From c8e03e6d6d782031dacb5291a05304b5c83cf700 Mon Sep 17 00:00:00 2001
From: Keenan Tims <ktims@gotroot.ca>
Date: Sat, 6 Dec 2025 00:44:36 -0800
Subject: [PATCH] day6: state machine solution, much cleaner

---
 src/day6.rs | 114 +++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 109 insertions(+), 5 deletions(-)

diff --git a/src/day6.rs b/src/day6.rs
index 264f7b0..7ff071e 100644
--- a/src/day6.rs
+++ b/src/day6.rs
@@ -1,9 +1,11 @@
 use aoc_runner_derive::{aoc, aoc_generator};
 use grid::Grid;
 use itertools::Itertools;
+use misc::POW10;
 use std::{
     fmt::{Display, Write},
     iter::repeat_n,
+    str::FromStr,
 };
 
 #[repr(u8)]
@@ -22,6 +24,16 @@ impl From<&str> for Op {
     }
 }
 
+impl From<&u8> for Op {
+    fn from(value: &u8) -> Self {
+        match value {
+            b'+' => Op::Add,
+            b'*' => Op::Mul,
+            c => panic!("Invalid op `{c}`"),
+        }
+    }
+}
+
 impl Display for Op {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
@@ -32,7 +44,7 @@ impl Display for Op {
 }
 
 impl Op {
-    fn f(&self, lhs: u64, rhs: u64) -> u64 {
+    fn f(&self, lhs: &u64, rhs: &u64) -> u64 {
         match self {
             Op::Add => lhs + rhs,
             Op::Mul => lhs * rhs,
@@ -71,7 +83,7 @@ fn part1((grid, ops): &(Grid<u64>, Vec<Op>)) -> u64 {
             grid.col_iter(x as i64)
                 .unwrap()
                 .cloned()
-                .reduce(|l, r| ops[x].f(l, r))
+                .reduce(|l, r| ops[x].f(&l, &r))
                 .unwrap()
         })
         .sum()
@@ -85,7 +97,7 @@ fn part1((grid, ops): &(Grid<u64>, Vec<Op>)) -> u64 {
 //         .collect()
 // }
 
-#[aoc_generator(day6, part2)]
+#[aoc_generator(day6, part2, Spaghett)]
 fn parse2(input: &str) -> (Grid<Vec<char>>, Vec<Op>) {
     let mut rows = input.lines().collect_vec();
     let ops = rows.pop().unwrap();
@@ -126,7 +138,7 @@ fn parse2(input: &str) -> (Grid<Vec<char>>, Vec<Op>) {
     (grid, ops)
 }
 
-#[aoc(day6, part2)]
+#[aoc(day6, part2, Spaghett)]
 fn part2((grid, ops): &(Grid<Vec<char>>, Vec<Op>)) -> u64 {
     let mut columns = Vec::new();
     for col in 0..grid.width() {
@@ -151,10 +163,97 @@ fn part2((grid, ops): &(Grid<Vec<char>>, Vec<Op>)) -> u64 {
     columns
         .iter()
         .enumerate()
-        .map(|(col, v)| v.iter().cloned().reduce(|l, r| ops[col].f(l, r)).unwrap())
+        .map(|(col, v)| v.iter().cloned().reduce(|l, r| ops[col].f(&l, &r)).unwrap())
         .sum::<u64>()
 }
 
+#[aoc_generator(day6, part2, StateMachine)]
+fn parse2_walk(input: &str) -> Grid<u8> {
+    Grid::from_str(input).unwrap()
+}
+
+enum NextPos {
+    Advance((usize, usize)),
+    NewCol((usize, usize)),
+    Done,
+}
+
+#[aoc(day6, part2, StateMachine)]
+fn part2_walk(g: &Grid<u8>) -> u64 {
+    let mut accum = 0;
+
+    // start at the bottom right
+    let mut pos = (g.width() - 1, g.height() - 1);
+    let mut digits = 0;
+    let mut cur_value = 0;
+    let mut cur_values = Vec::new();
+    let mut cur_op = Op::Add;
+    let mut last_col = false;
+
+    let height = g.height() - 1;
+
+    let next_pos = |(x, y)| -> NextPos {
+        if y == 0 {
+            if x == 0 {
+                return NextPos::Done;
+            }
+            return NextPos::NewCol((x - 1, height));
+        }
+        NextPos::Advance((x, y - 1))
+    };
+
+    while let Some(cur_char) = g.get(&pos) {
+        match cur_char {
+            b'*' | b'+' => {
+                last_col = true;
+                cur_op = cur_char.into()
+            }
+            cur_char if cur_char.is_ascii_digit() => {
+                cur_value += (*cur_char - b'0') as u64 * POW10[digits];
+                digits += 1;
+            }
+            _ => {}
+        }
+        pos = match next_pos(pos) {
+            NextPos::Advance(p) => p,
+            NextPos::NewCol(p) => {
+                // add the value
+                if cur_value != 0 {
+                    cur_values.push(cur_value);
+                    cur_value = 0;
+                }
+                digits = 0;
+                // this was a last col, so reduce and accumulate
+                if last_col {
+                    accum += cur_values
+                        .iter()
+                        .cloned()
+                        .reduce(|l, r| cur_op.f(&l, &r))
+                        .unwrap_or(0);
+                    // clear the state
+                    cur_values.clear();
+                    last_col = false;
+                }
+                p
+            }
+            NextPos::Done => {
+                if cur_value != 0 {
+                    cur_values.push(cur_value);
+                }
+                // accumulate the final result
+                accum += cur_values
+                    .iter()
+                    .cloned()
+                    .reduce(|l, r| cur_op.f(&l, &r))
+                    .unwrap_or(0);
+                return accum;
+            }
+        }
+    }
+
+    unreachable!()
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -173,4 +272,9 @@ mod tests {
     fn part2_example() {
         assert_eq!(part2(&parse2(EXAMPLE)), 3263827);
     }
+
+    #[test]
+    fn part2_walk_example() {
+        assert_eq!(part2_walk(&parse2_walk(EXAMPLE)), 3263827);
+    }
 }