From ca302578c9083da72b93dc09e87a931da4c91d52 Mon Sep 17 00:00:00 2001
From: Russell Champoux <Russell.Champoux@coupa.com>
Date: Tue, 9 Dec 2025 15:57:20 -0500
Subject: [PATCH] Solve Day 7: Tachyon beam splits and quantum timelines

---
 README.md            |   3 +-
 data/examples/07.txt |  17 ++++++
 src/bin/07.rs        | 132 +++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 151 insertions(+), 1 deletion(-)
 create mode 100644 data/examples/07.txt
 create mode 100644 src/bin/07.rs

diff --git a/README.md b/README.md
index d82020c..2c8d7e8 100644
--- a/README.md
+++ b/README.md
@@ -17,8 +17,9 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 | [Day 4](./src/bin/04.rs) | `244.2µs` | `4.1ms` |
 | [Day 5](./src/bin/05.rs) | `196.7µs` | `66.3µs` |
 | [Day 6](./src/bin/06.rs) | `109.0µs` | `194.2µs` |
+| [Day 7](./src/bin/07.rs) | `270.5µs` | `320.2µs` |
 
-**Total: 5.79ms**
+**Total: 6.38ms**
 <!--- benchmarking table --->
 
 ---
diff --git a/data/examples/07.txt b/data/examples/07.txt
new file mode 100644
index 0000000..5c4186a
--- /dev/null
+++ b/data/examples/07.txt
@@ -0,0 +1,17 @@
+.......S.......
+...............
+.......^.......
+...............
+......^.^......
+...............
+.....^.^.^.....
+...............
+....^.^...^....
+...............
+...^.^...^.^...
+...............
+..^...^.....^..
+...............
+.^.^.^.^.^...^.
+...............
+
diff --git a/src/bin/07.rs b/src/bin/07.rs
new file mode 100644
index 0000000..e856bba
--- /dev/null
+++ b/src/bin/07.rs
@@ -0,0 +1,132 @@
+use std::collections::{HashMap, HashSet};
+
+advent_of_code::solution!(7);
+
+/// Parse the grid and find the starting column
+fn parse_grid(input: &str) -> (Vec<Vec<char>>, usize) {
+    let grid: Vec<Vec<char>> = input
+        .lines()
+        .filter(|l| !l.is_empty())
+        .map(|l| l.chars().collect())
+        .collect();
+
+    let start_col = grid[0]
+        .iter()
+        .position(|&c| c == 'S')
+        .expect("No starting position 'S' found");
+
+    (grid, start_col)
+}
+
+/// Simulate tachyon beams through the manifold and count splits
+/// Beams merge when they occupy the same column (HashSet)
+fn count_splits(input: &str) -> u64 {
+    let (grid, start_col) = parse_grid(input);
+    if grid.is_empty() {
+        return 0;
+    }
+
+    let width = grid[0].len();
+    let mut beams: HashSet<usize> = HashSet::new();
+    beams.insert(start_col);
+
+    let mut split_count: u64 = 0;
+
+    for row in grid.iter().skip(1) {
+        let mut new_beams: HashSet<usize> = HashSet::new();
+
+        for &col in &beams {
+            match row[col] {
+                '^' => {
+                    split_count += 1;
+                    if col > 0 {
+                        new_beams.insert(col - 1);
+                    }
+                    if col + 1 < width {
+                        new_beams.insert(col + 1);
+                    }
+                }
+                _ => {
+                    new_beams.insert(col);
+                }
+            }
+        }
+
+        beams = new_beams;
+        if beams.is_empty() {
+            break;
+        }
+    }
+
+    split_count
+}
+
+/// Count the number of unique timelines (paths) through the manifold
+/// Each particle at a position represents independent timelines - they don't merge
+fn count_timelines(input: &str) -> u64 {
+    let (grid, start_col) = parse_grid(input);
+    if grid.is_empty() {
+        return 0;
+    }
+
+    let width = grid[0].len();
+
+    // Track number of particles (timelines) at each column position
+    let mut particles: HashMap<usize, u64> = HashMap::new();
+    particles.insert(start_col, 1);
+
+    for row in grid.iter().skip(1) {
+        let mut new_particles: HashMap<usize, u64> = HashMap::new();
+
+        for (&col, &count) in &particles {
+            match row[col] {
+                '^' => {
+                    // Each particle splits into 2 timelines (left and right)
+                    if col > 0 {
+                        *new_particles.entry(col - 1).or_insert(0) += count;
+                    }
+                    if col + 1 < width {
+                        *new_particles.entry(col + 1).or_insert(0) += count;
+                    }
+                }
+                _ => {
+                    // Particle continues straight down
+                    *new_particles.entry(col).or_insert(0) += count;
+                }
+            }
+        }
+
+        particles = new_particles;
+        if particles.is_empty() {
+            break;
+        }
+    }
+
+    // Sum all particles - each represents a unique timeline
+    particles.values().sum()
+}
+
+pub fn part_one(input: &str) -> Option<u64> {
+    Some(count_splits(input))
+}
+
+pub fn part_two(input: &str) -> Option<u64> {
+    Some(count_timelines(input))
+}
+
+#[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(21));
+    }
+
+    #[test]
+    fn test_part_two() {
+        let result = part_two(&advent_of_code::template::read_file("examples", DAY));
+        assert_eq!(result, Some(40));
+    }
+}