- New Aenebris.ML.IForest module: pure Isolation Forest scorer
implementing the Liu et al. 2008 ICDM formula
anomaly_score = 2^(-E[h(x)] / c(n))
where E[h(x)] is the average path length across iTrees and
c(n) = 2*H(n-1) - 2(n-1)/n (expected unsuccessful BST search depth).
ITree ADT (ITreeLeaf size | ITreeSplit featIdx threshold left right)
with leaf-size c(n) correction added to traversal depth at every
leaf, matching the original paper. Score-only mode (accept pre-
trained forests from Python sklearn or similar); fitting is deferred
to a later phase. Default constants from Liu et al.: 100 trees,
256 subsample size, max depth ceil(log2(256)) = 8.
- 24 tests covering: harmonicNumber edge cases (H(0), H(1), H(2),
H(3), large-n asymptotic), normalizationConstant for n in {0, 1,
2, 256}, single-split and deep-tree path length traversal,
scoreIForest edge cases (empty forest, subsample 0, subsample 1),
shorter-path-equals-higher-score invariant, default constants,
Euler-Mascheroni precision.
- aenebris.cabal: expose Aenebris.ML.IForest in the library stanza.
- test/Spec.hs: import Expectation explicitly from Test.Hspec for the
shouldBeApprox helper.
- 325 total examples passing, 0 GHC warnings on the new module.
The escalation-gate composition with the calibrated GBDT score
(per docs/research/phase-2.5-ml-synthesis.md correction over the
0.8/0.2 folk-wisdom blend) is intentionally NOT in this module --
it belongs in the downstream ML.Engine module that wires Loader +
Inference + Calibration + IForest into a single decision pipeline.
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