From 3a7cada1f9cee425291aa7c8392dc5238609f5b3 Mon Sep 17 00:00:00 2001
From: Glenn Griffin <ggriffiniii@gmail.com>
Date: Fri, 18 Dec 2020 16:03:23 -0800
Subject: [PATCH] day17 part1

---
 src/day17.rs | 74 +++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 73 insertions(+), 1 deletion(-)

diff --git a/src/day17.rs b/src/day17.rs
index e34e8a1..ede9bfe 100644
--- a/src/day17.rs
+++ b/src/day17.rs
@@ -1,6 +1,78 @@
 use aoc_runner_derive::aoc;
+use std::collections::HashSet;
 
 #[aoc(day17, part1)]
 fn solve_d17_p1(input: &str) -> usize {
-    42
+    let mut world = HashSet::new();
+    for (y, line) in input.split('\n').enumerate() {
+        for (x, _) in line.as_bytes().iter().copied().enumerate().filter(|&(_, byte)| {
+            byte == b'#'
+        }) {
+            world.insert((x as isize, y as isize, 0isize));
+        }
+    }
+    for _ in 0 .. 6 {
+        world = step(world);
+    }
+    world.len()
 }
+
+fn neighbors(world: &HashSet<(isize, isize, isize)>, (x, y, z): (isize, isize, isize)) -> usize {
+    let mut count = 0;
+    for xa in x-1..=x+1 {
+        for ya in y-1..=y+1 {
+            for za in z-1..=z+1 {
+                count += world.contains(&(xa, ya, za)) as usize;
+            }
+        }
+    }
+    count - world.contains(&(x, y, z)) as usize
+}
+
+fn step(world: HashSet<(isize, isize, isize)>) -> HashSet<(isize, isize, isize)> {
+    struct MinMax {
+        x_min: isize,
+        x_max: isize,
+        y_min: isize,
+        y_max: isize,
+        z_min: isize,
+        z_max: isize,
+    }
+    let MinMax{
+        x_min, x_max, y_min, y_max, z_min, z_max,
+    } = world.iter().fold(MinMax{
+        x_min: isize::MAX,
+        x_max: isize::MIN,
+        y_min: isize::MAX,
+        y_max: isize::MIN,
+        z_min: isize::MAX,
+        z_max: isize::MIN,
+    }, |minmax, &(x, y, z)| {
+        MinMax{
+            x_min: std::cmp::min(minmax.x_min, x),
+            x_max: std::cmp::max(minmax.x_max, x),
+            y_min: std::cmp::min(minmax.y_min, y),
+            y_max: std::cmp::max(minmax.y_max, y),
+            z_min: std::cmp::min(minmax.z_min, z),
+            z_max: std::cmp::max(minmax.z_max, z),
+        }
+    });
+    let mut new_world = HashSet::new();
+    for x in x_min -1 ..= x_max + 1 {
+        for  y in y_min -1 ..= y_max + 1 {
+            for z in z_min -1 ..= z_max + 1 {
+                let active = world.contains(&(x, y, z));
+                let n = neighbors(&world, (x, y, z));
+                if active && (n == 2 || n == 3) {
+                    new_world.insert((x, y, z));
+                }
+                if !active && n == 3 {
+                    new_world.insert((x, y, z));
+                }
+            }
+        }
+    }
+    new_world
+}
+
+