Choose random hot contracts from top 16 every 5 minutes.

common/util/random-string.ts

@@ -1 +0,0 @@
-export const randomString = () => Math.random().toString(16).substr(2, 14)

common/util/random.ts

@@ -0,0 +1,46 @@
+export const randomString = () => Math.random().toString(16).substr(2, 14)
+
+export function createRNG(seed: string) {
+  // https://stackoverflow.com/a/47593316/1592933
+
+  function genHash(str: string) {
+    // xmur3
+    for (var i = 0, h = 1779033703 ^ str.length; i < str.length; i++) {
+      h = Math.imul(h ^ str.charCodeAt(i), 3432918353)
+      h = (h << 13) | (h >>> 19)
+    }
+    return function () {
+      h = Math.imul(h ^ (h >>> 16), 2246822507)
+      h = Math.imul(h ^ (h >>> 13), 3266489909)
+      return (h ^= h >>> 16) >>> 0
+    }
+  }
+
+  const gen = genHash(seed)
+  let [a, b, c, d] = [gen(), gen(), gen(), gen()]
+
+  // sfc32
+  return function () {
+    a >>>= 0
+    b >>>= 0
+    c >>>= 0
+    d >>>= 0
+    var t = (a + b) | 0
+    a = b ^ (b >>> 9)
+    b = (c + (c << 3)) | 0
+    c = (c << 21) | (c >>> 11)
+    d = (d + 1) | 0
+    t = (t + d) | 0
+    c = (c + t) | 0
+    return (t >>> 0) / 4294967296
+  }
+}
+
+export const shuffle = (array: any[], rand: () => number) => {
+  for (let i = 0; i < array.length; i++) {
+    const swapIndex = Math.floor(rand() * (array.length - i))
+    const temp = array[i]
+    array[i] = array[swapIndex]
+    array[swapIndex] = temp
+  }
+}

functions/src/create-contract.ts

@@ -4,7 +4,7 @@ import * as admin from 'firebase-admin'
 import { chargeUser, getUser } from './utils'
 import { Contract } from '../../common/contract'
 import { slugify } from '../../common/util/slugify'
-import { randomString } from '../../common/util/random-string'
+import { randomString } from '../../common/util/random'
 import { getNewContract } from '../../common/new-contract'
 import { getAnteBets, MINIMUM_ANTE } from '../../common/antes'
 

web/lib/firebase/contracts.ts

@@ -19,6 +19,7 @@ import { app } from './init'
 import { getValues, listenForValues } from './utils'
 import { Contract } from '../../../common/contract'
 import { getProbability } from '../../../common/calculate'
+import { createRNG, shuffle } from '../../../common/util/random'
 export type { Contract }
 
 export function contractPath(contract: Contract) {
@@ -135,15 +136,24 @@ const hotContractsQuery = query(
   where('isResolved', '==', false),
   where('visibility', '==', 'public'),
   orderBy('volume24Hours', 'desc'),
-  limit(4)
+  limit(16)
 )
 
+function chooseHotContracts(contracts: Contract[]) {
+  const fiveMinutes = 5 * 60 * 1000
+  const seed = Math.round(Date.now() / fiveMinutes).toString()
+  shuffle(contracts, createRNG(seed))
+  return contracts.slice(0, 4)
+}
+
 export function listenForHotContracts(
   setHotContracts: (contracts: Contract[]) => void
 ) {
-  return listenForValues<Contract>(hotContractsQuery, setHotContracts)
+  return listenForValues<Contract>(hotContractsQuery, (contracts) =>
+    setHotContracts(chooseHotContracts(contracts))
+  )
 }
 
 export function getHotContracts() {
-  return getValues<Contract>(hotContractsQuery)
+  return getValues<Contract>(hotContractsQuery).then(chooseHotContracts)
 }

web/pages/index.tsx

@@ -17,6 +17,7 @@ import {
 import { Col } from '../components/layout/col'
 import { ContractCard } from '../components/contract-card'
 import { Bet, listAllBets } from '../lib/firebase/bets'
+import { useHotContracts } from '../hooks/use-contracts'
 
 export async function getStaticProps() {
   const [contracts, hotContracts, recentComments] = await Promise.all([
@@ -51,12 +52,9 @@ const Home = (props: {
   activeContractComments: Comment[][]
   hotContracts: Contract[]
 }) => {
-  const {
-    hotContracts,
-    activeContracts,
-    activeContractBets,
-    activeContractComments,
-  } = props
+  const { activeContracts, activeContractBets, activeContractComments } = props
+
+  const hotContracts = useHotContracts() ?? props.hotContracts
 
   return (
     <Page>
@@ -73,6 +71,8 @@ const Home = (props: {
 
 const HotMarkets = (props: { hotContracts: Contract[] }) => {
   const { hotContracts } = props
+  if (hotContracts.length < 4) return <></>
+
   const [c1, c2, c3, c4] = hotContracts
 
   return (