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

```asm
<strtoul>:
  endbr64
  push   %r15
  push   %r14
  push   %r13
  push   %r12
  push   %rbp
  push   %rbx
  mov    %edx,%ebx
  sub    $0x28,%rsp
  cmp    $0x1,%edx
  sete   %dl
  cmp    $0x24,%ebx
  mov    %rdi,0x8(%rsp)
  seta   %al
  mov    %rsi,0x10(%rsp)
  or     %al,%dl
  mov    %dl,0x1f(%rsp)
  je     L306
  test   %rsi,%rsi
  je     L193
  mov    %rdi,(%rsi)
L193:
  mov    D17(%rip),%rax
  xor    %r12d,%r12d
  mov    (%rax),%edx
  mov    D10(%rip),%rax
  mov    %edx,(%rax)
L270:
  add    $0x28,%rsp
  mov    %r12,%rax
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
  nopl   0x0(%rax)
L306:
  mov    0x8(%rsp),%r14
  nopl   (%rax)
L179:
  mov    %r14,%rbp
  movzbl (%r14),%r15d
  add    $0x1,%r14
  mov    %r15d,%edi
  call   <isspace@plt>
  test   %rax,%rax
  jne    L179
  cmp    $0x2d,%r15d
  je     L184
  cmp    $0x2b,%r15d
  je     L227
L281:
  test   $0xffffffef,%ebx
  jne    L236
  cmp    $0x30,%r15d
  je     L217
  test   %ebx,%ebx
  jne    L236
  movq   $0xa,(%rsp)
  mov    $0xa,%ebx
  jmp    L22
  nopl   0x0(%rax)
L236:
  movslq %ebx,%rax
  mov    %rax,(%rsp)
L22:
  mov    D47(%rip),%rax
  xor    %edx,%edx
  xor    %ebp,%ebp
  xor    %r12d,%r12d
  mov    (%rax),%rax
  divq   (%rsp)
  mov    %edx,0x18(%rsp)
  mov    %rax,%r13
  jmp    L287
  nopl   (%rax)
L230:
  cmp    %ecx,%ebx
  jle    L87
L154:
  cmp    $0xffffffff,%ebp
  je     L164
  cmp    %r13,%r12
  ja     L15
  jne    L279
  cmp    0x18(%rsp),%ecx
  jle    L279
L15:
  mov    D47(%rip),%rax
  mov    $0xffffffff,%ebp
  mov    (%rax),%r12
  mov    D36(%rip),%rax
  mov    (%rax),%edx
  mov    D10(%rip),%rax
  mov    %edx,(%rax)
L164:
  movzbl (%r14),%r15d
  add    $0x1,%r14
L287:
  mov    %r15d,%edi
  call   <isdigit@plt>
  lea    -0x30(%r15),%ecx
  test   %rax,%rax
  jne    L230
  mov    %r15d,%edi
  call   <isalpha@plt>
  test   %rax,%rax
  je     L87
  mov    %r15d,%edi
  call   <isupper@plt>
  mov    %r15d,%ecx
  cmp    $0x1,%rax
  sbb    %eax,%eax
  and    $0x20,%eax
  add    $0x37,%eax
  sub    %eax,%ecx
  cmp    %ecx,%ebx
  jg     L154
L87:
  test   %ebp,%ebp
  jle    L99
  cmpb   $0x0,0x1f(%rsp)
  je     L99
  neg    %r12
  cmpq   $0x0,0x10(%rsp)
  je     L270
L252:
  lea    -0x1(%r14),%rax
  mov    %rax,0x8(%rsp)
  jmp    L280
  nopw   0x0(%rax,%rax,1)
L279:
  imul   (%rsp),%r12
  movslq %ecx,%rcx
  mov    $0x1,%ebp
  add    %rcx,%r12
  jmp    L164
  nopl   (%rax)
L99:
  cmpq   $0x0,0x10(%rsp)
  je     L270
  test   %ebp,%ebp
  jne    L252
L280:
  mov    0x10(%rsp),%rax
  mov    0x8(%rsp),%rsi
  mov    %rsi,(%rax)
  jmp    L270
  nopw   0x0(%rax,%rax,1)
L227:
  movsbl (%r14),%r15d
  lea    0x2(%rbp),%r14
  jmp    L281
  nopl   (%rax)
L217:
  movzbl (%r14),%eax
  and    $0xffffffdf,%eax
  cmp    $0x58,%al
  je     L286
  test   %ebx,%ebx
  jne    L236
  movq   $0x8,(%rsp)
  mov    $0x8,%ebx
  jmp    L22
  nopl   (%rax)
L184:
  movsbl (%r14),%r15d
  movb   $0x1,0x1f(%rsp)
  lea    0x2(%rbp),%r14
  jmp    L281
L286:
  movsbl 0x1(%r14),%r15d
  mov    $0x10,%ebx
  add    $0x2,%r14
  movq   $0x10,(%rsp)
  jmp    L22
```

assistant: ```c
unsigned long strtoul(const char *nptr, char **endptr, int base) {
  const char *s;
  unsigned long acc, cutoff;
  int c;
  int neg, any, cutlim;
  if (base < 0 || base == 1 || base > 36) {
    if (endptr != 0)
      *endptr = (char *)nptr;
    errno = EINVAL;
    return 0;
  }
  s = nptr;
  do {
    c = (unsigned char)*s++;
  } while (isspace(c));
  if (c == '-') {
    neg = 1;
    c = *s++;
  } else {
    neg = 0;
    if (c == '+')
      c = *s++;
  }
  if ((base == 0 || base == 16) && c == '0' && (*s == 'x' || *s == 'X')) {
    c = s[1];
    s += 2;
    base = 16;
  }
  if (base == 0)
    base = c == '0' ? 8 : 10;
  cutoff = ULONG_MAX / (unsigned long)base;
  cutlim = ULONG_MAX % (unsigned long)base;
  for (acc = 0, any = 0;; c = (unsigned char)*s++) {
    if (isdigit(c))
      c -= '0';
    else if (isalpha(c))
      c -= isupper(c) ? 'A' - 10 : 'a' - 10;
    else
      break;
    if (c >= base)
      break;
    if (any < 0)
      continue;
    if (acc > cutoff || (acc == cutoff && c > cutlim)) {
      any = -1;
      acc = ULONG_MAX;
      errno = ERANGE;
    } else {
      any = 1;
      acc *= (unsigned long)base;
      acc += c;
    }
  }
  if (neg && any > 0)
    acc = -acc;
  if (endptr != 0)
    *endptr = (char *)(any ? s - 1 : nptr);
  return (acc);
}
```