Initial Solution

.gitignore

@@ -0,0 +1,2 @@
+.idea
+/target

Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "day3"
+version = "0.1.0"

Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "day3"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

src/input.txt

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src/main.rs

@@ -0,0 +1,96 @@
+use std::fs::File;
+use std::io;
+use std::io::BufRead;
+use std::path::Path;
+
+fn mark<F>(c: char, mut f: F)
+where
+    F: FnMut(usize),
+{
+    let p = match c {
+        'a'..='z' => Some(c as usize - 'a' as usize),
+        'A'..='Z' => Some(c as usize - 'A' as usize + 26),
+        _ => None,
+    };
+
+    if let Some(p) = p {
+        f(p);
+    }
+}
+
+fn main() {
+    let path = Path::new("input.txt");
+    let file = File::open(&path).expect("Could not find file");
+    let lines = io::BufReader::new(file).lines();
+
+    const COUNT: usize = 52;
+
+    let mut doubles = [0; COUNT + 1];
+
+    let mut badges = Vec::new();
+    let mut triads_items = [[0; 3]; COUNT];
+    let mut elf = 0;
+
+    for line in lines {
+        if let Ok(line) = line {
+            let mut flags_1 = [0; COUNT];
+            let mut flags_2 = [0; COUNT];
+
+            let line = line.as_bytes();
+            let end = line.len();
+            let middle = end >> 1;
+
+            for c in 0..middle {
+                mark(line[c] as char, |p| flags_1[p] += 1);
+                mark(line[end - 1 - c] as char, |p| flags_2[p] += 1);
+
+                // Look for the presence of an item in each backpack of
+                // the triads' elf
+                mark(line[c] as char, |p| triads_items[p][elf % 3] = 1);
+                mark(line[end - 1 - c] as char, |p| triads_items[p][elf % 3] = 1);
+            }
+
+            for i in 0..COUNT {
+                if flags_1[i] > 0 && flags_2[i] > 0 {
+                    // We have an item present two times
+                    // 'a' == 1, so let's update the COUNT at that position
+                    doubles[i + 1] += 1;
+                }
+            }
+        }
+
+        elf += 1;
+        if 0 == elf % 3 {
+            // for each item, sum the three elves' backpack
+            // presence marker. If it is 3, then this is the
+            // badge. Add this badge to the list of badges.
+            let badge = triads_items
+                .into_iter()
+                .enumerate()
+                .filter_map(|(p, v)| {
+                    if v.iter().sum::<i32>() == 3 {
+                        Some(p + 1) // 'a' == 1
+                    } else {
+                        None
+                    }
+                })
+                .take(1)
+                .collect::<Vec<_>>();
+
+            badges.push(badge[0]);
+
+            // reset the presence vector
+            triads_items = [[0; 3]; COUNT];
+        }
+    }
+
+    println!(
+        "first answer {}",
+        doubles
+            .iter()
+            .enumerate()
+            .map(|(p, c)| p * c)
+            .sum::<usize>()
+    );
+    println!("second answer {:?}", badges.iter().sum::<usize>());
+}