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

```asm
<dfct>:
  endbr64
  push   %r14
  push   %r13
  push   %r12
  push   %rbp
  push   %rbx
  sub    $0x10,%rsp
  mov    %edi,%r13d
  mov    %rsi,%rbp
  mov    %edi,%ebx
  sar    %ebx
  test   %ebx,%ebx
  jle    L43
  mov    %rsi,%rax
  movslq %edi,%rdx
  lea    (%rsi,%rdx,8),%rdx
  lea    -0x1(%rbx),%ecx
  lea    0x8(%rsi,%rcx,8),%rcx
L83:
  movsd  (%rax),%xmm0
  movsd  (%rdx),%xmm1
  movapd %xmm0,%xmm2
  subsd  %xmm1,%xmm2
  movsd  %xmm2,(%rax)
  addsd  %xmm1,%xmm0
  movsd  %xmm0,(%rdx)
  add    $0x8,%rax
  sub    $0x8,%rdx
  cmp    %rcx,%rax
  jne    L83
L43:
  movslq %r13d,%rax
  lea    0x0(%rbp,%rax,8),%r14
  movsd  (%r14),%xmm7
  movsd  %xmm7,0x8(%rsp)
  cmp    $0x1,%ebx
  jle    L1
  lea    0x8(%rbp),%r12
  jmp    L76
L47:
  mov    %ebx,%edi
  sar    %edi
  movslq %ebx,%rcx
  lea    0x0(%rbp,%rcx,8),%rdx
  movsd  (%rdx),%xmm0
  movsd  0x0(%rbp),%xmm1
  movsd  %xmm1,(%rdx)
  movsd  0x8(%rsp),%xmm4
  movapd %xmm4,%xmm1
  subsd  %xmm0,%xmm1
  movsd  %xmm1,0x0(%rbp)
  addsd  %xmm4,%xmm0
  movsd  %xmm0,0x8(%rsp)
  cmp    $0x3,%ebx
  jle    L92
  mov    %rbp,%rax
  lea    -0x2(%rdi),%esi
  lea    (%r12,%rsi,8),%rsi
L53:
  movsd  -0x8(%rdx,%rcx,8),%xmm0
  movsd  0x8(%rax,%rcx,8),%xmm1
  movsd  0x8(%rax),%xmm3
  movsd  -0x8(%rdx),%xmm2
  movsd  %xmm3,0x8(%rax,%rcx,8)
  movsd  %xmm2,-0x8(%rdx,%rcx,8)
  movapd %xmm0,%xmm2
  subsd  %xmm1,%xmm2
  movsd  %xmm2,0x8(%rax)
  addsd  %xmm1,%xmm0
  movsd  %xmm0,-0x8(%rdx)
  add    $0x8,%rax
  sub    $0x8,%rdx
  cmp    %rsi,%rax
  jne    L53
  movslq %edi,%rax
  lea    0x0(%rbp,%rax,8),%rdx
  movsd  (%rdx),%xmm0
  add    %edi,%ebx
  movslq %ebx,%rbx
  lea    0x0(%rbp,%rbx,8),%rax
  movsd  (%rax),%xmm1
  movsd  %xmm1,(%rdx)
  movsd  %xmm0,(%rax)
  mov    %edi,%ebx
L76:
  mov    %rbp,%rdx
  mov    $0x1,%esi
  mov    %ebx,%edi
  call   <ddct@plt>
  cmp    $0x2,%ebx
  jle    L47
  mov    %rbp,%rsi
  mov    %ebx,%edi
  call   <bitrv1@plt>
  jmp    L47
L1:
  movsd  0x8(%rbp),%xmm0
  movsd  0x0(%rbp),%xmm1
  movsd  %xmm1,0x8(%rbp)
  movsd  0x8(%rsp),%xmm6
  movapd %xmm6,%xmm1
  addsd  %xmm0,%xmm1
  movsd  %xmm1,0x0(%rbp)
  subsd  %xmm0,%xmm6
  movsd  %xmm6,(%r14)
  cmp    $0x2,%r13d
  jg     L72
L88:
  add    $0x10,%rsp
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  ret
L92:
  movslq %edi,%rax
  lea    0x0(%rbp,%rax,8),%rdx
  movsd  (%rdx),%xmm0
  lea    (%rdi,%rbx,1),%eax
  cltq
  lea    0x0(%rbp,%rax,8),%rax
  movsd  (%rax),%xmm1
  movsd  %xmm1,(%rdx)
  movsd  %xmm0,(%rax)
  movsd  0x8(%rbp),%xmm1
  movsd  0x0(%rbp),%xmm0
  movsd  %xmm0,0x8(%rbp)
  movsd  0x8(%rsp),%xmm5
  movapd %xmm5,%xmm0
  addsd  %xmm1,%xmm0
  movsd  %xmm0,0x0(%rbp)
  subsd  %xmm1,%xmm5
  movsd  %xmm5,(%r14)
L72:
  mov    %rbp,%rsi
  mov    %r13d,%edi
  call   <bitrv1@plt>
  jmp    L88
```

assistant: ```c
void dfct(int n, double *a) {
  void ddct(int n, int isgn, double *a);
  void bitrv1(int n, double *a);
  int j, k, m, mh;
  double xr, xi, yr, yi, an;
  m = n >> 1;
  for (j = 0; j < m; j++) {
    k = n - j;
    xr = a[j] + a[k];
    a[j] -= a[k];
    a[k] = xr;
  }
  an = a[n];
  while (m >= 2) {
    ddct(m, 1, a);
    if (m > 2) {
      bitrv1(m, a);
    }
    mh = m >> 1;
    xi = a[m];
    a[m] = a[0];
    a[0] = an - xi;
    an += xi;
    for (j = 1; j < mh; j++) {
      k = m - j;
      xr = a[m + k];
      xi = a[m + j];
      yr = a[j];
      yi = a[k];
      a[m + j] = yr;
      a[m + k] = yi;
      a[j] = xr - xi;
      a[k] = xr + xi;
    }
    xr = a[mh];
    a[mh] = a[m + mh];
    a[m + mh] = xr;
    m = mh;
  }
  xi = a[1];
  a[1] = a[0];
  a[0] = an + xi;
  a[n] = an - xi;
  if (n > 2) {
    bitrv1(n, a);
  }
}
```