Generate start and end values from time series

cpi_fetcher.rb

@@ -1,5 +1,6 @@
 # frozen_string_literal: true
 
+require 'csv'
 require 'json'
 require 'net/http'
 require 'net/https'
@@ -10,7 +11,20 @@ class CPIFetcher
     resp = fetch_raw_response
     return nil unless resp['Results']['series']
 
-    latest_cpi_value(resp['Results']['series'].first['data'])
+    data = resp['Results']['series'].first['data']
+    {
+      start: start_cpi_value(data),
+      latest: latest_cpi_value(data)
+    }
+  end
+
+  def cpi_csv
+    values = cpi
+    CSV.generate do |csv|
+      csv << ['year', 'value']
+      csv << [start_year, values.fetch(:start)]
+      csv << [end_year, values.fetch(:latest)]
+    end
   end
 
   private def url
@@ -22,7 +36,7 @@ class CPIFetcher
 
   private def series_id
     @series_id ||= ENV.fetch(
-      'CPI_SERIES_ID', 'CUURS000SA0,CUUSS000SA0'
+      'CPI_SERIES_ID', 'CUUR0200SA0'
     ).split(/[, ]/).map(&:strip).reject(&:empty?)
   end
 
@@ -38,6 +52,17 @@ class CPIFetcher
     @bls_token ||= ENV.fetch('BLS_TOKEN')
   end
 
+  private def start_cpi_value(data)
+    data = data.sort do |a, b|
+      record_key(a) <=> record_key(b)
+    end
+    data.first.fetch('value')
+  end
+
+  private def record_key(record)
+    "#{record['year']}.#{record['period']}"
+  end
+
   private def latest_cpi_value(data)
     data.find { |d| d['latest'] == 'true' }.fetch('value')
   end
@@ -66,6 +91,6 @@ class CPIFetcher
 end
 
 if $PROGRAM_NAME == __FILE__
-  puts CPIFetcher.new.cpi
+  puts CPIFetcher.new.cpi_csv
   exit 0
 end