advent-of-code/2024/src/day23.zig

const std = @import("std");
const aoc = @import("aoc.zig");

const Allocator = std.mem.Allocator;
const List = std.ArrayList;
const Map = std.AutoHashMap;
const Set = aoc.AutoHashSet;
const PriorityQueue = std.PriorityQueue;

pub fn main() !void {
    var gpa: std.heap.GeneralPurposeAllocator(.{}) = .{};
    defer _ = gpa.deinit();
    const allocator = gpa.allocator();

    const text = try aoc.readAll(allocator, "input/day23.txt");
    defer allocator.free(text);

    var edges = try aoc.parseAll(Edge, allocator, text, "\n");
    defer edges.deinit();

    var bw = std.io.bufferedWriter(std.io.getStdOut().writer());
    const stdout = bw.writer();

    try stdout.print("{}\n", .{try part1(allocator, edges.items)});
    try bw.flush();

    var password = try part2(allocator, edges.items);
    defer password.deinit();
    try stdout.print("{s}\n", .{password.items});
    try bw.flush();
}

// I really don't want to have to figure out string hashing again, so turn the
// strings into non-slice values. ("pre-hash" them?)
const Name = u16;

fn parseName(s: []const u8) Name {
    if (s.len != 2) unreachable;

    const hi: u16 = @intCast(s[0]);
    const lo: u16 = @intCast(s[1]);

    return (hi << 8) + lo;
}

fn nameStartsWith(n: Name, c: u8) bool {
    return n >> 8 == c;
}

fn formatName(n: Name) [2]u8 {
    return [2]u8{
        @intCast(n >> 8),
        @intCast(n & 0xff),
    };
}

const Edge = struct {
    a: Name,
    b: Name,

    pub fn parse(_: Allocator, line: []const u8) !Edge {
        var parts = std.mem.tokenizeScalar(u8, line, '-');
        const a = parts.next().?;
        const b = parts.next().?;

        return Edge{
            .a = parseName(a),
            .b = parseName(b),
        };
    }
};

const Graph = struct {
    allocator: Allocator,
    nodes: Set(Name),
    edges: Map(Name, Set(Name)),

    fn init(allocator: Allocator, edges: []const Edge) !Graph {
        var g = Graph{
            .allocator = allocator,
            .nodes = Set(Name).init(allocator),
            .edges = Map(Name, Set(Name)).init(allocator),
        };

        for (edges) |edge| {
            try g.nodes.put(edge.a);
            try g.nodes.put(edge.b);

            try g.addEdge(edge.a, edge.b);
            try g.addEdge(edge.b, edge.a);
        }

        return g;
    }

    fn deinit(self: *Graph) void {
        var it = self.edges.valueIterator();
        while (it.next()) |set| set.deinit();

        self.edges.deinit();
        self.nodes.deinit();
    }

    fn addEdge(self: *Graph, src: Name, dst: Name) !void {
        var res = try self.edges.getOrPut(src);
        if (!res.found_existing) {
            res.value_ptr.* = Set(Name).init(self.allocator);
        }
        try res.value_ptr.put(dst);
    }

    fn hasEdge(self: Graph, src: Name, dst: Name) bool {
        if (self.edges.get(src)) |set| {
            return set.contains(dst);
        }
        return false;
    }

    fn neighbors(self: Graph, src: Name) ?Set(Name).Iterator {
        if (self.edges.get(src)) |set| {
            return set.iterator();
        }
        return null;
    }

    fn popNode(self: *Graph) ?struct { Name, Set(Name) } {
        const src = self.nodes.pop() orelse return null;
        return .{ src, self.removeEdgeReferences(src) };
    }

    fn removeNode(self: *Graph, src: Name) ?Set(Name) {
        _ = self.nodes.remove(src) or return null;
        return self.removeEdgeReferences(src);
    }

    fn removeEdgeReferences(self: *Graph, src: Name) Set(Name) {
        var entry = self.edges.fetchRemove(src).?;
        var it = entry.value.iterator();
        while (it.next()) |dst| {
            if (self.edges.getPtr(dst.*)) |set| {
                _ = set.remove(src);
            }
        }

        return entry.value;
    }
};

fn part1(allocator: Allocator, edges: []const Edge) !usize {
    var graph = try Graph.init(allocator, edges);
    defer graph.deinit();

    var lans = Set([3]Name).init(allocator);
    defer lans.deinit();

    var nodes = graph.nodes.iterator();
    while (nodes.next()) |n| {
        if (!nameStartsWith(n.*, 't')) continue;

        var us = graph.neighbors(n.*).?;
        while (us.next()) |u| {
            var vs = graph.neighbors(u.*).?;
            while (vs.next()) |v| {
                if (graph.hasEdge(v.*, n.*)) {
                    var lan = [3]Name{ n.*, u.*, v.* };
                    std.mem.sort(Name, &lan, {}, comptime std.sort.asc(Name));

                    try lans.put(lan);
                }
            }
        }
    }

    return lans.count();
}

fn part2(allocator: Allocator, edges: []const Edge) !List(u8) {
    var graph = try Graph.init(allocator, edges);
    defer graph.deinit();

    var incl = Set(Name).init(allocator);
    defer incl.deinit();

    var prop = try graph.nodes.clone();
    defer prop.deinit();

    var excl = Set(Name).init(allocator);
    defer excl.deinit();

    var lan = Set(Name).init(allocator);
    try findCliques(graph, incl, prop, excl, &lan);
    defer lan.deinit();

    return try formatPassword(allocator, lan);
}

// https://en.wikipedia.org/wiki/Bron%E2%80%93Kerbosch_algorithm
fn findCliques(graph: Graph, included: Set(Name), proposed: Set(Name), excluded: Set(Name), best: *Set(Name)) !void {
    if (proposed.empty() and excluded.empty() and included.count() > best.count()) {
        best.deinit();
        best.* = try included.clone();
    }

    var p = try proposed.clone();
    defer p.deinit();

    var x = try excluded.clone();
    defer x.deinit();

    var it = proposed.iterator();
    while (it.next()) |v| {
        const neighbors = graph.edges.get(v.*).?;

        var incl = try included.with(v.*);
        defer incl.deinit();

        var prop = try p.intersect(neighbors);
        defer prop.deinit();

        var excl = try x.intersect(neighbors);
        defer excl.deinit();

        try findCliques(graph, incl, prop, excl, best);

        _ = p.remove(v.*);
        try x.put(v.*);
    }
}

fn formatPassword(allocator: Allocator, lan: Set(Name)) !List(u8) {
    var names = List(Name).init(allocator);
    defer names.deinit();
    {
        var it = lan.iterator();
        while (it.next()) |n| {
            try names.append(n.*);
        }
    }

    std.mem.sort(Name, names.items, {}, comptime std.sort.asc(Name));

    var buf = List(u8).init(allocator);
    var w = buf.writer();

    if (names.items.len > 0) {
        try w.print("{s}", .{formatName(names.items[0])});
    }

    if (names.items.len > 1) {
        for (names.items[1..]) |n| {
            try w.print(",{s}", .{formatName(n)});
        }
    }

    return buf;
}