From 61125c42a1495ecc92ffef8aa6387b743561dc20 Mon Sep 17 00:00:00 2001
From: Antoine Martin <antoine97.martin@gmail.com>
Date: Tue, 15 Dec 2020 09:56:43 +0100
Subject: [PATCH] 2020: day15: part 1

---
 aoc2020/benches/bench.rs         |  2 +
 aoc2020/input/day15.txt          |  1 +
 aoc2020/input/day15_provided.txt |  1 +
 aoc2020/src/day15.rs             | 83 ++++++++++++++++++++++++++++++++
 aoc2020/src/lib.rs               |  1 +
 aoc2020/src/main.rs              |  2 +
 6 files changed, 90 insertions(+)
 create mode 100644 aoc2020/input/day15.txt
 create mode 100644 aoc2020/input/day15_provided.txt
 create mode 100644 aoc2020/src/day15.rs

diff --git a/aoc2020/benches/bench.rs b/aoc2020/benches/bench.rs
index a5447c9..8e13e66 100644
--- a/aoc2020/benches/bench.rs
+++ b/aoc2020/benches/bench.rs
@@ -14,6 +14,7 @@ use aoc2020::day11;
 use aoc2020::day12;
 use aoc2020::day13;
 use aoc2020::day14;
+use aoc2020::day15;
 
 fn aoc2020_all(c: &mut Criterion) {
     c.bench_function("day01", |b| b.iter(|| day01::run().unwrap()));
@@ -30,6 +31,7 @@ fn aoc2020_all(c: &mut Criterion) {
     c.bench_function("day12", |b| b.iter(|| day12::run().unwrap()));
     c.bench_function("day13", |b| b.iter(|| day13::run().unwrap()));
     c.bench_function("day14", |b| b.iter(|| day14::run().unwrap()));
+    c.bench_function("day15", |b| b.iter(|| day15::run().unwrap()));
 }
 
 criterion_group! {
diff --git a/aoc2020/input/day15.txt b/aoc2020/input/day15.txt
new file mode 100644
index 0000000..7102600
--- /dev/null
+++ b/aoc2020/input/day15.txt
@@ -0,0 +1 @@
+8,11,0,19,1,2
diff --git a/aoc2020/input/day15_provided.txt b/aoc2020/input/day15_provided.txt
new file mode 100644
index 0000000..c84ffe7
--- /dev/null
+++ b/aoc2020/input/day15_provided.txt
@@ -0,0 +1 @@
+0,3,6
diff --git a/aoc2020/src/day15.rs b/aoc2020/src/day15.rs
new file mode 100644
index 0000000..6911a49
--- /dev/null
+++ b/aoc2020/src/day15.rs
@@ -0,0 +1,83 @@
+use std::collections::HashMap;
+use std::fmt::Write;
+
+use anyhow::Result;
+
+const INPUT: &str = include_str!("../input/day15.txt");
+
+pub fn run() -> Result<String> {
+    let mut res = String::with_capacity(128);
+
+    writeln!(res, "part 1: {}", part1(INPUT)?)?;
+
+    Ok(res)
+}
+
+fn part1(input: &str) -> Result<u64> {
+    let starting_numbers = input.trim_end()
+        .split(',')
+        .map(|num| num.parse().map_err(anyhow::Error::new))
+        .collect::<Result<Vec<u64>>>()?;
+
+    let mut spoken_numbers = HashMap::new();
+    let mut next_number: u64 = 0;
+    let mut turn: usize = 0;
+
+    for num in starting_numbers {
+        let last_turn = speak_number(num, turn, &mut spoken_numbers);
+        match last_turn {
+            Some(prev_turn) => next_number = (turn - prev_turn) as u64,
+            None => next_number = 0,
+        }
+
+        turn += 1;
+    }
+
+    let mut last_number: u64 = 0;
+
+    while turn < 2020 {
+        // store the number we're about to speak for the solution
+        last_number = next_number;
+
+        // get the previous time this number was spoken, if any
+        let last_turn = speak_number(next_number, turn, &mut spoken_numbers);
+        match last_turn {
+            // if it was spoken before, the number we'll say next turn is the difference between the
+            // two turns where it was last spoken
+            Some(prev_turn) => next_number = (turn - prev_turn) as u64,
+            // otherwise we'll say 0 next turn
+            None => next_number = 0,
+        }
+
+        turn += 1
+    }
+
+    Ok(last_number)
+}
+
+/// Inserts the turn a number was spoken in the map. Returns the previous turn the number was
+/// spoken, if any
+fn speak_number(n: u64, turn: usize, spoken_numbers: &mut HashMap<u64, usize>) -> Option<usize> {
+    let res = spoken_numbers.get(&n).copied();
+
+    spoken_numbers.insert(n, turn);
+
+    res
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const PROVIDED: &str = include_str!("../input/day15_provided.txt");
+
+    #[test]
+    fn part1_provided() {
+        assert_eq!(part1(PROVIDED).unwrap(), 436);
+    }
+
+    #[test]
+    fn part1_real() {
+        assert_eq!(part1(INPUT).unwrap(), 447);
+    }
+}
diff --git a/aoc2020/src/lib.rs b/aoc2020/src/lib.rs
index f4877db..c4cadc0 100644
--- a/aoc2020/src/lib.rs
+++ b/aoc2020/src/lib.rs
@@ -12,3 +12,4 @@ pub mod day11;
 pub mod day12;
 pub mod day13;
 pub mod day14;
+pub mod day15;
diff --git a/aoc2020/src/main.rs b/aoc2020/src/main.rs
index 104f8af..936890e 100644
--- a/aoc2020/src/main.rs
+++ b/aoc2020/src/main.rs
@@ -16,6 +16,7 @@ use aoc2020::day11;
 use aoc2020::day12;
 use aoc2020::day13;
 use aoc2020::day14;
+use aoc2020::day15;
 
 fn main() -> Result<()> {
     let days: &[DayFunc] = &[
@@ -33,6 +34,7 @@ fn main() -> Result<()> {
         day12::run,
         day13::run,
         day14::run,
+        day15::run,
     ];
 
     aoc::run(days)