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

```asm
<str_decode>:
  endbr64
  push   %rbp
  mov    %rsp,%rbp
  push   %r15
  push   %r14
  push   %r13
  push   %r12
  push   %rbx
  sub    $0x18,%rsp
  mov    %rdi,%r14
  mov    %rsi,%rbx
  mov    %rbx,%rdi
  mov    $0x0,%eax
  call   <ignorestr@plt>
  test   %eax,%eax
  je     L349
  mov    %rbx,%rsi
  mov    %r14,%rdi
  call   <strcpy@plt>
  jmp    L45
L349:
  mov    $0x0,%eax
  mov    %eax,-0x3c(%rbp)
  mov    %eax,-0x38(%rbp)
  mov    %eax,-0x34(%rbp)
  jmp    L271
L98:
  cmp    $0x3f,%r12d
  jne    L156
  lea    0x1(%rbx),%rax
  movzbl (%rax),%eax
  cmp    $0x3d,%al
  jne    L156
  add    $0x1,%rbx
  jmp    L250
L156:
  cmp    $0xa,%r12d
  jne    L224
  add    $0x1,%rbx
  mov    $0x0,%eax
  mov    %eax,-0x3c(%rbp)
  mov    %eax,-0x38(%rbp)
  mov    %eax,-0x34(%rbp)
  jmp    L250
L224:
  cmpl   $0x0,-0x38(%rbp)
  je     L291
  cmp    $0x3d,%r12d
  jne    L291
  add    $0x1,%rbx
  movzbl (%rbx),%eax
  movzbl %al,%r12d
  add    $0x1,%rbx
  movzbl (%rbx),%eax
  movzbl %al,%r13d
  mov    %r12d,%edi
  mov    $0x0,%eax
  call   <qp_code@plt>
  shl    $0x4,%eax
  mov    %eax,%r12d
  mov    %r13d,%edi
  mov    $0x0,%eax
  call   <qp_code@plt>
  mov    %r12d,%edx
  or     %eax,%edx
  mov    %r14,%rax
  lea    0x1(%rax),%r14
  mov    %dl,(%rax)
  jmp    L250
L291:
  cmpl   $0x0,-0x3c(%rbp)
  je     L277
  cmpl   $0x0,-0x34(%rbp)
  jne    L277
  jmp    L175
L179:
  add    $0x1,%rbx
  movzbl (%rbx),%eax
  movzbl %al,%r12d
L175:
  mov    %r12d,%edi
  call   <isspace@plt>
  test   %eax,%eax
  jne    L179
  test   %r12d,%r12d
  je     L139
L163:
  add    $0x1,%rbx
  movzbl (%rbx),%eax
  movzbl %al,%r13d
  mov    %r13d,%edi
  call   <isspace@plt>
  test   %eax,%eax
  jne    L163
  test   %r13d,%r13d
  je     L424
L481:
  add    $0x1,%rbx
  movzbl (%rbx),%eax
  movzbl %al,%r15d
  mov    %r15d,%edi
  call   <isspace@plt>
  test   %eax,%eax
  jne    L481
  test   %r15d,%r15d
  je     L251
L486:
  add    $0x1,%rbx
  movzbl (%rbx),%eax
  movzbl %al,%eax
  mov    %eax,-0x40(%rbp)
  mov    %eax,%edi
  call   <isspace@plt>
  test   %eax,%eax
  jne    L486
  cmpl   $0x0,-0x40(%rbp)
  je     L428
  cmp    $0x3d,%r12d
  je     L159
  cmp    $0x3d,%r13d
  jne    L484
L159:
  movl   $0x1,-0x34(%rbp)
  jmp    L250
L484:
  mov    %r13d,%edi
  mov    $0x0,%eax
  call   <base64_code@plt>
  mov    %eax,%r13d
  mov    %r12d,%edi
  mov    $0x0,%eax
  call   <base64_code@plt>
  shl    $0x2,%eax
  mov    %eax,%edx
  mov    %r13d,%eax
  sar    $0x4,%eax
  and    $0x3,%eax
  or     %eax,%edx
  mov    %r14,%rax
  lea    0x1(%rax),%r14
  mov    %dl,(%rax)
  cmp    $0x3d,%r15d
  jne    L326
  movl   $0x1,-0x34(%rbp)
  jmp    L250
L326:
  mov    %r15d,%edi
  mov    $0x0,%eax
  call   <base64_code@plt>
  mov    %eax,%r15d
  mov    %r13d,%eax
  shl    $0x4,%eax
  mov    %eax,%edx
  mov    %r15d,%eax
  sar    $0x2,%eax
  and    $0xf,%eax
  or     %eax,%edx
  mov    %r14,%rax
  lea    0x1(%rax),%r14
  mov    %dl,(%rax)
  cmpl   $0x3d,-0x40(%rbp)
  jne    L368
  movl   $0x1,-0x34(%rbp)
  jmp    L250
L368:
  mov    %r15d,%eax
  shl    $0x6,%eax
  mov    %eax,%r12d
  mov    -0x40(%rbp),%edi
  mov    $0x0,%eax
  call   <base64_code@plt>
  mov    %r12d,%edx
  or     %eax,%edx
  mov    %r14,%rax
  lea    0x1(%rax),%r14
  mov    %dl,(%rax)
  jmp    L250
L277:
  cmp    $0x3d,%r12d
  jne    L320
  lea    0x1(%rbx),%rax
  movzbl (%rax),%eax
  cmp    $0x3f,%al
  jne    L320
  mov    $0x0,%r15d
  mov    %r15d,%r13d
L367:
  add    $0x1,%rbx
  movzbl (%rbx),%eax
  movzbl %al,%r12d
  test   %r12d,%r12d
  je     L289
  cmp    $0x3f,%r12d
  je     L436
  cmp    $0x2,%r13d
  jne    L367
  mov    %r12d,%r15d
  or     $0x20,%r15d
  jmp    L367
L436:
  add    $0x1,%r13d
  cmp    $0x2,%r13d
  jg     L30
  jmp    L367
L289:
  nop
  jmp    L149
L30:
  nop
L149:
  cmp    $0x71,%r15d
  jne    L157
  movl   $0x1,-0x38(%rbp)
  jmp    L375
L157:
  cmp    $0x62,%r15d
  jne    L375
  movl   $0x1,-0x3c(%rbp)
  jmp    L375
L320:
  mov    %r14,%rax
  lea    0x1(%rax),%r14
  mov    %r12d,%edx
  mov    %dl,(%rax)
  jmp    L250
L375:
  nop
L250:
  add    $0x1,%rbx
L271:
  movzbl (%rbx),%eax
  movzbl %al,%r12d
  test   %r12d,%r12d
  jne    L98
  jmp    L101
L139:
  nop
  jmp    L101
L424:
  nop
  jmp    L101
L251:
  nop
  jmp    L101
L428:
  nop
L101:
  movb   $0x0,(%r14)
L45:
  add    $0x18,%rsp
  pop    %rbx
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  pop    %rbp
  ret
```

assistant: ```c
void str_decode(dst, src) register unsigned char *dst, *src;
{
  register int is_qp, is_base64, is_done;
  register int c1, c2, c3, c4;
  if (ignorestr(src)) {
    strcpy(dst, src);
    return;
  }
  for (is_done = is_qp = is_base64 = 0; (c1 = *src); src++) {
    if (c1 == '?' && src[1] == '=') {
      src++;
      continue;
    } else if (c1 == '\n') {
      src++;
      is_done = is_qp = is_base64 = 0;
      continue;
    } else if (is_qp && c1 == '=') {
      c1 = *++src;
      c2 = *++src;
      *dst++ = (qp_code(c1) << 4) | qp_code(c2);
    } else if (is_base64 && !is_done) {
      while (isspace(c1)) {
        c1 = *++src;
      }
      if (!c1)
        break;
      do {
        c2 = *++src;
      } while (isspace(c2));
      if (!c2)
        break;
      do {
        c3 = *++src;
      } while (isspace(c3));
      if (!c3)
        break;
      do {
        c4 = *++src;
      } while (isspace(c4));
      if (!c4)
        break;
      if (c1 == '=' || c2 == '=') {
        is_done = 1;
        continue;
      }
      c2 = base64_code(c2);
      *dst++ = (base64_code(c1) << 2) | ((c2 & 0x30) >> 4);
      if (c3 == '=')
        is_done = 1;
      else {
        c3 = base64_code(c3);
        *dst++ = ((c2 & 0xF) << 4) | ((c3 & 0x3c) >> 2);
        if (c4 == '=')
          is_done = 1;
        else {
          *dst++ = ((c3 & 0x03) << 6) | base64_code(c4);
        }
      }
    } else if ((c1 == '=') && (src[1] == '?')) {
      c2 = c3 = 0;
      for (;;) {
        c1 = *++src;
        if (c1 == 0)
          break;
        if (c1 != '?') {
          if (c2 == 2)
            c3 = c1 | 0x20;
        } else {
          if (++c2 >= 3)
            break;
        }
      }
      if (c3 == 'q')
        is_qp = 1;
      else if (c3 == 'b')
        is_base64 = 1;
    } else
      *dst++ = c1;
  }
  *dst = '\0';
}
```