From 2462a30b1d6bfe73a22bd9f01adac9d576c01178 Mon Sep 17 00:00:00 2001
From: George Thomas <georgefsthomas@gmail.com>
Date: Mon, 12 Jan 2026 18:23:58 +0000
Subject: [PATCH] Solve day 7 in Rust

---
 rust/main.rs         |  4 +-
 rust/puzzles/day7.rs | 91 ++++++++++++++++++++++++++++++++++++++++++++
 rust/puzzles/mod.rs  |  1 +
 3 files changed, 95 insertions(+), 1 deletion(-)
 create mode 100644 rust/puzzles/day7.rs

diff --git a/rust/main.rs b/rust/main.rs
index 58ff396..62a945c 100644
--- a/rust/main.rs
+++ b/rust/main.rs
@@ -7,16 +7,18 @@ use puzzles::day3;
 use puzzles::day4;
 use puzzles::day5;
 use puzzles::day6;
+use puzzles::day7;
 use std::fs;
 use std::time::{Duration, Instant};
 
-const PUZZLES: [&dyn SomePuzzle; 6] = [
+const PUZZLES: [&dyn SomePuzzle; 7] = [
     &day1::PUZZLE,
     &day2::PUZZLE,
     &day3::PUZZLE,
     &day4::PUZZLE,
     &day5::PUZZLE,
     &day6::PUZZLE,
+    &day7::PUZZLE,
 ];
 
 fn main() {
diff --git a/rust/puzzles/day7.rs b/rust/puzzles/day7.rs
new file mode 100644
index 0000000..e501fac
--- /dev/null
+++ b/rust/puzzles/day7.rs
@@ -0,0 +1,91 @@
+use crate::puzzle::Puzzle;
+use itertools::Itertools;
+use nom::{
+    Parser,
+    branch::alt,
+    character::complete::{char, newline},
+    combinator::{eof, value},
+    error::Error,
+    multi::{many1, separated_list1},
+    sequence::terminated,
+};
+use std::collections::{HashMap, HashSet};
+
+pub const PUZZLE: Puzzle<(usize, Vec<HashSet<usize>>), 2> = Puzzle {
+    number: 7,
+    parser: |input| {
+        terminated::<_, _, Error<&str>, _, _>(
+            terminated(
+                (
+                    terminated(
+                        many1(alt((value(false, char('.')), value(true, char('S'))))),
+                        newline,
+                    )
+                    .map_opt(|v| v.into_iter().position(|b| b)),
+                    separated_list1(
+                        newline,
+                        many1(alt((value(false, char('.')), value(true, char('^'))))),
+                    )
+                    .map(|rows| {
+                        rows.into_iter()
+                            .map(|row| {
+                                row.into_iter()
+                                    .enumerate()
+                                    .filter(|(_, b)| *b)
+                                    .map(|(i, _)| i)
+                                    .collect()
+                            })
+                            .collect()
+                    }),
+                ),
+                newline,
+            ),
+            eof,
+        )
+        .parse(input)
+        .unwrap()
+        .1
+    },
+    parts: [
+        |(start, splitter_rows)| {
+            let mut beams = HashSet::from([*start]);
+            let mut count = 0;
+            for splitters in splitter_rows {
+                beams = beams
+                    .iter()
+                    .flat_map(|x| {
+                        if splitters.contains(x) {
+                            count += 1;
+                            vec![x - 1, x + 1]
+                        } else {
+                            vec![*x]
+                        }
+                    })
+                    .collect();
+            }
+            count.to_string()
+        },
+        |(start, splitter_rows)| {
+            let mut beams = HashMap::from([(*start, 1)]);
+            for splitters in splitter_rows {
+                beams = beams
+                    .into_iter()
+                    .flat_map(|(x, n)| {
+                        if splitters.contains(&x) {
+                            vec![x - 1, x + 1]
+                        } else {
+                            vec![x]
+                        }
+                        .into_iter()
+                        .map(|x1| (x1, n))
+                        .collect_vec()
+                    })
+                    .into_group_map()
+                    .into_iter()
+                    .map(|(k, vs)| (k, vs.into_iter().sum()))
+                    .collect();
+            }
+            beams.values().sum::<usize>().to_string()
+        },
+    ],
+};
diff --git a/rust/puzzles/mod.rs b/rust/puzzles/mod.rs
index 92a3a4b..1613335 100644
--- a/rust/puzzles/mod.rs
+++ b/rust/puzzles/mod.rs
@@ -4,3 +4,4 @@ pub(crate) mod day3;
 pub(crate) mod day4;
 pub(crate) mod day5;
 pub(crate) mod day6;
+pub(crate) mod day7;