381 lines
18 KiB
Zig
381 lines
18 KiB
Zig
const std = @import("std");
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const print = std.debug.print;
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const util = @import("util.zig");
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const expect = std.testing.expect;
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pub fn main() !void {
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const response = try part_two(false);
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print("{}\n", .{response});
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}
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const Point = struct { x: usize, y: usize };
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const Heading = enum {
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north,
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east,
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south,
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west,
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pub fn turn_left(self: Heading) Heading {
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const int_val = @intFromEnum(self);
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return @enumFromInt(@as(u4, int_val) + 3 % 4);
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}
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pub fn turn_right(self: Heading) Heading {
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const int_val = @intFromEnum(self);
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return @enumFromInt(@as(u4, int_val) + 1 % 4);
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}
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};
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const Position = struct { point: Point, heading: Heading };
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// Sketch solution for Part Two:
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// * Extend data structure to keep track of which leading-in nodes generates the shortest distance to a given node. This
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// should be an ArrayList, because multiple leading-in nodes can lead to the same node with the same minimum distance
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// (if distance_so_far + cost is _equal_)
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// * Extend the loop to not just return when the target node is reached, but to keep running until `current_distance`
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// is _greater_ than the found minimum_distance_to_target
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// * Once the loop terminates (because current_distance is too large), build paths by iterating back from target_node
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// iteratively to all preceding nodes (branching when there are multiple).
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// * Keep track of all those points, then dedupe and count.
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fn part_one(is_test_case: bool) !u32 {
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var gpa = std.heap.GeneralPurposeAllocator(.{}){};
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defer _ = gpa.deinit();
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const allocator = gpa.allocator();
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const built = try build_map_and_start_and_end(is_test_case, allocator);
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const map = built.map;
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const start_position = built.start_position;
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const target_point = built.target_point;
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defer allocator.free(map);
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defer {
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for (map) |line| {
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allocator.free(line);
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}
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}
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var distances = std.AutoHashMap(Position, u32).init(allocator);
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defer distances.deinit();
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var visited_positions = std.AutoHashMap(Position, bool).init(allocator);
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defer visited_positions.deinit();
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try distances.put(start_position, 0);
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var current_position = start_position;
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var current_distance: u32 = 0;
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while (true) {
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// print("DEBUG - current_position is {}/{} ({}), and current_distance is {}\n", .{ current_position.point.x, current_position.point.y, current_position.heading, current_distance });
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if (std.meta.eql(current_position.point, target_point)) {
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return current_distance;
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}
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const moves = try find_valid_moves(map, current_position, allocator);
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for (moves) |move| {
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if (visited_positions.contains(move.position)) {
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continue;
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}
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const distance_from_here = current_distance + move.cost;
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// Below is an attempted implementation using `getOrPut`, which I still _really_ don't understand -
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// it required me to make `distance_from_here` a `var` so that I could do
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// `response.value_ptr = &distance_from_here`, and then was putting huge number values (probably - pointers,
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// not the actual values?) into the map. But if I tried `response.value_ptr = distance_from_here`, that gave
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// a type mismatch.
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// var response = try distances.getOrPut(move.position);
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// if (response.found_existing) {
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// if (distance_from_here < response.value_ptr.*) {
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// print("DEBUG - found a new lowest distance for {}/{} ({}) - moving from {} to {}\n", .{ move.position.point.x, move.position.point.y, move.position.heading, response.value_ptr.*, distance_from_here });
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// response.value_ptr = &distance_from_here;
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// }
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// } else {
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// print("DEBUG - found fresh lowest distance for {}/{} ({}) - {}\n", .{ move.position.point.x, move.position.point.y, move.position.heading, distance_from_here });
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// response.value_ptr = &distance_from_here;
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// }
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if (distances.contains(move.position)) {
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const current_lowest_distance = distances.get(move.position).?;
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if (distance_from_here < current_lowest_distance) {
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// print("DEBUG - found a new lowest distance for {}/{} ({}) - moving from {} to {}\n", .{ move.position.point.x, move.position.point.y, move.position.heading, current_lowest_distance, distance_from_here });
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try distances.put(move.position, distance_from_here);
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}
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} else {
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// print("DEBUG - found fresh lowest distance for {}/{} ({}) - {}\n", .{ move.position.point.x, move.position.point.y, move.position.heading, distance_from_here });
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try distances.put(move.position, distance_from_here);
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}
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}
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allocator.free(moves);
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try visited_positions.put(current_position, true);
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// Find the next candidate by iterating over all unvisited nodes with non-infinite distance, and picking the one
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// with lowest distance.
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// There would almost-certainly be a way to optimize this with a min-queue if we cared.
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var next_position: Position = undefined;
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// var lowest_distance_found: u32 = std.math.inf(u32);
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// above gives `error: reached unreachable code`
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var lowest_distance_found: u32 = 999999999;
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var dist_it = distances.iterator();
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while (dist_it.next()) |entry| {
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// print("DEBUG - checking whether {}/{}({}) is valid as next current_position - ", .{ entry.key_ptr.point.x, entry.key_ptr.point.y, entry.key_ptr.heading });
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if (visited_positions.contains(entry.key_ptr.*)) {
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// print("no, because it's been visited already\n", .{});
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continue;
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}
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if (entry.value_ptr.* > lowest_distance_found) {
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// print("no, because its distance ({}) is higher than the lowest found so far ({})\n", .{ entry.value_ptr.*, lowest_distance_found });
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continue;
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}
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// print("it is!\n", .{});
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// print("{}/{}({}) is a valid next-position\n", .{ entry.key_ptr.point.x, entry.key_ptr.point.y, entry.key_ptr.heading });
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next_position = entry.key_ptr.*;
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lowest_distance_found = entry.value_ptr.*;
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}
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current_position = next_position;
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current_distance = lowest_distance_found;
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}
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}
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// Existence of such a value in the map indicates that _all_ of the Positions listed in .predecessors can reach this
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// Position with a total path-length of length
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const PredecessorsAndLength = struct {
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predecessors: std.ArrayList(Position),
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length: u32,
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pub fn create(predecessor: Position, length: u32, allocator: std.mem.Allocator) !PredecessorsAndLength {
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var list = std.ArrayList(Position).init(allocator);
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try list.append(predecessor);
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return PredecessorsAndLength{ .predecessors = list, .length = length };
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}
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pub fn deinit(self: *PredecessorsAndLength) void {
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self.predecessors.deinit();
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}
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};
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fn part_two(is_test_case: bool) !u32 {
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var gpa = std.heap.GeneralPurposeAllocator(.{}){};
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defer _ = gpa.deinit();
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const allocator = gpa.allocator();
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const built = try build_map_and_start_and_end(is_test_case, allocator);
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const map = built.map;
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const start_position = built.start_position;
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const target_point = built.target_point;
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defer allocator.free(map);
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defer {
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for (map) |line| {
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allocator.free(line);
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}
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}
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var distances = std.AutoHashMap(Position, PredecessorsAndLength).init(allocator);
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defer distances.deinit();
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defer {
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var preds_it = distances.valueIterator();
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while (preds_it.next()) |val| {
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val.deinit();
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}
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}
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var visited_positions = std.AutoHashMap(Position, bool).init(allocator);
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defer visited_positions.deinit();
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try distances.put(start_position, PredecessorsAndLength{ .predecessors = std.ArrayList(Position).init(allocator), .length = 0 });
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var current_position = start_position;
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var current_distance: u32 = 0;
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var found_lowest_distance_to_target: ?u32 = null;
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while (true) {
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if (std.meta.eql(current_position.point, target_point) and found_lowest_distance_to_target == null) {
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found_lowest_distance_to_target = current_distance;
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}
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const moves = try find_valid_moves(map, current_position, allocator);
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for (moves) |move| {
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if (visited_positions.contains(move.position)) {
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continue;
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}
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const distance_from_here = current_distance + move.cost;
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if (distances.contains(move.position)) {
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const current_lowest_distance = distances.get(move.position).?.length;
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if (distance_from_here < current_lowest_distance) {
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// print("DEBUG - found a new lowest distance for {}/{} ({}) - moving from {} to {}\n", .{ move.position.point.x, move.position.point.y, move.position.heading, current_lowest_distance, distance_from_here });
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var stale_value = distances.get(move.position).?;
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stale_value.deinit();
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try distances.put(move.position, try PredecessorsAndLength.create(current_position, distance_from_here, allocator));
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}
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if (distance_from_here == current_lowest_distance) {
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try distances.getPtr(move.position).?.predecessors.append(current_position);
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}
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} else {
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// print("DEBUG - found fresh lowest distance for {}/{} ({}) - {}\n", .{ move.position.point.x, move.position.point.y, move.position.heading, distance_from_here });
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try distances.put(move.position, try PredecessorsAndLength.create(current_position, distance_from_here, allocator));
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}
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}
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allocator.free(moves);
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try visited_positions.put(current_position, true);
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// Find the next candidate by iterating over all unvisited nodes with non-infinite distance, and picking the one
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// with lowest distance.
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// There would almost-certainly be a way to optimize this with a min-queue if we cared.
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var next_position: Position = undefined;
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// var lowest_distance_found: u32 = std.math.inf(u32);
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// above gives `error: reached unreachable code`
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var lowest_distance_found: u32 = 999999999;
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var dist_it = distances.iterator();
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while (dist_it.next()) |entry| {
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// print("DEBUG - checking whether {}/{}({}) is valid as next current_position - ", .{ entry.key_ptr.point.x, entry.key_ptr.point.y, entry.key_ptr.heading });
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if (visited_positions.contains(entry.key_ptr.*)) {
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// print("no, because it's been visited already\n", .{});
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continue;
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}
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if (entry.value_ptr.*.length > lowest_distance_found) {
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// print("no, because its distance ({}) is higher than the lowest found so far ({})\n", .{ entry.value_ptr.*, lowest_distance_found });
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continue;
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}
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// print("it is!\n", .{});
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// print("{}/{}({}) is a valid next-position\n", .{ entry.key_ptr.point.x, entry.key_ptr.point.y, entry.key_ptr.heading });
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next_position = entry.key_ptr.*;
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lowest_distance_found = entry.value_ptr.*.length;
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}
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current_position = next_position;
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current_distance = lowest_distance_found;
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// If we are dealing with distances larger than _a_ found-distance-to-target, then (because all edge-lengths are
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// positive) no further paths to be found can be shorter - therefore we've found all possible shortest paths.
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if (found_lowest_distance_to_target != null and current_distance > found_lowest_distance_to_target.?) {
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break;
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}
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}
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print("DEBUG - finished finding all paths to target_point\n", .{});
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// Iterate back over the predecessors of paths that end at the target_point - all of those are on shortest-paths
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var points_on_shortest_paths = std.AutoHashMap(Point, bool).init(allocator);
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defer points_on_shortest_paths.deinit();
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var positions_to_be_processed = std.AutoHashMap(Position, bool).init(allocator);
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defer positions_to_be_processed.deinit();
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for (0..4) |i| {
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const target = Position{ .point = target_point, .heading = @enumFromInt(i) };
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if (distances.contains(target)) {
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try positions_to_be_processed.put(target, true);
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}
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}
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while (true) {
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var position_it = positions_to_be_processed.keyIterator();
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if (position_it.next()) |pos| {
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const actual_pos = pos.*;
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// print("DEBUG - found a position to be processed - it is {}/{}\n", .{ pos.point.x, pos.point.y });
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if (distances.contains(actual_pos)) {
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try points_on_shortest_paths.put(actual_pos.point, true);
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// print("DEBUG - it's on a shortest path, has been added\n", .{});
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for (distances.get(actual_pos).?.predecessors.items) |pred| {
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// print("DEBUG - adding {}/{} to the positions_to_be_processed\n", .{ pred.point.x, pred.point.y });
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try positions_to_be_processed.put(pred, true);
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}
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}
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_ = positions_to_be_processed.remove(actual_pos);
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} else {
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// `positions_to_be_processed` is empty - stop looping
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// print("DEBUG - there are no more positions_to_be_processed - stopping processing\n", .{});
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break;
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}
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}
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print("DEBUG - points_on_shortest_paths are: ", .{});
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var count: u32 = 0;
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var key_it = points_on_shortest_paths.keyIterator();
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while (key_it.next()) |point| {
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count += 1;
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print("{}/{}, ", .{ point.x, point.y });
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}
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print("\n", .{});
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return count;
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}
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fn build_map_and_start_and_end(is_test_case: bool, allocator: std.mem.Allocator) !struct { map: [][]u8, start_position: Position, target_point: Point } {
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const input_file = try util.getInputFile("16", is_test_case);
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const data = try util.readAllInputWithAllocator(input_file, allocator);
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defer allocator.free(data);
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var map_list = std.ArrayList([]u8).init(allocator);
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var it = std.mem.splitScalar(u8, data, '\n');
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var start_position: Position = undefined;
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var target_point: Point = undefined;
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var line_counter: usize = 0;
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while (it.next()) |line| : (line_counter += 1) {
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var line_list = std.ArrayList(u8).init(allocator);
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for (line) |c| {
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try line_list.append(c);
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}
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try map_list.append(try line_list.toOwnedSlice());
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const index_of_s = std.mem.indexOf(u8, line, "S");
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if (index_of_s != null) {
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start_position = Position{ .point = Point{ .x = index_of_s.?, .y = line_counter }, .heading = Heading.east };
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}
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const index_of_e = std.mem.indexOf(u8, line, "E");
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if (index_of_e != null) {
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target_point = Point{ .x = index_of_e.?, .y = line_counter };
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}
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}
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return .{ .map = try map_list.toOwnedSlice(), .start_position = start_position, .target_point = target_point };
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}
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const Move = struct { position: Position, cost: u32 };
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fn find_valid_moves(map: [][]u8, current_position: Position, allocator: std.mem.Allocator) ![]Move {
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var responses = std.ArrayList(Move).init(allocator);
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// First of three cases - move forward (if that's not a wall)
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var neighbour: Point = undefined;
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switch (current_position.heading) {
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Heading.north => {
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// print("DEBUG - north from {}/{} is ", .{ current_position.point.x, current_position.point.y });
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neighbour = Point{ .x = current_position.point.x, .y = current_position.point.y - 1 };
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},
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Heading.east => {
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// print("DEBUG - east from {}/{} is ", .{ current_position.point.x, current_position.point.y });
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neighbour = Point{ .x = current_position.point.x + 1, .y = current_position.point.y };
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},
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Heading.south => {
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// print("DEBUG - south from {}/{} is ", .{ current_position.point.x, current_position.point.y });
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neighbour = Point{ .x = current_position.point.x, .y = current_position.point.y + 1 };
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},
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Heading.west => {
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// print("DEBUG - west from {}/{} is ", .{ current_position.point.x, current_position.point.y });
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neighbour = Point{ .x = current_position.point.x - 1, .y = current_position.point.y };
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},
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}
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// print("{}/{}\n", .{ neighbour.x, neighbour.y });
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if (map[neighbour.y][neighbour.x] == '.' or map[neighbour.y][neighbour.x] == 'E') {
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try responses.append(Move{ .position = Position{ .point = neighbour, .heading = current_position.heading }, .cost = 1 });
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}
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// Second and third cases - turn left and right
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try responses.append(Move{ .position = Position{ .point = current_position.point, .heading = current_position.heading.turn_left() }, .cost = 1000 });
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try responses.append(Move{ .position = Position{ .point = current_position.point, .heading = current_position.heading.turn_right() }, .cost = 1000 });
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return responses.toOwnedSlice();
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}
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test "turn left and right" {
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try expect(Heading.north.turn_left() == Heading.west);
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try expect(Heading.north.turn_right() == Heading.east);
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try expect(Heading.north.turn_right().turn_right().turn_right().turn_right() == Heading.north);
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try expect(Heading.north.turn_left().turn_left().turn_left() == Heading.east);
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}
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test "part_one" {
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const part_one_response = try part_one(true);
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print("DEBUG - part_one_response is {}\n", .{part_one_response});
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try expect(part_one_response == 7036);
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}
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test "part_two" {
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const part_two_response = try part_two(true);
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print("DEBUG - part_two_response is {}\n", .{part_two_response});
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try expect(part_two_response == 45);
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}
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