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

```asm
<str_decode>:
  endbr64
  push   %r15
  mov    %rsi,%r15
  push   %r14
  push   %r13
  push   %r12
  push   %rbp
  push   %rbx
  sub    $0x138,%rsp
  mov    %rdi,0x8(%rsp)
  mov    %rsi,%rdi
  mov    %fs:0x28,%rax
  mov    %rax,0x128(%rsp)
  xor    %eax,%eax
  call   <ignorestr@plt>
  test   %rax,%rax
  jne    L406
  lea    0x20(%rsp),%rdi
  mov    $0x100,%edx
  mov    %r15,%rsi
  xor    %eax,%eax
  mov    %rdi,0x10(%rsp)
  mov    %r15,%rbx
  call   <memcpy@plt>
  movzbl (%r15),%r13d
  mov    0x8(%rsp),%r12
  mov    %r13d,%eax
  test   %r13d,%r13d
  je     L522
  movl   $0x0,0x18(%rsp)
  xor    %r14d,%r14d
  movl   $0x0,0x4(%rsp)
  jmp    L187
  nopw   0x0(%rax,%rax,1)
L232:
  cmp    $0xa,%r13d
  je     L528
  test   %r14d,%r14d
  je     L214
  cmp    $0x3d,%r13d
  jne    L214
  lea    0x1(%r15),%rax
  movzbl 0x1(%r15),%edi
  sub    %rbx,%rax
  cmp    $0xfe,%rax
  jg     L300
  lea    0x2(%r15),%r13
  movzbl 0x2(%r15),%r15d
  add    $0x1,%r12
  call   <qp_code@plt>
  mov    %eax,%ebp
  mov    %r15d,%edi
  call   <qp_code@plt>
  shl    $0x4,%ebp
  or     %eax,%ebp
  mov    %bpl,-0x1(%r12)
L170:
  lea    0x1(%r13),%r15
  movzbl 0x1(%r13),%r13d
  mov    %r13d,%eax
  test   %r13d,%r13d
  je     L522
  mov    %r15,%rdx
  sub    %rbx,%rdx
  cmp    $0xfe,%rdx
  jg     L300
L187:
  cmp    $0x3f,%r13d
  jne    L232
  cmpb   $0x3d,0x1(%r15)
  je     L415
  mov    0x4(%rsp),%ecx
  test   %ecx,%ecx
  je     L453
  mov    0x18(%rsp),%edx
  test   %edx,%edx
  je     L126
L453:
  mov    %al,(%r12)
  mov    %r15,%r13
  add    $0x1,%r12
  jmp    L170
  nopl   (%rax)
L559:
  mov    %r15,%rax
  sub    %rbx,%rax
  cmp    $0xfe,%rax
  jg     L300
  movzbl 0x1(%r15),%r13d
  add    $0x1,%r15
L126:
  mov    %r13d,%edi
  call   <isspace@plt>
  test   %rax,%rax
  jne    L559
  test   %r13d,%r13d
  jne    L54
  jmp    L522
  cs nopw 0x0(%rax,%rax,1)
L521:
  movzbl 0x1(%r15),%ebp
  add    $0x1,%r15
  mov    %ebp,%edi
  call   <isspace@plt>
  test   %rax,%rax
  je     L87
L54:
  mov    %r15,%rax
  sub    %rbx,%rax
  cmp    $0xfe,%rax
  jle    L521
L300:
  mov    0x10(%rsp),%rsi
  mov    0x8(%rsp),%rdi
  mov    $0x100,%edx
  xor    %eax,%eax
  call   <memcpy@plt>
L484:
  mov    0x128(%rsp),%rax
  sub    %fs:0x28,%rax
  jne    L161
  add    $0x138,%rsp
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
  cs nopw 0x0(%rax,%rax,1)
L528:
  movl   $0x0,0x18(%rsp)
  lea    0x1(%r15),%r13
  xor    %r14d,%r14d
  movl   $0x0,0x4(%rsp)
  jmp    L170
  nopl   0x0(%rax)
L214:
  mov    0x4(%rsp),%edi
  test   %edi,%edi
  je     L213
  mov    0x18(%rsp),%esi
  test   %esi,%esi
  je     L126
L213:
  cmp    $0x3d,%r13d
  jne    L453
  movzbl 0x1(%r15),%edx
  cmp    $0x3f,%dl
  jne    L453
  mov    %r15,%rax
  sub    %rbx,%rax
  cmp    $0xfe,%rax
  jg     L300
  xor    %edi,%edi
  xor    %ecx,%ecx
  jmp    L482
  nopl   0x0(%rax)
L348:
  or     $0x20,%edx
  cmp    $0x2,%ecx
  movzbl %dl,%edx
  cmove  %edx,%edi
L131:
  mov    %r15,%rax
  sub    %rbx,%rax
  cmp    $0xfe,%rax
  jg     L300
  movzbl 0x1(%r15),%edx
L482:
  add    $0x1,%r15
  cmp    $0x3f,%dl
  jne    L348
  add    $0x1,%ecx
  cmp    $0x2,%ecx
  jle    L131
  mov    %r15,%r13
  cmp    $0x71,%edi
  je     L421
  cmp    $0x62,%edi
  mov    $0x1,%eax
  cmovne 0x4(%rsp),%eax
  mov    %eax,0x4(%rsp)
  jmp    L170
  xchg   %ax,%ax
L406:
  mov    0x128(%rsp),%rax
  sub    %fs:0x28,%rax
  jne    L161
  mov    0x8(%rsp),%rdi
  add    $0x138,%rsp
  mov    %r15,%rsi
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  jmp    10c0 <strcpy@plt>
  nopl   (%rax)
L415:
  lea    0x1(%r15),%r13
  jmp    L170
  nopl   0x0(%rax)
L87:
  test   %ebp,%ebp
  jne    L412
  jmp    L522
  xchg   %ax,%ax
L320:
  movzbl 0x1(%r15),%r9d
  add    $0x1,%r15
  mov    %r9d,%edi
  mov    %r9d,0x18(%rsp)
  call   <isspace@plt>
  mov    0x18(%rsp),%r9d
  test   %rax,%rax
  je     L322
L412:
  mov    %r15,%rax
  sub    %rbx,%rax
  cmp    $0xfe,%rax
  jle    L320
  jmp    L300
  nopl   0x0(%rax,%rax,1)
L522:
  movb   $0x0,(%r12)
  jmp    L484
  nopw   0x0(%rax,%rax,1)
L322:
  test   %r9d,%r9d
  jne    L291
  jmp    L522
  nop
L349:
  movzbl 0x1(%r15),%r10d
  add    $0x1,%r15
  mov    %r10d,%edi
  mov    %r10d,0x1c(%rsp)
  call   <isspace@plt>
  mov    0x1c(%rsp),%r10d
  mov    0x18(%rsp),%r9d
  test   %rax,%rax
  je     L162
L291:
  mov    %r15,%rax
  mov    %r9d,0x18(%rsp)
  sub    %rbx,%rax
  cmp    $0xfe,%rax
  jle    L349
  jmp    L300
L162:
  test   %r10d,%r10d
  mov    %r9d,0x1c(%rsp)
  mov    %r10d,0x18(%rsp)
  je     L522
  cmp    $0x3d,%r13d
  je     L547
  cmp    $0x3d,%ebp
  je     L547
  mov    %ebp,%edi
  call   <base64_code@plt>
  mov    %r13d,%edi
  mov    %eax,%ebp
  call   <base64_code@plt>
  mov    0x1c(%rsp),%r9d
  mov    0x18(%rsp),%r10d
  mov    %eax,%r8d
  mov    %ebp,%eax
  sar    $0x4,%eax
  lea    0x0(,%r8,4),%edx
  and    $0x3,%eax
  or     %edx,%eax
  cmp    $0x3d,%r9d
  mov    %al,(%r12)
  jne    L533
  movl   $0x1,0x18(%rsp)
  add    $0x1,%r12
  mov    %r15,%r13
  jmp    L170
L547:
  movl   $0x1,0x18(%rsp)
  mov    %r15,%r13
  jmp    L170
L533:
  mov    %r9d,%edi
  mov    %r10d,0x18(%rsp)
  shl    $0x4,%ebp
  call   <base64_code@plt>
  mov    0x18(%rsp),%r10d
  mov    %eax,%r13d
  sar    $0x2,%eax
  and    $0xf,%eax
  or     %ebp,%eax
  mov    %al,0x1(%r12)
  cmp    $0x3d,%r10d
  jne    L152
  movl   $0x1,0x18(%rsp)
  add    $0x2,%r12
  mov    %r15,%r13
  jmp    L170
L421:
  mov    $0x1,%r14d
  jmp    L170
L152:
  mov    %r10d,%edi
  add    $0x3,%r12
  call   <base64_code@plt>
  mov    %r13d,%edx
  movl   $0x0,0x18(%rsp)
  mov    %r15,%r13
  shl    $0x6,%edx
  or     %eax,%edx
  mov    %dl,-0x1(%r12)
  jmp    L170
L161:
  call   <__stack_chk_fail@plt>
```

assistant: ```c
void str_decode(unsigned char *dst, unsigned char *src) {
  register int is_qp, is_base64, is_done;
  register int c1, c2, c3, c4;
  char *dsttmp;
  char *srctmp;
  char srcbuf[256];
  long pos;
  if (ignorestr((char *)src)) {
    strcpy((char *)dst, (char *)src);
    return;
  }
  memcpy(srcbuf, src, 256);
  dsttmp = (char *)dst;
  srctmp = (char *)src;
  pos = (long)src - (long)srctmp;
  for (is_done = is_qp = is_base64 = 0; (c1 = *src) != 0; src++) {
    pos = (long)src - (long)srctmp;
    if (pos > 254) {
      memcpy(dsttmp, srcbuf, 256);
      return;
    }
    if (c1 == '?' && src[1] == '=') {
      src++;
      continue;
    } else if (c1 == '\n') {
      src++;
      is_done = is_qp = is_base64 = 0;
      continue;
    } else if (is_qp && c1 == '=') {
      pos = (long)src - (long)srctmp;
      if (pos > 254) {
        memcpy(dsttmp, srcbuf, 256);
        return;
      }
      c1 = *++src;
      pos = (long)src - (long)srctmp;
      if (pos > 254) {
        memcpy(dsttmp, srcbuf, 256);
        return;
      }
      c2 = *++src;
      *dst++ = (qp_code(c1) << 4) | qp_code(c2);
    } else if (is_base64 && !is_done) {
      while (isspace(c1)) {
        pos = (long)src - (long)srctmp;
        if (pos > 254) {
          memcpy(dsttmp, srcbuf, 256);
          return;
        }
        c1 = *++src;
      }
      if (!c1)
        break;
      do {
        pos = (long)src - (long)srctmp;
        if (pos > 254) {
          memcpy(dsttmp, srcbuf, 256);
          return;
        }
        c2 = *++src;
      } while (isspace(c2));
      if (!c2)
        break;
      do {
        pos = (long)src - (long)srctmp;
        if (pos > 254) {
          memcpy(dsttmp, srcbuf, 256);
          return;
        }
        c3 = *++src;
      } while (isspace(c3));
      if (!c3)
        break;
      do {
        pos = (long)src - (long)srctmp;
        if (pos > 254) {
          memcpy(dsttmp, srcbuf, 256);
          return;
        }
        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 (;;) {
        pos = (long)src - (long)srctmp;
        if (pos > 254) {
          memcpy(dsttmp, srcbuf, 256);
          return;
        }
        c1 = *++src;
        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';
}
```