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

```asm
<str_decode>:
  endbr64
  push   %r15
  push   %r14
  push   %r13
  push   %r12
  push   %rbp
  push   %rbx
  sub    $0x218,%rsp
  mov    %rdi,%r15
  mov    %fs:0x28,%rax
  mov    %rax,0x208(%rsp)
  xor    %eax,%eax
  movzbl (%rdi),%r13d
  test   %r13b,%r13b
  je     L4
  mov    %rdi,%rbp
  mov    %rsp,%r12
  mov    $0x0,%r14d
  jmp    L205
L251:
  add    $0x1,%rbx
L143:
  movzbl (%rbx),%edi
  test   %dil,%dil
  je     L96
  movzbl %dil,%edi
  call   <isspace@plt>
  test   %eax,%eax
  jne    L251
L96:
  movzbl (%rbx),%eax
  cmp    $0x3d,%al
  je     L91
L62:
  add    $0x1,%rbp
  mov    %r13b,(%r12)
  lea    0x1(%r12),%r12
L232:
  movzbl 0x0(%rbp),%r13d
  test   %r13b,%r13b
  je     L36
L287:
  mov    %rsp,%rdx
  mov    %r12,%rax
  sub    %rdx,%rax
  cmp    $0x1fe,%rax
  ja     L36
L205:
  cmp    $0x3d,%r13b
  je     L66
  mov    %rbp,%rbx
  test   %r14d,%r14d
  jne    L143
  jmp    L62
L66:
  lea    0x1(%rbp),%rbx
  cmpb   $0x3f,0x1(%rbp)
  je     L235
L77:
  mov    %rbx,%rcx
  sub    %rbp,%rcx
  mov    $0x0,%eax
  cmp    %rbp,%rbx
  je     L232
L285:
  movzbl 0x0(%rbp,%rax,1),%edx
  mov    %dl,(%r12,%rax,1)
  add    $0x1,%rax
  cmp    %rcx,%rax
  jne    L285
  add    %rcx,%r12
  mov    %rbx,%rbp
  jmp    L232
L235:
  lea    0x2(%rbp),%rbx
  movzbl 0x2(%rbp),%eax
  test   %al,%al
  je     L249
  cmp    $0x3f,%al
  je     L249
L82:
  add    $0x1,%rbx
  movzbl (%rbx),%eax
  test   %al,%al
  je     L249
  cmp    $0x3f,%al
  jne    L82
L249:
  test   %al,%al
  je     L77
  movzbl 0x1(%rbx),%eax
  test   %al,%al
  je     L77
  cmpb   $0x3f,0x2(%rbx)
  jne    L77
  lea    0x3(%rbx),%r13
  movzbl %al,%esi
  mov    %r12,%rdx
  mov    %r13,%rdi
  mov    $0x0,%eax
  call   <mmdecode_header@plt>
  test   %eax,%eax
  js     L77
  jmp    L64
L54:
  mov    %rbp,%r13
L64:
  movzbl 0x0(%r13),%edx
  test   %dl,%dl
  je     L227
  lea    0x1(%r13),%rbp
  cmp    $0x3f,%dl
  jne    L54
  add    $0x2,%r13
  cmpb   $0x3d,0x0(%rbp)
  cmove  %r13,%rbp
  jmp    L169
L227:
  mov    %r13,%rbp
L169:
  cltq
  add    %rax,%r12
  mov    $0x1,%r14d
  jmp    L232
L91:
  mov    %eax,%r13d
  mov    %rbx,%rbp
  mov    $0x0,%r14d
  jmp    L287
L4:
  mov    %rsp,%r12
L36:
  movb   $0x0,(%r12)
  mov    %rsp,%rsi
  mov    %r15,%rdi
  call   <strcpy@plt>
  mov    0x208(%rsp),%rax
  sub    %fs:0x28,%rax
  jne    L117
  add    $0x218,%rsp
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
L117:
  call   <__stack_chk_fail@plt>
```

assistant: ```c
void str_decode(str) unsigned char *str;
{
  int adj;
  unsigned char *src, *dst;
  unsigned char buf[512];
  src = str;
  dst = buf;
  adj = 0;
  while (*src && (dst - buf) < sizeof(buf) - 1) {
    if (*src != '=') {
      unsigned char *tmp = src;
      while (adj && *tmp && isspace(*tmp))
        tmp++;
      if (adj && *tmp == '=') {
        adj = 0;
        src = tmp;
      } else
        *dst++ = *src++;
    } else {
      unsigned char *tmp = src + 1;
      if (*tmp == '?') {
        tmp++;
        while (*tmp && *tmp != '?')
          tmp++;
        if (*tmp && tmp[1] && tmp[2] == '?') {
          int i = mmdecode_header(tmp + 3, tmp[1], dst);
          if (i >= 0) {
            tmp += 3;
            while (*tmp && *tmp++ != '?')
              ;
            if (*tmp == '=')
              tmp++;
            src = tmp;
            dst += i;
            adj = 1;
          }
        }
      }
      while (src != tmp)
        *dst++ = *src++;
    }
  }
  *dst = 0;
  strcpy(str, buf);
}
```