From d81f83e63e19d8997c61e0eed9ff8d8feddbae8f Mon Sep 17 00:00:00 2001
From: Reiner Herrmann <reiner@reiner-h.de>
Date: Tue, 1 Dec 2020 18:27:00 +0100
Subject: day1

---
 src/main.rs | 76 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 76 insertions(+)
 create mode 100644 src/main.rs

(limited to 'src')

diff --git a/src/main.rs b/src/main.rs
new file mode 100644
index 0000000..ee10457
--- /dev/null
+++ b/src/main.rs
@@ -0,0 +1,76 @@
+#![allow(dead_code)]
+
+use std::fs;
+use std::collections::HashSet;
+
+fn read_file(file: &str) -> String {
+    fs::read_to_string(file).unwrap()
+}
+
+fn read_numbers(file: &str) -> Vec<i32> {
+    read_file(file).split('\n')
+                   .filter(|x| !x.is_empty())
+                   .map(|x| x.parse::<i32>().unwrap())
+                   .collect()
+}
+
+fn find_pair_with_sum(numbers: &HashSet<i32>, goal: i32) -> Option<(i32, i32)> {
+    for i in numbers {
+        if numbers.contains(&(goal - i)) {
+            return Some((*i, goal - i));
+        }
+    }
+    None
+}
+
+fn day1() {
+    let input = read_numbers("input01");
+    let mut numbers = HashSet::new();
+    for i in &input {
+        numbers.insert(*i);
+    }
+    let (x, y) = match find_pair_with_sum(&numbers, 2020) {
+        Some(pair) => pair,
+        None => panic!("nothing found"),
+    };
+    println!("1a: {}", x * y);
+
+    for i in &numbers {
+        let (x, y) = match find_pair_with_sum(&numbers, 2020 - i) {
+            Some(pair) => pair,
+            None => continue,
+        };
+        println!("1b: {}", i * x * y);
+        break;
+    }
+}
+
+fn main() {
+    day1();
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_day1() {
+        let input = [1721, 979, 366, 299, 675, 1456];
+        let mut numbers = HashSet::new();
+        for i in &input {
+            numbers.insert(*i);
+        }
+        let (x, y) = find_pair_with_sum(&numbers, 2020).unwrap();
+        assert_eq!(x * y, 514579);
+
+        for i in &numbers {
+            let (x, y) = match find_pair_with_sum(&numbers, 2020 - i) {
+                Some(pair) => pair,
+                None => continue,
+            };
+            assert_eq!(i + x + y, 2020);
+            assert_eq!(i * x * y, 241861950);
+            break;
+        }
+    }
+}
-- 
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