx];

    // Compute sum in Fenwick tree

    uint32_t penalty = 0;

    uint32_t i = s + 1;

    while (i != 0) {

      penalty += temp[i];

      i &= i - 1;  // clear lowest bit

    }

    JXL_ENSURE(s >= penalty);

    code[idx] = s - penalty;

    i = s + 1;

    // Add operation in Fenwick tree

    while (i < n + 1) {

      temp[i] += 1;

      i += ValueOfLowest1Bit(i);

    }

  }

  return true;

}

// Decodes the Lehmer code in code[0..n) into permutation[0..n).

// temp must have 1 << CeilLog2(n) elements but need not be initialized.

template <typename PermutationT>

Status DecodeLehmerCode(const LehmerT* JXL_RESTRICT code,

                        uint32_t* JXL_RESTRICT temp, size_t n,

                        PermutationT* JXL_RESTRICT permutation) {

  JXL_ENSURE(n != 0);

  const size_t log2n = CeilLog2Nonzero(n);

  const size_t padded_n = 1ull << log2n;

  for (size_t i = 0; i < padded_n; i++) {

    const int32_t i1 = static_cast<int32_t>(i + 1);

    temp[i] = static_cast<uint32_t>(ValueOfLowest1Bit(i1));

  }

  for (size_t i = 0; i < n; i++) {

    JXL_ENSURE(code[i] + i < n);

    uint32_t rank = code[i] + 1;

    // Extract i-th unused element via implicit order-statistics tree.

    size_t bit = padded_n;

    size_t next = 0;

    for (size_t b = 0; b <= log2n; b++) {

      const size_t cand = next + bit;

      JXL_ENSURE(cand >= 1);

      bit >>= 1;

      if (temp[cand - 1] < rank) {

        next = cand;

        rank -= temp[cand - 1];

      }

    }

    permutation[i] = next;

    // Mark as used

    next += 1;

    while (next <= padded_n) {

      temp[next - 1] -= 1;

      next += ValueOfLowest1Bit(next);

    }

  }

  return true;

}

}  // namespace jxl

#endif  // LIB_JXL_LEHMER_CODE_H_