ARIMA_SAX_Hybrid_Forecaster/DEFECT_LOG.md

# DEFECT_LOG.md

Every discovered implementation defect is recorded here with its impact,
fix commit, and re-run status. Original (invalid) evidence is preserved and
never overwritten; corrected runs are clearly re-labelled.

---

## DEFECT-001 — SAX candidate windows not z-normalized (frozen analog path)

**Status:** FIXED (commit `ff95e11`), re-run completed.

### Discovery
During R1 (real-data transfer), the fast SAX path (which z-normalizes candidate
windows, matching the SAX reference methodology) was validated against the
frozen `SaxAnalogForecaster`. The equivalence check FAILED: frozen output was
**constant across forecast origins** (e.g. identical median outcome and selected
analog set j = [23..32] at every origin), while the fast path produced
origin-dependent analog selection.

### Root cause
In `src/sax/analog.py` the candidate windows were encoded as:

```python
cand = close[j - W + 1:j + 1]
cand_word = sx.sax_encode(cand, ...)   # NOT z-normalized
```

The query pattern was z-normalized, but the candidate windows were not.
SAX breakpoints are only valid for z-normalized series; encoding raw levels
collapses almost all candidate windows to the same symbol sequence, so
`mindist_midpoints` returns a near-constant distance for every candidate.
`argsort` then always selects the first `top_k` candidates (the earliest
bars), yielding identical, origin-independent outcomes.

### Impact
- **All SAX component outputs and all ARIMA+SAX Hybrid outputs produced before
  this fix are INVALID.**
- Specifically affected evidence: E1–E8 synthetic benchmark (SAX and Hybrid
  rows), any prior SAX unit/integration checks that relied on analog selection.
- NOT affected: Naive and Drift baselines, ARIMA component, hybrid state
  classification logic itself, walk-forward machinery, no-lookahead guards.

### Fix
`src/sax/analog.py` now z-normalizes candidate windows before SAX encoding
(identical to the reference methodology and to the validated fast path):

```python
cand = sx.z_normalize(close[j - W + 1:j + 1])
cand_word = sx.sax_encode(cand, ...)
```

A regression test (`tests/test_sax_analog_zscore.py`) asserts that analog
selection is origin-dependent and that identical windows yield zero distance
while perturbed windows yield larger distances.

### Evidence re-run policy
Per the frozen governance: the affected experiments are re-run from the
beginning after the fix. Old result artifacts are preserved and marked
`INVALID (DEFECT-001)` in the relevant summary/manifest files.

### Re-run status
- [x] R1 (real XAUUSDc M1/M5/M15) — re-run after fix (this session)
- [x] E1–E8 synthetic benchmark — SAX/Hybrid rows regenerated after fix
      (ARIMA/Naive rows unchanged and remain valid)