Year 2015 Day 8 Part 2

src/Years/Y2015/Day8.md

@@ -199,7 +199,7 @@ string = do
   pure xs
 ```
 
-## Running a parser
+### Running a parser
 
 Run a parser, with some debug logging, by peeling the parsing effects of of the
 type. The order is important here, remember that function composition "runs"
@@ -284,6 +284,23 @@ runParser x str = do
     lazyRights (Right x :: xs) = x :: lazyRights xs
 ```
 
+## Escaping characters
+
+This is much more boring the the parsing, we just do simple recursive pattern
+matching on the characters of the provided string, escaping `"` and `\`, and
+surround the resulting string with quotes.
+
+```idris
+escape : String -> String
+escape str = "\"\{pack . escape' . unpack $ str}\""
+  where
+  escape' : List Char -> List Char
+  escape' [] = []
+  escape' ('"' :: xs) = '\\' :: '"' :: escape' xs
+  escape' ('\\' :: xs) = '\\' :: '\\' :: escape' xs
+  escape' (x :: xs) = x :: escape' xs
+```
+
 ## Part Functions
 
 ### Part 1
@@ -293,7 +310,7 @@ possible failures by sequencing the `List (Maybe String)` into a
 `Maybe (List String)`, and the compute the difference in character counts.
 
 ```idris
-part1 : Eff (PartEff String) (Int, ())
+part1 : Eff (PartEff String) (Int, List String)
 part1 = do
   inputs <- map lines $ askAt "input"
   outputs <- traverse (runParser string) inputs
@@ -301,11 +318,34 @@ part1 = do
     | _ => throw "Some strings failed to parse"
   let difference = 
     sum $ zipWith (\x, y => strLength x - strLength y) inputs outputs
-  pure (difference, ())
+  pure (difference, inputs)
+```
+
+### Part 2
+
+Map our character escaping function over our input string and compute the
+difference in character counts.
+
+Make a little stop along the way to ensure that escape -> parse round trips
+without changing the content of the string.
+
+```idris
+part2 : List String -> Eff (PartEff String) Int
+part2 inputs = do
+  let outputs = map escape inputs
+  Just reversed_outputs <- map sequence . traverse (runParser string) $ outputs
+    | _ => throw "Reversing escaping of the inputs failed"
+  unless (all id $ zipWith (==) inputs reversed_outputs) $ do
+    debug . delay . joinBy "\n" . map show . filter (\(x, y, z) => x /= z) $ 
+      zip3 inputs outputs reversed_outputs
+    throw "Parsed outputs were not identical to inputs"
+  let difference = 
+    sum $ zipWith (\x, y => strLength y - strLength x) inputs outputs
+  pure difference
 ```
 
 <!-- idris
 public export
 day8 : Day
-day8 = First 8 part1
+day8 = Both 8 part1 part2
 -->