A safer way to implement boolean flags

Introduction

Boolean flags are commonly used to disable a block of code while another is running: for example,

  private bool in_use;
  
  private void Process() {
    in_use = true;
    (...)
    in_use = false;
  }
  
  private void OnEvent() {
    if (in_use)
      return;
      
    (...)
  }
  Private InUse As Boolean;
  
  Private Sub Process()
    InUse = True
    (...)
    InUse = False
  End Sub
  
  Private Sub Process()
    If InUse Then Return
    
    (...)
  End Sub

This design has a major drawback : in_use blocks cannot be nested, since nesting two such blocks will turn in_use to false too early. In the following excerpt, the (...) section in the Process function will not run correctly, because in_use will have been set to false when it runs.

  private void Process() {
    in_use = true;
    Action1();
    Action2();
    (...) // in_use == false here (!)
    in_use = false;
  }

  private void Action1() {
    in_use = true;
    (...)
    in_use = false;
  }

  private void Action2() {
    in_use = true;
    (...)
    in_use = false;
  }
  Private Sub Process()
    InUse = True
    Action1()
    Action2()
    (...) ' InUse = False here (!)
    InUse = False
  End Sub

  Private Sub Action1()
    InUse = True
    (...)
    InUse = False
  End Sub

  Private Sub Action2()
    InUse = True
    (...)
    InUse = False
  End Sub

Such errors are difficult to spot in large applications, and often lead to hard-to-track bugs.

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Unscrambling shuffled text

A story which surfaced a few years ago, and met quite some success in the press and on the internet, pretended Cambridge University had been conducing research on some of the most amazing faculties of the human brain. According to a supposed study, the order in which letters were laid out when writing a word mattered very little, provided the first and last letter be kept in place : this conclusion was supported by a short excerpt of shuffled text, which anyone could easily decipher. As a short example, consider the following sentence:

Narlmloy, radneig tihs shdulon’t be too hrad.

As many commentators pointed out at the time, the trick works well because the words used are relatively short; the following passage should be much harder to understand:

The eofrft rureieqd to sfssllcceuuy dhiepecr sbecmrald pgsaases daiaarclltmy ianserecs as wdors get lehegtinr.

This article presents an efficient algorithm to unshuffle scrambled text.

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