2 files changed, 103 insertions, 0 deletions
diff --git a/2020/rust/day10/Cargo.toml b/2020/rust/day10/Cargo.toml
new file mode 100644
index 0000000..75eae97
--- /dev/null
+++ b/2020/rust/day10/Cargo.toml
@@ -0,0 +1,9 @@
+[package]
+name = "day10"
+version = "0.1.0"
+authors = ["cos <cos>"]
+edition = "2018"
+
+[dependencies]
+aoc = { path = "../aoc" }
+anyhow = "1.0"
diff --git a/2020/rust/day10/src/main.rs b/2020/rust/day10/src/main.rs
new file mode 100644
index 0000000..905c525
--- /dev/null
+++ b/2020/rust/day10/src/main.rs
@@ -0,0 +1,94 @@
+use anyhow::Result;
+use std::collections::HashMap;
+use std::env::args;
+use std::fs::File;
+use std::io::{BufRead, BufReader};
+use std::path::Path;
+
+fn read_input<T: AsRef<Path>>(filename: T) -> Result<Vec<usize>> {
+    let f = File::open(filename)?;
+    let reader = BufReader::new(f);
+
+    let values = reader.lines()
+        .map(|v| v?.parse::<usize>().map_err(anyhow::Error::new)).collect();
+
+    values
+}
+
+fn part1(input: &[usize]) -> Option<usize> {
+    let mut sorted = input.to_vec();
+    sorted.push(0);
+    sorted.sort();
+    let max = sorted[sorted.len()-1];
+    sorted.push(max+3);
+    let (_prev, count) = sorted.iter().fold((None, vec![0, 0, 0, 0]), |acc, jolt| {
+        let mut res = acc.1.clone();
+        if let Some(prev) = acc.0 {
+            let diff = jolt - prev;
+            res[diff] += 1;
+        }
+        (Some(jolt), res)
+    });
+
+    Some(count[1] * count[3])
+}
+
+fn part2(input: &[usize]) -> Option<u128> {
+    let mut sorted = input.to_vec();
+    sorted.push(0);
+    sorted.sort();
+    let max = sorted[sorted.len()-1];
+    sorted.push(max+3);
+    let mut paths: HashMap<usize, u128> = HashMap::new();
+    paths.insert(max+3, 1);
+    sorted.iter().rev().fold(0, |_, elem| {
+        let count = (1..=3).map(|jump| {
+            let pos = *elem + jump;
+            if sorted.contains(&pos) {
+                paths.get(&pos).copied().unwrap_or(0)
+            } else {
+                0
+            }
+        }).sum();
+        let lm = paths.entry(*elem).or_insert(0); 
+        *lm += count;
+        count
+    });
+
+    Some(paths[&0])
+}
+
+fn main() {
+    let ( do_part_1, do_part_2 ) = aoc::do_parts();
+
+    let filename = match args().nth(1) {
+        Some(f) => f,
+        None => {
+            eprintln!("Missing input filename");
+            std::process::exit(1);
+        },
+    };
+    match read_input(filename) {
+        Ok(input) => {
+            if do_part_1 {
+                match part1(&input) {
+                    Some(solution) => println!("Part1: {}", solution),
+                    None => {
+                        eprintln!("Part1, no solution found");
+                        std::process::exit(1);
+                    }
+                };
+            }
+            if do_part_2 {
+                match part2(&input) {
+                    Some(solution) => println!("Part2: {}", solution),
+                    None => {
+                        eprintln!("Part2, no solution found");
+                        std::process::exit(1);
+                    }
+                };
+            }
+        },
+        Err(err) => eprintln!("Could not read input: {}", err),
+    }
+}