user: What is the c source code of the assembly code below:

```asm
<qbase64_decode>:
  endbr64
  push   %rbx
  sub    $0x110,%rsp
  mov    %fs:0x28,%rax
  mov    %rax,0x108(%rsp)
  xor    %eax,%eax
  movabs $0x4040404040404040,%rax
  movabs $0x4040404040404040,%rdx
  mov    %rax,(%rsp)
  mov    %rdx,0x8(%rsp)
  mov    %rax,0x10(%rsp)
  mov    %rdx,0x18(%rsp)
  movabs $0x4040404040404040,%rcx
  movabs $0x3f4040403e404040,%rbx
  mov    %rcx,0x20(%rsp)
  mov    %rbx,0x28(%rsp)
  movabs $0x3b3a393837363534,%rcx
  movabs $0x4040404040403d3c,%rbx
  mov    %rcx,0x30(%rsp)
  mov    %rbx,0x38(%rsp)
  movabs $0x605040302010040,%rcx
  movabs $0xe0d0c0b0a090807,%rbx
  mov    %rcx,0x40(%rsp)
  mov    %rbx,0x48(%rsp)
  movabs $0x161514131211100f,%rcx
  movabs $0x4040404040191817,%rbx
  mov    %rcx,0x50(%rsp)
  mov    %rbx,0x58(%rsp)
  movabs $0x201f1e1d1c1b1a40,%rcx
  movabs $0x2827262524232221,%rbx
  mov    %rcx,0x60(%rsp)
  mov    %rbx,0x68(%rsp)
  movabs $0x302f2e2d2c2b2a29,%rcx
  movabs $0x4040404040333231,%rbx
  mov    %rcx,0x70(%rsp)
  mov    %rbx,0x78(%rsp)
  mov    %rax,0x80(%rsp)
  mov    %rdx,0x88(%rsp)
  mov    %rax,0x90(%rsp)
  mov    %rdx,0x98(%rsp)
  mov    %rax,0xa0(%rsp)
  mov    %rdx,0xa8(%rsp)
  mov    %rax,0xb0(%rsp)
  mov    %rdx,0xb8(%rsp)
  mov    %rax,0xc0(%rsp)
  mov    %rdx,0xc8(%rsp)
  mov    %rax,0xd0(%rsp)
  mov    %rdx,0xd8(%rsp)
  mov    %rax,0xe0(%rsp)
  mov    %rdx,0xe8(%rsp)
  mov    %rax,0xf0(%rsp)
  mov    %rdx,0xf8(%rsp)
  movzbl (%rdi),%edx
  test   %dl,%dl
  je     L102
  mov    %rdi,%rax
  mov    %rdi,%rcx
  mov    $0x0,%esi
  mov    $0x0,%ebx
  mov    $0x1,%r8d
  mov    $0x0,%r9d
  mov    $0x3,%r11d
  mov    $0x2,%r10d
  jmp    L7
L31:
  shl    $0x2,%esi
  mov    %edx,%ebx
  sar    $0x4,%bl
  or     %ebx,%esi
  mov    %sil,(%rax)
  mov    %edx,%esi
  lea    0x1(%rax),%rax
  mov    %r10d,%ebx
  jmp    L83
L77:
  shl    $0x4,%esi
  mov    %edx,%ebx
  sar    $0x2,%bl
  or     %ebx,%esi
  mov    %sil,(%rax)
  mov    %edx,%esi
  lea    0x1(%rax),%rax
  mov    %r11d,%ebx
  jmp    L83
L34:
  mov    %edx,%esi
  mov    %r8d,%ebx
L83:
  add    $0x1,%rcx
  movzbl (%rcx),%edx
  test   %dl,%dl
  je     L106
L7:
  movzbl %dl,%edx
  movzbl (%rsp,%rdx,1),%edx
  cmp    $0x40,%dl
  je     L83
  test   %ebx,%ebx
  je     L34
  cmp    $0x1,%ebx
  je     L31
  cmp    $0x2,%ebx
  je     L77
  shl    $0x6,%esi
  or     %edx,%esi
  mov    %sil,(%rax)
  mov    %edx,%esi
  lea    0x1(%rax),%rax
  mov    %r9d,%ebx
  jmp    L83
L102:
  mov    %rdi,%rax
L106:
  movb   $0x0,(%rax)
  sub    %rdi,%rax
  mov    0x108(%rsp),%rdx
  sub    %fs:0x28,%rdx
  jne    L38
  add    $0x110,%rsp
  pop    %rbx
  ret
L38:
  call   <__stack_chk_fail@plt>
```

assistant: ```c
size_t qbase64_decode(char *str) {
  const char B64MAPTBL[16 * 16] = {64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 62, 64, 64, 64, 63, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 64, 64, 64, 64, 64, 64, 64, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 64, 64, 64, 64, 64, 64, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64};
  char *pEncPt, *pBinPt = str;
  int nIdxOfFour = 0;
  char cLastByte = 0;
  for (pEncPt = str; *pEncPt != '\0'; pEncPt++) {
    char cByte = B64MAPTBL[(unsigned char)(*pEncPt)];
    if (cByte == 64)
      continue;
    if (nIdxOfFour == 0) {
      nIdxOfFour++;
    } else if (nIdxOfFour == 1) {
      *pBinPt++ = ((cLastByte << 2) | (cByte >> 4));
      nIdxOfFour++;
    } else if (nIdxOfFour == 2) {
      *pBinPt++ = ((cLastByte << 4) | (cByte >> 2));
      nIdxOfFour++;
    } else {
      *pBinPt++ = ((cLastByte << 6) | cByte);
      nIdxOfFour = 0;
    }
    cLastByte = cByte;
  }
  *pBinPt = '\0';
  return (pBinPt - str);
}
```