Day 17, part 2

src/bin/d17p2.rs

@@ -0,0 +1,13 @@
+use aoc22::{day17, util};
+
+const BLOCKS: usize = 1_000_000_000_000;
+
+pub fn main() {
+    let jets = day17::parse_jets(&util::parse_input());
+
+    println!(
+        "Height after {} blocks: {}",
+        BLOCKS,
+        day17::do_moves(&jets, BLOCKS)
+    );
+}

src/day17.rs

@@ -1,3 +1,5 @@
+use std::collections::HashMap;
+
 use once_cell::sync::OnceCell;
 
 use crate::util;
@@ -148,14 +150,45 @@ impl Block {
 
 type VentRow = u8;
 
+const CYCLE_SIZE: usize = 5;
+
 pub fn do_moves(jets: &Vec<Jet>, n_blocks: usize) -> usize {
     let mut block_i = 0;
     let mut current_block: Option<Block> = None;
     let mut vent = vec![0; 4];
 
+    // {jet_i: {round: (height, block_i)}}
+    let mut block_cycles: HashMap<usize, HashMap<usize, (usize, usize)>> = HashMap::new();
+    let mut skipped_height: Option<usize> = None;
+
     let mut round = 0;
     while block_i <= n_blocks {
+        let jet_i = round % jets.len();
+
         if current_block.is_none() {
+            if block_i % N_BLOCK_TYPES == 0 {
+                if skipped_height.is_none() {
+                    let entry = block_cycles.entry(jet_i).or_insert_with(HashMap::new);
+                    entry.insert(round, (vent.len() - empty_rows(&vent), block_i));
+                    if let Some((cycle_height, cycle_blocks)) = find_cycle(entry, CYCLE_SIZE) {
+                        println!(
+                            "Found cycle ({} height per {} blocks) after {} rounds!",
+                            cycle_height, cycle_blocks, round
+                        );
+
+                        let remaining_blocks = n_blocks - block_i;
+                        let cycles = remaining_blocks / cycle_blocks;
+                        block_i += cycles * cycle_blocks;
+                        skipped_height = Some(cycles * cycle_height);
+
+                        println!(
+                            "Skipping {} blocks, {} height",
+                            cycles * cycle_blocks,
+                            cycles * cycle_height
+                        );
+                    }
+                }
+            }
             current_block = Some(spawn_block(&mut vent, block_i % N_BLOCK_TYPES));
             // println!("Vent as block {} begins falling:", block_i);
             // print_vent(&vent, current_block.as_ref());
@@ -165,7 +198,7 @@ pub fn do_moves(jets: &Vec<Jet>, n_blocks: usize) -> usize {
         let block = current_block.as_mut().unwrap();
 
         // Push left/right
-        let jet = jets[round % jets.len()];
+        let jet = jets[jet_i];
         if block.can_push(&vent, jet) {
             let (_, ref mut col) = block.pos;
             *col = ((*col as isize) - jet.shift()) as usize;
@@ -182,7 +215,7 @@ pub fn do_moves(jets: &Vec<Jet>, n_blocks: usize) -> usize {
         round += 1;
     }
 
-    vent.len() - empty_rows(&vent)
+    vent.len() - empty_rows(&vent) + skipped_height.unwrap_or(0)
 }
 
 const SPAWN_OFFSET_X: usize = 2;
@@ -244,3 +277,41 @@ pub fn print_vent(vent: &Vec<VentRow>, block: Option<&Block>) {
     }
     println!("+-------+");
 }
+
+fn find_cycle(
+    jet_heights: &HashMap<usize, (usize, usize)>,
+    n_cycles: usize,
+) -> Option<(usize, usize)> {
+    if jet_heights.len() < n_cycles {
+        return None;
+    }
+
+    for (init_round, (init_height, init_blocks)) in jet_heights {
+        'next_loop: for (next_round, (next_height, next_blocks)) in jet_heights {
+            if init_round == next_round || *next_height < *init_height {
+                continue;
+            }
+
+            let cycle = next_round - init_round;
+            let cycle_height = next_height - init_height;
+            let cycle_blocks = next_blocks - init_blocks;
+            for n in 2..=n_cycles {
+                let round = init_round + n * cycle;
+                let entry = jet_heights.get(&round);
+                if let Some((height, blocks)) = entry {
+                    let expected_height = init_height + n * cycle_height;
+                    let expected_blocks = init_blocks + n * cycle_blocks;
+                    if *height != expected_height || *blocks != expected_blocks {
+                        continue 'next_loop;
+                    }
+                } else {
+                    continue 'next_loop;
+                }
+            }
+
+            return Some((cycle_height, cycle_blocks));
+        }
+    }
+
+    None
+}