From 3daa7a1ad11faacea165b394c63ff575fa2142ad Mon Sep 17 00:00:00 2001
From: George Thomas <georgefsthomas@gmail.com>
Date: Fri, 9 Jan 2026 13:39:14 +0000
Subject: [PATCH] Solve day 6 in Rust

---
 rust/main.rs         |   4 +-
 rust/puzzles/day6.rs | 137 +++++++++++++++++++++++++++++++++++++++++++
 rust/puzzles/mod.rs  |   1 +
 3 files changed, 141 insertions(+), 1 deletion(-)
 create mode 100644 rust/puzzles/day6.rs

diff --git a/rust/main.rs b/rust/main.rs
index 96909a8..58ff396 100644
--- a/rust/main.rs
+++ b/rust/main.rs
@@ -6,15 +6,17 @@ use puzzles::day2;
 use puzzles::day3;
 use puzzles::day4;
 use puzzles::day5;
+use puzzles::day6;
 use std::fs;
 use std::time::{Duration, Instant};
 
-const PUZZLES: [&dyn SomePuzzle; 5] = [
+const PUZZLES: [&dyn SomePuzzle; 6] = [
     &day1::PUZZLE,
     &day2::PUZZLE,
     &day3::PUZZLE,
     &day4::PUZZLE,
     &day5::PUZZLE,
+    &day6::PUZZLE,
 ];
 
 fn main() {
diff --git a/rust/puzzles/day6.rs b/rust/puzzles/day6.rs
new file mode 100644
index 0000000..e7643df
--- /dev/null
+++ b/rust/puzzles/day6.rs
@@ -0,0 +1,137 @@
+use crate::puzzle::Puzzle;
+use itertools::Itertools;
+use nom::{
+    Parser,
+    branch::alt,
+    character::complete::{char, newline, satisfy, space1},
+    combinator::{eof, value},
+    error::Error,
+    multi::{many1, separated_list1},
+    sequence::terminated,
+};
+use std::ops::{Add, Mul};
+
+pub const PUZZLE: Puzzle<(Vec<Op>, Vec<Vec<Option<u8>>>), 2> = Puzzle {
+    number: 6,
+    parser: |input| {
+        terminated::<_, _, Error<&str>, _, _>(
+            (
+                terminated(
+                    separated_list1(
+                        newline,
+                        many1(alt((
+                            satisfy(|c| c.is_digit(10)).map(|c| Some(c as u8 - '0' as u8)),
+                            value(None, char(' ')),
+                        ))),
+                    ),
+                    newline,
+                ),
+                terminated(
+                    terminated(
+                        separated_list1(
+                            space1,
+                            alt((value(Op::Multiply, char('*')), value(Op::Add, char('+')))),
+                        ),
+                        space1,
+                    ),
+                    newline,
+                ),
+            ),
+            eof,
+        )
+        .map(|(grid, ops)| (ops, grid))
+        .parse(input)
+        .unwrap()
+        .1
+    },
+    parts: [
+        |(ops, grid)| {
+            transpose(
+                &grid
+                    .iter()
+                    .map(|row| {
+                        row.group_somes()
+                            .iter()
+                            .filter(|v| !v.is_empty())
+                            .map(|digits| digits_to_int(&digits))
+                            .collect()
+                    })
+                    .collect::<Vec<_>>(),
+            )
+            .iter()
+            .zip(ops.iter())
+            .map(|(list, op)| op.apply_to_list(list))
+            .sum::<usize>()
+            .to_string()
+        },
+        |(ops, grid)| {
+            transpose(grid)
+                .iter()
+                .map(|l| {
+                    if l.iter().all(|x| x.is_none()) {
+                        None
+                    } else {
+                        Some(digits_to_int(
+                            &l.iter().filter_map(|x| *x).collect::<Vec<_>>(),
+                        ))
+                    }
+                })
+                .collect::<Vec<_>>()
+                .group_somes()
+                .iter()
+                .zip(ops.iter())
+                .map(|(list, op)| op.apply_to_list(list))
+                .sum::<usize>()
+                .to_string()
+        },
+    ],
+};
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum Op {
+    Add,
+    Multiply,
+}
+impl Op {
+    fn apply<N: Add<Output = N> + Mul<Output = N>>(self, a: N, b: N) -> N {
+        match self {
+            Op::Add => a + b,
+            Op::Multiply => a * b,
+        }
+    }
+    fn unit<N: From<u8>>(self) -> N {
+        match self {
+            Op::Add => N::from(0),
+            Op::Multiply => N::from(1),
+        }
+    }
+    fn apply_to_list(self, list: &[usize]) -> usize {
+        list.iter().fold(self.unit(), |acc, &x| self.apply(acc, x))
+    }
+}
+
+trait GroupSomes<T> {
+    fn group_somes(&self) -> Vec<Vec<T>>;
+}
+impl<T: Clone> GroupSomes<T> for [Option<T>] {
+    fn group_somes(&self) -> Vec<Vec<T>> {
+        self.iter()
+            .chunk_by(|x| x.is_some())
+            .into_iter()
+            .filter_map(|(is_some, group)| {
+                is_some.then(|| group.filter_map(|x| x.clone()).collect())
+            })
+            .collect()
+    }
+}
+
+fn transpose<T: Clone>(grid: &[Vec<T>]) -> Vec<Vec<T>> {
+    let w = grid.iter().map(|r| r.len()).max().unwrap_or(0);
+    (0..w)
+        .map(|x| grid.iter().filter_map(|r| r.get(x).cloned()).collect())
+        .collect()
+}
+
+fn digits_to_int(digits: &[u8]) -> usize {
+    digits.iter().fold(0, |acc, &d| acc * 10 + (d as usize))
+}
diff --git a/rust/puzzles/mod.rs b/rust/puzzles/mod.rs
index f5551d6..92a3a4b 100644
--- a/rust/puzzles/mod.rs
+++ b/rust/puzzles/mod.rs
@@ -3,3 +3,4 @@ pub(crate) mod day2;
 pub(crate) mod day3;
 pub(crate) mod day4;
 pub(crate) mod day5;
+pub(crate) mod day6;