day22: problem 2 solution.

no help but annoyingly i misread the hint about low/high and didn't submit my *correct* answer for over an hour while I bug hunted!

22/Cargo.lock

@@ -14,6 +14,7 @@ version = "0.1.0"
 dependencies = [
  "itertools",
  "ndarray",
+ "petgraph",
 ]
 
 [[package]]
@@ -22,6 +23,34 @@ version = "1.9.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a26ae43d7bcc3b814de94796a5e736d4029efb0ee900c12e2d54c993ad1a1e07"
 
+[[package]]
+name = "equivalent"
+version = "1.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5443807d6dff69373d433ab9ef5378ad8df50ca6298caf15de6e52e24aaf54d5"
+
+[[package]]
+name = "fixedbitset"
+version = "0.4.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0ce7134b9999ecaf8bcd65542e436736ef32ddca1b3e06094cb6ec5755203b80"
+
+[[package]]
+name = "hashbrown"
+version = "0.14.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "290f1a1d9242c78d09ce40a5e87e7554ee637af1351968159f4952f028f75604"
+
+[[package]]
+name = "indexmap"
+version = "2.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d530e1a18b1cb4c484e6e34556a0d948706958449fca0cab753d649f2bce3d1f"
+dependencies = [
+ "equivalent",
+ "hashbrown",
+]
+
 [[package]]
 name = "itertools"
 version = "0.12.0"
@@ -82,6 +111,16 @@ dependencies = [
  "autocfg",
 ]
 
+[[package]]
+name = "petgraph"
+version = "0.6.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e1d3afd2628e69da2be385eb6f2fd57c8ac7977ceeff6dc166ff1657b0e386a9"
+dependencies = [
+ "fixedbitset",
+ "indexmap",
+]
+
 [[package]]
 name = "rawpointer"
 version = "0.2.1"

22/Cargo.toml

@@ -8,3 +8,4 @@ edition = "2021"
 [dependencies]
 itertools = "0.12.0"
 ndarray = "0.15.6"
+petgraph = "0.6.4"

22/src/main.rs

@@ -1,7 +1,8 @@
 use itertools::Itertools;
 use ndarray::prelude::*;
-use std::borrow::BorrowMut;
+use petgraph::prelude::*;
-use std::collections::{BinaryHeap, HashMap, VecDeque};
+use petgraph::visit::{IntoNodeReferences, Walker};
+use std::collections::BinaryHeap;
 use std::fmt::{Display, Write};
 use std::fs::File;
 use std::io::{BufRead, BufReader, Lines};
@@ -67,16 +68,6 @@ impl std::fmt::Debug for BrickBlock {
 }
 
 impl BrickBlock {
-    fn map_remove(&self, mut map: BlockMap) -> BlockMap {
-        // loop over the (inclusive) bounding coordinates and remove them all from the map
-        map.slice_mut(s![
-            std::cmp::min(self.c1.x, self.c2.x)..std::cmp::max(self.c1.x, self.c2.x) + 1,
-            std::cmp::min(self.c1.y, self.c2.y)..std::cmp::max(self.c1.y, self.c2.y) + 1,
-            std::cmp::min(self.c1.z, self.c2.z)..std::cmp::max(self.c1.z, self.c2.z) + 1
-        ])
-        .fill(None);
-        map
-    }
     fn map_into(&self, mut map: BlockMap) -> BlockMap {
         // loop over the (inclusive) bounding coordinates and add them all to the map
         map.slice_mut(s![
@@ -137,6 +128,7 @@ struct BlockPile {
     blocks: Vec<BrickBlock>,
     block_map: Array3<MapTile>,
     bounds: (usize, usize, usize),
+    graph: Graph<BrickBlock, (), Directed, usize>,
 }
 
 impl BlockPile {
@@ -172,31 +164,26 @@ impl BlockPile {
             block.top_z_plane() + 1..std::cmp::min(block.top_z_plane() + 2, self.bounds.2)
         ]);
 
-        directly_above.iter().filter_map(|v| v.clone()).dedup().collect()
+        directly_above.iter().filter_map(|v| v.clone()).unique().collect()
     }
     fn supported_by(&self, block: &BrickBlock) -> usize {
-        println!("{:?} is supported by:", block);
         let z_plane = std::cmp::min(block.c1.z, block.c2.z);
-        println!(" z plane: {}", z_plane);
         // get the slice of tiles below us
         let directly_below = self.block_map.slice(s![
             block.bottom_x_plane()..block.top_x_plane() + 1,
             block.bottom_y_plane()..block.top_y_plane() + 1,
-            z_plane - 1..z_plane // don't include our own plane
+            z_plane - 1..z_plane // the layer below
         ]);
-        println!(
+        directly_below.iter().filter_map(|v| v.clone()).unique().count()
-            "  {} directly below {:?}",
-            directly_below.iter().filter_map(|v| v.clone()).dedup().count(),
-            block
-        );
-        directly_below.iter().filter_map(|v| v.clone()).dedup().count()
     }
     fn blocks_above_will_move_if_we_are_gone(&mut self, block: &BrickBlock) -> bool {
-        println!("checking directly above block {:?}", block);
         self.blocks_directly_above(&block)
             .iter()
             .map(|b| self.supported_by(b))
-            .any(|b| b == 1) // 0 can happen at the origin, and an empty iterator (falsy) will happen at the top
+            .any(|b| b == 1) // block we support will move if we are their only support
+    }
+    fn blocks_supported_by_at_all(&self, block: &BrickBlock) -> Vec<BrickBlock> {
+        self.blocks_directly_above(&block).iter().map(|b| b.clone()).collect()
     }
     /// Find the plane of the first block directly below us
     fn supporting_plane(&self, block: &BrickBlock) -> Option<usize> {
@@ -222,51 +209,38 @@ impl BlockPile {
     }
 
     fn drop_blocks(&mut self) {
+        // VecDeque doesn't sort and Vec isn't convenient for pushback and popfront, so eh... use a heap.
         let mut blocks_to_move = BinaryHeap::from(self.blocks.clone());
         while let Some(mut block) = blocks_to_move.pop() {
-            match self.supporting_plane(&block) {
+            let z_move = match self.supporting_plane(&block) {
-                Some(z) if z + 1 != block.bottom_z_plane() => {
+                Some(z) if z + 1 != block.bottom_z_plane() => block.bottom_z_plane() - (z + 1),
-                    // we can move down to this position
+                None if block.bottom_z_plane() != 1 => block.bottom_z_plane() - 1,
-                    // set old position to None
-                    self.remove_block(&block);
-                    // set new positions
-                    let z_height = block.c1.z.abs_diff(block.c2.z);
-                    block.c1.z = z + 1;
-                    block.c2.z = z + 1 + z_height;
-                    self.add_block(&block);
-                    // queue the block again
-                    blocks_to_move.push(block);
-                }
-                None if block.bottom_z_plane() != 1 => {
-                    // we can move down to this position
-                    // set old position to None
-                    let z = 1;
-                    self.remove_block(&block);
-                    // set new positions
-                    let z_height = block.c1.z.abs_diff(block.c2.z);
-                    block.c1.z = z;
-                    block.c2.z = z + z_height;
-                    self.add_block(&block);
-                    // queue the block again
-                    blocks_to_move.push(block);
-                }
                 _ => {
                     continue;
                 } // we are in position already with nothing below us
-            }
+            };
+            self.remove_block(&block);
+            block.c1.z -= z_move;
+            block.c2.z -= z_move;
+            self.add_block(&block);
+            blocks_to_move.push(block);
         }
     }
-    fn can_move(&self, block: &BrickBlock) -> bool {
+    fn build_graph(&mut self) {
-        match self.supporting_plane(&block) {
+        self.blocks.sort_by_key(|b| b.bottom_z_plane());
-            Some(z) if z + 1 != block.bottom_z_plane() => {
+        for b in 0..self.blocks.len() {
-                return true;
+            self.graph.add_node(self.blocks[b].clone());
+        }
+        for b in 0..self.blocks.len() {
+            let block = &self.blocks[b];
+            let depends_on_us = self.blocks_supported_by_at_all(block);
+            for dependent in depends_on_us {
+                self.graph.add_edge(
+                    b.into(),
+                    self.blocks.iter().position(|b| b == &dependent).unwrap().into(),
+                    (),
+                );
             }
-            None if block.bottom_z_plane() != 1 => {
-                return true;
-            }
-            _ => {
-                return false;
-            } // we are in position already with nothing below us
         }
     }
 }
@@ -277,6 +251,7 @@ impl<T: BufRead> From<Lines<T>> for BlockPile {
             blocks: lines.map(|line| BrickBlock::from(line.unwrap().as_str())).collect(),
             block_map: Array3::from_elem([0, 0, 0], None),
             bounds: (0, 0, 0),
+            graph: Graph::default(),
         };
 
         for block in &new.blocks {
@@ -338,25 +313,63 @@ fn problem1<T: BufRead>(input: Lines<T>) -> u64 {
     println!("{}", pile);
 
     let blocks = pile.blocks.clone();
-    let mut must_keep: Vec<_> = blocks
+    let removable: Vec<_> = blocks
-        .iter()
-        .filter(|b| pile.blocks_above_will_move_if_we_are_gone(*b))
-        .collect();
-    let mut removable: Vec<_> = blocks
         .iter()
         .filter(|b| !pile.blocks_above_will_move_if_we_are_gone(*b))
         .collect();
-    removable.sort_by(|a, b| a.bottom_z_plane().cmp(&b.bottom_z_plane()));
-    must_keep.sort_by(|a, b| a.bottom_z_plane().cmp(&b.bottom_z_plane()));
-
-    println!("remove: {:?}", removable);
 
     removable.len() as u64
 }
 
 // PROBLEM 2 solution
 fn problem2<T: BufRead>(input: Lines<T>) -> u64 {
-    0
+    let mut pile = BlockPile::from(input);
+    pile.drop_blocks();
+    pile.build_graph();
+
+    println!("{}", pile);
+    println!("block 0 {:?}", pile.blocks[0]);
+
+    let mut accum = 0;
+    let fixed_nodes = pile
+        .graph
+        .node_references()
+        .filter(|(_idx, b)| b.bottom_z_plane() == 1)
+        .map(|(idx, _b)| idx)
+        .collect_vec();
+    for node in pile.graph.node_indices() {
+        // remove links to node's neighbors
+        let dependents = pile.graph.neighbors(node).collect_vec();
+        let edges = pile.graph.edges(node).map(|v| v.id()).collect_vec();
+        for edge in edges {
+            pile.graph.remove_edge(edge);
+        }
+
+        // find how many nodes are reachable from z = 1 - these won't move
+        let safe_blocks = fixed_nodes
+            .iter()
+            .flat_map(|origin| {
+                Bfs::new(&pile.graph, *origin)
+                    .iter(&pile.graph)
+                    .map(|n| pile.graph[n].clone())
+            })
+            .unique()
+            .count();
+        // we are looking for the nodes that *will* disintegrate
+        println!(
+            "From {}, {} safe, {} disintegrate",
+            node.index(),
+            safe_blocks,
+            pile.graph.node_count() - safe_blocks
+        );
+        accum += pile.graph.node_count() - safe_blocks;
+        // put the graph back how it was
+        for neigh in dependents {
+            pile.graph.add_edge(node, neigh, ());
+        }
+    }
+    println!("blocks: {} nodes: {}", pile.blocks.len(), pile.graph.node_count());
+    accum as u64
 }
 
 #[cfg(test)]
@@ -381,6 +394,6 @@ mod tests {
     #[test]
     fn problem2_example() {
         let c = Cursor::new(EXAMPLE);
-        assert_eq!(problem2(c.lines()), 0);
+        assert_eq!(problem2(c.lines()), 7);
     }
 }