From d6a2225e01424b8dccb6e4663bdfd42f1f0e44ed Mon Sep 17 00:00:00 2001
From: Ishan Jain <ishan.jain@zscaler.com>
Date: Wed, 18 Dec 2024 18:29:58 +0530
Subject: [PATCH] optimized day 18 p2

---
 src/day18/2.rs | 25 ++++++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)

diff --git a/src/day18/2.rs b/src/day18/2.rs
index 798edfd..2984728 100644
--- a/src/day18/2.rs
+++ b/src/day18/2.rs
@@ -30,12 +30,21 @@ pub fn run(input: &str) -> (i64, i64) {
     let mut l = 0;
     let mut r = bytes.len()-1;
 
+    let mut last_set = 0;
+
     while l <= r {
         let mid = (l + r) / 2;
 
-        for b in bytes[0..mid].iter() {
-            obstacles.set(b.0, b.1);
+        if last_set > mid {
+            for b in bytes[mid..last_set].iter() {
+                obstacles.reset(b.0, b.1);
+            }
+        } else {
+            for b in bytes[last_set..mid].iter() {
+                obstacles.set(b.0, b.1);
+            }
         }
+        last_set = mid;
 
         if dfs(&obstacles, &mut visited, (0,0), (70,70)) {
             l = mid + 1;
@@ -43,7 +52,6 @@ pub fn run(input: &str) -> (i64, i64) {
             r = mid - 1;
         }
 
-        obstacles.clear();
         visited.clear();
     }
 
@@ -85,6 +93,7 @@ fn dfs(obstacles: &Bitmap, visited: &mut Bitmap, (x,y): (i64, i64), (ex, ey): (i
 
 const WORD_SIZE: usize = 64;
 
+#[derive(Debug, PartialEq, Eq)]
 struct Bitmap {
     inner: [u64; (71 * 71) / WORD_SIZE + 1],
     size: i64,
@@ -115,6 +124,16 @@ impl Bitmap {
         }
     }
 
+    fn reset(&mut self, x: i64, y: i64) {
+        let offset = (x * self.size + y) as usize;
+        let index = offset / WORD_SIZE;
+        let bit_offset = offset % WORD_SIZE;
+
+        unsafe {
+            *self.inner.get_unchecked_mut(index) &= !(1 << bit_offset);
+        }
+    }
+
     fn clear(&mut self) {
         self.inner.fill(0);
     }