user: Translate the below assembly code into C programming language:

```asm
<css_decode>:
  endbr64
  test   %rdi,%rdi
  je     L38
  push   %r15
  push   %r14
  mov    %rsi,%r14
  push   %r13
  push   %r12
  mov    %rdi,%r12
  push   %rbp
  mov    %rdi,%rbp
  push   %rbx
  sub    $0x28,%rsp
  test   %rsi,%rsi
  jle    L278
  xor    %ebx,%ebx
  jmp    L166
  nopl   0x0(%rax)
L156:
  mov    %al,0x0(%rbp)
  mov    %r13,%rbx
  add    $0x1,%rbp
L314:
  cmp    %r14,%rbx
  jge    L102
L166:
  movzbl (%r12,%rbx,1),%eax
  mov    %rbx,%rsi
  lea    0x1(%rbx),%r13
  cmp    $0x5c,%al
  jne    L156
  cmp    %r13,%r14
  jg     L229
L279:
  mov    %ebp,%eax
  sub    %r12d,%eax
L327:
  movb   $0x0,0x0(%rbp)
  add    $0x28,%rsp
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
  nopl   0x0(%rax,%rax,1)
L229:
  mov    %r13,%rax
  xor    %r15d,%r15d
L100:
  movzbl (%r12,%rax,1),%ecx
  lea    -0x30(%rcx),%edi
  cmp    $0x9,%dil
  jbe    L96
  mov    %ecx,%edi
  and    $0xffffffdf,%edi
  sub    $0x41,%edi
  cmp    $0x5,%dil
  ja     L220
L96:
  add    $0x1,%r15
  cmp    $0x6,%r15
  je     L55
  add    $0x1,%rax
  cmp    %r14,%rax
  jne    L100
L236:
  lea    (%r15,%r13,1),%rcx
  lea    0x1(%rbp),%r8
  mov    %r8,0x8(%rsp)
  mov    %rcx,%rbx
  cmp    $0x4,%r15
  je     L309
  cmp    $0x5,%r15
  jne    L104
  lea    0x4(%r12,%rsi,1),%rdi
  mov    %r8,0x18(%rsp)
  mov    %rcx,0x10(%rsp)
  call   <x2c@plt>
  mov    0x10(%rsp),%rcx
  mov    0x18(%rsp),%r8
  mov    %al,0x0(%rbp)
  cmpb   $0x30,(%r12,%r13,1)
  mov    %eax,%esi
  jne    L14
L293:
  lea    -0x1(%rsi),%eax
  mov    %rcx,%rbx
  cmp    $0x5d,%al
  ja     L290
  movzbl -0x3(%r12,%rcx,1),%eax
  and    $0xffffffdf,%eax
  cmp    $0x46,%al
  jne    L290
  movzbl -0x4(%r12,%rcx,1),%eax
  and    $0xffffffdf,%eax
  cmp    $0x46,%al
  jne    L290
  add    $0x20,%esi
  mov    %sil,0x0(%rbp)
  cs nopw 0x0(%rax,%rax,1)
L290:
  mov    %r8,0x8(%rsp)
  cmp    %rbx,%r14
  jle    L101
L70:
  movzbl (%r12,%rbx,1),%edi
  call   <isspace@plt>
  test   %rax,%rax
  je     L292
  mov    0x8(%rsp),%rbp
  lea    0x1(%r15,%r13,1),%rbx
  jmp    L314
  nopl   0x0(%rax)
L104:
  lea    -0x2(%r15),%rax
  cmp    $0x1,%rax
  ja     L121
  lea    -0x2(%r12,%rcx,1),%rdi
  call   <x2c@plt>
  mov    %al,0x0(%rbp)
L14:
  cmp    %rbx,%r14
  jg     L70
L101:
  mov    0x8(%rsp),%rbp
  jmp    L279
  cs nopw 0x0(%rax,%rax,1)
L220:
  test   %r15,%r15
  jne    L236
  add    $0x2,%rbx
  cmp    $0xa,%cl
  je     L314
  mov    %cl,0x0(%rbp)
  add    $0x1,%rbp
  jmp    L314
  nopw   0x0(%rax,%rax,1)
L121:
  lea    (%r12,%r13,1),%rdi
  call   <xsingle2c@plt>
  mov    %al,0x0(%rbp)
  jmp    L14
  cs nopw 0x0(%rax,%rax,1)
L278:
  xor    %eax,%eax
  jmp    L327
L309:
  lea    0x3(%r12,%rsi,1),%rdi
  mov    %r8,0x10(%rsp)
  mov    %rcx,0x8(%rsp)
  call   <x2c@plt>
  mov    0x8(%rsp),%rcx
  mov    0x10(%rsp),%r8
  mov    %al,0x0(%rbp)
  mov    %eax,%esi
  jmp    L293
  nop
L55:
  lea    0x5(%r12,%rbx,1),%rdi
  call   <x2c@plt>
  mov    %al,0x0(%rbp)
  cmpb   $0x30,(%r12,%r13,1)
  mov    %eax,%esi
  je     L114
L4:
  lea    0x1(%rbp),%rax
  add    $0x7,%rbx
  mov    %rax,0x8(%rsp)
  jmp    L14
L292:
  mov    0x8(%rsp),%rbp
  jmp    L166
L114:
  cmpb   $0x30,0x2(%r12,%rbx,1)
  lea    0x6(%r13),%rcx
  lea    0x1(%rbp),%r8
  jne    L4
  jmp    L293
L38:
  mov    $0xffffffff,%eax
  ret
L102:
  mov    %rbp,0x8(%rsp)
  jmp    L101
```

assistant: ```c
int css_decode(unsigned char *input, long int input_len) {
  unsigned char *d = (unsigned char *)input;
  long int i, j, count;
  if (input == NULL)
    return -1;
  i = count = 0;
  while (i < input_len) {
    if (input[i] == '\\') {
      if (i + 1 < input_len) {
        i++;
        j = 0;
        while ((j < 6) && (i + j < input_len) && ((((input[i + j] >= '0') && (input[i + j] <= '9')) || ((input[i + j] >= 'a') && (input[i + j] <= 'f')) || ((input[i + j] >= 'A') && (input[i + j] <= 'F'))))) {
          j++;
        }
        if (j > 0) {
          int fullcheck = 0;
          switch (j) {
          case 1:
            *d++ = xsingle2c(&input[i]);
            break;
          case 2:
          case 3:
            *d++ = x2c(&input[i + j - 2]);
            break;
          case 4:
            *d = x2c(&input[i + j - 2]);
            fullcheck = 1;
            break;
          case 5:
            *d = x2c(&input[i + j - 2]);
            if (input[i] == '0') {
              fullcheck = 1;
            } else {
              d++;
            }
            break;
          case 6:
            *d = x2c(&input[i + j - 2]);
            if ((input[i] == '0') && (input[i + 1] == '0')) {
              fullcheck = 1;
            } else {
              d++;
            }
            break;
          }
          if (fullcheck) {
            if ((*d > 0x00) && (*d < 0x5f) && ((input[i + j - 3] == 'f') || (input[i + j - 3] == 'F')) && ((input[i + j - 4] == 'f') || (input[i + j - 4] == 'F'))) {
              (*d) += 0x20;
            }
            d++;
          }
          if ((i + j < input_len) && isspace(input[i + j])) {
            j++;
          }
          count++;
          i += j;
        } else if (input[i] == '\n') {
          i++;
        } else {
          *d++ = input[i++];
          count++;
        }
      } else {
        i++;
      }
    } else {
      *d++ = input[i++];
      count++;
    }
  }
  *d = '\0';
  return d - input;
}
```