Solve day 4

README.md

@@ -9,3 +9,4 @@ Below are a list of the programming languages used to solve each day:
 - Day 1 - Haskell
 - Day 2 - Haskell
 - Day 3 - Rust
+- Day 4 - Rust

day4/Cargo.lock

@@ -0,0 +1,25 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "day4"
+version = "0.1.0"
+dependencies = [
+ "itertools",
+]
+
+[[package]]
+name = "either"
+version = "1.13.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "60b1af1c220855b6ceac025d3f6ecdd2b7c4894bfe9cd9bda4fbb4bc7c0d4cf0"
+
+[[package]]
+name = "itertools"
+version = "0.13.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "413ee7dfc52ee1a4949ceeb7dbc8a33f2d6c088194d9f922fb8318faf1f01186"
+dependencies = [
+ "either",
+]

day4/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "day4"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+itertools = "0.13.0"

day4/README.md

@@ -0,0 +1,17 @@
+# Day 4
+
+## Instructions
+
+From the root directory of the crate (where this README is) just execute:
+
+```
+cargo run
+```
+
+The solution will be printed to the terminal output.
+
+## Notes:
+
+Implemented this using a pseudo-directed graph data structure. That took some extra time to implement but leads to a more efficient implementation. 
+Rust is certainly slower to develop in (the cost of all that safety) so I may swap to another one for future problems since my time is limited.
+

day4/input.txt

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day4/src/lib.rs

@@ -0,0 +1,265 @@
+use std::{
+    collections::HashSet,
+    error::Error,
+    fmt::Display,
+    fs::File,
+    io::{BufRead, BufReader},
+};
+
+use itertools::Itertools;
+
+/// Basic error type
+#[derive(Debug)]
+enum ParsingError {
+    ParserFailure,
+    IndexOutOfBounds,
+    InconsistentGrid,
+}
+
+impl Display for ParsingError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            ParsingError::ParserFailure => write!(f, "Failed to parse segment"),
+            ParsingError::IndexOutOfBounds => write!(f, "Index out of bounds for CharGrid"),
+            ParsingError::InconsistentGrid => write!(f, "File is not a consistent char grid"),
+        }
+    }
+}
+
+type MyResult<T> = Result<T, Box<dyn Error>>;
+
+impl Error for ParsingError {}
+
+/// Struct that flattens out the input grid
+pub struct CharGrid {
+    pub chars: Vec<char>,
+    pub m: usize,
+    pub n: usize,
+}
+
+impl CharGrid {
+    /// Take a file and split it into a CharGrid. Since input has only ASCII characters this should
+    /// be fine
+    pub fn new(input_file: &str) -> MyResult<CharGrid> {
+        let file = File::open(input_file)?;
+        let reader = BufReader::new(file);
+
+        let mut m = 0;
+        let mut n: Option<usize> = None;
+        let chars: Vec<char> = reader
+            .lines()
+            .map(|line| -> MyResult<Vec<char>> {
+                let parsed_line = Vec::from_iter(line?.chars());
+                m += 1;
+                if n.is_none() {
+                    n = Some(parsed_line.len());
+                } else if n.unwrap() != parsed_line.len() {
+                    return Err(Box::new(ParsingError::InconsistentGrid));
+                }
+                Ok(parsed_line)
+            })
+            .collect::<MyResult<Vec<Vec<char>>>>()?
+            .into_iter()
+            .flatten()
+            .collect();
+
+        // Safe to unwrap n because it is validated if we get to this point
+        if chars.len() != m * n.unwrap() {
+            Err(Box::new(ParsingError::ParserFailure))
+        } else {
+            Ok(CharGrid {
+                chars,
+                m,
+                n: n.unwrap(),
+            })
+        }
+    }
+
+    /// Get an index within the grid
+    pub fn get(&self, i: i32, j: i32) -> MyResult<char> {
+        let idx = self.two2one_index(i, j)?;
+        Ok(self.chars[idx])
+    }
+
+    // Convert the one dimensional index to two dimensional index
+    pub fn one2two_index(&self, idx: usize) -> (usize, usize) {
+        let i = idx / self.n;
+        let j = idx % self.n;
+        (i, j)
+    }
+
+    // Convert the two dimensional index to one dimensional index
+    pub fn two2one_index(&self, i: i32, j: i32) -> MyResult<usize> {
+        if i < 0 || j < 0 {
+            Err(Box::new(ParsingError::IndexOutOfBounds))
+        } else if i as usize >= self.m || j as usize >= self.n {
+            Err(Box::new(ParsingError::IndexOutOfBounds))
+        } else {
+            Ok((i as usize) * self.n + (j as usize))
+        }
+    }
+}
+
+impl From<CharGrid> for CharGraph {
+    fn from(value: CharGrid) -> Self {
+        // Build up the CharGraph
+        let mut edges: Vec<Edge> = Vec::new();
+        let mut node_idx: Vec<usize> = vec![0];
+        let mut node_char: Vec<char> = Vec::new();
+
+        for (idx, c) in value.chars.iter().enumerate() {
+            let (i, j) = value.one2two_index(idx);
+            for (edge_type, (offset_i, offset_j)) in [-1, 0, 1]
+                .into_iter()
+                .cartesian_product([-1, 0, 1])
+                .enumerate()
+            {
+                let nei_i: i32 = i as i32 + offset_i;
+                let nei_j: i32 = j as i32 + offset_j;
+                if let Ok(nei_char) = value.get(nei_i, nei_j) {
+                    if (*c == 'X' && nei_char == 'M')
+                        || (*c == 'M' && nei_char == 'A')
+                        || (*c == 'A' && nei_char == 'S')
+                    {
+                        edges.push(Edge {
+                            start_idx: idx,
+                            // Safe to unwrap here since this index has been validated already
+                            end_idx: value.two2one_index(nei_i, nei_j).unwrap(),
+                            start_char: *c,
+                            edge_type,
+                        })
+                    }
+                }
+            }
+            node_idx.push(edges.len());
+            node_char.push(*c);
+        }
+        CharGraph {
+            edges,
+            node_idx,
+            node_char,
+        }
+    }
+}
+
+/// Simple graph-like data structure to represent continuous XMAS's
+pub struct CharGraph {
+    edges: Vec<Edge>,
+    node_idx: Vec<usize>,
+    node_char: Vec<char>,
+}
+
+impl CharGraph {
+    /// Iterate through all edges. If it's an X count all the full XMAS segments that can be made
+    pub fn count_xmas(&self) -> usize {
+        let mut segments = 0;
+        for (idx, edge) in self.edges.iter().enumerate() {
+            if edge.start_char == 'X' {
+                segments = self.get_full_segments(idx, ' ', segments);
+            }
+        }
+        segments
+    }
+
+    /// recursive call to get XMAS segments
+    fn get_full_segments(&self, edge_idx: usize, prev_char: char, mut segments: usize) -> usize {
+        let Edge {
+            end_idx,
+            start_char,
+            edge_type,
+            ..
+        } = &self.edges[edge_idx];
+
+        // If this is an AS segment increment segment count and return
+        if *start_char == 'A' && prev_char == 'M' {
+            segments + 1
+        } else {
+            if *end_idx < self.node_idx.len() {
+                for next_edge_idx in self.node_idx[*end_idx]..self.node_idx[*end_idx + 1] {
+                    if self.edges[next_edge_idx].edge_type == *edge_type {
+                        segments = self.get_full_segments(next_edge_idx, *start_char, segments)
+                    }
+                }
+            }
+            segments
+        }
+    }
+
+    /// Count all X-MAS shapes for part 2. Trying to reuse code from part 1 as much as possible so
+    /// this might be a little hacky. Running out of time here.
+    pub fn count_part2_xmas(&self) -> usize {
+        let mut a_we_met: HashSet<usize> = HashSet::new();
+        let mut xmas_count = 0;
+        for (idx, edge) in self.edges.iter().enumerate() {
+            if edge.start_char == 'M' && [0, 8, 2, 6].contains(&edge.edge_type) {
+                let segments = self.get_full_segments(idx, ' ', 0);
+                if segments == 1 {
+                    if a_we_met.contains(&edge.end_idx) {
+                        xmas_count += 1;
+                    } else {
+                        a_we_met.insert(edge.end_idx);
+                    }
+                }
+            }
+        }
+        xmas_count
+    }
+}
+/// Representation of a directed edge where the edge only exists for:
+/// start = X end = M
+/// start = M end = A
+/// start = A end = S
+/// Edge type just represents the direction of the edge
+/// |     0   1   2
+/// |      \  |  /
+/// |    3 -start- 5
+/// |      /  |  \
+/// |     6   7    8
+///
+/// Should've used an enum but trying to rush here
+pub struct Edge {
+    start_idx: usize,
+    start_char: char,
+    end_idx: usize,
+    edge_type: usize,
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_char_grid() {
+        let grid = CharGrid::new("test_inputs/test_input.txt").unwrap();
+        assert_eq!(grid.m, 2);
+        assert_eq!(grid.n, 8);
+
+        if CharGrid::new("test_inputs/bad_input.txt").is_ok() {
+            panic!("Should've failed to parse bad input")
+        };
+    }
+
+    #[test]
+    fn test_char_graph() {
+        let grid = CharGrid::new("test_inputs/test_input.txt").unwrap();
+        let graph = CharGraph::from(grid);
+        assert_eq!(graph.edges.len(), 20);
+    }
+
+    #[test]
+    fn test_count_xmas() {
+        let grid = CharGrid::new("test_inputs/test_input.txt").unwrap();
+        let graph = CharGraph::from(grid);
+        assert_eq!(graph.count_xmas(), 2);
+        let grid = CharGrid::new("test_inputs/test_advent_sample.txt").unwrap();
+        let graph = CharGraph::from(grid);
+        assert_eq!(graph.count_xmas(), 18);
+    }
+
+    #[test]
+    fn test_count_xmas_part2() {
+        let grid = CharGrid::new("test_inputs/test_advent_sample.txt").unwrap();
+        let graph = CharGraph::from(grid);
+        assert_eq!(graph.count_part2_xmas(), 9);
+    }
+}

day4/src/main.rs

@@ -0,0 +1,14 @@
+use day4::{CharGraph, CharGrid};
+
+fn main() {
+    let grid = match CharGrid::new("input.txt") {
+        Ok(grid) => grid,
+        Err(err) => {
+            println!("Failed becuase of \n{err}");
+            panic!("Program failed to execute")
+        }
+    };
+    let graph = CharGraph::from(grid);
+    println!("Solution to part 1 is {}", graph.count_xmas());
+    println!("Solution to part 1 is {}", graph.count_part2_xmas());
+}

day4/test_inputs/bad_input.txt

@@ -0,0 +1,3 @@
+XMASXMASS
+XMASXMAS
+XMASXMA

day4/test_inputs/test_advent_sample.txt

@@ -0,0 +1,10 @@
+MMMSXXMASM
+MSAMXMSMSA
+AMXSXMAAMM
+MSAMASMSMX
+XMASAMXAMM
+XXAMMXXAMA
+SMSMSASXSS
+SAXAMASAAA
+MAMMMXMMMM
+MXMXAXMASX

day4/test_inputs/test_input.txt

@@ -0,0 +1,2 @@
+XMASXMAS
+SAASMMAS