From ed3d8a86cac30561e16c5d68f5342fc47e8e7b30 Mon Sep 17 00:00:00 2001
From: Nathan McCarty <thatonelutenist@stranger.systems>
Date: Sun, 19 Jan 2025 18:46:24 -0500
Subject: [PATCH] Year 2015 Day 9 Part 2
---
src/Years/Y2015/Day9.md | 52 ++++++++++++++++++++++++++++++++++-------
1 file changed, 44 insertions(+), 8 deletions(-)
diff --git a/src/Years/Y2015/Day9.md b/src/Years/Y2015/Day9.md
index acb702c..8abf4b2 100644
--- a/src/Years/Y2015/Day9.md
+++ b/src/Years/Y2015/Day9.md
@@ -210,11 +210,9 @@ lengths.
```idris
shortestPath : Has (Reader DistanceMap) fs => Has (Except String) fs =>
- (paths : List (List Location)) -> Eff fs (List Location, Integer)
+ (paths : List (List Location, Integer)) -> Eff fs (List Location, Integer)
shortestPath paths = do
- lens <- traverse pathLen paths
- let pairs = zip paths lens
- shortest pairs Nothing
+ shortest paths Nothing
where
shortest : List (a, Integer) -> (acc : Maybe (a, Integer))
-> Eff fs (a, Integer)
@@ -228,6 +226,27 @@ shortestPath paths = do
else shortest xs (Just y)
```
+Calculate the longest path from a list of paths by recursively comparing
+lengths.
+
+```idris
+longestPath : Has (Reader DistanceMap) fs => Has (Except String) fs =>
+ (paths : List (List Location, Integer)) -> Eff fs (List Location, Integer)
+longestPath paths = do
+ longest paths Nothing
+ where
+ longest : List (a, Integer) -> (acc : Maybe (a, Integer))
+ -> Eff fs (a, Integer)
+ longest [] acc = note "No paths" acc
+ longest (x :: xs) acc =
+ case acc of
+ Nothing => longest xs (Just x)
+ Just y =>
+ if (snd x) > (snd y)
+ then longest xs (Just x)
+ else longest xs (Just y)
+```
+
## Part Functions
### Part 1
@@ -236,7 +255,8 @@ Parse our locations, turn them into a `DistanceMap`, load that into our reader,
generate our paths, then find the one with the shortest length.
```idris
-part1 : Eff (PartEff String) (Integer, ())
+part1 :
+ Eff (PartEff String) (Integer, (DistanceMap, List (List Location, Integer)))
part1 = do
locations <- map lines (askAt "input") >>= traverse parseLocationPair
info "Locations: \{show locations}"
@@ -246,13 +266,29 @@ part1 = do
paths <- map Prelude.toList allPaths
trace "Paths: \{show paths}"
info "Found \{show $ length paths} paths"
- (path, len) <- shortestPath paths
+ distances <- traverse pathLen paths
+ let pairs = zip paths distances
+ (path, len) <- shortestPath pairs
info "Shortest Path: \{show path} \{show len}"
- pure (len, ())
+ pure (len, (distance_map, pairs))
+```
+
+### Part 2
+
+Feed the locations back into a longest path function
+
+```idris
+part2 : (DistanceMap, List (List Location, Integer))
+ -> Eff (PartEff String) Integer
+part2 (distance_map, pairs) = do
+ runReader distance_map {fs = Reader DistanceMap :: PartEff String} $ do
+ (path, len) <- longestPath pairs
+ info "Longest Path: \{show path} \{show len}"
+ pure len
```
<!-- idris
public export
day9 : Day
-day9 = First 9 part1
+day9 = Both 9 part1 part2
-->