diff --git a/src/device/input/random.go b/src/device/input/random.go
index 3dc3375..e5998a9 100644
--- a/src/device/input/random.go
+++ b/src/device/input/random.go
@@ -40,3 +40,32 @@ func RandomCharFromRange(start, stop byte) func() byte {
 		return start + byte(rand.Int()%int(1+stop-start))
 	}
 }
+
+func RandomCharFromWeights(m map[byte]int) func() byte {
+	type pair struct {
+		b byte
+		i int
+	}
+	result := make([]pair, 0, len(m))
+	sum := 0
+	for k, v := range m {
+		result = append(result, pair{b: k, i: v})
+		if v < 0 {
+			panic("each weight must each be natural")
+		}
+		sum += v
+	}
+	if sum <= 0 {
+		panic("weights must total nonzero")
+	}
+	return func() byte {
+		n := rand.Int() % sum
+		for _, v := range result {
+			n -= v.i
+			if n <= 0 {
+				return v.b
+			}
+		}
+		panic("how")
+	}
+}
diff --git a/src/device/input/random_test.go b/src/device/input/random_test.go
index 8b50b0c..9824f04 100644
--- a/src/device/input/random_test.go
+++ b/src/device/input/random_test.go
@@ -14,3 +14,18 @@ func TestRandom(t *testing.T) {
 		}
 	}
 }
+
+func TestRandomCharFromWeights(t *testing.T) {
+	weights := map[byte]int{
+		'a': 1,
+		'b': 99,
+	}
+	foo := input.RandomCharFromWeights(weights)
+	for {
+		got := foo()
+		t.Logf("%c", got)
+		if got == 'a' {
+			break
+		}
+	}
+}