user: Converte this assembly snippet into its equivalent C code:

```asm
<decode_base64>:
  endbr64
  push   %rbp
  mov    %rsp,%rbp
  mov    %rdi,-0x18(%rbp)
  movl   $0x0,-0x8(%rbp)
  movl   $0x0,-0x4(%rbp)
  jmp    L16
L4:
  mov    -0x4(%rbp),%edx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  mov    $0x40,%edx
  mov    %edx,%ecx
  sub    %eax,%ecx
  mov    -0x4(%rbp),%edx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  sub    $0x5b,%eax
  and    %ecx,%eax
  sar    $0x8,%eax
  mov    %eax,%ecx
  mov    -0x4(%rbp),%edx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  sub    $0x40,%eax
  and    %ecx,%eax
  lea    -0x1(%rax),%ecx
  mov    -0x4(%rbp),%edx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  mov    $0x60,%edx
  mov    %edx,%esi
  sub    %eax,%esi
  mov    -0x4(%rbp),%edx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  sub    $0x7b,%eax
  and    %esi,%eax
  sar    $0x8,%eax
  mov    %eax,%esi
  mov    -0x4(%rbp),%edx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  sub    $0x46,%eax
  and    %esi,%eax
  add    %eax,%ecx
  mov    -0x4(%rbp),%edx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  mov    $0x2f,%edx
  mov    %edx,%esi
  sub    %eax,%esi
  mov    -0x4(%rbp),%edx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  sub    $0x3a,%eax
  and    %esi,%eax
  sar    $0x8,%eax
  mov    %eax,%esi
  mov    -0x4(%rbp),%edx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  add    $0x5,%eax
  and    %esi,%eax
  add    %eax,%ecx
  mov    -0x4(%rbp),%edx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  mov    $0x2a,%edx
  mov    %edx,%esi
  sub    %eax,%esi
  mov    -0x4(%rbp),%edx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  sub    $0x2c,%eax
  and    %esi,%eax
  sar    $0x8,%eax
  and    $0x3f,%eax
  add    %eax,%ecx
  mov    -0x4(%rbp),%edx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  mov    $0x2e,%edx
  mov    %edx,%esi
  sub    %eax,%esi
  mov    -0x4(%rbp),%edx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  sub    $0x30,%eax
  and    %esi,%eax
  sar    $0x8,%eax
  and    $0x40,%eax
  lea    (%rcx,%rax,1),%esi
  mov    -0x4(%rbp),%edx
  mov    %edx,%eax
  add    %eax,%eax
  add    %edx,%eax
  add    %eax,%eax
  mov    %eax,%edx
  mov    $0x12,%eax
  sub    %edx,%eax
  mov    %eax,%ecx
  shl    %cl,%esi
  mov    %esi,%eax
  or     %eax,-0x8(%rbp)
  addl   $0x1,-0x4(%rbp)
L16:
  cmpl   $0x3,-0x4(%rbp)
  jbe    L4
  mov    -0x8(%rbp),%eax
  pop    %rbp
  ret
```

assistant: ```c
int decode_base64(const char src[4]) {
  int val = 0;
  for (unsigned int i = 0; i < 4; ++i)
    val |= (-1 + ((((('A' - 1) - src[i]) & (src[i] - ('Z' + 1))) >> 8) & (src[i] - 64)) + ((((('a' - 1) - src[i]) & (src[i] - ('z' + 1))) >> 8) & (src[i] - 70)) + ((((('0' - 1) - src[i]) & (src[i] - ('9' + 1))) >> 8) & (src[i] + 5)) + ((((('+' - 1) - src[i]) & (src[i] - ('+' + 1))) >> 8) & 63) + ((((('/' - 1) - src[i]) & (src[i] - ('/' + 1))) >> 8) & 64)) << (18 - 6 * i);
  return val;
}
```