user: What would the C source look like for the assembly code provided here:

```asm
<nmea_calc_crc>:
  endbr64
  mov    %rdi,%rcx
  mov    %esi,%edx
  test   %esi,%esi
  jle    L61
  lea    -0x1(%rsi),%eax
  cmp    $0xe,%eax
  jbe    L63
  shr    $0x4,%esi
  pxor   %xmm2,%xmm2
  pxor   %xmm6,%xmm6
  mov    %rdi,%rax
  sub    $0x1,%esi
  pxor   %xmm4,%xmm4
  shl    $0x4,%rsi
  lea    0x10(%rdi,%rsi,1),%rsi
  nopl   0x0(%rax,%rax,1)
L102:
  movdqu (%rax),%xmm0
  movdqa %xmm6,%xmm3
  add    $0x10,%rax
  pcmpgtb %xmm0,%xmm3
  movdqa %xmm0,%xmm1
  punpcklbw %xmm3,%xmm1
  punpckhbw %xmm3,%xmm0
  movdqa %xmm4,%xmm3
  pcmpgtw %xmm0,%xmm3
  movdqa %xmm0,%xmm5
  punpcklwd %xmm3,%xmm5
  punpckhwd %xmm3,%xmm0
  movdqa %xmm4,%xmm3
  pcmpgtw %xmm1,%xmm3
  pxor   %xmm5,%xmm0
  movdqa %xmm1,%xmm5
  punpcklwd %xmm3,%xmm5
  punpckhwd %xmm3,%xmm1
  pxor   %xmm5,%xmm1
  pxor   %xmm1,%xmm0
  pxor   %xmm0,%xmm2
  cmp    %rsi,%rax
  jne    L102
  movdqa %xmm2,%xmm0
  mov    %edx,%esi
  psrldq $0x8,%xmm0
  and    $0xfffffff0,%esi
  pxor   %xmm0,%xmm2
  movdqa %xmm2,%xmm0
  psrldq $0x4,%xmm0
  pxor   %xmm0,%xmm2
  movd   %xmm2,%eax
  test   $0xf,%dl
  je     L55
L138:
  movslq %esi,%rdi
  movsbl (%rcx,%rdi,1),%edi
  xor    %edi,%eax
  lea    0x1(%rsi),%edi
  cmp    %edi,%edx
  jle    L75
  movslq %edi,%rdi
  movsbl (%rcx,%rdi,1),%edi
  xor    %edi,%eax
  lea    0x2(%rsi),%edi
  cmp    %edi,%edx
  jle    L75
  movslq %edi,%rdi
  movsbl (%rcx,%rdi,1),%edi
  xor    %edi,%eax
  lea    0x3(%rsi),%edi
  cmp    %edi,%edx
  jle    L75
  movslq %edi,%rdi
  movsbl (%rcx,%rdi,1),%edi
  xor    %edi,%eax
  lea    0x4(%rsi),%edi
  cmp    %edi,%edx
  jle    L75
  movslq %edi,%rdi
  movsbl (%rcx,%rdi,1),%edi
  xor    %edi,%eax
  lea    0x5(%rsi),%edi
  cmp    %edi,%edx
  jle    L75
  movslq %edi,%rdi
  movsbl (%rcx,%rdi,1),%edi
  xor    %edi,%eax
  lea    0x6(%rsi),%edi
  cmp    %edi,%edx
  jle    L75
  movslq %edi,%rdi
  movsbl (%rcx,%rdi,1),%edi
  xor    %edi,%eax
  lea    0x7(%rsi),%edi
  cmp    %edi,%edx
  jle    L75
  movslq %edi,%rdi
  movsbl (%rcx,%rdi,1),%edi
  xor    %edi,%eax
  lea    0x8(%rsi),%edi
  cmp    %edi,%edx
  jle    L75
  movslq %edi,%rdi
  movsbl (%rcx,%rdi,1),%edi
  xor    %edi,%eax
  lea    0x9(%rsi),%edi
  cmp    %edi,%edx
  jle    L75
  movslq %edi,%rdi
  movsbl (%rcx,%rdi,1),%edi
  xor    %edi,%eax
  lea    0xa(%rsi),%edi
  cmp    %edi,%edx
  jle    L75
  movslq %edi,%rdi
  movsbl (%rcx,%rdi,1),%edi
  xor    %edi,%eax
  lea    0xb(%rsi),%edi
  cmp    %edi,%edx
  jle    L75
  movslq %edi,%rdi
  movsbl (%rcx,%rdi,1),%edi
  xor    %edi,%eax
  lea    0xc(%rsi),%edi
  cmp    %edi,%edx
  jle    L75
  movslq %edi,%rdi
  movsbl (%rcx,%rdi,1),%edi
  xor    %edi,%eax
  lea    0xd(%rsi),%edi
  cmp    %edi,%edx
  jle    L75
  movslq %edi,%rdi
  add    $0xe,%esi
  movsbl (%rcx,%rdi,1),%edi
  xor    %edi,%eax
  cmp    %esi,%edx
  jle    L75
  movslq %esi,%rsi
  movsbl (%rcx,%rsi,1),%edx
  xor    %edx,%eax
  ret
  nopl   0x0(%rax,%rax,1)
L61:
  xor    %eax,%eax
L75:
  ret
L63:
  xor    %esi,%esi
  xor    %eax,%eax
  jmp    L138
L55:
  ret
```

assistant: ```c
int nmea_calc_crc(const char *buff, int buff_sz) {
  int chsum = 0, it;
  for (it = 0; it < buff_sz; ++it)
    chsum ^= (int)buff[it];
  return chsum;
}
```