Implement real single-source shortest-paths
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abd6bbcfd7
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7d15b70c2d
2 changed files with 128 additions and 74 deletions
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@ -123,3 +123,73 @@ pub fn readAll(allocator: std.mem.Allocator, path: []const u8) ![]u8 {
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return try file.readToEndAlloc(allocator, comptime 1 << 30);
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}
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pub fn shortestPath(
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comptime State: type,
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allocator: std.mem.Allocator,
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ctx: State.Context,
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start: State,
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) !?u64 {
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const Entry = struct {
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state: State,
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cost: u64,
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const Self = @This();
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fn compare(_: void, a: Self, b: Self) std.math.Order {
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return std.math.order(a.cost, b.cost);
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}
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};
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var queue = std.PriorityQueue(Entry, void, comptime Entry.compare).init(allocator, {});
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defer queue.deinit();
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try queue.add(Entry{
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.state = start,
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.cost = 0,
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});
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var prev = std.AutoHashMap(State, State).init(allocator);
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defer prev.deinit();
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var dist = std.AutoHashMap(State, u64).init(allocator);
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defer dist.deinit();
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try dist.put(start, 0);
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while (queue.removeOrNull()) |entry| {
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const u, const cost = .{ entry.state, entry.cost };
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if (dist.get(u)) |best| {
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if (best < cost) {
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continue;
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}
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}
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if (u.isGoal(ctx)) {
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return cost;
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}
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var neighbors: std.ArrayList(State.Neighbor) = try u.neighbors(ctx);
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defer neighbors.deinit();
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for (neighbors.items) |neighbor| {
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const v, const extra = .{ neighbor.next, neighbor.extra };
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const alt = cost + extra;
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if (dist.get(v)) |dv| {
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if (alt >= dv) {
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continue;
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}
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}
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try prev.put(v, u);
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try dist.put(v, alt);
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try queue.add(Entry{
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.state = v,
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.cost = alt,
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});
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}
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}
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return null;
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}
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@ -28,7 +28,7 @@ pub fn main() !void {
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}
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fn part1(allocator: Allocator, grid: Grid) !u64 {
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const start, const goal = v: {
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const pos, const goal = v: {
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var start: Pos = undefined;
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var end: Pos = undefined;
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@ -46,65 +46,37 @@ fn part1(allocator: Allocator, grid: Grid) !u64 {
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const State = struct {
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pos: Pos,
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dir: Direction,
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};
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const Entry = struct {
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path: List(State),
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cost: u64,
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const Self = @This();
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fn compare(_: void, a: Self, b: Self) std.math.Order {
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return std.math.order(a.cost, b.cost);
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}
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pub const Context = struct {
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allocator: Allocator,
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grid: Grid,
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goal: Pos,
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};
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var queue = PriorityQueue(Entry, void, comptime Entry.compare).init(allocator, {});
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defer queue.deinit();
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defer while (queue.removeOrNull()) |*entry| entry.path.deinit();
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pub fn isGoal(self: Self, ctx: Context) bool {
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return std.meta.eql(self.pos, ctx.goal);
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}
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try queue.add(v: {
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var path = List(State).init(allocator);
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try path.append(State{
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.pos = start,
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.dir = Direction.e,
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});
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break :v Entry{
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.path = path,
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.cost = 0,
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pub const Neighbor = struct {
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next: Self,
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extra: u64,
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};
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});
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var expanded = Set(State).init(allocator);
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defer expanded.deinit();
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while (queue.removeOrNull()) |entry| {
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const path, const cost = .{ entry.path, entry.cost };
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defer path.deinit();
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const state = path.getLast();
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if (expanded.contains(state)) {
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continue;
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}
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try expanded.put(state);
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if (std.meta.eql(state.pos, goal)) {
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return cost;
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}
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pub fn neighbors(self: Self, ctx: Context) !List(Neighbor) {
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var list = List(Neighbor).init(ctx.allocator);
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// Move once in the direction we're already going.
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{
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const move = state.pos.move(state.dir);
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if (grid.get(move) != '#') {
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var next = try path.clone();
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try next.append(State{
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const move = self.pos.move(self.dir);
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if (ctx.grid.get(move) != '#') {
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try list.append(Neighbor{
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.next = Self{
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.pos = move,
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.dir = state.dir,
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});
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try queue.add(Entry{
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.path = next,
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.cost = cost + 1,
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.dir = self.dir,
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},
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.extra = 1,
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});
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}
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}
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@ -114,25 +86,37 @@ fn part1(allocator: Allocator, grid: Grid) !u64 {
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//
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// As a consequence, there's never *ever* a reason to turn twice in a row
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// and go back the way we came in.
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for ([_]Direction{ state.dir.clockwise(), state.dir.counterclockwise() }) |dir| {
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const move = state.pos.move(dir);
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if (grid.get(move) == '#') {
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for ([_]Direction{ self.dir.clockwise(), self.dir.counterclockwise() }) |dir| {
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const move = self.pos.move(dir);
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if (ctx.grid.get(move) == '#') {
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continue;
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}
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var next = try path.clone();
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try next.append(State{
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try list.append(Neighbor{
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.next = Self{
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.pos = move,
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.dir = dir,
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},
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.extra = 1000 + 1,
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});
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try queue.add(Entry{
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.path = next,
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.cost = cost + 1000 + 1,
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});
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}
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}
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return 0;
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return list;
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}
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};
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const start = State{
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.pos = pos,
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.dir = Direction.e,
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};
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const ctx = State.Context{
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.allocator = allocator,
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.grid = grid,
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.goal = goal,
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};
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return (try aoc.shortestPath(State, allocator, ctx, start)).?;
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}
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const Pos = struct {
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