From 7a3c36000c95c95bd956c8b21ea43da889142109 Mon Sep 17 00:00:00 2001
From: Tyler Hallada <tyler@hallada.net>
Date: Wed, 3 Dec 2025 22:03:24 -0500
Subject: [PATCH] Solve day 3

---
 README.md                 |  1 +
 src/day03/input/test1.txt |  4 ++
 src/day03/mod.rs          | 93 +++++++++++++++++++++++++++++++++++++++
 src/days.rs               |  1 +
 src/lib.rs                |  1 +
 5 files changed, 100 insertions(+)
 create mode 100644 src/day03/input/test1.txt
 create mode 100644 src/day03/mod.rs

diff --git a/README.md b/README.md
index 56c0aea..449beae 100644
--- a/README.md
+++ b/README.md
@@ -34,3 +34,4 @@ Timings are given as: [lower-bound **best-estimate** upper-bound]
 |-----|--------|--------|
 | 01  | [101.34 µs **101.95 µs** 102.61 µs] | [105.90 µs **106.40 µs** 106.95 µs] |
 | 02  | [2.0990 ms **2.1113 ms** 2.1236 ms] | [2.0954 ms **2.1055 ms** 2.1157 ms] |
+| 03  | [623.89 µs **631.99 µs** 641.14 µs] | [786.95 µs **793.43 µs** 800.13 µs] |
diff --git a/src/day03/input/test1.txt b/src/day03/input/test1.txt
new file mode 100644
index 0000000..7255fca
--- /dev/null
+++ b/src/day03/input/test1.txt
@@ -0,0 +1,4 @@
+987654321111111
+811111111111119
+234234234234278
+818181911112111
diff --git a/src/day03/mod.rs b/src/day03/mod.rs
new file mode 100644
index 0000000..7918d50
--- /dev/null
+++ b/src/day03/mod.rs
@@ -0,0 +1,93 @@
+use color_eyre::{Result, eyre::eyre};
+use tracing::{debug, instrument};
+
+pub const INPUT: &str = include_str!("input/input.txt");
+
+#[derive(Debug, Clone, Copy)]
+struct Battery {
+    column: usize,
+    joltage: u32,
+}
+
+impl Default for Battery {
+    fn default() -> Self {
+        Battery {
+            column: 0,
+            joltage: 0,
+        }
+    }
+}
+
+fn largest_output_joltage<const N: usize>(input: &str) -> Result<u64> {
+    let mut output_joltage: u64 = 0;
+    for (line_num, line) in input.trim().split('\n').enumerate() {
+        let mut batteries: [Battery; N] = [Battery::default(); N];
+        let line_len = line.len();
+        for (column, c) in line.bytes().enumerate() {
+            let joltage = (c as char).to_digit(10).ok_or_else(|| {
+                eyre!(
+                    "Invalid character '{}' at line {}, column {}:\n  {}\n  {}^",
+                    c as char,
+                    line_num + 1,
+                    column + 1,
+                    line,
+                    " ".repeat(column)
+                )
+            })?;
+            let min = N.saturating_sub(line_len - column);
+            for i in min..N {
+                if joltage > batteries[i].joltage {
+                    batteries[i].column = column;
+                    batteries[i].joltage = joltage;
+                    batteries[i + 1..].fill(Battery::default());
+                    break;
+                }
+            }
+        }
+        debug!(line, batteries = ?batteries.map(|b| b.joltage));
+        output_joltage += batteries
+            .iter()
+            .fold(0u64, |acc, &b| acc * 10 + b.joltage as u64);
+    }
+    Ok(output_joltage)
+}
+
+#[instrument(skip(input))]
+pub fn part1(input: &str) -> Result<u64> {
+    largest_output_joltage::<2>(input)
+}
+
+#[instrument(skip(input))]
+pub fn part2(input: &str) -> Result<u64> {
+    largest_output_joltage::<12>(input)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use test_log::test;
+
+    const TEST_INPUT1: &str = include_str!("input/test1.txt");
+
+    #[test]
+    fn test_invalid_character_error() {
+        let input = "1234\n5678\n1234x678\n9012";
+        let result = part1(input);
+        assert!(result.is_err());
+        let error_msg = result.unwrap_err().to_string();
+        assert_eq!(
+            error_msg,
+            "Invalid character 'x' at line 3, column 5:\n  1234x678\n      ^"
+        );
+    }
+
+    #[test]
+    fn test_part1() {
+        assert_eq!(part1(TEST_INPUT1).unwrap(), 357);
+    }
+
+    #[test]
+    fn test_part2() {
+        assert_eq!(part2(TEST_INPUT1).unwrap(), 3121910778619);
+    }
+}
diff --git a/src/days.rs b/src/days.rs
index ec8584f..e3734d2 100644
--- a/src/days.rs
+++ b/src/days.rs
@@ -7,6 +7,7 @@ macro_rules! all_days {
         $macro_name! {
             1 => day01,
             2 => day02,
+            3 => day03,
         }
     };
 }
diff --git a/src/lib.rs b/src/lib.rs
index cd2013a..a000027 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,3 +1,4 @@
 pub mod day01;
 pub mod day02;
+pub mod day03;
 pub mod days;