Year 2015 Day 7 Part 2

src/Years/Y2015/Day7.md

@@ -197,11 +197,11 @@ solveWire wire = do
 
 ### Part 1
 
-Parse the input, then feed it and an initial empty cache into our `solveWire` function, passing the produced value as the output and the circut, represented as a dependent map from wires to gates, as the context for part 2.
+Parse the input, then feed it and an initial empty cache into our `solveWire` function, passing the produced value as the output and the circut, represented as a dependent map from wires to gates, as well as the output signal from wire 'a' as the context for part 2.
 
 ```idris
 covering
-part1 : Eff (PartEff String) (Bits16, SortedDMap Wire Gate)
+part1 : Eff (PartEff String) (Bits16, (SortedDMap Wire Gate, Literal))
 part1 = do
   circut <- askAt "input" >>= parseGates
   (value, _) <- 
@@ -209,11 +209,28 @@ part1 = do
       {fs = State (SortedMap Wire Literal) 
             :: Reader (SortedDMap Wire Gate) 
             :: PartEff String }
-  pure (value, circut)
+  pure (value, (circut, value))
+```
+
+### Part 2
+
+Override the value for the 'b' wire to the output from the 'a' wire in part 1, then rerun our calcuation to find the new output for the 'a' wire.
+
+```idris
+covering
+part2 : (SortedDMap Wire Gate, Literal) -> Eff (PartEff String) Bits16
+part2 (circut, value) = do
+  let circut = insert "b" (Constant (Left value)) circut
+  (value, _) <- 
+    runState empty . runReader circut $ solveWire "a" 
+      {fs = State (SortedMap Wire Literal) 
+            :: Reader (SortedDMap Wire Gate) 
+            :: PartEff String }
+  pure value
 ```
 
 <!-- idris
 public export covering
 day7 : Day
-day7 = First 7 part1
+day7 = Both 7 part1 part2
 -->