feat: added day 20 part 1 and 2

Signed-off-by: Louis Vallat <contact@louis-vallat.dev>

day20/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "day20"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

day20/src/main.rs

@@ -0,0 +1,125 @@
+use std::collections::HashSet;
+use std::{env, fs};
+
+fn read_input(path: &str) -> String {
+    fs::read_to_string(path).expect("Cannot read file.")
+}
+
+fn parse_input(input: &str) -> (Vec<Vec<i32>>, (usize, usize), (usize, usize)) {
+    let mut map = Vec::new();
+    let (mut start, mut end) = ((0, 0), (0, 0));
+
+    for (row, line) in input.lines().filter(|s| !s.is_empty()).enumerate() {
+        map.push(Vec::new());
+        for (col, char) in line.chars().enumerate() {
+            match char {
+                '#' => map[row].push(-1),
+                '.' => map[row].push(0),
+                'E' => {
+                    map[row].push(0);
+                    end = (row, col);
+                }
+                'S' => {
+                    map[row].push(0);
+                    start = (row, col);
+                }
+                _ => unreachable!(),
+            }
+        }
+    }
+
+    (map, start, end)
+}
+
+fn get_if_in_map(
+    map: &Vec<Vec<i32>>,
+    row: usize,
+    offset_r: i32,
+    col: usize,
+    offset_c: i32,
+) -> Option<(usize, usize)> {
+    let (row, col) = (row as i32 + offset_r, col as i32 + offset_c);
+    if !(0..map.len() as i32).contains(&row) || !(0..map[row as usize].len() as i32).contains(&col)
+    {
+        None
+    } else {
+        Some((row as usize, col as usize))
+    }
+}
+
+fn race_without_cheating(
+    map: &mut Vec<Vec<i32>>,
+    start: &(usize, usize),
+    end: &(usize, usize),
+) -> HashSet<(usize, usize)> {
+    let mut positions = HashSet::new();
+    positions.insert(*start);
+
+    let mut position = *start;
+    let mut cost = 0;
+    while position != *end {
+        for dir in [(0, 1), (-1, 0), (1, 0), (0, -1)] {
+            if let Some((row, col)) = get_if_in_map(map, position.0, dir.0, position.1, dir.1) {
+                if map[row][col] == 0 && (row, col) != *start {
+                    (position, cost) = ((row, col), cost + 1);
+                    map[row][col] = cost;
+                    positions.insert(position);
+                    break;
+                }
+            }
+        }
+    }
+
+    positions
+}
+
+fn get_cheating_shortcuts(
+    map: &Vec<Vec<i32>>,
+    path: &HashSet<(usize, usize)>,
+    radius: i32,
+) -> usize {
+    let mut savings = 0;
+    for &position in path {
+        let source = map[position.0][position.1];
+        for i in -radius..=radius {
+            for j in -radius..=radius {
+                let distance = i.abs() + j.abs();
+                if distance > radius {
+                    continue;
+                }
+
+                if let Some((row, col)) = get_if_in_map(map, position.0, i, position.1, j) {
+                    let destination = map[row][col];
+
+                    if source >= destination || destination == -1 {
+                        continue;
+                    }
+
+                    let saving = destination - source - distance;
+                    if distance <= radius && saving >= 100 {
+                        savings += 1;
+                    }
+                }
+            }
+        }
+    }
+    savings
+}
+
+fn main() {
+    let args: Vec<String> = env::args().collect();
+    for arg in args.iter().skip(1) {
+        let input = read_input(&arg);
+        let (mut map, start, end) = parse_input(&input);
+        let race = race_without_cheating(&mut map, &start, &end);
+        println!("[{}]", &arg);
+        println!(
+            "\t[Part 1] => Answer is '{}'.",
+            get_cheating_shortcuts(&map, &race, 2)
+        );
+        println!(
+            "\t[Part 2] => Answer is '{}'.",
+            get_cheating_shortcuts(&map, &race, 20)
+        );
+    }
+}