From a1e1bb6eed68ff56bb4fdf619caa33bd98ae935e Mon Sep 17 00:00:00 2001
From: Leandro Lisboa Penz <lpenz@lpenz.org>
Date: Fri, 22 Dec 2023 15:43:33 +0000
Subject: [PATCH] Day 22b

---
 day22/src/bin/day22a.rs | 13 +------------
 day22/src/bin/day22b.rs | 32 ++++++++++++++++++++++++++++++++
 day22/src/lib.rs        | 19 +++++++++++++++++++
 3 files changed, 52 insertions(+), 12 deletions(-)
 create mode 100644 day22/src/bin/day22b.rs

diff --git a/day22/src/bin/day22a.rs b/day22/src/bin/day22a.rs
index f7bb694..2196012 100644
--- a/day22/src/bin/day22a.rs
+++ b/day22/src/bin/day22a.rs
@@ -17,18 +17,7 @@ fn can_disintegrate(bricks: &[Brick], b: &Brick) -> bool {
 
 fn process(bufin: impl BufRead) -> Result<usize> {
     let mut bricks = parser::parse(bufin)?;
-    let mut changed = true;
-    while changed {
-        let old = bricks.clone();
-        changed = false;
-        for b in &mut bricks {
-            if let Some(z) = falls_to(&old, b) {
-                let height = b.2.max() - b.2.min();
-                b.2 = Range::new(z, z + height);
-                changed = true;
-            }
-        }
-    }
+    settle_bricks(&mut bricks);
     Ok(bricks
         .par_iter()
         .filter(|b| can_disintegrate(&bricks, b))
diff --git a/day22/src/bin/day22b.rs b/day22/src/bin/day22b.rs
new file mode 100644
index 0000000..330b5dd
--- /dev/null
+++ b/day22/src/bin/day22b.rs
@@ -0,0 +1,32 @@
+// Copyright (C) 2023 Leandro Lisboa Penz <lpenz@lpenz.org>
+// This file is subject to the terms and conditions defined in
+// file 'LICENSE', which is part of this source code package.
+
+use day22::*;
+
+use rayon::prelude::*;
+
+fn would_fall(bricks: &[Brick], b: &Brick) -> usize {
+    let mut bricks = bricks
+        .iter()
+        .filter(|o| o != &b)
+        .copied()
+        .collect::<Vec<_>>();
+    settle_bricks(&mut bricks)
+}
+
+fn process(bufin: impl BufRead) -> Result<usize> {
+    let mut bricks = parser::parse(bufin)?;
+    settle_bricks(&mut bricks);
+    Ok(bricks.par_iter().map(|b| would_fall(&bricks, b)).sum())
+}
+
+#[test]
+fn test() -> Result<()> {
+    assert_eq!(process(EXAMPLE.as_bytes())?, 7);
+    Ok(())
+}
+
+fn main() -> Result<()> {
+    do_main(|| process(stdin().lock()))
+}
diff --git a/day22/src/lib.rs b/day22/src/lib.rs
index 2b68c5d..61f9996 100644
--- a/day22/src/lib.rs
+++ b/day22/src/lib.rs
@@ -5,6 +5,7 @@
 pub use aoc::*;
 
 use std::cmp::min;
+use std::collections::HashSet;
 
 pub const EXAMPLE: &str = "1,0,1~1,2,1
 0,0,2~2,0,2
@@ -114,3 +115,21 @@ pub fn falls_to(bricks: &[Brick], b: &Brick) -> Option<i64> {
         (b.2.min() != 1).then_some(1)
     }
 }
+
+pub fn settle_bricks(bricks: &mut [Brick]) -> usize {
+    let mut changed = true;
+    let mut fell = HashSet::<usize>::new();
+    while changed {
+        let old = bricks.to_vec();
+        changed = false;
+        for (i, b) in bricks.iter_mut().enumerate() {
+            if let Some(z) = falls_to(&old, b) {
+                let height = b.2.max() - b.2.min();
+                b.2 = Range::new(z, z + height);
+                fell.insert(i);
+                changed = true;
+            }
+        }
+    }
+    fell.len()
+}