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📜 WAYPOINT LESSON

A diff against intention

⭐⭐⭐ advanced⏳ 10 min read📍 Lesson 217 of 256

Spec first, code second: the bug is always 'code does X, spec says Y' — trace the cheapest failing case.

Debugging is a diff against intention. The broken method was written by someone competent; the bug is ONE decision, and the symptom usually appears far from the cause. The exam-style procedure:

  1. Read the spec first, the code second. The bug is always "code does X, spec says Y" — you cannot see the diff without both sides.
  2. Pick the cheapest failing case from the examples. Trace it by hand through the code — not the spec, the CODE.
  3. Locate the first line where trace and spec disagree. That line is the bug; everything after it is consequence.
  4. Name the bug class before fixing: boundary, wrong accumulator, inverted condition, missing reset, wrong tie-break. Naming prevents the "fixed it by accident" failure.

The classic subtle bugs, ranked by exam frequency: off-by-one loop bounds, < where the spec said <=, forgetting to reset a run-counter, >= where a tie-break demanded >, and mutating while iterating forward.