= Test oracle
A <test oracle> determines whether an observed test result is acceptable. It can be an explicitly expected <output>, a mathematical property, a trusted reference implementation or another justified source of comparison. Generating many <inputs> without an oracle does not by itself establish whether the results are correct.
For a <sorting algorithm>[sorting routine], an oracle can check both <sortedness> and preservation of the <multiset>[input multiset]. Returning an empty <sequence> passes a <sortedness> check but fails preservation for a nonempty <input>. A <differential test> instead compares implementations; a disagreement exposes a problem without necessarily identifying which implementation is wrong. Hypothesis documents both reference-oracle comparisons and differential comparisons in its equivalence-test generator.<Integrations Reference in the Hypothesis documentation>[equivalent, oracle and differential testing examples]{ref}
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