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

```asm
<base64decode>:
  endbr64
  push   %rbp
  mov    %rsp,%rbp
  sub    $0x40,%rsp
  mov    %rdi,-0x28(%rbp)
  mov    %rsi,-0x30(%rbp)
  mov    %edx,-0x34(%rbp)
  cmpq   $0x0,-0x28(%rbp)
  je     L227
  cmpq   $0x0,-0x30(%rbp)
  jne    L127
L227:
  mov    $0xffffffff,%eax
  jmp    L107
L127:
  mov    -0x30(%rbp),%rax
  mov    %rax,%rdi
  call   <strlen@plt>
  mov    %eax,-0x4(%rbp)
  movl   $0x0,-0x18(%rbp)
  movl   $0x0,-0x14(%rbp)
  movl   $0x0,-0x8(%rbp)
  movl   $0x0,-0xc(%rbp)
  jmp    L39
L115:
  mov    -0xc(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x30(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  cmp    $0x40,%al
  jle    L116
  mov    -0xc(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x30(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  cmp    $0x5a,%al
  jg     L116
  mov    -0xc(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x30(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  sub    $0x41,%eax
  mov    %eax,-0x10(%rbp)
  jmp    L55
L116:
  mov    -0xc(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x30(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  cmp    $0x60,%al
  jle    L209
  mov    -0xc(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x30(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  cmp    $0x7a,%al
  jg     L209
  mov    -0xc(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x30(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  sub    $0x47,%eax
  mov    %eax,-0x10(%rbp)
  jmp    L55
L209:
  mov    -0xc(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x30(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  cmp    $0x2f,%al
  jle    L151
  mov    -0xc(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x30(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  cmp    $0x39,%al
  jg     L151
  mov    -0xc(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x30(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  add    $0x4,%eax
  mov    %eax,-0x10(%rbp)
  jmp    L55
L151:
  mov    -0xc(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x30(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  cmp    $0x2b,%al
  jne    L106
  movl   $0x3e,-0x10(%rbp)
  jmp    L55
L106:
  mov    -0xc(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x30(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  cmp    $0x2d,%al
  jne    L27
  movl   $0x3f,-0x10(%rbp)
  jmp    L55
L27:
  movl   $0xffffffff,-0x10(%rbp)
L55:
  cmpl   $0x0,-0x10(%rbp)
  js     L175
  shll   $0x6,-0x18(%rbp)
  mov    -0x10(%rbp),%eax
  add    %eax,-0x18(%rbp)
  addl   $0x6,-0x14(%rbp)
  jmp    L199
L121:
  mov    -0x8(%rbp),%eax
  cmp    -0x34(%rbp),%eax
  jge    L8
  mov    -0x14(%rbp),%eax
  sub    $0x8,%eax
  mov    -0x18(%rbp),%edx
  mov    %eax,%ecx
  sar    %cl,%edx
  mov    %edx,%ecx
  mov    -0x8(%rbp),%eax
  lea    0x1(%rax),%edx
  mov    %edx,-0x8(%rbp)
  movslq %eax,%rdx
  mov    -0x28(%rbp),%rax
  add    %rdx,%rax
  mov    %ecx,%edx
  mov    %dl,(%rax)
L8:
  subl   $0x8,-0x14(%rbp)
  mov    -0x14(%rbp),%eax
  mov    $0x1,%edx
  mov    %eax,%ecx
  shl    %cl,%edx
  mov    %edx,%eax
  sub    $0x1,%eax
  and    %eax,-0x18(%rbp)
L199:
  cmpl   $0x7,-0x14(%rbp)
  jg     L121
L175:
  addl   $0x1,-0xc(%rbp)
L39:
  mov    -0xc(%rbp),%eax
  cmp    -0x4(%rbp),%eax
  jl     L115
  mov    -0x8(%rbp),%eax
  cmp    -0x34(%rbp),%eax
  jge    L167
  mov    -0x8(%rbp),%eax
  lea    0x1(%rax),%edx
  mov    %edx,-0x8(%rbp)
  movslq %eax,%rdx
  mov    -0x28(%rbp),%rax
  add    %rdx,%rax
  movb   $0x0,(%rax)
L167:
  mov    -0x8(%rbp),%eax
L107:
  leave
  ret
```

assistant: ```c
int base64decode(void *dst, char *src, int maxlen) {
  int bitval;
  int bits;
  int val;
  int len;
  int x;
  int y;
  if (!dst || !src)
    return -1;
  len = strlen(src);
  bitval = 0;
  bits = 0;
  y = 0;
  for (x = 0; x < len; x++) {
    if ((src[x] >= 'A') && (src[x] <= 'Z'))
      val = src[x] - 'A';
    else if ((src[x] >= 'a') && (src[x] <= 'z'))
      val = src[x] - 'a' + 26;
    else if ((src[x] >= '0') && (src[x] <= '9'))
      val = src[x] - '0' + 52;
    else if ('+' == src[x])
      val = 62;
    else if ('-' == src[x])
      val = 63;
    else
      val = -1;
    if (val >= 0) {
      bitval <<= 6;
      bitval += val;
      bits += 6;
      while (bits >= 8) {
        if (y < maxlen)
          ((char *)dst)[y++] = (bitval >> (bits - 8)) & 0xff;
        bits -= 8;
        bitval &= (1 << bits) - 1;
      }
    }
  }
  if (y < maxlen)
    ((char *)dst)[y++] = 0;
  return y;
}
```