Amey-Thakur Scientific debugging
A bug is a disagreement between what you believe the program does and what it actually does. Editing at random until the symptom moves papers over that disagreement: the test goes green and you still cannot say why. Treating each belief as a claim you actively try to disprove keeps every step informative and every green result earned.
Method
- Pin the symptom in exact values. Record the precise input, the observed
output, and the expected output on one line: "
discount(100, 0.2)returns 0, expected 80." A numeric symptom is testable; "discounts look off" is not. - Write one hypothesis that can be false. "The rate arrives as a string and multiplies to 0" is refutable. "Something with types" is not. Hold to a single hypothesis per cycle, or no probe can tell you which claim it answered.
- Predict the reading before you run. Commit out loud: if this is right,
type(rate)isstrand the product is"". Writing the expected value first stops you from rationalizing whatever the probe happens to show. - Probe with the sharpest discriminating test. Add one assertion or one log at the exact boundary the hypothesis names. If a probe prints the same thing whether you are right or wrong, it discriminates nothing: replace it.
- Bisect the cause space between known-good points. You trust the input and distrust the output. Probe the midpoint of the call chain, keep the half still showing the fault, and repeat. Each cycle halves suspects, never shifts them sideways.
- Keep a three-column log: hypothesis, prediction, result. A refuted hypothesis is progress because it deletes a region of the search space for good. The log is what stops you retesting the same dead end at 2 a.m.
- Close only when the fix reproduces your prediction. The patch is done when the value you predicted appears and the symptom is gone, not when the symptom happens to vanish and you shrug.
Litmus tests
- Can you state the single observation that would prove your current hypothesis wrong, and have you gone and looked for it?
- Does your log show the suspect region strictly shrinking, probe by probe?
- When an edit made the bug disappear, can you name which hypothesis it confirmed?
Boundaries
A hypothesis you cannot trigger on demand is a guess with better grammar: stabilize the failure with reproduction-first before you reason about it. When the question is which commit introduced the regression, hand the search to git-bisect rather than probing by hand.