Day 9

src/day9.rs

@@ -0,0 +1,137 @@
+use std::{
+    fs::File,
+    io::{BufReader, Read},
+    iter,
+};
+
+use itertools::Itertools;
+
+#[derive(Copy, Clone)]
+enum Block {
+    Free,
+    File { id: usize },
+}
+
+impl Block {
+    fn is_free(&self) -> bool {
+        matches!(self, Self::Free)
+    }
+
+    fn file_id(&self) -> usize {
+        match self {
+            Self::File { id } => *id,
+            Self::Free => unreachable!(),
+        }
+    }
+}
+
+fn get_data() -> Vec<Block> {
+    let mut result = vec![];
+    let reader = BufReader::new(File::open("input.txt").unwrap());
+    for (i, b) in reader.bytes().enumerate() {
+        let num = b.unwrap() - b'0';
+        let record = if i % 2 == 0 {
+            Block::File { id: i / 2 }
+        } else {
+            Block::Free
+        };
+        result.extend(iter::repeat(record).take(num as usize));
+    }
+
+    result
+}
+
+#[derive(Clone, Copy)]
+struct LBlock {
+    start: usize,
+    end: usize,
+}
+
+impl LBlock {
+    fn len(&self) -> usize {
+        self.end - self.start
+    }
+}
+
+fn checksum(mem: &[Block]) -> usize {
+    mem.iter()
+        .enumerate()
+        .filter(|(_i, block)| !block.is_free())
+        .map(|(i, block)| i * block.file_id())
+        .sum()
+}
+
+pub fn task1() {
+    let mut data = get_data();
+    let mut lo = 0;
+    let mut ro = data.len() - 1;
+
+    while lo < ro {
+        while lo < data.len() && !data[lo].is_free() {
+            lo += 1;
+        }
+
+        while ro > lo && data[ro].is_free() {
+            ro -= 1;
+        }
+
+        data.swap(lo, ro);
+        lo += 1;
+        ro = ro.saturating_sub(1);
+    }
+
+    println!("{}", checksum(&data));
+}
+
+pub fn task2() {
+    let mut data = get_data();
+    let mut free_mem = vec![];
+    let mut files = vec![];
+    for (is_free, mut chunk) in &data
+        .iter()
+        .enumerate()
+        .chunk_by(|(_i, block)| block.is_free())
+    {
+        if is_free {
+            let (fbi, fb) = chunk.next().unwrap();
+            let (last_block_index, _last_block) = chunk.last().unwrap_or((fbi, fb));
+            let c_block = LBlock {
+                start: fbi,
+                end: last_block_index + 1,
+            };
+            free_mem.push(c_block);
+        } else {
+            for (_file_id, mut chunk) in &chunk.chunk_by(|(_i, block)| block.file_id()) {
+                let (fbi, fb) = chunk.next().unwrap();
+                let (lbi, _last_block) = chunk.last().unwrap_or((fbi, fb));
+                let c_block = LBlock {
+                    start: fbi,
+                    end: lbi + 1,
+                };
+                files.push(c_block);
+            }
+        };
+    }
+
+    for file in files.into_iter().rev() {
+        let size = file.len();
+        let Some((free_index, _)) = free_mem
+            .iter()
+            .find_position(|block| block.len() >= size && block.start < file.start)
+        else {
+            continue;
+        };
+
+        let free_mem_block = &mut free_mem[free_index];
+
+        for (file_index, free_index) in
+            (file.start..file.end).zip(free_mem_block.start..free_mem_block.end)
+        {
+            data.swap(file_index, free_index);
+        }
+
+        free_mem_block.start += file.len();
+    }
+
+    println!("{}", checksum(&data));
+}

src/main.rs

@@ -6,6 +6,7 @@ mod day5;
 mod day6;
 mod day7;
 mod day8;
+mod day9;
 
 use std::{
     fs::File,
@@ -23,6 +24,6 @@ fn parse_line(line: &str, pattern: char) -> impl Iterator<Item = i32> + '_ {
 }
 
 fn main() {
-    day8::task1();
-    day8::task2();
+    day9::task1();
+    day9::task2();
 }