From c12a5464475c4b993ae5fc758fd943f2125e3653 Mon Sep 17 00:00:00 2001
From: Ian Gulliver <git@flamingcow.io>
Date: Sat, 20 Nov 2021 15:20:42 -1000
Subject: [PATCH] Biased random generator

---
 gen/rand.go       | 10 ++++++++++
 test/rand_test.go | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+)
 create mode 100644 gen/rand.go
 create mode 100644 test/rand_test.go

diff --git a/gen/rand.go b/gen/rand.go
new file mode 100644
index 0000000..1f3c523
--- /dev/null
+++ b/gen/rand.go
@@ -0,0 +1,10 @@
+package gen
+
+import "math/rand"
+
+// Generate a random uint64 with an even distribution of bits.Len64()
+func RandBiasedUint64() uint64 {
+	// The shift-right by up to 64 (shifting it to 0) makes up for randomness
+        // lost by setting the high bit.
+	return (rand.Uint64() | 0x8000000000000000) >> rand.Int31n(65)
+}
diff --git a/test/rand_test.go b/test/rand_test.go
new file mode 100644
index 0000000..5821bab
--- /dev/null
+++ b/test/rand_test.go
@@ -0,0 +1,33 @@
+package test
+
+import "math"
+import "math/bits"
+import "testing"
+
+import "github.com/firestuff/subcoding/gen"
+
+func TestRandBiasedUint64(t *testing.T) {
+	buckets := [65]uint64{}
+
+	for i := 0; i < 1000000; i++ {
+		val := gen.RandBiasedUint64()
+		buckets[bits.Len64(val)]++
+	}
+
+	var max uint64 = 0
+	var min uint64 = math.MaxUint64
+
+	for _, count := range buckets {
+		if count > max {
+			max = count
+		}
+
+		if count < min {
+			min = count
+		}
+	}
+
+	if max - min > max / 10 {
+		t.Fatalf("Variance greater than allowed: %d > %d (max=%d min=%d)", max - min, max / 10, max, min)
+	}
+}