From 89800584f4351a43dcbb92446985b89d05665a0f Mon Sep 17 00:00:00 2001
From: Dory <dory@dory.moe>
Date: Wed, 18 Dec 2024 12:34:43 -0800
Subject: [PATCH] bfs and binary search

---
 day18/d18p2.swift | 49 ++++++++++++++++++++++++++---------------------
 1 file changed, 27 insertions(+), 22 deletions(-)

diff --git a/day18/d18p2.swift b/day18/d18p2.swift
index 55285d1..b630d6a 100644
--- a/day18/d18p2.swift
+++ b/day18/d18p2.swift
@@ -33,27 +33,25 @@ struct Map : CustomStringConvertible {
     func heur(_ a: Coord, _ b: Coord) -> Int {
         return abs(a.x - b.x) + abs(a.y - b.y)
     }
-    func astar(from start: Coord, to end: Coord) -> Int? {
-        var q = Heap<(Coord, Int)>(comparator: { l, r in l.1 < r.1 })
-        var seen: [Coord: Int] = [start: 0]
-        q.insert((start, heur(start, end)))
-        while let (cell, _) = q.pop() {
+    func bfs(from start: Coord, to end: Coord) -> Bool {
+        var q: Set<Coord> = [start]
+        var seen: Set<Coord> = [start]
+        while !q.isEmpty {
+            let cell = q.removeFirst()
             if cell == end {
-                return seen[cell]
+                return true
             }
             [(0, -1), (0, 1), (-1, 0), (1, 0)].map { dy, dx in
                 Coord(x: cell.x + dx, y: cell.y + dy)}
                 .filter { c in c.x >= 0 && c.y >= 0 && c.x < w && c.y < h }
                 .filter { !walls.contains($0) }
+                .filter { !seen.contains($0) }
                 .forEach { nb in
-                    let newCost = seen[cell]! + 1
-                    if seen[nb, default: w*h] > newCost {
-                        seen[nb] = newCost
-                        q.insert((nb, newCost + heur(nb, end)))
-                    }
+                    seen.insert(nb)
+                    q.insert(nb)
                 }
         }
-        return nil
+        return false
     }
 }
 
@@ -64,19 +62,26 @@ struct AoC {
         let w = Int(CommandLine.arguments[2]) ?? 71
         let h = Int(CommandLine.arguments[3]) ?? 71
         let startLimit = Int(CommandLine.arguments[4]) ?? 1024
-        for limit in startLimit..<bytes.count {
-            let map = Map(walls: Set(bytes.prefix(limit)), w: w, h: h)
-            if let cost = map.astar(
-                from: Coord(x: 0, y: 0),
-                to: Coord(x: map.w - 1, y: map.h - 1)
-            ) {
-                print("\(limit): \(cost)")
+        var range = Array(startLimit..<bytes.count)
+        while range.count > 1 {
+            let center = range.count/2
+            print("Trying \(range[center])...", terminator: "")
+            let map = Map(walls: Set(bytes.prefix(range[center])), w: w, h: h)
+            if map.bfs(from: Coord(x: 0, y: 0), to: Coord(x: w-1, y: h-1)) {
+                print(" works")
+                range = Array(range[center+1..<range.count])
             } else {
-                print(map)
-                print("found byte \(limit-1): \(bytes[limit-1])")
-                return
+                print(" blocked")
+                range = Array(range[0..<center])
             }
         }
+        let map = Map(walls: Set(bytes.prefix(range[0])), w: w, h: h)
+        print(map)
+        if map.bfs(from: Coord(x: 0, y: 0), to: Coord(x: w-1, y: h-1)) {
+            print("at \(range[0]): \(bytes[range[0]])")
+        } else {
+            print("at \(range[0] + 1): \(bytes[range[0] + 1])")
+        }
     }
 }