By: Tom Sydney Kerckhove <syd@cs-syd.eu>
Mutation: order covering tests cheapest-first in the mutation child When a mutation child runs the tests that cover a mutation, it runs them synchronously under fail-fast, so the first covering test that fails kills the mutant. Ordering the covering tests cheapest-first, using the per-test baseline the coverage phase already measures (and previously discarded into a single timeout), reaches a killing test sooner and cuts the mutation phase's wall-clock, since killed mutants are the overwhelming majority of a run. On two sample projects the mutation phase ran ~6% and ~24% faster, with identical verdicts. - reorderTestForestByTiming: a pure, tree-aware, stable reorder. It sorts siblings by ascending baseline (missing = first) and orders each subtree by the minimum cost of any leaf it contains, so a shared-setup group reorders as a unit and setup is never split. It mirrors randomiseTestForest: inside a DoNotRandomiseExecutionOrder scope it leaves the subtree untouched at every depth. So it is a drop-in alternative to execution-order randomisation, legal under exactly the same conditions. Only sibling order changes, so which tests run (and every verdict) is unaffected. - reorderForMutationChild: the pure gate the child uses -- reorder only when the suite randomises execution order and a baseline is available. Keeping it out of the child's IO makes the gate unit-testable. - The coverage phase persists the merged per-test baselines as baseline.json next to the manifest; the run and diff paths union them into the augmented dir the children read. - The child reads that baseline and reorders the filtered forest after filtering, so the coverage-order TestIds the filter needs are untouched. Tests: ReorderSpec covers ascending order, stable ties + missing-as-first, that a sequential (parallelism) subtree is reordered but a doNotRandomiseExecutionOrder one is not (incl. no descent into a nested randomise block), that a beforeAll_ group reorders as a unit without splitting setup, that the test-id set is unchanged, and the gate (on/off/no-baseline).