Fixing the Sorting Algorithm for Android, Java and Python

Tim Peters developed the Timsort hybrid sorting algorithm in 2002. TimSort was first developed for Python, a popular programming language, but later ported to Java (where it appears as java.util.Collections.sort and java.util.Arrays.sort). TimSort is today used as the default sorting algorithm in Java, in Android (a widely used platform by Google for mobile devices), in Python and many other programming languages and frameworks. Given the popularity of these platforms this means that the number of computers, cloud services and mobile phones that use TimSort for sorting is well into the billions. After we had successfully verified Counting and Radix sort implementations in Java [1] with a formal verification tool called KeY, we were looking for a new challenge. TimSort seemed to fit the bill, as it is rather complex and widely used. Unfortunately, we weren’t able to prove its correctness. A closer analysis showed that this was, quite simply, because

Fixing the Sorting Algorithm for Android, Java and Python

Algorithms and the Foundations of Software technolog

Fixing the Sorting Algorithm for Android, Java and Python

Tim Peters developed the Timsort hybrid sorting algorithm in 2002. TimSort was first developed for Python, a popular programming language, but later ported to Java (where it appears as java.util.Collections.sort and java.util.Arrays.sort). TimSort is today used as the default sorting algorithm in Java, in Android (a widely used platform by Google for mobile devices), in Python and many other programming languages and frameworks. Given the popularity of these platforms this means that the number of computers, cloud services and mobile phones that use TimSort for sorting is well into the billions. After we had successfully verified Counting and Radix sort implementations in Java [1] with a formal verification tool called KeY, we were looking for a new challenge. TimSort seemed to fit the bill, as it is rather complex and widely used. Unfortunately, we weren’t able to prove its correctness. A closer analysis showed that this was, quite simply, because TimSort was broken and our theo