The code snippet contains two functions, `buildIntLittleEndian` and `buildIntBigEndian`, that...

The code snippet contains two functions, buildIntLittleEndian and buildIntBigEndian, that construct a 4-byte int (assuming int is 4 bytes on the platform) from a char array input using either little-endian or big-endian encoding. Here's what each function does in detail:

1. buildIntLittleEndian

This function interprets a sequence of bytes in little-endian order and constructs an integer from them.

int buildIntLittleEndian(const char *input) {
  int res = 0;
  for (int i = 0; i < sizeof(res); ++i) {
    res |= input[i] << (8 * i);
  }
  return res;
}

• How it works:

  • res is initialized to 0.
  • A for loop iterates over each byte of the 4-byte int (or however many bytes sizeof(res) indicates, typically 4).
  • In each iteration:
    • The function takes the i-th byte of input (input[i]).
    • Shifts it left by 8 * i bits to move it into the correct position for the little-endian layout.
    • Combines it into the result res using a bitwise OR operation (|).
  • The first byte (input[0]) is placed in the least significant byte of res, the second byte in the next least significant byte, and so on, consistent with the little-endian order.

• What it does:

  • Constructs an integer by treating the input bytes as if they are stored in little-endian format: least significant byte first.

2. buildIntBigEndian

This function interprets a sequence of bytes in big-endian order and constructs an integer from them.

int buildIntBigEndian(const char *input) {
  int res = 0;
  for (int i = 0; i < sizeof(res); ++i) {
    res <<= 8;
    res |= input[i];
  }
  return res;
}

• How it works:

  • res is initialized to 0.
  • A for loop iterates over each byte of the 4-byte int.
  • In each iteration:
    • The function shifts the existing bits of res left by 8 bits (res <<= 8) to make room for the next byte.
    • Combines the current byte (input[i]) into res using a bitwise OR operation (|).
  • The first byte (input[0]) is placed in the most significant byte of res, the second byte in the next most significant byte, and so on, consistent with the big-endian order.

• What it does:

  • Constructs an integer by treating the input bytes as if they are stored in big-endian format: most significant byte first.

Summary:

1. buildIntLittleEndian:

   • Converts the byte sequence in the input into an integer assuming the bytes are in little-endian order.
   • Example: If input = {0x01, 0x02, 0x03, 0x04}, the result is 0x04030201.

2. buildIntBigEndian:

   • Converts the byte sequence in the input into an integer assuming the bytes are in big-endian order.
   • Example: If input = {0x01, 0x02, 0x03, 0x04}, the result is 0x01020304.