Fix: Interpolate missing ENTSOE price data points

Handle missing hourly price data from ENTSOE API by interpolating
values from adjacent time slots instead of returning nil.

.gitignore

@@ -2,3 +2,4 @@
 .config
 .project
 *.pid
+data

app/models/entsoe.rb

@@ -104,14 +104,60 @@ class Entsoe
       
       doc = Nokogiri::XML(URI.open(url))
 
+      #p "Entsoe prices: %s" % doc
+      
       prices = doc.xpath('.//xmlns:Point').map{|p| parse_point(p)}
       begin
         # get start_time (in UTC) from XML docment
         start_time = DateTime.parse(doc.xpath('.//xmlns:period.timeInterval//xmlns:start').text)
         
+        # Get resolution to determine expected number of positions
+        resolution = doc.xpath('.//xmlns:resolution').text
+        interval_minutes = resolution == 'PT15M' ? 15 : 60
+        expected_positions = (24 * 60) / interval_minutes
+        
+        # Create hash from available prices
+        price_hash = prices.to_h
+        
+        # Fill in missing positions by interpolating from adjacent values
+        complete_prices = {}
+        (1..expected_positions).each do |position|
+          if price_hash.key?(position)
+            complete_prices[position] = price_hash[position]
+          else
+            # Find previous and next available prices for interpolation
+            prev_price = (position-1).downto(1).find { |p| price_hash.key?(p) }
+            next_price = (position+1).upto(expected_positions).find { |p| price_hash.key?(p) }
+            
+            if prev_price && next_price
+              # Interpolate between previous and next
+              complete_prices[position] = ((price_hash[prev_price] + price_hash[next_price]) / 2.0).round(5)
+            elsif prev_price
+              # Use previous price as fallback
+              complete_prices[position] = price_hash[prev_price]
+            elsif next_price
+              # Use next price as fallback
+              complete_prices[position] = price_hash[next_price]
+            end
+            
+            # Calculate the formatted hour for the warning message
+            hour_offset = interval_minutes == 15 ? (position - 1) / 4 : (position - 1)
+            missing_hour = start_time.advance(hours: hour_offset).in_time_zone(zone).strftime("%F %H")
+            p "Warning: Missing Entsoe data for #{missing_hour}, interpolated value: #{complete_prices[position]}"
+          end
+        end
+        
         #returns a hash with keys formatted "yyyy-mm-dd hr" and values price (per kwh)
-        # <position> tag runs from 1-24. We need hours from 00-23, therefore substracting 1
-        prices.map{|p| [start_time.advance(hours: (p[0]-1)), p[1]]}.to_h
+        # <position> tag runs from 1-96 for 15min intervals. Convert to hourly by taking first interval of each hour
+        if interval_minutes == 15
+          # For 15-minute intervals, use the first interval of each hour (positions 1, 5, 9, 13, ...)
+          complete_prices.select { |pos, _| (pos - 1) % 4 == 0 }
+                        .map { |pos, price| [start_time.advance(hours: ((pos - 1) / 4)), price] }
+                        .to_h
+        else
+          # For hourly data (position runs from 1-24, we need hours from 00-23)
+          complete_prices.map { |pos, price| [start_time.advance(hours: (pos - 1)), price] }.to_h
+        end
       rescue Date::Error => e
         p e.message
         {}