From 21332905656d44c830b819e2b408d94a0d89e95f Mon Sep 17 00:00:00 2001
From: Joseph Montanaro <jfmontanaro@modg.org>
Date: Thu, 9 Dec 2021 13:18:32 -0800
Subject: [PATCH] simplify day 7

---
 2021/src/day7.rs | 73 ++++++++++++++++++++++--------------------------
 1 file changed, 34 insertions(+), 39 deletions(-)

diff --git a/2021/src/day7.rs b/2021/src/day7.rs
index 1f9fdb1..b9e5263 100644
--- a/2021/src/day7.rs
+++ b/2021/src/day7.rs
@@ -1,59 +1,54 @@
 use color_eyre::eyre;
 
 
-#[derive(Default)]
-struct Map {
-    counts: Vec<usize>, // index is position, value is how many at that position
-    index: Vec<usize>, // values are indices of `counts`, sorted from most to least
+fn distance_between(a: usize, b: usize) -> usize {
+    // can't do the fancypants (a - b).abs() thing here because these are unsigned
+    if a > b {
+        a - b
+    }
+    else {
+        b - a
+    }
 }
 
-impl Map {
-    fn from(nums: &[usize]) -> Map {
-        let hi = nums.iter().max().unwrap_or(&0);
-        let mut counts = vec![0; hi + 1];
-        for n in nums {
-            counts[*n] += 1;
-        }
-        let mut index: Vec<usize> = (0..counts.len()).collect();
-        index.sort_by(|&a, &b| counts[b].cmp(&counts[a])); // flip the comparison function to sort in reverse
 
-        Map {counts, index}
-    }
+fn cost_simple(counts: &[usize], pos: usize) -> usize {
+    counts.iter()
+        .enumerate()
+        .fold(0, |total, (i, count)| {
+            total + (distance_between(pos, i) * count) // yes, these parens are unnecessary
+        })
+}
 
-    fn cost_simple(&self, pos: usize) -> usize {
-        let mut total = 0;
-        for (i, count) in self.counts.iter().enumerate() {
-            let distance = if pos > i {pos - i} else {i - pos};
-            total += distance * count;
-        }
-        total
-    }
 
-    fn cost_complex(&self, pos: usize) -> usize {
-        let mut total = 0;
-        for (i, count) in self.counts.iter().enumerate() {
-            let distance = if pos > i {pos -i} else {i - pos};
-            let cost_single = distance * (distance + 1) / 2; // Gauss' method for summing numbers 0 to n
-            total += cost_single * count;
-        }
-        total
-    }
+fn cost_complex(counts: &[usize], pos: usize) -> usize {
+    counts.iter()
+        .enumerate()
+        .fold(0, |total, (i, count)| {
+            let dx = distance_between(pos, i);
+            let cost_single = dx * (dx + 1) / 2; // Gauss' method for summing numbers from 1 to n
+            total + (cost_single * count)
+        })
 }
 
 
 pub fn run(data: &str) -> eyre::Result<(usize, usize)> {
-    let nums = data.trim().split(',')
-        .map(|s| s.parse::<usize>())
-        .collect::<Result<Vec<_>, _>>()?;
-    let map = Map::from(&nums);
+    let mut counts = Vec::new();
+    for s in data.trim().split(',') {
+        let n = s.parse::<usize>()?;
+        if n >= counts.len() {
+            counts.resize(n + 1, 0);
+        }
+        counts[n] += 1;
+    }
 
     let mut min_simple = usize::MAX;
     let mut min_complex = usize::MAX;
-    for i in 0..map.counts.len() {
-        let simple = map.cost_simple(i);
+    for i in 0..counts.len() {
+        let simple = cost_simple(&counts, i);
         if simple < min_simple {min_simple = simple;}
 
-        let complex = map.cost_complex(i);
+        let complex = cost_complex(&counts, i);
         if complex < min_complex {min_complex = complex;}
     }