feat: coincap forex api fallback

lib/forex.js

@@ -1,5 +1,62 @@
+const _ = require('lodash/fp')
 const axios = require('axios')
+const BN = require('./bn')
+
+const MAX_ROTATIONS = 5
 
 const getFiatRates = () => axios.get('https://bitpay.com/api/rates').then(response => response.data)
 
-module.exports = { getFiatRates }
+const API_QUEUE = [
+  { api: getBitPayFxRate, name: 'bitpay', fiatCodeProperty: 'code', rateProperty: 'rate' },
+  { api: getCoinCapFxRate, name: 'coincap', fiatCodeProperty: 'symbol', rateProperty: 'rateUsd' }
+]
+
+function getBitPayFxRate (fiatCode, fiatCodeProperty, rateProperty) {
+  return axios.get('https://bitpay.com/rates')
+    .then(response => {
+      const fxRates = response.data.data
+      const usdRate = findCurrencyRates(fxRates, 'USD', fiatCodeProperty, rateProperty)
+      const fxRate = findCurrencyRates(fxRates, fiatCode, fiatCodeProperty, rateProperty).div(usdRate)
+      return {
+        fxRate
+      }
+    })
+}
+
+function getCoinCapFxRate (fiatCode, fiatCodeProperty, rateProperty) {
+  return axios.get('https://api.coincap.io/v2/rates')
+    .then(response => {
+      const fxRates = response.data.data
+      const fxRate = findCurrencyRates(fxRates, fiatCode, fiatCodeProperty, rateProperty)
+      return {
+        fxRate
+      }
+    })
+}
+
+function findCurrencyRates (fxRates, fiatCode, fiatCodeProperty, rateProperty) {
+  const rates = _.find(_.matchesProperty(fiatCodeProperty, fiatCode), fxRates)
+  if (!rates || !rates[rateProperty]) throw new Error(`Unsupported currency: ${fiatCode}`)
+  return new BN(rates[rateProperty].toString())
+}
+
+const getRate = (retries = 1, fiatCode) => {
+  const selected = _.first(API_QUEUE).name
+  const activeAPI = _.first(API_QUEUE).api
+  const fiatCodeProperty = _.first(API_QUEUE).fiatCodeProperty
+  const rateProperty = _.first(API_QUEUE).rateProperty
+
+  if (!activeAPI) throw new Error(`FOREX api ${selected} does not exist.`)
+
+  return activeAPI(fiatCode, fiatCodeProperty, rateProperty).then(res => res)
+    .catch(() => {
+      // Switch service
+      const erroredService = API_QUEUE.shift()
+      API_QUEUE.push(erroredService)
+      if (retries >= MAX_ROTATIONS) throw new Error(`FOREX API error from ${erroredService.name}`)
+
+      return getRate(++retries, fiatCode)
+    })
+}
+
+module.exports = { getFiatRates, getRate }