From 19e219bcc33108ceca17b5048a66a419f8f65621 Mon Sep 17 00:00:00 2001
From: Joseph Montanaro <jfmontanaro@modg.org>
Date: Thu, 9 Dec 2021 12:56:44 -0800
Subject: [PATCH] day 7

---
 2021/src/day7.rs | 61 ++++++++++++++++++++++++++++++++++++++++++++++++
 2021/src/main.rs |  6 ++---
 2 files changed, 64 insertions(+), 3 deletions(-)
 create mode 100644 2021/src/day7.rs

diff --git a/2021/src/day7.rs b/2021/src/day7.rs
new file mode 100644
index 0000000..1f9fdb1
--- /dev/null
+++ b/2021/src/day7.rs
@@ -0,0 +1,61 @@
+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
+}
+
+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(&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
+    }
+}
+
+
+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 min_simple = usize::MAX;
+    let mut min_complex = usize::MAX;
+    for i in 0..map.counts.len() {
+        let simple = map.cost_simple(i);
+        if simple < min_simple {min_simple = simple;}
+
+        let complex = map.cost_complex(i);
+        if complex < min_complex {min_complex = complex;}
+    }
+
+    Ok((min_simple, min_complex))
+}
diff --git a/2021/src/main.rs b/2021/src/main.rs
index 979da0d..ecb01fe 100644
--- a/2021/src/main.rs
+++ b/2021/src/main.rs
@@ -4,14 +4,14 @@ use color_eyre::eyre;
 
 mod lib;
 use lib::load;
-mod day6;
+mod day7;
 
 
 fn main() -> eyre::Result<()> {
-    let data = load("data/06.txt")?;
+    let data = load("data/07.txt")?;
 
     let start = Instant::now();
-    let (one, two) = day6::run(&data)?;
+    let (one, two) = day7::run(&data)?;
     let (dur, unit) = format_ns(start.elapsed().as_nanos());
 
     let precision = 2.0 - dur.log10().floor();