Implements rough draft of order-book

orderBook.js

@@ -0,0 +1,83 @@
+function reconcileOrder(existingBook, incomingOrder) {
+  // first thing we do is try to find a deal, if we find one
+  // return the index of that order, if not then just combine
+  // our orders together
+  let dealFoundIndex = findDeal(existingBook, incomingOrder)
+
+  // let's check our index, if it's less than 0 then we didnt find a deal
+  // so simply concat our lists
+  if (dealFoundIndex < 0) {
+    // no deal found, concat our order into our book and return it
+    return existingBook.concat({ ...incomingOrder })
+  }
+
+  // now let's make our deal using our indexed order and generate a new pending order
+  const newOrder = makeDeal(existingBook[dealFoundIndex], incomingOrder)
+
+  // now let's trim our existing order from the book, if the quantity isn't 0
+  // it will be reinserted on the next recursion
+  existingBook.splice(dealFoundIndex, 1)
+
+  // now let's check if our order has any quantity, if it doesnt then we are done
+  if (newOrder.quantity > 0) {
+    // we still have some quantity left, so let's recall the function using our
+    // new data causing a recursion that wont be stopped until the quantity is 0
+    // or no deal is found on line 11
+    return reconcileOrder(existingBook, newOrder)
+  } else {
+    // quantity is 0, we are done! no need to concat because it's not a valid order
+    // just return our book which has been spliced of the order that was processed 
+    return existingBook
+  }
+}
+
+
+// our function which first finds a deal, and returns the index of that deal
+function findDeal(existingBook, incomingOrder) {
+  // we want to return the index of a deal by making a callback function
+  // find index will cycle through each order until it finds one
+  return existingBook.findIndex((existingOrder) => {
+    // if our existingOrder type isn't the same
+    // as our incoming order, then its a match
+    // we also need to make sure the prices work
+    // if both are true, return this order
+    // this will then make the function return the index
+    // if no order is found it will return -1
+    return existingOrder.type !== incomingOrder.type && goodDeal(existingOrder, incomingOrder) === true
+  })
+}
+
+// this function checks the prices to see if both sides benefit
+function goodDeal(existingOrder, incomingOrder) {
+  // let's check if the incoming order is a buy, if not then its a sell
+  if (incomingOrder.type === 'buy') {
+    // if the incoming order is a buy, we need to check if the price
+    // of the existing order is >= to it
+    return incomingOrder.price >= existingOrder.price
+  } else {
+    // if it's not a buy, it must be a sell, so now we want to check if
+    // the existing order is <= to incoming order
+    return incomingOrder.price <= existingOrder.price
+  }
+}
+
+// this function makes a deal and returns our newOrder
+function makeDeal(existingOrder, incomingOrder) {
+  // let's see which order will be sent back depending on the quantity
+  if (incomingOrder.quantity >= existingOrder.quantity) {
+    // our incoming order has more quantity, so subtract the existing order from it
+    // then return it as our newest order to be fulfilled
+    incomingOrder.quantity -= existingOrder.quantity
+
+    return { ...incomingOrder }
+  } else {
+    // now our incoming order has less quantity, so let's take its quantity from
+    // our existing order then return it to be processed through
+    existingOrder.quantity -= incomingOrder.quantity
+
+    return { ...existingOrder }
+  }
+}
+
+// export our module to tests and the front end
+module.exports = reconcileOrder

package.json

@@ -5,7 +5,7 @@
   "main": ".eslintrc.js",
   "scripts": {
     "lint": "./node_modules/.bin/eslint --format codeframe .",
-    "test": "./node_modules/.bin/mocha ./tests.js"
+    "test": "./node_modules/.bin/mocha ./tests/*.js"
   },
   "repository": {
     "type": "git",

tests.js → tests/orderBook.test.js

@@ -1,6 +1,6 @@
 /* eslint-disable max-len */
 const { expect } = require('chai')
-const reconcileOrder = require('./orderBook')
+const reconcileOrder = require('../orderBook')
 
 describe('Order Book', () => {
   describe('reconcileOrder', () => {
@@ -91,7 +91,7 @@ describe('Order Book', () => {
       expect(updatedBook).to.deep.equal([{ type: 'sell', quantity: 12, price: 6950 }, { type: 'sell', quantity: 5, price: 6150 }])
     })
 
-    it.skip('Extra Credit: it fulfills a mismatched order when both parties benefit', () => {
+    it('Extra Credit: it fulfills a mismatched order when both parties benefit', () => {
       const existingBook = [{ type: 'buy', quantity: 15, price: 6000 }, { type: 'sell', quantity: 12, price: 6950 }]
       const incomingOrder = { type: 'sell', quantity: 15, price: 5900 }
 
@@ -100,7 +100,7 @@ describe('Order Book', () => {
       expect(updatedBook).to.deep.equal([{ type: 'sell', quantity: 12, price: 6950 }])
     })
 
-    it.skip('Extra Credit: it does not fulfill a mismatched order when it does not benefit both parties', () => {
+    it('Extra Credit: it does not fulfill a mismatched order when it does not benefit both parties', () => {
       const existingBook = [{ type: 'buy', quantity: 15, price: 5900 }, { type: 'sell', quantity: 12, price: 6950 }]
       const incomingOrder = { type: 'sell', quantity: 15, price: 6000 }