user: 这段汇编代码用C语言表示是什么样的?

```asm
<base64_decode>:
  endbr64
  push   %r15
  push   %r14
  push   %r13
  push   %r12
  push   %rbp
  push   %rbx
  sub    $0x28,%rsp
  mov    %rdi,%r12
  mov    %rsi,%rbp
  mov    %edx,0xc(%rsp)
  mov    %fs:0x28,%rax
  mov    %rax,0x18(%rsp)
  xor    %eax,%eax
  mov    $0x0,%esi
  mov    %rbp,%rdi
  call   <memset@plt>
  mov    %r12,%rdi
  call   <strlen@plt>
  test   %eax,%eax
  je     L11
  mov    %r12,%rbx
  lea    -0x1(%rax),%eax
  lea    0x1(%r12,%rax,1),%r13
  mov    $0x0,%r12d
  lea    0x18(%rsp),%r14
  jmp    L74
L94:
  mov    $0x0,%r12d
L122:
  add    $0x1,%rbx
  cmp    %r13,%rbx
  je     L138
L74:
  mov    %rbx,%r15
  movzbl (%rbx),%edi
  cmp    $0x3d,%dil
  je     L138
  movzbl %dil,%edi
  call   <is_base64@plt>
  test   %rax,%rax
  je     L138
  lea    0x1(%r12),%eax
  movslq %r12d,%r12
  movzbl (%r15),%edx
  mov    %dl,0x14(%rsp,%r12,1)
  mov    %eax,%r12d
  cmp    $0x4,%eax
  jne    L122
  lea    0x14(%rsp),%r12
L33:
  movzbl (%r12),%edi
  call   <base64_chars_find@plt>
  mov    %al,(%r12)
  add    $0x1,%r12
  cmp    %r14,%r12
  jne    L33
  movzbl 0x15(%rsp),%ecx
  mov    %ecx,%edx
  sar    $0x4,%edx
  and    $0x3,%edx
  movzbl 0x14(%rsp),%eax
  lea    (%rdx,%rax,4),%eax
  mov    %al,0x11(%rsp)
  movzbl 0x16(%rsp),%eax
  mov    %eax,%edx
  sar    $0x2,%edx
  and    $0xf,%edx
  shl    $0x4,%ecx
  add    %ecx,%edx
  mov    %dl,0x12(%rsp)
  shl    $0x6,%eax
  add    0x17(%rsp),%al
  mov    %al,0x13(%rsp)
  lea    0x11(%rsp),%r12
  lea    0x14(%rsp),%r15
  mov    0xc(%rsp),%eax
  mov    %rax,(%rsp)
L167:
  mov    %rbp,%rdi
  call   <strlen@plt>
  cmp    (%rsp),%rax
  jae    L94
  mov    %rbp,%rdi
  call   <strlen@plt>
  movzbl (%r12),%edx
  mov    %dl,0x0(%rbp,%rax,1)
  add    $0x1,%r12
  cmp    %r15,%r12
  jne    L167
  mov    $0x0,%r12d
  jmp    L122
L138:
  test   %r12d,%r12d
  je     L11
  cmp    $0x3,%r12d
  jg     L141
  movslq %r12d,%rax
L114:
  movb   $0x0,0x14(%rsp,%rax,1)
  add    $0x1,%rax
  cmp    $0x3,%eax
  jle    L114
L141:
  lea    0x14(%rsp),%rbx
  lea    0x18(%rsp),%r13
L41:
  movzbl (%rbx),%edi
  call   <base64_chars_find@plt>
  mov    %al,(%rbx)
  add    $0x1,%rbx
  cmp    %r13,%rbx
  jne    L41
  movzbl 0x15(%rsp),%ecx
  mov    %ecx,%edx
  sar    $0x4,%edx
  and    $0x3,%edx
  movzbl 0x14(%rsp),%eax
  lea    (%rdx,%rax,4),%eax
  mov    %al,0x11(%rsp)
  movzbl 0x16(%rsp),%eax
  mov    %eax,%edx
  sar    $0x2,%edx
  and    $0xf,%edx
  shl    $0x4,%ecx
  add    %ecx,%edx
  mov    %dl,0x12(%rsp)
  shl    $0x6,%eax
  add    0x17(%rsp),%al
  mov    %al,0x13(%rsp)
  cmp    $0x1,%r12d
  jle    L11
  lea    0x11(%rsp),%rbx
  lea    -0x2(%r12),%eax
  lea    0x12(%rsp,%rax,1),%r12
  mov    0xc(%rsp),%r13d
L106:
  mov    %rbp,%rdi
  call   <strlen@plt>
  cmp    %r13,%rax
  jae    L11
  mov    %rbp,%rdi
  call   <strlen@plt>
  movzbl (%rbx),%edx
  mov    %dl,0x0(%rbp,%rax,1)
  add    $0x1,%rbx
  cmp    %rbx,%r12
  jne    L106
L11:
  mov    %rbp,%rdi
  call   <strlen@plt>
  mov    0x18(%rsp),%rdx
  sub    %fs:0x28,%rdx
  jne    L29
  add    $0x28,%rsp
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
L29:
  call   <__stack_chk_fail@plt>
```

assistant: ```c
size_t base64_decode(unsigned char const *encoded_string, unsigned char *ret, unsigned int retsize) {
  int in_len, i = 0, j = 0, in_ = 0;
  unsigned char char_array_4[4], char_array_3[3];
  memset(ret, 0, retsize);
  in_len = strlen(encoded_string);
  while (in_len-- && (encoded_string[in_] != '=') && is_base64(encoded_string[in_])) {
    char_array_4[i++] = encoded_string[in_];
    in_++;
    if (i == 4) {
      for (i = 0; i < 4; i++)
        char_array_4[i] = base64_chars_find(char_array_4[i]);
      char_array_3[0] = (char_array_4[0] << 2) + ((char_array_4[1] & 0x30) >> 4);
      char_array_3[1] = ((char_array_4[1] & 0xf) << 4) + ((char_array_4[2] & 0x3c) >> 2);
      char_array_3[2] = ((char_array_4[2] & 0x3) << 6) + char_array_4[3];
      for (i = 0; (i < 3) && strlen(ret) < retsize; i++)
        ret[strlen(ret)] = char_array_3[i];
      i = 0;
    }
  }
  if (i) {
    for (j = i; j < 4; j++)
      char_array_4[j] = 0;
    for (j = 0; j < 4; j++)
      char_array_4[j] = base64_chars_find(char_array_4[j]);
    char_array_3[0] = (char_array_4[0] << 2) + ((char_array_4[1] & 0x30) >> 4);
    char_array_3[1] = ((char_array_4[1] & 0xf) << 4) + ((char_array_4[2] & 0x3c) >> 2);
    char_array_3[2] = ((char_array_4[2] & 0x3) << 6) + char_array_4[3];
    for (j = 0; (j < i - 1) && strlen(ret) < retsize; j++) {
      ret[strlen(ret)] = char_array_3[j];
    }
  }
  return strlen(ret);
}
```