Another shortest path impl

Author: mnvr

17.wip.swift

@@ -17,10 +17,54 @@ func readInput() -> Grid {
             x += 1
         }
         y += 1
+    }
+    return Grid(items: items, lastNode: .init(x: x - 1, y: y - 1))
+}
+
+func shortestPath(grid: Grid) -> Int? {
+    let startNode = Node(x: 0, y: 0)
+    func isEnd(node: Node) -> Bool { node == grid.lastNode }
 
+    var dist: [Node: Int] = [startNode: 0]
+    var seen: Set<Node> = Set()
+    //var invDist: [Int: [Node]] = []
+    //var minDist = 0
+
+    func popNearest() -> (Node, Int)? {
+        var u: Node?
+        var du: Int = .max
+        for (v, dv) in dist {
+            if dv < du && !seen.contains(v) {
+                u = v
+                du = dv
+            }
+        }
+        if let u { return (u, du) }
+        return nil
     }
-    return Grid(items: items, lastNode: .init(x: x, y: y))
+
+    while let (u, du) = popNearest() {
+        if !seen.insert(u).inserted { continue }
+        if isEnd(node: u) { return du }
+        for v in neighbours(grid: grid, node: u) {
+            let d = grid.items[v]!
+            let dv = dist[v] ?? .max
+            if du + d < dv {
+                dist[v] = du + d
+            }
+        }
+    }
+
+    return nil
+}
+
+func neighbours(grid: Grid, node: Node) -> [Node] {
+    let (x, y) = (node.x, node.y)
+    return [(x - 1, y), (x + 1, y), (x, y - 1), (x, y + 1)]
+      .map { Node(x: $0.0, y: $0.1) }
+      .filter { grid.items[$0] != nil }
 }
 
 let grid = readInput()
-print(grid)
+let sp = shortestPath(grid: grid)
+print(sp ?? 0)