[BUG] num_scalings is always overcounted by exactly 1 (shift(-1) makes the last row NaN)

Bug

calculate_metrics in src/vasim/simulator/analysis/plot_utils.py:104 counts scaling events with a backward shift:

num_changes = (merged["CURR_LIMIT"] != merged["CURR_LIMIT"].shift(-1)).sum()

shift(-1) leaves the last element NaN, and NaN != x is always True in pandas, so the final row is always counted as a scaling event. The result is off by exactly +1 on every run.

Repro

import pandas as pd
d = pd.DataFrame({"CURR_LIMIT": [4, 4, 4, 4, 4]})   # never scales
(d["CURR_LIMIT"] != d["CURR_LIMIT"].shift(-1)).sum()
# 1   <- expected 0

shift(-1): [4.0, 4.0, 4.0, 4.0, nan]
!= gives : [False, False, False, False, True]   <- NaN != 4 is always True

Across cases:

case	CURR_LIMIT	reported	expected
never scales	[4, 4, 4, 4, 4]	1	0
one change	[4, 4, 8, 8, 8]	2	1
two changes	[4, 8, 8, 2, 2]	3	2
single row	[4]	1	0
alternating	[4, 8, 4, 8, 4]	5	4

A configuration that never scales at all reports 1 scaling.

Why it matters

num_scalings is not just displayed — it feeds ParetoFrontier normalization (num_scalings / num_scalings.max()) and the Pareto ranking, so tuning comparisons between configurations are skewed by it.

Suggested fix

-    num_changes = (merged["CURR_LIMIT"] != merged["CURR_LIMIT"].shift(-1)).sum()
+    num_changes = (merged["CURR_LIMIT"] != merged["CURR_LIMIT"].shift(1)).sum() - 1

Verified this returns 0 / 1 / 2 / 0 / 4 on the cases above.

Companion crash the fix exposes

ParetoFrontier.preprocess_df (src/vasim/simulator/analysis/ParetoFrontier.py:64-69) creates num_scalings_norm only inside a guard, then uses it unconditionally:

if df["num_scalings"].max() > 0:
    df["num_scalings_norm"] = df["num_scalings"] / df["num_scalings"].max()

df = df[df["num_scalings_norm"] <= np.percentile(df["num_scalings_norm"], 90)]   # KeyError when max == 0

Today num_scalings is never 0 because of the bug above, so this is unreachable. Fixing the off-by-one makes it reachable for any sweep where no configuration scales. The sum_slack / sum_insufficient_cpu guards above it are fine — those columns aren't referenced later — so this is specifically a num_scalings_norm oversight. Worth fixing in the same change:

         if df["num_scalings"].max() > 0:
             df["num_scalings_norm"] = df["num_scalings"] / df["num_scalings"].max()
-
-        df = df[df["num_scalings_norm"] <= np.percentile(df["num_scalings_norm"], 90)]
+            df = df[df["num_scalings_norm"] <= np.percentile(df["num_scalings_norm"], 90)]
         return df

Note on existing tests

Four e2e expectations currently hardcode the inflated values and would each need decrementing by 1 alongside the fix:

• tests/test_e2e_single_run_sim.py:110 (466) and :157 (347)
• tests/test_e2e_multi_run_tune_with_strategy.py:134 (109) and :209 (133)

Happy to open a PR with the fix plus those four updates if this looks right to you.

https://claude.ai/code/session_01CFRv5kvesfnKYgHmNw9Vpf

Start in src/vasim/simulator/analysis/plot_utils.py:104 and inspect how calculate_metrics derives num_scalings, then read ParetoFrontier.preprocess_df at src/vasim/simulator/analysis/ParetoFrontier.py:64-69. Update the affected expectations in tests/test_e2e_single_run_sim.py:110,157 and tests/test_e2e_multi_run_tune_with_strategy.py:134,209; done means zero-scale cases work and all four expected values match the corrected counts.