// SPDX-FileCopyrightText: Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
// SPDX-License-Identifier: BSD-3-Clause

#include "vtkSegYIOUtils.h"

#include <cmath>
#include <cstdint>
#include <cstring>
#include <sys/types.h>

//------------------------------------------------------------------------------
VTK_ABI_NAMESPACE_BEGIN
vtkSegYIOUtils::vtkSegYIOUtils()
{
  this->IsBigEndian = checkIfBigEndian();
}

//------------------------------------------------------------------------------
vtkSegYIOUtils* vtkSegYIOUtils::Instance()
{
  static vtkSegYIOUtils vtkSegYIOUtils;
  return &vtkSegYIOUtils;
}

//------------------------------------------------------------------------------
short vtkSegYIOUtils::readShortInteger(std::streamoff pos, std::istream& in)
{
  in.seekg(pos, std::istream::beg);
  return readShortInteger(in);
}

//------------------------------------------------------------------------------
short vtkSegYIOUtils::readShortInteger(std::istream& in)
{
  char buffer[2];
  in.read(buffer, sizeof(buffer));

  if (!this->IsBigEndian)
  {
    swap(buffer, buffer + 1);
  }

  short num;
  memcpy(&num, buffer, 2);
  return num;
}

//------------------------------------------------------------------------------
int vtkSegYIOUtils::readLongInteger(std::streamoff pos, std::istream& in)
{
  in.seekg(pos, std::istream::beg);
  return readLongInteger(in);
}

//------------------------------------------------------------------------------
int vtkSegYIOUtils::readLongInteger(std::istream& in)
{
  char buffer[4];
  in.read(buffer, sizeof(buffer));

  if (!this->IsBigEndian)
  {
    swap(buffer, buffer + 3);
    swap(buffer + 1, buffer + 2);
  }

  int num;
  memcpy(&num, buffer, 4);
  return num;
}

//------------------------------------------------------------------------------
float vtkSegYIOUtils::readFloat(std::istream& in)
{
  char buffer[4];
  in.read(buffer, sizeof(buffer));

  if (!this->IsBigEndian)
  {
    swap(buffer, buffer + 3);
    swap(buffer + 1, buffer + 2);
  }

  float num;
  memcpy(&num, buffer, 4);
  return num;
}

//------------------------------------------------------------------------------
float vtkSegYIOUtils::readIBMFloat(std::istream& in)
{
  char buffer[4];
  in.read(buffer, sizeof(buffer));

  if (!this->IsBigEndian)
  {
    swap(buffer, buffer + 3);
    swap(buffer + 1, buffer + 2);
  }

  // The IBM Hex single precision floating point representation:
  //
  //  1      7                           24                    (width in bits)
  // +-+----------------+-----------------------------------------+
  // |S|   Exponent     |                Fraction                 |
  // +-+----------------+-----------------------------------------+
  // 31 30           24 23                                        0 (bit index)
  //
  //     Value = (-1^S) (0.F) (16^(E - 64))
  //
  // Key points:
  // - S = sign: 0 = Positive, 1 = Negative
  // - Exponent = power of 16 with a bias of 64
  // - Fraction = Normalized F portion of 24 bit fraction 0.F
  // - Value = 0 if F = 0
  // More details at
  // https://en.m.wikipedia.org/wiki/IBM_Floating_Point_Architecture

  const uint32_t* longbuffer = reinterpret_cast<uint32_t*>(buffer);
  int sign = longbuffer[0] >> 31 & 0x01;
  int exponent = longbuffer[0] >> 24 & 0x7F;
  float fraction = (longbuffer[0] & 0x00ffffff) / powf(2.0f, 24.0f);
  if (fraction == 0.0f)
  {
    // Value is 0
    return 0.0f;
  }
  float num = (1 - 2 * sign) * fraction * powf(16.0f, exponent - 64.0f);
  return num;
}

//------------------------------------------------------------------------------
char vtkSegYIOUtils::readChar(std::istream& in)
{
  char buffer;
  in.read(&buffer, sizeof(buffer));
  return buffer;
}

//------------------------------------------------------------------------------
unsigned char vtkSegYIOUtils::readUChar(std::istream& in)
{
  char buffer;
  in.read(&buffer, sizeof(buffer));
  return buffer;
}

//------------------------------------------------------------------------------
void vtkSegYIOUtils::swap(char* a, char* b) noexcept
{
  char temp = *a;
  *a = *b;
  *b = temp;
}

//------------------------------------------------------------------------------
std::streamoff vtkSegYIOUtils::getFileSize(std::istream& in)
{
  in.seekg(0, std::istream::end);
  return in.tellg();
}
VTK_ABI_NAMESPACE_END
