From d80911a0cf243cc3439497fc35bcd420d3ee911c Mon Sep 17 00:00:00 2001
From: Wesley Moore <wes@wezm.net>
Date: Mon, 2 Dec 2019 21:45:25 +1100
Subject: [PATCH] Day 2 part 2

---
 2019/src/bin/day2.rs | 43 +++++++++++++++++++++++++++++++++++--------
 1 file changed, 35 insertions(+), 8 deletions(-)

diff --git a/2019/src/bin/day2.rs b/2019/src/bin/day2.rs
index 5b88abb..1d5cb97 100644
--- a/2019/src/bin/day2.rs
+++ b/2019/src/bin/day2.rs
@@ -4,20 +4,32 @@ use advent_of_code::input;
 
 fn main() -> io::Result<()> {
     let input = fs::read_to_string("input/day2.txt")?;
-    let mut program = input::read_separated_line(',', &input)?;
+    let program = input::read_separated_line(',', &input)?;
 
     // To do this, before running the program, replace position 1 with the value 12 and replace
     // position 2 with the value 2. What value is left at position 0 after the program halts?
-    program[1] = 12;
-    program[2] = 2;
-    run_program(&mut program);
+    let mut part1 = program.clone();
+    run_program(&mut part1, 12, 2);
 
-    println!("Part 1: {}", program[0]);
+    println!("Part 1: {}", part1[0]);
+
+    // Part 2
+    // Determine what pair of inputs produces the output 19690720."
+    // Each of the two input values will be between 0 and 99, inclusive
+    let (noun, verb) = part2(19690720, &program);
+    println!(
+        "Part 2: noun={} verb={} result={}",
+        noun,
+        verb,
+        100 * noun + verb
+    );
 
     Ok(())
 }
 
-fn run_program(program: &mut [i32]) {
+fn run_program(program: &mut [i32], noun: i32, verb: i32) {
+    program[1] = noun;
+    program[2] = verb;
     let mut addr = 0;
 
     loop {
@@ -38,6 +50,21 @@ fn run_program(program: &mut [i32]) {
     }
 }
 
+fn part2(target: i32, program: &[i32]) -> (i32, i32) {
+    for noun in 0..=99 {
+        for verb in 0..=99 {
+            let mut candidate = program.to_vec();
+            run_program(&mut candidate, noun, verb);
+
+            if candidate[0] == target {
+                return (noun, verb);
+            }
+        }
+    }
+
+    panic!("Did not find solution");
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -47,7 +74,7 @@ mod tests {
         let input = "1,0,0,0,99";
         let mut program = input::read_separated_line(',', &input).unwrap();
 
-        run_program(&mut program);
+        run_program(&mut program, 0, 0);
 
         assert_eq!(program, &[2, 0, 0, 0, 99])
     }
@@ -56,7 +83,7 @@ mod tests {
         let input = "1,1,1,4,99,5,6,0,99";
         let mut program = input::read_separated_line(',', &input).unwrap();
 
-        run_program(&mut program);
+        run_program(&mut program, 1, 1);
 
         assert_eq!(program, &[30, 1, 1, 4, 2, 5, 6, 0, 99])
     }