user: 请将这段汇编代码片段转换成等效的C代码：

```asm
<ffts1>:
  endbr64
  push   %r15
  push   %r14
  push   %r13
  push   %r12
  push   %rbp
  mov    %rdx,%rbp
  push   %rbx
  mov    %rdi,%rbx
  sub    $0x18,%rsp
  cmp    $0x2,%rsi
  je     L73
  mov    %rsi,%r12
  mov    %rcx,%r13
  mov    %r8,%r15
  jg     L192
  test   %rsi,%rsi
  je     L87
  cmp    $0x1,%rsi
  jne    L244
  test   %rdx,%rdx
  jle    L87
  nopl   0x0(%rax)
L225:
  mov    %rbx,%rdi
  xor    %eax,%eax
  add    $0x10,%rbx
  call   <fft2pt@plt>
  sub    $0x1,%rbp
  jne    L225
L87:
  add    $0x18,%rsp
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
  nopl   0x0(%rax,%rax,1)
L192:
  cmp    $0x3,%rsi
  jne    L244
  test   %rdx,%rdx
  jle    L87
  nopl   0x0(%rax,%rax,1)
L300:
  mov    %rbx,%rdi
  xor    %eax,%eax
  add    $0x40,%rbx
  call   <fft8pt@plt>
  sub    $0x1,%rbp
  jne    L300
  jmp    L87
  xchg   %ax,%ax
L73:
  test   %rdx,%rdx
  jle    L87
  nopl   (%rax)
L91:
  mov    %rbx,%rdi
  xor    %eax,%eax
  add    $0x20,%rbx
  call   <fft4pt@plt>
  sub    $0x1,%rbp
  jne    L91
  jmp    L87
  xchg   %ax,%ax
L244:
  test   %rbp,%rbp
  jle    L87
  movabs $0x5555555555555556,%rdx
  lea    -0x1(%r12),%rcx
  mov    %rcx,%rax
  imul   %rdx
  mov    %rcx,%rax
  sar    $0x3f,%rax
  sub    %rax,%rdx
  mov    %rdx,%r14
  lea    0x0(,%rdx,4),%rdx
  mov    %r14,%rax
  sub    %rdx,%rax
  mov    $0x8,%edx
  add    %rcx,%rax
  mov    %r12d,%ecx
  shl    %cl,%rdx
  mov    %rdx,0x8(%rsp)
  cmp    $0x1,%rax
  je     L17
  cmp    $0xb,%r12
  jbe    L295
  cmp    $0x2,%rax
  je     L176
  nopw   0x0(%rax,%rax,1)
L94:
  mov    %rbx,%rdi
  mov    %r15,%rdx
  mov    %r12,%rsi
  xor    %eax,%eax
  call   <bitrevR2@plt>
  mov    %rbx,%rdi
  mov    %r14,%r9
  mov    $0x1,%ecx
  mov    $0x2,%r8d
  mov    %r13,%rdx
  mov    %r12,%rsi
  xor    %eax,%eax
  call   <fftrecurs@plt>
  add    0x8(%rsp),%rbx
  sub    $0x1,%rbp
  jne    L94
  jmp    L87
  cs nopw 0x0(%rax,%rax,1)
L176:
  mov    %rbx,%rdi
  mov    %r15,%rdx
  mov    %r12,%rsi
  xor    %eax,%eax
  call   <bitrevR2@plt>
  mov    %rbx,%rdi
  mov    $0x2,%edx
  mov    %r12,%rsi
  xor    %eax,%eax
  call   <bfR4@plt>
  mov    %rbx,%rdi
  mov    %r14,%r9
  mov    $0x1,%ecx
  mov    $0x8,%r8d
  mov    %r13,%rdx
  mov    %r12,%rsi
  xor    %eax,%eax
  call   <fftrecurs@plt>
  add    0x8(%rsp),%rbx
  sub    $0x1,%rbp
  jne    L176
  jmp    L87
L295:
  cmp    $0x2,%rax
  je     L287
  cs nopw 0x0(%rax,%rax,1)
L64:
  mov    %rbx,%rdi
  mov    %r15,%rdx
  mov    %r12,%rsi
  xor    %eax,%eax
  call   <bitrevR2@plt>
  mov    %rbx,%rdi
  mov    %r14,%r9
  mov    $0x1,%ecx
  mov    $0x2,%r8d
  mov    %r13,%rdx
  mov    %r12,%rsi
  xor    %eax,%eax
  call   <bfstages@plt>
  add    0x8(%rsp),%rbx
  sub    $0x1,%rbp
  jne    L64
  jmp    L87
  xchg   %ax,%ax
L287:
  mov    %rbx,%rdi
  mov    %r15,%rdx
  mov    %r12,%rsi
  xor    %eax,%eax
  call   <bitrevR2@plt>
  mov    %rbx,%rdi
  mov    $0x2,%edx
  mov    %r12,%rsi
  xor    %eax,%eax
  call   <bfR4@plt>
  mov    %rbx,%rdi
  mov    %r14,%r9
  mov    $0x1,%ecx
  mov    $0x8,%r8d
  mov    %r13,%rdx
  mov    %r12,%rsi
  xor    %eax,%eax
  call   <bfstages@plt>
  add    0x8(%rsp),%rbx
  sub    $0x1,%rbp
  jne    L287
  jmp    L87
L17:
  cmp    $0xb,%r12
  jbe    L195
  cs nopw 0x0(%rax,%rax,1)
L180:
  mov    %rbx,%rdi
  mov    %r15,%rdx
  mov    %r12,%rsi
  xor    %eax,%eax
  call   <bitrevR2@plt>
  mov    %rbx,%rdi
  mov    $0x2,%edx
  mov    %r12,%rsi
  xor    %eax,%eax
  call   <bfR2@plt>
  mov    %rbx,%rdi
  mov    %r14,%r9
  mov    $0x1,%ecx
  mov    $0x4,%r8d
  mov    %r13,%rdx
  mov    %r12,%rsi
  xor    %eax,%eax
  call   <fftrecurs@plt>
  add    0x8(%rsp),%rbx
  sub    $0x1,%rbp
  jne    L180
  jmp    L87
L195:
  mov    %rbx,%rdi
  mov    %r15,%rdx
  mov    %r12,%rsi
  xor    %eax,%eax
  call   <bitrevR2@plt>
  mov    %rbx,%rdi
  mov    $0x2,%edx
  mov    %r12,%rsi
  xor    %eax,%eax
  call   <bfR2@plt>
  mov    %rbx,%rdi
  mov    %r14,%r9
  mov    $0x1,%ecx
  mov    $0x4,%r8d
  mov    %r13,%rdx
  mov    %r12,%rsi
  xor    %eax,%eax
  call   <bfstages@plt>
  add    0x8(%rsp),%rbx
  sub    $0x1,%rbp
  jne    L195
  jmp    L87
```

assistant: ```c
void ffts1(float *ioptr, long M, long Rows, float *Utbl, short *BRLow) {
  long StageCnt;
  long NDiffU;
  switch (M) {
  case 0:
    break;
  case 1:
    for (; Rows > 0; Rows--) {
      fft2pt(ioptr);
      ioptr += 2 * ((unsigned long)1 << (M));
    }
    break;
  case 2:
    for (; Rows > 0; Rows--) {
      fft4pt(ioptr);
      ioptr += 2 * ((unsigned long)1 << (M));
    }
    break;
  case 3:
    for (; Rows > 0; Rows--) {
      fft8pt(ioptr);
      ioptr += 2 * ((unsigned long)1 << (M));
    }
    break;
  default:
    for (; Rows > 0; Rows--) {
      bitrevR2(ioptr, M, BRLow);
      StageCnt = (M - 1) / 3;
      NDiffU = 2;
      if ((M - 1 - (StageCnt * 3)) == 1) {
        bfR2(ioptr, M, NDiffU);
        NDiffU *= 2;
      }
      if ((M - 1 - (StageCnt * 3)) == 2) {
        bfR4(ioptr, M, NDiffU);
        NDiffU *= 4;
      }
      if (M <= (11 - (sizeof(float) / 8)))
        bfstages(ioptr, M, Utbl, 1, NDiffU, StageCnt);
      else {
        fftrecurs(ioptr, M, Utbl, 1, NDiffU, StageCnt);
      }
      ioptr += 2 * ((unsigned long)1 << (M));
    }
  }
}
```