day 8 part 2 dp

src/bin/day8.rs

@@ -1,18 +1,9 @@
-#![forbid(elided_lifetimes_in_paths)]
+#![forbid(elided_lifetimes_in_paths, unsafe_code)]
 
 use aoc23::read;
 use chumsky::prelude::*;
-use std::{
-	collections::{HashMap, HashSet},
-	sync::{
-		atomic::{AtomicBool, Ordering},
-		Mutex
-	},
-	thread,
-	time::Duration
-};
+use std::collections::{HashMap, HashSet, VecDeque};
 
-#[derive(Clone, Copy, Debug)]
 enum Instruction {
 	Left,
 	Right
@@ -27,7 +18,7 @@ impl Instruction {
 	}
 }
 
-#[derive(Clone, Debug)]
+#[derive(Debug)]
 struct Node {
 	left: String,
 	right: String
@@ -51,7 +42,7 @@ impl Node {
 	}
 }
 
-#[derive(Clone, Debug)]
+#[derive(Debug)]
 struct Network {
 	nodes: HashMap<String, Node>
 }
@@ -92,15 +83,93 @@ fn parser() -> impl Parser<char, (Vec<Instruction>, Network), Error = Simple<cha
 		.then_ignore(end())
 }
 
-static STOP: AtomicBool = AtomicBool::new(false);
-static mut RESULTS: Vec<Mutex<Vec<usize>>> = Vec::new();
+#[derive(Copy, Clone, Debug)]
+struct Ghost<'a> {
+	node: &'a str,
+	i: usize,
+	steps: u128
+}
+
+impl<'a> Ghost<'a> {
+	fn new(node: &'a str) -> Self {
+		Self {
+			node,
+			i: 0,
+			steps: 0
+		}
+	}
+
+	fn advance(&mut self, ghost: Ghost<'a>) {
+		self.node = ghost.node;
+		self.i = ghost.i;
+		self.steps += ghost.steps;
+	}
+}
 
 fn main() -> anyhow::Result<()> {
 	let (instructions, network) = read("inputs/day8.txt", parser())?;
 
+	// build a lookup map of left/right predecessors
+	let mut left_pred: HashMap<&str, Vec<&str>> = HashMap::new();
+	let mut right_pred: HashMap<&str, Vec<&str>> = HashMap::new();
+	for (node, next) in &network.nodes {
+		left_pred.entry(&next.left).or_default().push(node);
+		right_pred.entry(&next.right).or_default().push(node);
+	}
+	eprintln!("Built pred lookup map");
+
+	// build a lookup map from instruction position and node with the length to the next
+	// finish node
+	let mut dp: HashMap<(&str, usize), Ghost<'_>> = HashMap::new();
+	let mut q: VecDeque<(&str, usize)> = VecDeque::new();
+	for node in network
+		.nodes
+		.keys()
+		.map(|node| node.as_str())
+		.filter(|node| node.ends_with('Z'))
+	{
+		for i in 0 .. instructions.len() {
+			dp.insert((node, i), Ghost { node, i, steps: 0 });
+			q.push_back((node, i));
+		}
+	}
+	while let Some((node, i)) = q.pop_front() {
+		let ghost = dp[&(node, i)];
+		let i_pred = match i {
+			0 => instructions.len() - 1,
+			i => i - 1
+		};
+		let pred = match instructions[i_pred] {
+			Instruction::Left => left_pred.get(node),
+			Instruction::Right => right_pred.get(node)
+		};
+		for node_pred in pred.into_iter().flatten() {
+			let new = (*node_pred, i_pred);
+			#[allow(clippy::map_entry)] // lint is opinionated garbage
+			if !dp.contains_key(&new) {
+				dp.insert(new, Ghost {
+					node: ghost.node,
+					i: ghost.i,
+					steps: ghost.steps + 1
+				});
+				q.push_back(new);
+				if dp.len() % 1_000_000 == 0 {
+					dbg!(dp.len());
+				}
+			}
+		}
+		if ghost.steps == 0 {
+			dp.remove(&(node, i));
+		}
+	}
+	eprintln!("Built ghost lookup map");
+	// for (key, value) in &dp {
+	// eprintln!("\t{key:?}\t = \t{value:?}");
+	// }
+
 	// let mut curr_node = "AAA";
 	// let mut i = 0;
-	// let mut steps = 0;
+	// let mut steps = 0_u128;
 	// while curr_node != "ZZZ" {
 	// 	curr_node = match instructions[i] {
 	// 		Instruction::Left => network.left(curr_node),
@@ -111,84 +180,39 @@ fn main() -> anyhow::Result<()> {
 	// }
 	// println!("{steps}");
 
-	let curr_nodes: HashSet<&str> = network
+	let mut ghosts: Vec<Ghost<'_>> = network
 		.nodes
 		.keys()
-		.map(|node| node.as_str())
 		.filter(|node| node.ends_with('A'))
+		.map(|node| Ghost::new(node))
 		.collect();
-	dbg!(curr_nodes.len());
-
-	for (thread_index, node) in curr_nodes
+	let mut progress = 0;
+	loop {
+		if ghosts[0].steps > 0
+			&& ghosts
 				.iter()
-		.map(|node| String::from(*node))
-		.enumerate()
+				.skip(1)
+				.all(|ghost| ghost.steps == ghosts[0].steps)
 		{
-		unsafe {
-			RESULTS.push(Mutex::new(Vec::new()));
-		}
-		let network = network.clone();
-		let instructions = instructions.clone();
-		thread::spawn(move || {
-			let mut curr_node = node.as_str();
-			let mut i = 0;
-			let mut steps = 0;
-			while !STOP.load(Ordering::Relaxed) {
-				curr_node = match instructions[i] {
-					Instruction::Left => network.left(curr_node),
-					Instruction::Right => network.right(curr_node)
-				};
-				i = (i + 1) % instructions.len();
-				steps += 1;
-				if steps % 1_000_000 == 0 {
-					dbg!(steps);
-				}
-				if curr_node.ends_with('Z') {
-					unsafe {
-						RESULTS[thread_index].lock().unwrap().push(steps);
-					}
-				}
-			}
-		});
-	}
-	'outer: loop {
-		thread::sleep(Duration::from_secs(1));
-		unsafe {
-			let res0 = RESULTS[0].lock().unwrap();
-			'i: for i in 0 .. res0.len() {
-				't: for t in 1 .. RESULTS.len() {
-					let rest = RESULTS[t].lock().unwrap();
-					for j in 0 .. rest.len() {
-						if res0[i] == rest[j] {
-							continue 't;
-						}
-					}
-					continue 'i;
-				}
-				println!("{}", res0[i]);
-				STOP.store(true, Ordering::Relaxed);
-				break 'outer;
-			}
-		}
+			println!("{}", ghosts[0].steps);
+			break;
 		}
 
-	// let mut i = 0;
-	// let mut steps = 0;
-	// while curr_nodes.iter().any(|node| !node.ends_with('Z')) {
-	// 	curr_nodes = curr_nodes
-	// 		.into_iter()
-	// 		.map(|node| match instructions[i] {
-	// 			Instruction::Left => network.left(node),
-	// 			Instruction::Right => network.right(node)
-	// 		})
-	// 		.collect();
-	// 	i = (i + 1) % instructions.len();
-	// 	steps += 1;
-	// 	if steps % 10000 == 0 {
-	// 		dbg!(steps);
-	// 	}
-	// }
-	// println!("{steps}");
+		let (ghost_index, ghost) = ghosts
+			.iter()
+			.enumerate()
+			.min_by_key(|(_, ghost)| ghost.steps)
+			.unwrap();
+		if ghost.steps - progress >= 1_000_000_000_000 {
+			progress = ghost.steps;
+			eprintln!("progress: {progress}");
+		}
+
+		// eprint!("{ghost:?}\t -> \t");
+		let ghost = dp[&(ghost.node, ghost.i)];
+		// eprintln!("{ghost:?}");
+		ghosts[ghost_index].advance(ghost);
+	}
 
 	Ok(())
 }