From 0893f9a3c187a4e6e7d74764f2c5dbd6579391d7 Mon Sep 17 00:00:00 2001
From: Gard Spreemann <gspr@nonempty.org>
Date: Wed, 8 Dec 2021 16:26:53 +0100
Subject: Improve an embarassingly stupid day 6

---
 06/src/part-1.rs |  6 ++++++
 06/src/part-2.rs | 36 ++++++++----------------------------
 2 files changed, 14 insertions(+), 28 deletions(-)

diff --git a/06/src/part-1.rs b/06/src/part-1.rs
index 5a1660e..f1723e1 100644
--- a/06/src/part-1.rs
+++ b/06/src/part-1.rs
@@ -1,5 +1,11 @@
 use std::io::{BufRead};
 
+/*
+ *  This is an embarassingly naïve solution based on poor reading of
+ *  the problem and thinking that the actual example printout had to
+ *  be reproduced. Part 2 is solved in a much saner way.
+ */
+
 pub fn main() {
     let args: Vec<String> = std::env::args().collect();
     if args.len() != 2 { panic!("Need exactly 1 argument"); }
diff --git a/06/src/part-2.rs b/06/src/part-2.rs
index 37d095e..c9ea499 100644
--- a/06/src/part-2.rs
+++ b/06/src/part-2.rs
@@ -1,11 +1,5 @@
 use std::io::{BufRead};
 
-#[derive(Clone, Copy)]
-struct FishSchool {
-    state: u64,
-    size: usize
-}
-
 pub fn main() {
     let args: Vec<String> = std::env::args().collect();
     if args.len() != 2 { panic!("Need exactly 1 argument"); }
@@ -20,30 +14,16 @@ pub fn main() {
         String::from(buf.trim_end())
     };
 
-    let mut schools: Vec<FishSchool> = input.split(',').map(|w| FishSchool {
-        state: w.parse().expect("Malformed input"),
-        size: 1} ).collect();
+    // counts[i] is the number of fish that breed on days that are divisible by i.
+    let mut counts: [usize; 7] = [0; 7];
+    let mut newborn_counts: [usize; 9] = [0; 9];
+    input.split(',').map(|x| x.parse::<usize>().expect("Malformed input")).for_each(|x| counts[x % 7] += 1);
 
     for i in 0..num_days {
-        let mut new_fish_count: usize = 0;
-        for school in (&mut schools).into_iter() {
-            if school.state == 0 {
-                new_fish_count += school.size;
-                school.state = 6;
-            }
-            else { school.state -= 1; }
-        }
-        schools.push(FishSchool { state: 8, size: new_fish_count });
-
-        /*
-        print!("After day {:>width$}: ", i, width = format!("{}", num_days).len());
-        for school in (& schools).into_iter() {
-            for _ in 0..school.size { print!("{} ", school.state); }
-        }
-        println!("");
-        */
+        let tmp = newborn_counts[i % 9];
+        newborn_counts[i % 9] += counts[i % 7];
+        counts[i % 7] += tmp;
     }
 
-    let n: usize = schools.into_iter().map(|school| school.size).sum();
-    println!("After {} days there are {} fishies", num_days, n);
+    println!("After {} days there are {} fishiess", num_days, counts.into_iter().sum::<usize>() + newborn_counts.into_iter().sum::<usize>());
 }
-- 
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