Detect Infinite Loop Program

Aidan Dyga, Molly Suppo, Javier Bejarano-Jimenez

September 8, 2025

Introduction

• Goal: Implement DetectInfiniteLoop to flag source code that will never terminate.
• Motivation: Infinite loops waste computational resources.
• Examples of loops:
  • while(True)
  • for i in range(10)

What is an Infinite Loop?

• Definition: Code that repeats forever without reaching a stopping point.
• Key signs: No reachable exit condition, exit is impossible due to logic, waiting for an event that never happens
• Common causes: Loop variable never changes, unreachable return statement, waiting for an event that never occurs

Is this Problem Tractable, Intractable, or Uncomputable?

• Answer: Uncomputable in the general case.
• Halting Problem: It is mathematically proven that no algorithm can always decide whether an arbitrary program will halt or run forever.
• In Practice: We can only detect some infinite loops using patterns and heuristics.

Solutions

• Static Analyzer: A tool that examines source code without running it, searching for bugs, unreachable code, or infinite loops. (ex. Vulture)
• How They Work: Use pattern matching and heuristics to flag suspicious code (e.g., while True, loops with constant conditions).
• Our Approach: Implement our very own basic static analyzer.

Python Implementation

Limitations

• Not complete: Our program cannot detect every possible infinite loop (Halting Problem).
• False positives: Code may look infinite (e.g., while True) but actually breaks out later.
• False negatives: Some infinite loops depend on logic that’s too complex to analyze (e.g., recursion, user input, random values).

More Limitations

• Example False Negative:

  i = 1
  while i != 0:
      i = (i * 2) % 5

  Looks like it will end, but runs forever.
  Our program would incorrectly say: “No infinite loop detected.”

• No runtime awareness: The analyzer only looks at code patterns — it does not execute the program.

• Scope: Focuses only on simple patterns (while True, infinite iterators) and may miss more subtle cases.

Conclusion

• Impossible: Detecting all infinite loops is impossible due to the Halting Problem.
• Partial Solution: Heuristics can still catch many real bugs.
• Takeaway: Use static analysis to flag obvious cases, but undecidable in general.

Sources

• Course Textbook
• GitHub Copilot
• Microsoft Copilot