Helpers and utilities

Warning

This page is a work in progress.

We use the following utilities to solve common programming problems.

Pseudo-random number generator

If you need a one-time use random number (which will never be stored), there’s nothing wrong with JavaScript’s built-in Math.random(). However, there are times where we need a seed for a pseudo-random number generator (PRNG). Seeds generate a random-looking sequence of numbers, but they have the benefit of remaining consistent across renders. For example, this is useful on our sidewalk segment, where we want to generate a random set of pedestrians but we want those pedestrians to remain the same between page refreshes or React component updates.

We can generate seeds by calling generateRandSeed(), then by feeding those seeds into the seedrandom library:

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 import seedrandom from 'seedrandom'
 import { generateRandSeed } from './util/random'

 function doThingsWithThePRNG () {
   const seed = generateRandSeed()
   const prng1 = seedrandom(seed)

   // Generate pseudo-random numbers. Each time rng() is called, it returns
   // the next pseudo-random number in the sequence.
   console.log(prng1()) // -> first pseudo-random number
   console.log(prng1()) // -> second pseudo-random number
   // .. etc

   // Create another generator with the same seed
   const prng2 = seedrandom(seed)
   console.log(prng2()) // -> same value as the first call to prng1()
 }

When seedrandom() is called, it returns a function that will return a consistent sequence of numbers each time it is called, for the given seed. The sequence returned is local to that function.

Note

Although the seed can be any type of value, like a string, generateRandSeed() returns only integer values so that when we store seeds in a data model we can check for type consistency.

In the above example, we call seedrandom() twice, with the same seed, so that we can create the same sequence of pseudo-random numbers in two different scenarios. This example is contrived because the pseudo-random number generators are used in the same function, and in reality the seedrandom() call may happen in separate parts of the application.

References