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

```asm
<permute_letters>:
  endbr64
  push   %rbp
  mov    %rsp,%rbp
  push   %r15
  push   %r14
  push   %r13
  push   %r12
  push   %rbx
  sub    $0x48,%rsp
  mov    %rdi,-0x68(%rbp)
  mov    %esi,-0x6c(%rbp)
  mov    %fs:0x28,%rax
  mov    %rax,-0x38(%rbp)
  xor    %eax,%eax
  mov    %rsp,%rax
  mov    %rax,%rbx
  mov    -0x6c(%rbp),%eax
  add    $0x1,%eax
  movslq %eax,%rdx
  sub    $0x1,%rdx
  mov    %rdx,-0x58(%rbp)
  movslq %eax,%rdx
  mov    %rdx,%r10
  mov    $0x0,%r11d
  movslq %eax,%rdx
  mov    %rdx,%r8
  mov    $0x0,%r9d
  cltq
  mov    $0x10,%edx
  sub    $0x1,%rdx
  add    %rdx,%rax
  mov    $0x10,%edi
  mov    $0x0,%edx
  div    %rdi
  imul   $0x10,%rax,%rax
  mov    %rax,%rcx
  and    $0xfffffffffffff000,%rcx
  mov    %rsp,%rdx
  sub    %rcx,%rdx
L7:
  cmp    %rdx,%rsp
  je     L24
  sub    $0x1000,%rsp
  orq    $0x0,0xff8(%rsp)
  jmp    L7
L24:
  mov    %rax,%rdx
  and    $0xfff,%edx
  sub    %rdx,%rsp
  mov    %rax,%rdx
  and    $0xfff,%edx
  test   %rdx,%rdx
  je     L25
  and    $0xfff,%eax
  sub    $0x8,%rax
  add    %rsp,%rax
  orq    $0x0,(%rax)
L25:
  mov    %rsp,%rax
  add    $0x0,%rax
  mov    %rax,-0x50(%rbp)
  mov    -0x50(%rbp),%rdx
  mov    -0x6c(%rbp),%eax
  cltq
  movb   $0x0,(%rdx,%rax,1)
  mov    -0x68(%rbp),%rdx
  mov    -0x50(%rbp),%rax
  mov    %rdx,%rsi
  mov    %rax,%rdi
  call   <strcpy@plt>
  mov    -0x6c(%rbp),%eax
  movslq %eax,%rdx
  sub    $0x1,%rdx
  mov    %rdx,-0x48(%rbp)
  movslq %eax,%rdx
  mov    %rdx,%r14
  mov    $0x0,%r15d
  movslq %eax,%rdx
  mov    %rdx,%r12
  mov    $0x0,%r13d
  cltq
  lea    0x0(,%rax,4),%rdx
  mov    $0x10,%eax
  sub    $0x1,%rax
  add    %rdx,%rax
  mov    $0x10,%esi
  mov    $0x0,%edx
  div    %rsi
  imul   $0x10,%rax,%rax
  mov    %rax,%rcx
  and    $0xfffffffffffff000,%rcx
  mov    %rsp,%rdx
  sub    %rcx,%rdx
L67:
  cmp    %rdx,%rsp
  je     L87
  sub    $0x1000,%rsp
  orq    $0x0,0xff8(%rsp)
  jmp    L67
L87:
  mov    %rax,%rdx
  and    $0xfff,%edx
  sub    %rdx,%rsp
  mov    %rax,%rdx
  and    $0xfff,%edx
  test   %rdx,%rdx
  je     L54
  and    $0xfff,%eax
  sub    $0x8,%rax
  add    %rsp,%rax
  orq    $0x0,(%rax)
L54:
  mov    %rsp,%rax
  add    $0x3,%rax
  shr    $0x2,%rax
  shl    $0x2,%rax
  mov    %rax,-0x40(%rbp)
  movl   $0x0,-0x5c(%rbp)
  jmp    L3
L0:
  mov    -0x40(%rbp),%rax
  mov    -0x5c(%rbp),%edx
  movslq %edx,%rdx
  movl   $0x0,(%rax,%rdx,4)
  addl   $0x1,-0x5c(%rbp)
L3:
  mov    -0x5c(%rbp),%eax
  cmp    -0x6c(%rbp),%eax
  jl     L0
  mov    -0x50(%rbp),%rcx
  mov    -0x40(%rbp),%rdx
  mov    -0x6c(%rbp),%esi
  mov    -0x68(%rbp),%rax
  mov    $0x0,%r8d
  mov    %rax,%rdi
  call   <do_permute_letters@plt>
  mov    %rbx,%rsp
  nop
  mov    -0x38(%rbp),%rax
  sub    %fs:0x28,%rax
  je     L53
  call   <__stack_chk_fail@plt>
L53:
  lea    -0x28(%rbp),%rsp
  pop    %rbx
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  pop    %rbp
  ret
```

assistant: ```c
void permute_letters(const char *letters, int len) {
  char buffer[len + 1];
  buffer[len] = '\0';
  strcpy(buffer, letters);
  bool used[len];
  for (int i = 0; i < len; i++) {
    used[i] = false;
  }
  do_permute_letters(letters, len, used, buffer, 0);
}
```