week_5: Module C (The Librarian) — C.3 confidence calibration (temperature scaling)

Author: PRAteek-singHWY

application/tests/librarian/temperature_test.py

@@ -0,0 +1,169 @@
+"""Tests for C.3 temperature-scaling calibration (Week 5).
+
+Hermetic and deterministic: synthetic candidate-logit shortlists + 0/1 labels
+only — no cross-encoder, DB, or embedding key. Covers the softmax-over-shortlist
+confidence, temperature flatten/sharpen behaviour, the NLL fit recovering a known
+T and reducing NLL, ECE on perfectly- and mis-calibrated data (hand-checked), and
+every guard.
+"""
+
+import math
+import unittest
+
+import numpy as np
+from scipy.special import softmax
+
+from application.utils.librarian.calibration.temperature import (
+    CALIBRATOR_NAME,
+    CalibrationError,
+    DegenerateLabelsError,
+    TemperatureScaler,
+    expected_calibration_error,
+    fit_temperature,
+    negative_log_likelihood,
+)
+
+
+class TemperatureScalerTest(unittest.TestCase):
+    def test_confidence_is_softmax_top_mass(self) -> None:
+        logits = [2.0, 1.0, 0.0]
+        expected = float(softmax(np.array(logits)).max())
+        self.assertAlmostEqual(TemperatureScaler(1.0).confidence(logits), expected, 9)
+
+    def test_probabilities_sum_to_one_and_peak_at_top(self) -> None:
+        p = TemperatureScaler(1.0).probabilities([3.0, 1.0, -2.0])
+        self.assertAlmostEqual(float(p.sum()), 1.0, places=9)
+        self.assertEqual(int(np.argmax(p)), 0)  # highest logit -> highest prob
+
+    def test_high_temperature_flattens_toward_uniform(self) -> None:
+        logits = [3.0, 1.0, 0.0]
+        hot = TemperatureScaler(1000.0).confidence(logits)
+        base = TemperatureScaler(1.0).confidence(logits)
+        self.assertLess(hot, base)
+        self.assertAlmostEqual(hot, 1.0 / 3.0, places=2)  # ~uniform over 3
+
+    def test_low_temperature_sharpens_toward_one(self) -> None:
+        logits = [3.0, 1.0, 0.0]
+        cold = TemperatureScaler(0.1).confidence(logits)
+        base = TemperatureScaler(1.0).confidence(logits)
+        self.assertGreater(cold, base)
+        self.assertGreater(cold, 0.99)
+
+    def test_single_candidate_confidence_is_one(self) -> None:
+        # softmax over one element is always 1.0, at any temperature.
+        self.assertAlmostEqual(TemperatureScaler(3.0).confidence([0.42]), 1.0, 9)
+
+    def test_empty_shortlist_rejected(self) -> None:
+        with self.assertRaises(CalibrationError):
+            TemperatureScaler(1.0).confidence([])
+
+    def test_non_positive_or_nonfinite_temperature_rejected(self) -> None:
+        for bad in (0.0, -1.0, float("inf"), float("nan")):
+            with self.assertRaises(CalibrationError):
+                TemperatureScaler(bad)
+
+
+class NegativeLogLikelihoodTest(unittest.TestCase):
+    def test_matches_hand_computed_single_pair(self) -> None:
+        # one shortlist, top-1 correct: loss = -log(softmax([2,0]).max())
+        conf = float(softmax(np.array([2.0, 0.0])).max())
+        self.assertAlmostEqual(
+            negative_log_likelihood([[2.0, 0.0]], [1.0], 1.0), -math.log(conf), 6
+        )
+
+    def test_length_mismatch_rejected(self) -> None:
+        with self.assertRaises(CalibrationError):
+            negative_log_likelihood([[1.0, 0.0], [2.0, 0.0]], [1.0], 1.0)
+
+    def test_non_positive_temperature_rejected(self) -> None:
+        with self.assertRaises(CalibrationError):
+            negative_log_likelihood([[1.0, 0.0]], [1.0], 0.0)
+
+
+class FitTemperatureTest(unittest.TestCase):
+    def _synthetic(self, true_t: float, n: int = 4000, k: int = 5):
+        """Shortlists whose top-1 correctness is drawn from softmax(logits/true_t).
+
+        Data generated with true_t flattens the softmax, so the raw (T=1)
+        confidence is over-confident and the fit should recover T ~ true_t.
+        Seeded -> stable.
+        """
+        rng = np.random.default_rng(0)
+        sets, labels = [], []
+        for _ in range(n):
+            z = rng.normal(0.0, 3.0, size=k)
+            z[::-1].sort()  # descending so index 0 is the top-1 (argmax)
+            p_correct = softmax(z / true_t).max()
+            sets.append(z.tolist())
+            labels.append(1.0 if rng.random() < p_correct else 0.0)
+        return sets, labels
+
+    def test_recovers_known_temperature(self) -> None:
+        sets, labels = self._synthetic(true_t=2.0)
+        scaler = fit_temperature(sets, labels)
+        self.assertAlmostEqual(scaler.temperature, 2.0, delta=0.5)
+
+    def test_fit_reduces_nll_versus_T1(self) -> None:
+        sets, labels = self._synthetic(true_t=2.0)
+        scaler = fit_temperature(sets, labels)
+        self.assertLess(
+            negative_log_likelihood(sets, labels, scaler.temperature),
+            negative_log_likelihood(sets, labels, 1.0),
+        )
+
+    def test_single_class_labels_raise_degenerate(self) -> None:
+        with self.assertRaises(DegenerateLabelsError):
+            fit_temperature([[2.0, 0.0], [3.0, 1.0]], [1.0, 1.0])
+
+    def test_non_binary_labels_rejected(self) -> None:
+        with self.assertRaises(CalibrationError):
+            fit_temperature([[2.0, 0.0], [3.0, 1.0]], [0.0, 2.0])
+
+
+class ExpectedCalibrationErrorTest(unittest.TestCase):
+    def test_perfectly_calibrated_is_near_zero(self) -> None:
+        # bin [0.5,0.6): five correct, five wrong -> acc 0.5 == conf 0.5.
+        self.assertAlmostEqual(
+            expected_calibration_error([0.5] * 10, [1.0, 0.0] * 5), 0.0, places=6
+        )
+
+    def test_confidently_wrong_is_large(self) -> None:
+        self.assertAlmostEqual(
+            expected_calibration_error([0.9] * 10, [0.0] * 10), 0.9, places=6
+        )
+
+    def test_hand_checked_two_bin_value(self) -> None:
+        # [0.2,0.3): conf .2 acc 0 gap .2 ; [0.8,0.9): conf .8 acc 1 gap .2
+        # ECE = 0.5*0.2 + 0.5*0.2 = 0.20
+        self.assertAlmostEqual(
+            expected_calibration_error([0.2, 0.2, 0.8, 0.8], [0.0, 0.0, 1.0, 1.0]),
+            0.20,
+            places=6,
+        )
+
+    def test_length_mismatch_and_empty_rejected(self) -> None:
+        with self.assertRaises(CalibrationError):
+            expected_calibration_error([0.5, 0.6], [1.0])
+        with self.assertRaises(CalibrationError):
+            expected_calibration_error([], [])
+        with self.assertRaises(CalibrationError):
+            expected_calibration_error([0.5], [1.0], n_bins=0)
+
+
+class EndToEndTest(unittest.TestCase):
+    def test_flat_shortlists_are_low_confidence(self) -> None:
+        # A near-tie shortlist (bad match) -> low top-1 confidence.
+        self.assertLess(TemperatureScaler(1.0).confidence([0.1, 0.0, -0.1]), 0.45)
+
+    def test_peaked_shortlist_is_high_confidence(self) -> None:
+        # A clear winner -> high top-1 confidence.
+        self.assertGreater(TemperatureScaler(1.0).confidence([6.0, -1.0, -3.0]), 0.9)
+
+
+class MetadataTest(unittest.TestCase):
+    def test_calibrator_name_is_versioned(self) -> None:
+        self.assertEqual(CALIBRATOR_NAME, "temperature-scaling/0.2.0")
+
+
+if __name__ == "__main__":
+    unittest.main()

application/utils/librarian/__init__.py

@@ -16,7 +16,9 @@
              explicit-link resolution).
   W3 (C.1):  candidate retriever (in-memory + pgvector) + pipeline switch.
   W4 (C.2):  cross-encoder reranker — re-sorts the C.1 shortlist, fills reranked[].
-Calibration + decision routing (C.3-C.4, W5-W6) onward is not built yet.
+  W5 (C.3):  confidence calibration — temperature scaling maps a rerank logit to
+             an honest probability (fit by NLL on the golden set, gated ECE < 0.10).
+Decision routing (C.4, W6) onward is not built yet.
 
 Vendored RFC JSON schemas live under ``_rfc_schemas/``. They are pinned to
 upstream/owasp-graph @ 2b1437987768d5ed20fe9ee721ab9a898c4b84af (PR #734).

application/utils/librarian/calibration/__init__.py

@@ -0,0 +1,13 @@
+"""Module C.3 — confidence calibration (Week 5).
+
+C.2 (the cross-encoder, W4) emits a raw ranking logit per candidate — great for
+ordering, meaningless as confidence (a +1.5 is not "82% sure"). C.3 turns that
+logit into an honest probability via **temperature scaling**: ``p = sigmoid(z/T)``
+with a single scalar ``T`` fit by negative-log-likelihood on the golden set, and
+proves the result honest with **ECE < 0.10**.
+
+The W6 decision engine thresholds that probability (auto-link vs. human review),
+so calibration is what makes the threshold trustworthy. Kept dependency-light
+(numpy + scipy) and model-free so it stays hermetically testable — mirrors the
+C.1/C.2 seams.
+"""