Since we will work a lot with arrays, we can use some utility functions. First, we create a function that flattens a two dimensional array into a one dimensional array:

export function flatten<U>(array: U[][]) { 
  return (<U[]>[]).concat(...array); 
} 

Next, we create a function that replaces a single element of an array with a specified value. We will use functional programming in this chapter again, so we must use immutable data structures. We can use map for this, since this function provides both the element and the index to the callback. With this index, we can determine whether that element should be replaced:

export function arrayModify<U>(array: U[], index: number, newValue: U) { 
  return array.map((oldValue, currentIndex) => 
    currentIndex === index ? newValue : oldValue); 
} 

We also create a function that returns a random integer under a certain upper bound:

export function randomInt(max: number...