day 9

data/examples/09.txt

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+2333133121414131402

src/bin/09.rs

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+advent_of_code::solution!(9);
+
+pub fn part_one(input: &str) -> Option<usize> {
+    let input: String = input.chars().filter(|c| c.is_ascii_digit()).collect();
+    let mut blocks: Vec<Option<usize>> = Vec::new();
+    let mut file_id: usize = 0;
+
+    for (i, c) in input.chars().enumerate() {
+        let count = c.to_digit(10)? as usize;
+        if i % 2 == 0 {
+            for _ in 0..count {
+                blocks.push(Some(file_id));
+            }
+            file_id = file_id.wrapping_add(1);
+        } else {
+            for _ in 0..count {
+                blocks.push(None);
+            }
+        }
+    }
+
+    let mut i = 0;
+    while i < blocks.len() {
+        if blocks[i].is_none() {
+            if let Some(j) = blocks.iter().rposition(|b| b.is_some()) {
+                if j <= i {
+                    break;
+                }
+                blocks[i] = blocks[j];
+                blocks[j] = None;
+            } else {
+                break;
+            }
+        }
+        i += 1;
+    }
+
+    let checksum: u128 = blocks
+        .iter()
+        .enumerate()
+        .filter_map(|(pos, &block)| block.map(|id| (id as u128).saturating_mul(pos as u128)))
+        .sum();
+
+    usize::try_from(checksum).ok()
+}
+
+pub fn part_two(input: &str) -> Option<usize> {
+    let input: String = input.chars().filter(|c| c.is_ascii_digit()).collect();
+    let mut blocks: Vec<Option<usize>> = Vec::new();
+    let mut file_id: usize = 0;
+    let mut file_ranges = Vec::new();
+
+    let mut current_pos = 0;
+    for (i, c) in input.chars().enumerate() {
+        let count = c.to_digit(10)? as usize;
+        if i % 2 == 0 {
+            if count > 0 {
+                let start = current_pos;
+                for _ in 0..count {
+                    blocks.push(Some(file_id));
+                    current_pos += 1;
+                }
+                file_ranges.push((file_id, start, count));
+            } else {
+                file_ranges.push((file_id, current_pos, 0));
+            }
+            file_id = file_id.wrapping_add(1);
+        } else {
+            for _ in 0..count {
+                blocks.push(None);
+                current_pos += 1;
+            }
+        }
+    }
+
+    if let Some(max_id) = file_ranges.iter().map(|f| f.0).max() {
+        for id in (0..=max_id).rev() {
+            let (_, start, length) = file_ranges[id];
+            if length == 0 {
+                continue;
+            }
+            if let Some(free_start) = find_free_span(&blocks, length, start) {
+                for offset in 0..length {
+                    blocks[free_start + offset] = blocks[start + offset];
+                    blocks[start + offset] = None;
+                }
+                file_ranges[id] = (id, free_start, length);
+            }
+        }
+    }
+
+    let checksum: u128 = blocks
+        .iter()
+        .enumerate()
+        .filter_map(|(pos, &b)| b.map(|id| (id as u128) * (pos as u128)))
+        .sum();
+
+    usize::try_from(checksum).ok()
+}
+
+fn find_free_span(blocks: &[Option<usize>], length: usize, end_limit: usize) -> Option<usize> {
+    let mut consecutive = 0;
+
+    for (i, _block) in blocks.iter().enumerate().take(end_limit) {
+        if blocks[i].is_none() {
+            consecutive += 1;
+            if consecutive == length {
+                return Some(i - length + 1);
+            }
+        } else {
+            consecutive = 0;
+        }
+    }
+
+    None
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_part_one() {
+        let result = part_one(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(1928));
+    }
+
+    #[test]
+    fn test_part_two() {
+        let result = part_two(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(2858));
+    }
+}