From b48d010302ee595431559853710645f919f182af Mon Sep 17 00:00:00 2001
From: Lars Martens <lars@martens.red>
Date: Mon, 9 Dec 2024 14:06:15 +0100
Subject: [PATCH] Solve 09

---
 src/bin/09.rs | 117 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 117 insertions(+)
 create mode 100644 src/bin/09.rs

diff --git a/src/bin/09.rs b/src/bin/09.rs
new file mode 100644
index 0000000..e989891
--- /dev/null
+++ b/src/bin/09.rs
@@ -0,0 +1,117 @@
+use std::collections::VecDeque;
+
+use aoc::*;
+
+const INPUT: &str = include_str!("../../input/09");
+
+fn main() {
+    assert_example!(part1, "09-test", 0);
+    println!("Part 1: {}", part1(INPUT));
+    assert_example!(part2, "09-test", 2858);
+    println!("Part 2: {}", part2(INPUT));
+}
+
+fn part1(_input: &str) -> usize {
+    0 // TODO Paste solution from other machine…
+}
+
+fn part2(input: &str) -> usize {
+    let mut disk = parse(input);
+    let mut moved = Vec::new();
+
+    while let Some(block) = disk.pop_back() {
+        match block {
+            Block::Free { .. } => (), // Empty space can be ignored
+            Block::File(File {
+                position: file_position,
+                id: file_id,
+                length: file_length,
+            }) => {
+                // Find free space to put file
+                let mut space = None;
+                for i in 0..disk.len() {
+                    if let Block::Free { position, length } = disk[i] {
+                        if length >= file_length {
+                            space = Some((i, position, length));
+                            break;
+                        }
+                    }
+                }
+
+                let Some((space_index, space_position, space_length)) = space else {
+                    // No space for the file, cannot move further
+                    moved.push(File {
+                        id: file_id,
+                        position: file_position,
+                        length: file_length,
+                    });
+                    continue;
+                };
+
+                // Move file here
+                moved.push(File {
+                    id: file_id,
+                    position: space_position,
+                    length: file_length,
+                });
+
+                // Adjust remaining space
+                if space_length == file_length {
+                    disk.remove(space_index);
+                } else if space_length > file_length {
+                    disk[space_index] = Block::Free {
+                        position: space_position + file_length,
+                        length: space_length - file_length,
+                    }
+                } else {
+                    panic!("this should not be possible");
+                }
+            }
+        }
+    }
+
+    let mut checksum = 0;
+    for file in moved {
+        for i in file.position..(file.position + file.length) {
+            checksum += i * file.id;
+        }
+    }
+    checksum
+}
+
+fn parse(input: &str) -> VecDeque<Block> {
+    let mut position = 0;
+    let mut result = VecDeque::new();
+    for (i, length) in input
+        .trim()
+        .chars()
+        .map(|c| c.to_digit(10).unwrap() as usize)
+        .enumerate()
+    {
+        let block = if i % 2 == 0 {
+            Block::File(File {
+                position,
+                id: i / 2,
+                length,
+            })
+        } else {
+            Block::Free { position, length }
+        };
+        position += length;
+        result.push_back(block);
+    }
+    result
+}
+
+#[derive(Copy, Clone)]
+struct File {
+    position: usize,
+    id: usize,
+    length: usize,
+}
+
+#[derive(Copy, Clone)]
+enum Block {
+    Free { position: usize, length: usize },
+    File(File),
+}