nbg and auto fill GEL exchange rate

lib/gel_rate.rb

@@ -0,0 +1,61 @@
+require 'net/http'
+require 'json'
+require 'uri'
+require 'date'
+require 'bigdecimal'
+
+# Fetches the official GEL exchange rate from the National Bank of Georgia.
+# https://nbg.gov.ge/en/monetary-policy/currency
+#
+# API: GET .../currencies/en/json/?currencies=USD&date=YYYY-MM-DD
+# Response is a top-level array; rate is quoted per `quantity` units, so the
+# per-unit rate is rate / quantity. NBG returns the rate valid on or before the
+# requested date (weekends/holidays roll back to the last published rate).
+module GelRate
+  ENDPOINT = 'https://nbg.gov.ge/gw/api/ct/monetarypolicy/currencies/en/json/'.freeze
+  TIMEOUT = 6 # seconds
+
+  class Error < StandardError; end
+
+  module_function
+
+  # Returns a BigDecimal: how many GEL per 1 unit of `currency` on `date`.
+  # `date` may be a Date or YYYY-MM-DD string; omitted ⇒ latest published rate.
+  def fetch(currency = 'USD', date: nil)
+    currency = currency.to_s.upcase
+    uri = URI(ENDPOINT)
+    query = { currencies: currency }
+    query[:date] = normalize_date(date) if date
+    uri.query = URI.encode_www_form(query)
+
+    res = Net::HTTP.start(uri.host, uri.port, use_ssl: true,
+                          open_timeout: TIMEOUT, read_timeout: TIMEOUT) do |http|
+      http.get(uri.request_uri, 'Accept' => 'application/json')
+    end
+    raise Error, "NBG HTTP #{res.code}" unless res.is_a?(Net::HTTPSuccess)
+
+    data = JSON.parse(res.body)
+    entry = Array(data).first or raise Error, 'Empty NBG response'
+    cur = Array(entry['currencies']).find { |c| c['code'] == currency } \
+      or raise Error, "No #{currency} in NBG response"
+
+    rate = BigDecimal(cur.fetch('rate').to_s)
+    qty  = BigDecimal(cur.fetch('quantity', 1).to_s)
+    raise Error, 'NBG quantity is zero' if qty.zero?
+
+    (rate / qty).round(8)
+  rescue JSON::ParserError => e
+    raise Error, "Bad JSON from NBG: #{e.message}"
+  rescue Net::OpenTimeout, Net::ReadTimeout
+    raise Error, 'NBG timed out'
+  rescue KeyError => e
+    raise Error, "Missing field in NBG response: #{e.message}"
+  end
+
+  def normalize_date(date)
+    d = date.is_a?(Date) ? date : Date.parse(date.to_s)
+    d.strftime('%Y-%m-%d')
+  rescue ArgumentError
+    raise Error, "Invalid date: #{date}"
+  end
+end