user: Please show me the C implementation that matches the assembly code listed below:

```asm
<str_decode>:
  endbr64
  push   %r15
  push   %r14
  push   %r13
  push   %r12
  push   %rbp
  push   %rbx
  sub    $0x138,%rsp
  mov    %rdi,0x10(%rsp)
  mov    %rsi,%rbx
  mov    %fs:0x28,%rax
  mov    %rax,0x128(%rsp)
  xor    %eax,%eax
  mov    %rsi,%rdi
  call   <ignorestr@plt>
  test   %rax,%rax
  jne    L46
  lea    0x20(%rsp),%rdi
  mov    $0x100,%edx
  mov    %rbx,%rsi
  mov    $0x0,%eax
  call   <memcpy@plt>
  mov    %rbx,%r12
  movzbl (%rbx),%eax
  movzbl %al,%ebp
  test   %ebp,%ebp
  je     L272
  mov    0x10(%rsp),%r13
  movl   $0x0,0x18(%rsp)
  movl   $0x0,0xc(%rsp)
  mov    $0x0,%r15d
  jmp    L521
L46:
  mov    %rbx,%rsi
  mov    0x10(%rsp),%rdi
  call   <strcpy@plt>
L536:
  mov    0x128(%rsp),%rax
  sub    %fs:0x28,%rax
  jne    L438
  add    $0x138,%rsp
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
L272:
  mov    0x10(%rsp),%r13
L273:
  movb   $0x0,0x0(%r13)
  jmp    L536
L355:
  lea    0x20(%rsp),%rsi
  mov    $0x100,%edx
  mov    0x10(%rsp),%rdi
  mov    $0x0,%eax
  call   <memcpy@plt>
  jmp    L536
L523:
  cmp    $0x3d,%ebp
  sete   %dl
  cmpb   $0x3d,0x1(%rbx)
  je     L238
L313:
  cmpl   $0x0,0xc(%rsp)
  je     L409
  cmpl   $0x0,0x18(%rsp)
  je     L228
L409:
  test   %dl,%dl
  jne    L33
L8:
  mov    %al,0x0(%r13)
  mov    %rbx,%rbp
  lea    0x1(%r13),%r13
L454:
  lea    0x1(%rbp),%rbx
  movzbl 0x1(%rbp),%eax
  movzbl %al,%ebp
  test   %ebp,%ebp
  je     L273
  mov    %rbx,%rdx
  sub    %r12,%rdx
  cmp    $0xfe,%rdx
  jg     L355
L521:
  cmp    $0x3f,%ebp
  je     L523
  cmp    $0xa,%ebp
  je     L67
  cmp    $0x3d,%ebp
  sete   %dl
  test   %r15d,%r15d
  je     L313
  test   %dl,%dl
  je     L313
  movzbl 0x1(%rbx),%edi
  lea    0x1(%rbx),%rax
  sub    %r12,%rax
  cmp    $0xfe,%rax
  jg     L538
  lea    0x2(%rbx),%rbp
  movzbl 0x2(%rbx),%ebx
  call   <qp_code@plt>
  mov    %eax,%r14d
  mov    %ebx,%edi
  call   <qp_code@plt>
  shl    $0x4,%r14d
  or     %eax,%r14d
  mov    %r14b,0x0(%r13)
  lea    0x1(%r13),%r13
  jmp    L454
L238:
  lea    0x1(%rbx),%rbp
  jmp    L454
L67:
  lea    0x1(%rbx),%rbp
  movl   $0x0,0x18(%rsp)
  movl   $0x0,0xc(%rsp)
  mov    $0x0,%r15d
  jmp    L454
L538:
  lea    0x20(%rsp),%rsi
  mov    $0x100,%edx
  mov    0x10(%rsp),%rdi
  mov    $0x0,%eax
  call   <memcpy@plt>
  jmp    L536
L375:
  lea    0x20(%rsp),%rsi
  mov    $0x100,%edx
  mov    0x10(%rsp),%rdi
  mov    $0x0,%eax
  call   <memcpy@plt>
  jmp    L536
L228:
  mov    %ebp,%edi
  call   <isspace@plt>
  test   %rax,%rax
  je     L518
  mov    %rbx,%rax
  sub    %r12,%rax
  cmp    $0xfe,%rax
  jg     L375
  add    $0x1,%rbx
  movzbl (%rbx),%ebp
  jmp    L228
L518:
  test   %ebp,%ebp
  je     L273
L95:
  mov    %rbx,%rax
  sub    %r12,%rax
  cmp    $0xfe,%rax
  jg     L247
  add    $0x1,%rbx
  movzbl (%rbx),%r14d
  mov    %r14d,%edi
  call   <isspace@plt>
  test   %rax,%rax
  jne    L95
  test   %r14d,%r14d
  je     L273
L261:
  mov    %rbx,%rax
  sub    %r12,%rax
  cmp    $0xfe,%rax
  jg     L322
  add    $0x1,%rbx
  movzbl (%rbx),%eax
  mov    %eax,0x18(%rsp)
  mov    %eax,%edi
  call   <isspace@plt>
  test   %rax,%rax
  jne    L261
  cmpl   $0x0,0x18(%rsp)
  je     L273
L484:
  mov    %rbx,%rax
  sub    %r12,%rax
  cmp    $0xfe,%rax
  jg     L217
  add    $0x1,%rbx
  movzbl (%rbx),%eax
  mov    %eax,0x1c(%rsp)
  mov    %eax,%edi
  call   <isspace@plt>
  test   %rax,%rax
  jne    L484
  cmpl   $0x0,0x1c(%rsp)
  je     L273
  cmp    $0x3d,%ebp
  je     L556
  cmp    $0x3d,%r14d
  je     L556
  mov    %r14d,%edi
  call   <base64_code@plt>
  mov    %eax,%r14d
  mov    %ebp,%edi
  call   <base64_code@plt>
  mov    %eax,%edx
  mov    %r14d,%eax
  sar    $0x4,%eax
  and    $0x3,%eax
  shl    $0x2,%edx
  or     %edx,%eax
  mov    %al,0x0(%r13)
  cmpl   $0x3d,0x18(%rsp)
  jne    L134
  add    $0x1,%r13
  mov    %rbx,%rbp
  movl   $0x1,0x18(%rsp)
  jmp    L454
L247:
  lea    0x20(%rsp),%rsi
  mov    $0x100,%edx
  mov    0x10(%rsp),%rdi
  mov    $0x0,%eax
  call   <memcpy@plt>
  jmp    L536
L322:
  lea    0x20(%rsp),%rsi
  mov    $0x100,%edx
  mov    0x10(%rsp),%rdi
  mov    $0x0,%eax
  call   <memcpy@plt>
  jmp    L536
L217:
  lea    0x20(%rsp),%rsi
  mov    $0x100,%edx
  mov    0x10(%rsp),%rdi
  mov    $0x0,%eax
  call   <memcpy@plt>
  jmp    L536
L134:
  mov    0x18(%rsp),%edi
  call   <base64_code@plt>
  mov    %eax,%ebp
  sar    $0x2,%eax
  and    $0xf,%eax
  shl    $0x4,%r14d
  or     %r14d,%eax
  mov    %al,0x1(%r13)
  cmpl   $0x3d,0x1c(%rsp)
  jne    L444
  add    $0x2,%r13
  mov    %rbx,%rbp
  movl   $0x1,0x18(%rsp)
  jmp    L454
L444:
  mov    0x1c(%rsp),%edi
  call   <base64_code@plt>
  shl    $0x6,%ebp
  or     %eax,%ebp
  mov    %bpl,0x2(%r13)
  mov    %rbx,%rbp
  lea    0x3(%r13),%r13
  movl   $0x0,0x18(%rsp)
  jmp    L454
L33:
  cmpb   $0x3f,0x1(%rbx)
  jne    L8
  mov    %rbx,%rax
  sub    %r12,%rax
  cmp    $0xfe,%rax
  jg     L62
  mov    $0x0,%ecx
  mov    $0x0,%edx
  jmp    L10
L62:
  lea    0x20(%rsp),%rsi
  mov    $0x100,%edx
  mov    0x10(%rsp),%rdi
  mov    $0x0,%eax
  call   <memcpy@plt>
  jmp    L536
L7:
  add    $0x1,%edx
  cmp    $0x2,%edx
  jg     L224
L19:
  mov    %rbx,%rax
  sub    %r12,%rax
  cmp    $0xfe,%rax
  jg     L62
L10:
  add    $0x1,%rbx
  movzbl (%rbx),%eax
  cmp    $0x3f,%al
  je     L7
  or     $0x20,%eax
  movzbl %al,%eax
  cmp    $0x2,%edx
  cmove  %eax,%ecx
  jmp    L19
L224:
  cmp    $0x71,%ecx
  je     L190
  mov    %rbx,%rbp
  cmp    $0x62,%ecx
  mov    $0x1,%eax
  cmovne 0xc(%rsp),%eax
  mov    %eax,0xc(%rsp)
  jmp    L454
L556:
  mov    %rbx,%rbp
  movl   $0x1,0x18(%rsp)
  jmp    L454
L190:
  mov    %rbx,%rbp
  mov    $0x1,%r15d
  jmp    L454
L438:
  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';
}
```