#include <cstdint>
#include <cstdio>
#include <cstring>
#include <cstdlib>

namespace crc32
{
  void generate_table(uint32_t(&table)[256])
  {
    for (uint32_t i = 0; i < 256; i++)
    {
      auto c = i;
      for (size_t j = 0; j < 8; j++)
        if (c & 1)
          c = 0xEDB88320 ^ (c >> 1);
        else
          c >>= 1;
      table[i] = c;
    }
  }

  static uint32_t update(uint32_t(&table)[256], uint32_t initial, const void* buf, size_t len)
  {
    auto c = initial ^ 0xFFFFFFFF;
    const auto* u = static_cast<const uint8_t*>(buf);
    for (size_t i = 0; i < len; ++i)
    {
      c = table[(c ^ u[i]) & 0xFF] ^ (c >> 8);
    }
    return c ^ 0xFFFFFFFF;
  }
};

int main(int argc, char** argv)
{
  const auto fp = fopen(argv[1], "rb");
  fseek(fp, 0, SEEK_END);
  const auto sz = ftell(fp);
  rewind(fp);
  const auto png = new std::uint8_t[sz];
  fread(png, 1, sz, fp);
  fclose(fp);
  auto it = png + sz - 4;
  while (*(uint32_t*)it != 'DNEI')
    --it;
  const auto new_sz = atoll(argv[3]);
  const auto new_png = new std::uint8_t[new_sz];
  memset(new_png, 'a', new_sz);
  memcpy(new_png, png, it - png);
  auto inv = (uint32_t*)(it - png + new_png);
  *inv++ = new_sz - sz - 12;
  *inv++ = 'iTXt';
  const auto start = inv;
  (uint8_t*&)inv += new_sz - sz - 12;
  uint32_t table[256];
  crc32::generate_table(table);
  *inv = crc32::update(table, 0, start, (char*)inv - (char*)start);
  memcpy(new_png + new_sz - (png + sz - it), it, png + sz - it);
  const auto ofp = fopen(argv[2], "wb");
  fwrite(new_png, new_sz, 1, ofp);
  fclose(fp);
  delete[] png;
  delete[] new_png;
}