From 829d69d81f60d0a222bf5d1a38f78e07ae69dfeb Mon Sep 17 00:00:00 2001
From: Stephen Paul Weber <singpolyma@singpolyma.net>
Date: Tue, 20 Apr 2021 10:34:58 -0500
Subject: [PATCH] Add helper to fetch current BTC sell prices

Scrapes the sell price for Bitcoin from canadianbitcoins.com
USD price is done by converting this CAD sell price to USD via openexchangerates
---
 Gemfile                      |  2 ++
 lib/btc_sell_prices.rb       | 57 ++++++++++++++++++++++++++++++++++++
 test/test_btc_sell_prices.rb | 31 ++++++++++++++++++++
 3 files changed, 90 insertions(+)
 create mode 100644 lib/btc_sell_prices.rb
 create mode 100644 test/test_btc_sell_prices.rb

diff --git a/Gemfile b/Gemfile
index e777509..c32ccea 100644
--- a/Gemfile
+++ b/Gemfile
@@ -6,9 +6,11 @@ gem "blather", git: "https://github.com/singpolyma/blather.git", branch: "ergono
 gem "braintree"
 gem "dhall"
 gem "em-hiredis"
+gem "em-http-request"
 gem "em-pg-client", git: "https://github.com/royaltm/ruby-em-pg-client"
 gem "em_promise.rb"
 gem "eventmachine"
+gem "money-open-exchange-rates"
 
 group(:development) do
 	gem "pry-reload"
diff --git a/lib/btc_sell_prices.rb b/lib/btc_sell_prices.rb
new file mode 100644
index 0000000..3262110
--- /dev/null
+++ b/lib/btc_sell_prices.rb
@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+require "em-http"
+require "money/bank/open_exchange_rates_bank"
+require "nokogiri"
+
+require_relative "em"
+
+class BTCSellPrices
+	def initialize(redis, oxr_app_id)
+		@redis = redis
+		@oxr = Money::Bank::OpenExchangeRatesBank.new(
+			Money::RatesStore::Memory.new
+		)
+		@oxr.app_id = oxr_app_id
+	end
+
+	def cad
+		fetch_canadianbitcoins.then do |http|
+			canadianbitcoins = Nokogiri::HTML.parse(http.response)
+
+			bitcoin_row = canadianbitcoins.at("#ticker > table > tbody > tr")
+			raise "Bitcoin row has moved" unless bitcoin_row.at("td").text == "Bitcoin"
+
+			BigDecimal.new(
+				bitcoin_row.at("td:nth-of-type(3)").text.match(/^\$(\d+\.\d+)/)[1]
+			)
+		end
+	end
+
+	def usd
+		EMPromise.all([cad, cad_to_usd]).then { |(a, b)| a * b }
+	end
+
+protected
+
+	def fetch_canadianbitcoins
+		EM::HttpRequest.new(
+			"https://www.canadianbitcoins.com",
+			tls: { verify_peer: true }
+		).get
+	end
+
+	def cad_to_usd
+		@redis.get("cad_to_usd").then do |rate|
+			next rate.to_f if rate
+
+			EM.promise_defer {
+				# OXR gem is not async, so defer to threadpool
+				oxr.update_rates
+				oxr.get_rate("CAD", "USD")
+			}.then do |orate|
+				@redis.set("cad_to_usd", orate, ex: 60 * 60).then { orate }
+			end
+		end
+	end
+end
diff --git a/test/test_btc_sell_prices.rb b/test/test_btc_sell_prices.rb
new file mode 100644
index 0000000..dc53a1d
--- /dev/null
+++ b/test/test_btc_sell_prices.rb
@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+require "em-hiredis"
+require "test_helper"
+require "btc_sell_prices"
+
+class BTCSellPricesTest < Minitest::Test
+	def setup
+		@redis = Minitest::Mock.new
+		@subject = BTCSellPrices.new(@redis, "")
+	end
+
+	def test_cad
+		stub_request(:get, "https://www.canadianbitcoins.com").to_return(
+			body: "<div id='ticker'><table><tbody><tr>" \
+			      "<td>Bitcoin</td><td></td><td>$123.00</td>"
+		)
+		assert_equal BigDecimal.new(123), @subject.cad.sync
+	end
+	em :test_cad
+
+	def test_usd
+		stub_request(:get, "https://www.canadianbitcoins.com").to_return(
+			body: "<div id='ticker'><table><tbody><tr>" \
+			      "<td>Bitcoin<td></td><td>$123.00</td>"
+		)
+		@redis.expect(:get, EMPromise.resolve("0.5"), ["cad_to_usd"])
+		assert_equal BigDecimal.new(123) / 2, @subject.usd.sync
+	end
+	em :test_usd
+end
-- 
2.38.5