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

```asm
<flip_H4>:
  endbr64
  push   %r12
  push   %rbp
  push   %rbx
  mov    %rdi,%r12
  mov    %rsi,%rbx
  movabs $0x200000000,%rbp
  add    %rsi,%rbp
  and    %rdi,%rbp
  movabs $0xfc00000000,%rax
  and    %rax,%rbp
  je     L19
  sub    $0x1,%rbp
  movabs $0xfe00000000,%rax
  and    %rax,%rbp
L19:
  mov    %r12,%rdi
  and    $0x40810,%edi
  mov    %rbx,%rax
  and    $0x2040800,%eax
  mov    $0xfdfbf000,%edx
  or     %rdx,%rax
  mov    %rdi,%rdx
  shl    $0x7,%rdx
  lea    (%rax,%rdx,1),%rax
  shr    $0x20,%rax
  jne    L21
L14:
  movabs $0x4020000000000,%rax
  and    %rbx,%rax
  movabs $0x7fbfc0000000000,%rdx
  or     %rdx,%rax
  movabs $0x20000000000,%rdx
  add    %rdx,%rax
  and    %r12,%rax
  movabs $0x804000000000000,%rdx
  and    %rdx,%rax
  je     L38
  sub    $0x1,%rax
  movabs $0x4020000000000,%rdx
  and    %rdx,%rax
  or     %rax,%rbp
L38:
  mov    %r12,%rdi
  and    $0x10101,%edi
  mov    %rbx,%rax
  and    $0x1010100,%eax
  mov    $0xfefefe00,%edx
  or     %rdx,%rax
  mov    %rdi,%rdx
  shl    $0x8,%rdx
  lea    (%rax,%rdx,1),%rax
  shr    $0x20,%rax
  jne    L56
L4:
  movabs $0x1010000000000,%rax
  and    %rax,%rbx
  movabs $0xfefe0000000000,%rax
  or     %rax,%rbx
  movabs $0x10000000000,%rax
  add    %rax,%rbx
  and    %r12,%rbx
  movabs $0x101000000000000,%rax
  and    %rax,%rbx
  je     L51
  sub    $0x1,%rbx
  movabs $0x1010000000000,%rax
  and    %rax,%rbx
  or     %rbx,%rbp
L51:
  mov    %rbp,%rax
  pop    %rbx
  pop    %rbp
  pop    %r12
  ret
L21:
  call   <last_bit@plt>
  mov    $0xfffffffffffffffe,%rdx
  mov    %eax,%ecx
  shl    %cl,%rdx
  and    $0x2040800,%edx
  or     %rdx,%rbp
  jmp    L14
L56:
  call   <last_bit@plt>
  mov    $0xfffffffffffffffe,%rdx
  mov    %eax,%ecx
  shl    %cl,%rdx
  and    $0x1010100,%edx
  or     %rdx,%rbp
  jmp    L4
```

assistant: ```c
unsigned long long flip_H4(const unsigned long long discs_player, const unsigned long long discs_opponent) {
  unsigned long long my_discs, opp_discs, r, flipped = 0ULL;
  my_discs = discs_player & 0X000000FC00000000ULL;
  r = (discs_opponent + 0X0000000200000000ULL) & my_discs;
  if (r != 0) {
    flipped |= (r - 1) & 0X000000FE00000000ULL;
  }
  my_discs = discs_player & 0X0000000000040810ULL;
  opp_discs = (discs_opponent & 0X0000000002040800ULL) | 0X00000000FDFBF000ULL;
  r = ((my_discs << 7) + opp_discs) & 0X0000000100000000ULL;
  if (r != 0) {
    flipped |= (0X0000000100000000ULL - (0X2ULL << last_bit(my_discs))) & 0X0000000002040800ULL;
  }
  my_discs = discs_player & 0X0804000000000000ULL;
  opp_discs = (discs_opponent & 0X0004020000000000ULL) | 0X07FBFC0000000000ULL;
  r = (opp_discs + 0X0000020000000000ULL) & my_discs;
  if (r != 0) {
    flipped |= (r - 1) & 0X0004020000000000ULL;
  }
  my_discs = discs_player & 0X0000000000010101ULL;
  opp_discs = (discs_opponent & 0X0000000001010100ULL) | 0X00000000FEFEFE00ULL;
  r = ((my_discs << 8) + opp_discs) & 0X0000000100000000ULL;
  if (r != 0) {
    flipped |= (0X0000000100000000ULL - (0X2ULL << last_bit(my_discs))) & 0X0000000001010100ULL;
  }
  my_discs = discs_player & 0X0101000000000000ULL;
  opp_discs = (discs_opponent & 0X0001010000000000ULL) | 0X00FEFE0000000000ULL;
  r = (opp_discs + 0X0000010000000000ULL) & my_discs;
  if (r != 0) {
    flipped |= (r - 1) & 0X0001010000000000ULL;
  }
  return flipped;
}
```