Year 2015 Day 13 Part 2

src/Years/Y2015/Day13.md

@@ -89,7 +89,8 @@ a dependent pair of the name of the individual this record describes, and the
 change described by that record.
 
 ```idris
-parseChange : Has (Except String) fs => String -> Eff fs (name ** Change name)
+parseChange : Has (Except String) fs => 
+  String -> Eff fs (name ** Change name)
 parseChange str = do
   changee ::: [_, direction, amount, _, _, _, _, _, _, other] 
     <- pure $ split (== ' ') str
@@ -110,10 +111,11 @@ that the conditions encoded by the auto-implict proofs in `Changes` are met.
 
 ```idris
 parseChanges : Has (Except String) fs =>
-  List String -> Eff fs Changes
+  List String -> (seed : SortedDMap Name (\n => List (Change n)))
-parseChanges strs = do
+  -> Eff fs Changes
+parseChanges strs seed = do
   changes <- traverse parseChange strs
-  let change_map = insertChanges changes empty
+  let change_map = insertChanges changes seed
   case isItSucc (length (keys change_map)) of
     Yes prf => pure $ MkChanges change_map (fromList (keys change_map))
     No contra => throw "Empty table, not very interesting"
@@ -225,15 +227,35 @@ Notice how, since we are outside the `parameters` block, we have to provide the
 part1 : Eff (PartEff String) (Happiness, ())
 part1 = do
   input <- map lines $ askAt "input"
-  changes <- parseChanges input
+  changes <- parseChanges input empty
   (max, arrangement) <- maxHappiness changes
   pure (max, ())
 ```
 
+### Part 2
+
+Our implementation already replaces missing relationships with 0, so we can
+cheese this by injecting ourself with an empty relationship list into the
+`change_map : SortedDMap Name (\n => (List n))`.
+
+The overall `Changes` data structure isn't easy to modify, and since our data
+set is quite small here, we'll just inject this into parsing and reparse our
+data.
+
+```idris
+part2 : () -> Eff (PartEff String) Happiness
+part2 x = do
+  input <- map lines $ askAt "input"
+  let seed = insert "ME!!!!" [] empty
+  changes <- parseChanges input seed
+  (max, arrangement) <- maxHappiness changes
+  pure max
+```
+
 ```idris hide
 public export
 day13 : Day
-day13 = First 13 part1
+day13 = Both 13 part1 part2
 ```
 
 ## References