diff --git a/src/hidden_markov_music/random.clj b/src/hidden_markov_music/random.clj
new file mode 100644
index 0000000..07570d4
--- /dev/null
+++ b/src/hidden_markov_music/random.clj
@@ -0,0 +1,21 @@
+(ns hidden-markov-music.random)
+
+(defn rand*
+  "Returns a random floating point number between 0 (exclusive) and
+  n (default 1) (inclusive)."
+  ([]
+     (rand* 1.0))
+  ([n]
+     (- n (rand n))))
+
+(defn select-random-key
+  "Takes a collection of `[key prob]` pairs, whose `prob`s sum to 1.0, and
+  selects a `key` randomly based on its `prob`."
+  [key->prob]
+  (loop [key->prob       key->prob
+         selection-index (rand*)]
+    (let [[k p] (first key->prob)
+          new-selection-index (- selection-index p)]
+      (if (pos? new-selection-index)
+        (recur (rest key->prob) new-selection-index)
+        k))))