diff --git a/DeepDataMiningLearning/ngperception/lane/culane_metric.py b/DeepDataMiningLearning/ngperception/lane/culane_metric.py
index 4b789c9b..a53de79e 100644
--- a/DeepDataMiningLearning/ngperception/lane/culane_metric.py
+++ b/DeepDataMiningLearning/ngperception/lane/culane_metric.py
@@ -51,13 +51,23 @@ def _match(preds, gts, H, W, width, iou_thresh):
             inter = np.logical_and(a, b).sum()
             union = asum + b.sum() - inter
             iou[i, j] = inter / union if union > 0 else 0.0
+    # Greedy one-to-one over the whole matrix, best pair first. Offering each
+    # prediction only its argmax GT (and dropping it when that GT is taken)
+    # discards pairs this same matrix says clear the threshold: with two lanes
+    # 6 px apart and the 15 px stamp, p1 can have IoU 0.889 with a used g0 and
+    # 0.545 with a free g1, and be counted as a false positive anyway -- while
+    # g1 is counted as a false negative.
     tp = 0
-    used_g = set()
-    for i in np.argsort(-iou.max(axis=1)):          # preds by best-IoU first
-        j = int(iou[i].argmax())
-        if j not in used_g and iou[i, j] >= iou_thresh:
-            tp += 1
-            used_g.add(j)
+    used_p, used_g = set(), set()
+    for flat in np.argsort(-iou, axis=None):
+        i, j = np.unravel_index(flat, iou.shape)
+        if iou[i, j] < iou_thresh:
+            break                                   # sorted: nothing later qualifies
+        if i in used_p or j in used_g:
+            continue
+        tp += 1
+        used_p.add(int(i))
+        used_g.add(int(j))
     fp = len(preds) - tp
     fn = len(gts) - tp
     return tp, fp, fn
diff --git a/DeepDataMiningLearning/ngperception/lane/tests/test_culane_metric.py b/DeepDataMiningLearning/ngperception/lane/tests/test_culane_metric.py
new file mode 100644
index 00000000..20dfbcd4
--- /dev/null
+++ b/DeepDataMiningLearning/ngperception/lane/tests/test_culane_metric.py
@@ -0,0 +1,204 @@
+"""What `_match` counts, and what the IoU matrix it built says it should.
+
+The module docstring promises "greedily one-to-one match". These pin that
+down: no ground-truth lane is used twice, no prediction is used twice, and a
+prediction is not discarded while a ground-truth lane it clears the threshold
+against is still free.
+
+Pure numpy, no dataset, no model, no GPU.
+
+    pytest DeepDataMiningLearning/ngperception/lane/tests
+"""
+import numpy as np
+import pytest
+
+from DeepDataMiningLearning.ngperception.lane.culane_metric import (
+    CULaneF1, _match, rasterize,
+)
+
+H, W = 320, 800                  # CULaneF1's defaults
+WIDTH = max(4, round(W / 55))    # its own width rule: 15 px
+T = 0.5
+
+
+def lane(x_top, x_bottom=None, n=40):
+    """A vertical-ish lane as the decoder emits it, top to bottom."""
+    ys = np.linspace(0, H - 1, n)
+    xs = np.linspace(x_top, x_top if x_bottom is None else x_bottom, n)
+    return np.stack([xs, ys], axis=1)
+
+
+def iou_matrix(preds, gts):
+    pm = [rasterize(p, H, W, WIDTH) for p in preds]
+    gm = [rasterize(g, H, W, WIDTH) for g in gts]
+    m = np.zeros((len(pm), len(gm)))
+    for i, a in enumerate(pm):
+        for j, b in enumerate(gm):
+            inter = np.logical_and(a, b).sum()
+            union = a.sum() + b.sum() - inter
+            m[i, j] = inter / union if union else 0.0
+    return m
+
+
+def best_possible_tp(m, thresh=T):
+    """Upper bound on any one-to-one matching, by exhaustive search."""
+    from itertools import permutations
+    n_p, n_g = m.shape
+    best = 0
+    if n_p <= n_g:
+        for perm in permutations(range(n_g), n_p):
+            best = max(best, sum(1 for i, j in enumerate(perm) if m[i, j] >= thresh))
+    else:
+        for perm in permutations(range(n_p), n_g):
+            best = max(best, sum(1 for j, i in enumerate(perm) if m[i, j] >= thresh))
+    return best
+
+
+# ------------------------------------------------------------ the defect
+def test_a_prediction_is_not_dropped_while_a_lane_it_matches_is_free():
+    """Found by sweeping lane spacing against prediction offset."""
+    gts = [lane(300), lane(306)]
+    preds = [lane(300), lane(301)]
+    m = iou_matrix(preds, gts)
+
+    # the premise: p1's best is the same lane as p0's, and it also clears the
+    # threshold against the other lane
+    assert m[0].argmax() == 0 and m[0, 0] >= T
+    assert m[1].argmax() == 0 and m[1, 1] >= T
+
+    tp, fp, fn = _match(preds, gts, H, W, WIDTH, T)
+    assert (tp, fp, fn) == (2, 0, 0)
+
+
+def test_the_sweep_that_found_it_reports_no_under_counting():
+    lost = 0
+    for gap in range(4, 18):
+        for off in np.arange(0.0, gap, 0.5):
+            gts = [lane(300), lane(300 + gap)]
+            preds = [lane(300), lane(300 + off)]
+            m = iou_matrix(preds, gts)
+            tp, _, _ = _match(preds, gts, H, W, WIDTH, T)
+            if tp < best_possible_tp(m):
+                lost += 1
+    assert lost == 0
+
+
+# ------------------------------------------------ one-to-one is preserved
+def test_two_predictions_cannot_claim_one_lane():
+    gts = [lane(300)]
+    preds = [lane(300), lane(301)]
+    tp, fp, fn = _match(preds, gts, H, W, WIDTH, T)
+    assert (tp, fp, fn) == (1, 1, 0)
+
+
+def test_one_prediction_cannot_claim_two_lanes():
+    gts = [lane(300), lane(301)]
+    preds = [lane(300)]
+    tp, fp, fn = _match(preds, gts, H, W, WIDTH, T)
+    assert (tp, fp, fn) == (1, 0, 1)
+
+
+def test_counts_always_add_up():
+    rng = np.random.default_rng(3)
+    for _ in range(30):
+        base = np.sort(rng.uniform(120, 680, size=4))
+        gts = [lane(x, x - 90) for x in base]
+        preds = [lane(x + rng.normal(0, 8), x - 90 + rng.normal(0, 8)) for x in base]
+        tp, fp, fn = _match(preds, gts, H, W, WIDTH, T)
+        assert tp + fp == len(preds)
+        assert tp + fn == len(gts)
+        assert tp <= min(len(preds), len(gts))
+
+
+# ------------------------------------------------------ threshold respected
+def test_a_pair_below_the_threshold_is_never_matched():
+    gts = [lane(300)]
+    preds = [lane(360)]                      # far enough apart that IoU is 0
+    m = iou_matrix(preds, gts)
+    assert m[0, 0] < T
+    assert _match(preds, gts, H, W, WIDTH, T) == (0, 1, 1)
+
+
+def test_a_higher_threshold_matches_strictly_fewer():
+    gts = [lane(300), lane(306)]
+    preds = [lane(300), lane(301)]
+    loose = _match(preds, gts, H, W, WIDTH, 0.5)[0]
+    tight = _match(preds, gts, H, W, WIDTH, 0.95)[0]
+    assert tight <= loose
+
+
+# ------------------------------------------------------------ empty sides
+def test_no_ground_truth_makes_every_prediction_a_false_positive():
+    assert _match([lane(300)], [], H, W, WIDTH, T) == (0, 1, 0)
+
+
+def test_no_predictions_makes_every_lane_a_false_negative():
+    assert _match([], [lane(300)], H, W, WIDTH, T) == (0, 0, 1)
+
+
+def test_both_empty_is_all_zeros():
+    assert _match([], [], H, W, WIDTH, T) == (0, 0, 0)
+
+
+# ------------------------------------------------------- the public class
+def test_perfect_predictions_score_one():
+    gts = [lane(200, 150), lane(400, 350), lane(600, 550)]
+    metric = CULaneF1(img_h=H, img_w=W, iou_thresh=T)
+    metric.update([gts], [gts])
+    r = metric.compute()
+    assert r["tp"] == 3 and r["fp"] == 0 and r["fn"] == 0
+    assert r["f1"] == pytest.approx(1.0, abs=1e-6)
+
+
+def _reference_greedy(m, thresh=T):
+    """An independent greedy one-to-one matcher, written from the docstring."""
+    pairs = sorted(((m[i, j], i, j) for i in range(m.shape[0])
+                    for j in range(m.shape[1]) if m[i, j] >= thresh), reverse=True)
+    tp, used_p, used_g = 0, set(), set()
+    for _, i, j in pairs:
+        if i in used_p or j in used_g:
+            continue
+        tp += 1
+        used_p.add(i)
+        used_g.add(j)
+    return tp
+
+
+def _argmax_only(m, thresh=T):
+    """What the module did before: each prediction gets only its argmax GT."""
+    tp, used_g = 0, set()
+    for i in np.argsort(-m.max(axis=1)):
+        j = int(m[i].argmax())
+        if j not in used_g and m[i, j] >= thresh:
+            tp += 1
+            used_g.add(j)
+    return tp
+
+
+def test_the_class_agrees_with_an_independent_greedy_matcher():
+    """Greedy is what the docstring specifies. It is not always the maximum:
+    greedy can commit a high pair that blocks two lower ones. The claim here is
+    only that the module implements greedy, and that greedy never scores below
+    the argmax-only rule it replaces."""
+    rng = np.random.default_rng(0)
+    metric = CULaneF1(img_h=H, img_w=W, iou_thresh=T)
+    greedy_total = optimal_total = argmax_total = 0
+    for _ in range(40):
+        base = np.sort(rng.uniform(120, 680, size=4))
+        gts = [lane(x, x - 90) for x in base]
+        preds = []
+        for k, x in enumerate(base):
+            drift = rng.normal(0, 4)
+            if rng.random() < 0.25 and k + 1 < len(base):
+                drift += (base[k + 1] - x) * 0.35
+            preds.append(lane(x + drift, x - 90 + drift))
+        metric.update([preds], [gts])
+        m = iou_matrix(preds, gts)
+        greedy_total += _reference_greedy(m)
+        optimal_total += best_possible_tp(m)
+        argmax_total += _argmax_only(m)
+
+    got = metric.compute()["tp"]
+    assert got == greedy_total, "the module is not doing greedy one-to-one"
+    assert argmax_total <= got <= optimal_total
+    assert got > argmax_total, "this sample should show the improvement"