From caf307bf2d45725ff691e48e216128c17ddafd3e Mon Sep 17 00:00:00 2001
From: Johannes <freelon@gmx.net>
Date: Sun, 12 Nov 2023 23:12:30 +0100
Subject: [PATCH] Day 19 part 2 (manually reverse engineered the assembler
 instructions).

---
 src/main.rs        |  2 +-
 src/tasks/day19.rs | 49 ++++++++++++++++++++++++++++++++++++++++++----
 2 files changed, 46 insertions(+), 5 deletions(-)

diff --git a/src/main.rs b/src/main.rs
index 4fc4acd..9d553c6 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,3 +1,3 @@
 fn main() {
-    aoc_2018::tasks::day19::task1();
+    aoc_2018::tasks::day19::task2();
 }
diff --git a/src/tasks/day19.rs b/src/tasks/day19.rs
index 40f63d7..cb82e1b 100644
--- a/src/tasks/day19.rs
+++ b/src/tasks/day19.rs
@@ -2,25 +2,64 @@ use std::ops::{BitAnd, BitOr};
 
 pub fn task1() {
     let input = include_str!("../../input/day19.txt");
-    let result = run1(input);
+    let result = run1(input, 1);
 
     println!("Day 19 part 1: {result}");
 }
 
-fn run1(input: &str) -> i32 {
+pub fn task2() {
+    let mut a = 1i64;
+    let mut d = 948;
+    if a == 1 {
+        d += 10550400;
+        a = 0;
+    }
+
+    let mut f = 1;
+    'outer: loop {
+        let mut c = 1;
+        'inner: loop {
+            let b = f * c;
+            if b == d {
+                a = a + f;
+            } else if b > d {
+                // this is the important part: break the loop because otherwise we count and count
+                // and count ... but cannot reach the b == d condition without resetting c.
+                break 'inner;
+            }
+            c += 1;
+            if c > d {
+                break 'inner;
+            }
+        }
+        f += 1;
+        if f > d {
+            break 'outer;
+        }
+    }
+
+    println!("Day 19 part 2: {a}");
+}
+
+fn run1(input: &str, start_value: i32) -> i32 {
     let (config, program) = input.split_once("\n").unwrap();
     let ip_register: usize = config.split_once(" ").unwrap().1.parse().unwrap();
 
     let program: Vec<Instruction> = program.lines().map(parse_instruction).collect();
 
-    let mut registers: Registers = [0i32; 6];
+    let mut registers: Registers = [start_value, 0, 0, 0, 0, 0];
     let mut ip = 0;
+    let mut count = 0;
     loop {
         //print!("ip={ip}, {registers:?}");
 
         registers[ip_register] = ip as i32;
         let instruction = program.get(ip).unwrap();
 
+        if ip == 2 {
+            println!("{registers:?}");
+        }
+
         //print!("{instruction:?}");
 
         let op: Op = instruction.opcode;
@@ -34,7 +73,9 @@ fn run1(input: &str) -> i32 {
         if program.get(ip).is_none() {
             break;
         }
+        count += 1;
     }
+    println!("executed instructions: {count}");
     registers[0]
 }
 
@@ -197,6 +238,6 @@ mod test {
                           setr 1 0 0\n\
                           seti 8 0 4\n\
                           seti 9 0 5";
-        assert_eq!(run1(input), 6);
+        assert_eq!(run1(input, 0), 6);
     }
 }