Add multiplication practice

src/prob.ts

@@ -0,0 +1,16 @@
+namespace Prob {
+  export type Probability<T> = [ number, T ]
+  type NonEmptyArray<T> = [ T, ...T[] ]
+  export type ProbabilitySet<T> = NonEmptyArray<Probability<T>>
+  export const chooseRandom = <T>(probabilities : ProbabilitySet<T>) : T => {
+    let r = Math.random(), accum = 0
+    for (let i = 0; i < probabilities.length; i++) {
+      if ((r >= accum) && (r < accum + probabilities[i][0]))
+        return probabilities[i][1]
+      accum += probabilities[i][0]
+    }
+    return probabilities[probabilities.length - 1][1]
+  }
+}
+
+export default Prob