aoc-2023/src/day18.zig

const std = @import("std");
const assert = std.debug.assert;
const aoc = @import("./aoc.zig");
const Allocator = std.mem.Allocator;
const PointLib = @import("./point.zig");
const PointI32 = PointLib.PointI32;
const PointU32 = PointLib.PointU32;

const Direction = enum {
    Up,
    Down,
    Left,
    Right,

    fn from_str(str: []const u8) ?Direction {
        if (std.mem.eql(u8, str, "U")) {
            return .Up;
        } else if (std.mem.eql(u8, str, "D")) {
            return .Down;
        } else if (std.mem.eql(u8, str, "L")) {
            return .Left;
        } else if (std.mem.eql(u8, str, "R")) {
            return .Right;
        } else {
            return null;
        }
    }

    fn to_offset(self: Direction) PointI32 {
        return switch (self) {
            .Up    => PointI32{ .x =  0, .y = -1 },
            .Down  => PointI32{ .x =  0, .y =  1 },
            .Left  => PointI32{ .x = -1, .y =  0 },
            .Right => PointI32{ .x =  1, .y =  0 },
        };
    }
};

const DigInstruction = struct {
    dir: Direction,
    steps: u32
};

const Instructions = std.ArrayList(DigInstruction);

fn parse_dig_instruction1(line: []const u8) !DigInstruction {
    var iter = std.mem.splitScalar(u8, line, ' ');

    const dir_str = iter.next() orelse return error.MissingDirection;
    const steps_str = iter.next() orelse return error.MissingSteps;

    return DigInstruction{
        .dir = Direction.from_str(dir_str) orelse return error.InvalidDirection,
        .steps = try std.fmt.parseUnsigned(u32, steps_str, 10)
    };
}

fn parse_input1(allocator: Allocator, lines: []const []const u8) !Instructions {
    var result = Instructions.init(allocator);
    errdefer result.deinit();

    for (lines) |line| {
        try result.append(try parse_dig_instruction1(line));
    }

    return result;
}

fn get_bounds(insts: Instructions) std.meta.Tuple(&.{ PointI32, PointI32 }) {
    var upper_left = PointI32.zero();
    var lower_right = PointI32.zero();

    var current = PointI32.zero();
    for (insts.items) |inst| {
        const step = inst.dir.to_offset().mul(@intCast(inst.steps));
        current = current.add(step);

        upper_left.x = @min(upper_left.x, current.x);
        upper_left.y = @min(upper_left.y, current.y);
        lower_right.x = @max(lower_right.x, current.x);
        lower_right.y = @max(lower_right.y, current.y);
    }

    return .{ upper_left, lower_right };
}

fn flood_fill(allocator: Allocator, map: []bool, map_size: PointU32, from: PointU32) !void {
    var stack = std.ArrayList(PointU32).init(allocator);
    defer stack.deinit();

    try stack.append(from);
    map[from.y * map_size.x + from.x] = true;

    while (stack.popOrNull()) |point| {
        const directions = [_]Direction{
            .Up,
            .Right,
            .Down,
            .Left
        };
        for (directions) |dir| {
            const offset = dir.to_offset();
            const neighbour_x = @as(i32, @intCast(point.x)) + offset.x;
            const neighbour_y = @as(i32, @intCast(point.y)) + offset.y;

            if (!(0 <= neighbour_x and neighbour_x < map_size.x)) continue;
            if (!(0 <= neighbour_y and neighbour_y < map_size.y)) continue;

            const neighbour = PointU32{
                .x = @intCast(neighbour_x),
                .y = @intCast(neighbour_y)
            };
            const neighbour_idx = neighbour.y * map_size.x + neighbour.x;
            if (!map[neighbour_idx]) {
                map[neighbour_idx] = true;
                try stack.append(neighbour);
            }
        }
    }
}

fn find_flood_fill_start(map: []bool, width: u32, height: u32) ?PointU32 {
    assert(height >= 2);

    var found_wall = false;
    for (1..height) |y| {
        for (0..width) |x| {
            const idx = y * width + x;
            if (!found_wall) {
                if (map[idx]) {
                    found_wall = true;
                }
            } else {
                if (!map[idx]) {
                    return PointU32{ .x = @intCast(x), .y = @intCast(y) };
                }
            }
        }
    }

    return null;
}

fn get_lagoon_size(allocator: Allocator, instructions: Instructions) !usize {
    const bounds = get_bounds(instructions);
    const upper_left: PointI32 = bounds[0];
    const lower_right: PointI32 = bounds[1];
    const size = PointU32{
        .x = @abs(lower_right.x - upper_left.x) + 1,
        .y = @abs(lower_right.y - upper_left.y) + 1
    };

    std.debug.print("size: {any}\n", .{size});

    const map = try allocator.alloc(bool, size.x * size.y);
    defer allocator.free(map);
    @memset(map, false);

    var current = PointI32.zero();
    for (instructions.items) |inst| {
        const dir_offset = inst.dir.to_offset();

        for (0..inst.steps) |i| {
            const pos = current.add(dir_offset.mul(@intCast(i)));
            const idx = (pos.y - upper_left.y) * @as(i32, @intCast(size.x)) + (pos.x - upper_left.x);
            map[@intCast(idx)] = true;
        }

        const step = dir_offset.mul(@intCast(inst.steps));
        current = current.add(step);
    }

    const flood_fill_start = find_flood_fill_start(map, size.x, size.y) orelse return error.NoFloodFillStart;
    try flood_fill(allocator, map, size, flood_fill_start);

    // for (0..size.y) |y| {
    //     for (0..size.x) |x| {
    //         const idx = size.x * y + x;
    //         if (map[idx]) {
    //             std.debug.print("#", .{});
    //         } else {
    //             std.debug.print(".", .{});
    //         }
    //     }
    //     std.debug.print("\n", .{});
    // }

    return std.mem.count(bool, map, &.{ true });
}

pub fn part1(input: *aoc.Input) !aoc.Result {
    const allocator = input.allocator;

    var instructions = Instructions.init(allocator);
    defer instructions.deinit();
    for (input.lines) |line| {
        try instructions.append(try parse_dig_instruction1(line));
    }

    return .{ .uint = try get_lagoon_size(allocator, instructions) };
}

fn parse_dig_instruction2(line: []const u8) !DigInstruction {
    var iter = std.mem.splitScalar(u8, line, ' ');

    _ = iter.next();
    _ = iter.next();

    const color_str = iter.next() orelse return error.MissingColor;
    const color_trimmed = std.mem.trim(u8, color_str, "()#");
    if (color_trimmed.len != 6) return error.ColorTooShort;

    const dir: Direction = switch (color_trimmed[5]) {
        '0' => .Up,
        '1' => .Down,
        '2' => .Left,
        '3' => .Up,
        else => return error.InvalidDirection
    };

    return DigInstruction{
        .dir = dir,
        .steps = try std.fmt.parseUnsigned(u32, color_trimmed[1..5], 16)
    };
}

pub fn part2(input: *aoc.Input) !aoc.Result {
    const allocator = input.allocator;

    var instructions = Instructions.init(allocator);
    defer instructions.deinit();
    for (input.lines) |line| {
        try instructions.append(try parse_dig_instruction2(line));
    }

    return .{ .uint = try get_lagoon_size(allocator, instructions) };
}

const example_input = [_][]const u8{
    "R 6 (#70c710)",
    "D 5 (#0dc571)",
    "L 2 (#5713f0)",
    "D 2 (#d2c081)",
    "R 2 (#59c680)",
    "D 2 (#411b91)",
    "L 5 (#8ceee2)",
    "U 2 (#caa173)",
    "L 1 (#1b58a2)",
    "U 2 (#caa171)",
    "R 2 (#7807d2)",
    "U 3 (#a77fa3)",
    "L 2 (#015232)",
    "U 2 (#7a21e3)",
};

test "part 1 example" {
    try aoc.expectAnswerUInt(part1, 62, &example_input);
}

test "part 2 example" {
    try aoc.expectAnswerUInt(part2, 952408144115, &example_input);
}