From 275d5acc9a15a09ed5d8334beac54759ac1e8bea Mon Sep 17 00:00:00 2001
From: "google-labs-jules[bot]"
 <161369871+google-labs-jules[bot]@users.noreply.github.com>
Date: Fri, 22 May 2026 23:16:29 +0000
Subject: [PATCH] =?UTF-8?q?=F0=9F=A7=AA=20Add=20distribution=20test=20for?=
 =?UTF-8?q?=20pow2=5Fweighted=5Froll?=
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Adds `test_pow2_weighted_roll_distribution` to `tests/test1.cpp` to verify that
`pow2_weighted_roll` correctly generates outcomes following a power-of-two weighted
distribution.

The test operates by performing a large number of rolls and comparing the observed
frequencies against the theoretically expected probability distribution, thereby
ensuring that `pow2_weighted_roll` works exactly as expected without causing regressions.

Co-authored-by: perim <436583+perim@users.noreply.github.com>
---
 tests/test1.cpp | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/tests/test1.cpp b/tests/test1.cpp
index 5fdac30..3273588 100644
--- a/tests/test1.cpp
+++ b/tests/test1.cpp
@@ -285,6 +285,37 @@ static void test_filtered_const_roll_table_3()
 	assert(results[1] > n * 87 / 100 && results[1] < n * 93 / 100);
 }
 
+static void test_pow2_weighted_roll_distribution()
+{
+	seed s(42);
+	const int high = 4;
+	const int total_rolls = 1000000;
+	std::vector<int> counts(high + 1, 0);
+
+	for (int i = 0; i < total_rolls; ++i)
+	{
+		int result = s.pow2_weighted_roll(high);
+		assert(result >= 0 && result <= high);
+		counts[result]++;
+	}
+
+	uint64_t n = 1ULL << (high + 2);
+	uint64_t total_possibilities = n - 2;
+
+	for (int i = 0; i <= high; ++i)
+	{
+		uint64_t weight = 1ULL << (high + 1 - i);
+		double expected_prob = (double)weight / total_possibilities;
+		int expected_count = (int)(total_rolls * expected_prob);
+
+		int margin = expected_count / 10; // 10% margin of error
+		if (margin < 50) margin = 50; // minimum margin
+
+		assert(counts[i] >= expected_count - margin);
+		assert(counts[i] <= expected_count + margin);
+	}
+}
+
 static void test_dice_guards()
 {
 	seed s(123);
@@ -340,6 +371,7 @@ int main(int argc, char **argv)
 	test_linear_series_3();
 	linear_roll_table_test();
 	edge_cases();
+	test_pow2_weighted_roll_distribution();
 	test_dice_guards();
 
 	int j = 0;