From 4fcd930a3203001e3c8a0cb8b4a7e506d189c9ee Mon Sep 17 00:00:00 2001
From: Nilstrieb <48135649+Nilstrieb@users.noreply.github.com>
Date: Mon, 4 Dec 2023 21:35:48 +0100
Subject: [PATCH] more opts

---
 2023/day04/src/lib.rs              |  2 ++
 2023/day04/src/p2chunks.rs         | 17 +++++-----
 2023/day04/src/p2manual_slicing.rs | 54 ++++++++++++++++++++++++++++++
 3 files changed, 65 insertions(+), 8 deletions(-)
 create mode 100644 2023/day04/src/p2manual_slicing.rs

diff --git a/2023/day04/src/lib.rs b/2023/day04/src/lib.rs
index 46c32fc..567e7c0 100644
--- a/2023/day04/src/lib.rs
+++ b/2023/day04/src/lib.rs
@@ -1,6 +1,7 @@
 mod p2ascii;
 mod p2cache;
 mod p2chunks;
+mod p2manual_slicing;
 mod p2no_alloc;
 
 use helper::{Day, Variants};
@@ -22,6 +23,7 @@ helper::define_variants! {
         no_alloc => crate::p2no_alloc::part2;
         ascii => crate::p2ascii::part2;
         chunks => crate::p2chunks::part2;
+        manual_slicing => crate::p2manual_slicing::part2;
     }
 }
 
diff --git a/2023/day04/src/p2chunks.rs b/2023/day04/src/p2chunks.rs
index ca647bd..ce44d2e 100644
--- a/2023/day04/src/p2chunks.rs
+++ b/2023/day04/src/p2chunks.rs
@@ -1,21 +1,22 @@
 fn line_match_count(line: &str) -> usize {
-    let mut numbers = line.split(':').nth(1).unwrap().split("|");
+    let line = line.as_bytes();
+    fn pack(chunk: &[u8]) -> u16 {
+        ((chunk[1] as u16) << 8) | (chunk[2] as u16)
+    }
+
+    let mut numbers = line.split(|&b| b == b':').nth(1).unwrap().split(|&b| b == b'|');
     let winning = numbers
         .next()
         .unwrap()
-        .as_bytes()
         // Chunks of double digit numbers with a leading space. We don't care about the trailing space.
         .chunks_exact(3)
+        .map(pack)
         .collect::<arrayvec::ArrayVec<_, 16>>();
 
-    let you_have = numbers.next().unwrap().as_bytes().chunks_exact(3);
+    let you_have = numbers.next().unwrap().chunks_exact(3).map(pack);
 
     you_have
-        .filter(|have| {
-            winning.iter().any(|w| {
-                w == have
-            })
-        })
+        .filter(|have| winning.iter().any(|w| w == have))
         .count()
 }
 
diff --git a/2023/day04/src/p2manual_slicing.rs b/2023/day04/src/p2manual_slicing.rs
new file mode 100644
index 0000000..c6bfff6
--- /dev/null
+++ b/2023/day04/src/p2manual_slicing.rs
@@ -0,0 +1,54 @@
+use nom::InputIter;
+
+fn line_match_count(line: &str) -> usize {
+    let line = line.as_bytes();
+    fn pack(chunk: &[u8]) -> u16 {
+        ((chunk[1] as u16) << 8) | (chunk[2] as u16)
+    }
+
+    let colon = line.position(|b| b == b':').unwrap();
+    let numbers = &line[(colon + 1)..];
+    let pipe = numbers.position(|b| b == b'|').unwrap();
+    let winning = &numbers[..(pipe - 1)];
+    let you_have = &numbers[(pipe + 1)..];
+
+    let winning = winning
+        .chunks_exact(3)
+        .map(pack)
+        .collect::<arrayvec::ArrayVec<_, 16>>();
+
+    let you_have = you_have.chunks_exact(3).map(pack);
+
+    you_have
+        .filter(|have| winning.iter().any(|w| w == have))
+        .count()
+}
+
+pub fn part2(input: &str) -> u64 {
+    let lines = input.lines().map(line_match_count).collect::<Vec<_>>();
+
+    let mut cache = vec![0; lines.len()];
+
+    let mut processed = 0;
+
+    // By iterating backwards, we ensure the cache is always populated for every subsequent line.
+    for (i, result) in lines.iter().copied().enumerate().rev() {
+        let before = processed;
+        processed += 1; // Every card gives us one card.
+                        // Now, let's see how many cards this card will expand to.
+        for expand in (i + 1)..((i + 1) + result) {
+            #[cfg(debug_assertions)]
+            eprintln!(
+                "{} expands to {} which is worth {}",
+                i + 1,
+                expand + 1,
+                cache[expand]
+            );
+            // Since the value is bigger than i, it must be cached!
+            processed += cache[expand];
+        }
+        cache[i] = processed - before;
+    }
+
+    processed
+}