3 files changed, 148 insertions, 1 deletions
diff --git a/2021/rust/Cargo.toml b/2021/rust/Cargo.toml
index 5badeb6..49ed1d7 100644
--- a/2021/rust/Cargo.toml
+++ b/2021/rust/Cargo.toml
@@ -6,7 +6,7 @@ members = [
     "day04",
     "day05",
     "day06",
-#    "day07",
+    "day07",
 #    "day08",
 #    "day09",
 #    "day10",
diff --git a/2021/rust/day07/Cargo.toml b/2021/rust/day07/Cargo.toml
new file mode 100644
index 0000000..645ee26
--- /dev/null
+++ b/2021/rust/day07/Cargo.toml
@@ -0,0 +1,9 @@
+[package]
+name = "day07"
+version = "0.1.0"
+authors = ["cos <cos>"]
+edition = "2021"
+
+[dependencies]
+aoc = { path = "../../../common/rust/aoc" }
+anyhow = "1.0"
diff --git a/2021/rust/day07/src/main.rs b/2021/rust/day07/src/main.rs
new file mode 100644
index 0000000..56fcde1
--- /dev/null
+++ b/2021/rust/day07/src/main.rs
@@ -0,0 +1,138 @@
+use {
+    anyhow::{
+        anyhow,
+        Context,
+        Result,
+    },
+    std::{
+        collections::BTreeMap,
+        env::args,
+        fs::File,
+        io::{
+            BufRead,
+            BufReader,
+        },
+        path::Path,
+    },
+};
+
+fn read_input<T: AsRef<Path>>(filename: T) -> Result<Vec<usize>> {
+    let reader = BufReader::new(File::open(filename)?);
+
+    reader.lines().next().ok_or(anyhow!(""))??.split(',').map(
+        |s| {
+            let n = s.parse()?;
+            Ok(n)
+        }
+    ).collect()
+}
+
+fn part1<'a, I: Copy + IntoIterator<Item = &'a usize>>(input: I) -> Result<usize> {
+    let mut subs: BTreeMap<usize, usize> = BTreeMap::new();
+    let mut fuel = 0;
+    for position in input {
+        *subs.entry(*position).or_insert(0) += 1;
+    }
+    
+    while subs.len() > 1 {
+        let first_position = *subs.keys().next().ok_or(anyhow!("Couldn't get first position."))?;
+        let last_position = *subs.keys().rev().next()
+            .ok_or(anyhow!("Couldn't get last position."))?;
+        let first_value = *subs.get(&first_position).ok_or(anyhow!("Couldn't get first value."))?;
+        let last_value = *subs.get(&last_position).ok_or(anyhow!("Couldn't get last value."))?;
+        if first_value <= last_value {
+            let value = subs.remove(&first_position)
+                .ok_or(anyhow!("Couldn't remove first value"))?;
+            let new_position = first_position + 1;
+            *subs.entry(new_position).or_insert(0) += value;
+            fuel += value;
+        }
+        if first_value >= last_value {
+            let value = subs.remove(&last_position).ok_or(anyhow!("Couldn't remove last value"))?;
+            let new_position = last_position - 1;
+            *subs.entry(new_position).or_insert(0) += value;
+            fuel += value;
+        }
+    }
+    Ok(fuel)
+}
+
+fn part2<'a, I: Copy + IntoIterator<Item = &'a usize>>(input: I) -> Result<usize> {
+    #[derive(Debug)]
+    struct Occupancy {
+        cost: usize,
+        vessel_count: usize,
+    }
+
+    impl Occupancy {
+        fn new() -> Self {
+            let cost = 0;
+            let vessel_count = 0;
+            Self {
+                cost,
+                vessel_count,
+            }
+        }
+
+        fn add_vessel(&mut self) {
+            self.cost += 1;
+            self.vessel_count += 1;
+        }
+
+        fn merge(&mut self, other: Occupancy) -> Result<()> {
+            self.cost += other.cost + other.vessel_count;
+            self.vessel_count += other.vessel_count;
+            Ok(())
+        }
+    }
+
+    let mut subs: BTreeMap<usize, Occupancy> = BTreeMap::new();
+    let mut fuel = 0;
+    for position in input {
+        subs.entry(*position).or_insert_with(Occupancy::new).add_vessel();
+    }
+    
+    while subs.len() > 1 {
+        let first_position = *subs.keys().next().ok_or(anyhow!("Couldn't get first position."))?;
+        let last_position = *subs.keys().rev().next()
+            .ok_or(anyhow!("Couldn't get last position."))?;
+        let first_costs = (*subs.get(&first_position).ok_or(anyhow!("Couldn't get first value."))?)
+            .cost;
+        let first_fuel = first_costs;
+        let last_costs = (*subs.get(&last_position).ok_or(anyhow!("Couldn't get last value."))?)
+            .cost;
+        let last_fuel = last_costs;
+        if first_fuel <= last_fuel {
+            let value = subs.remove(&first_position)
+                .ok_or(anyhow!("Couldn't remove first value"))?;
+            let new_position = first_position + 1;
+            let costs = subs.entry(new_position).or_insert_with(Occupancy::new);
+            costs.merge(value)?;
+            fuel += first_fuel;
+        }
+        if first_fuel >= last_fuel {
+            let value = subs.remove(&last_position).ok_or(anyhow!("Couldn't remove last value"))?;
+            let new_position = last_position - 1;
+            let costs = subs.entry(new_position).or_insert_with(Occupancy::new);
+            costs.merge(value)?;
+            fuel += last_fuel;
+        }
+    }
+    Ok(fuel)
+}
+
+fn main() -> Result<()> {
+    let ( do_part_1, do_part_2 ) = aoc::do_parts();
+
+    let filename = args().nth(1).ok_or(anyhow!("Missing input filename"))?;
+    let input = read_input(filename).context("Could not read input")?;
+    if do_part_1 {
+        let solution = part1(&input).context("No solution for part 1")?;
+        println!("Part1, solution found to be: {}", solution);
+    }
+    if do_part_2 {
+        let solution = part2(&input).context("No solution for part 2")?;
+        println!("Part2, solution found to be: {}", solution);
+    }
+    Ok(())
+}