From 3aff343f90a31c4dcf94c2f8348de211fd6d10fb Mon Sep 17 00:00:00 2001
From: timeshifter <timeshifter@sumatrae>
Date: Wed, 1 Jul 2026 19:31:30 +0200
Subject: [PATCH] add day 12

---
 day12/Cargo.lock  |   7 +++
 day12/Cargo.toml  |   6 ++
 day12/example.txt |   6 ++
 day12/input.txt   |  23 +++++++
 day12/src/main.rs | 154 ++++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 196 insertions(+)
 create mode 100644 day12/Cargo.lock
 create mode 100644 day12/Cargo.toml
 create mode 100644 day12/example.txt
 create mode 100644 day12/input.txt
 create mode 100644 day12/src/main.rs

diff --git a/day12/Cargo.lock b/day12/Cargo.lock
new file mode 100644
index 0000000..f8bfcac
--- /dev/null
+++ b/day12/Cargo.lock
@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "day12"
+version = "0.1.0"
diff --git a/day12/Cargo.toml b/day12/Cargo.toml
new file mode 100644
index 0000000..ed2fcd9
--- /dev/null
+++ b/day12/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "day12"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
diff --git a/day12/example.txt b/day12/example.txt
new file mode 100644
index 0000000..01a004b
--- /dev/null
+++ b/day12/example.txt
@@ -0,0 +1,6 @@
+cpy 41 a
+inc a
+inc a
+dec a
+jnz a 2
+dec a
diff --git a/day12/input.txt b/day12/input.txt
new file mode 100644
index 0000000..3729d83
--- /dev/null
+++ b/day12/input.txt
@@ -0,0 +1,23 @@
+cpy 1 a
+cpy 1 b
+cpy 26 d
+jnz c 2
+jnz 1 5
+cpy 7 c
+inc d
+dec c
+jnz c -2
+cpy a c
+inc a
+dec b
+jnz b -2
+cpy c b
+dec d
+jnz d -6
+cpy 19 c
+cpy 11 d
+inc a
+dec d
+jnz d -2
+dec c
+jnz c -5
diff --git a/day12/src/main.rs b/day12/src/main.rs
new file mode 100644
index 0000000..5f5c642
--- /dev/null
+++ b/day12/src/main.rs
@@ -0,0 +1,154 @@
+#[cfg(debug_assertions)]
+const FILENAME: &str = "example.txt";
+#[cfg(not(debug_assertions))]
+const FILENAME: &str = "input.txt";
+
+fn main() {
+    let program: Vec<_> = std::fs::read_to_string(FILENAME)
+        .unwrap()
+        .lines()
+        .map(Instruction::parse)
+        .collect();
+
+    part1(&program);
+    part2(&program);
+}
+
+struct Registers {
+    registers: [usize; 4],
+}
+
+impl Registers {
+    fn set(&mut self, register: char, source: &Source) {
+        let address = Self::get_address(register);
+        match source {
+            Source::Register(from_register) => {
+                let from_address = Self::get_address(*from_register);
+                self.registers[address] = self.registers[from_address];
+            }
+            Source::Value(value) => self.registers[address] = *value,
+        }
+    }
+
+    fn inc(&mut self, register: char) {
+        let address = Self::get_address(register);
+        self.registers[address] += 1;
+    }
+
+    fn dec(&mut self, register: char) {
+        let address = Self::get_address(register);
+        self.registers[address] -= 1;
+    }
+
+    fn is_nonzero(&self, value: &Source) -> bool {
+        match value {
+            Source::Register(register) => {
+                let address = Self::get_address(*register);
+                self.registers[address] != 0
+            }
+            Source::Value(value) => *value != 0,
+        }
+    }
+
+    fn get_address(register: char) -> usize {
+        match register {
+            'a' => 0,
+            'b' => 1,
+            'c' => 2,
+            'd' => 3,
+            _ => panic!("unknown register: {register}"),
+        }
+    }
+
+    fn new() -> Self {
+        Self { registers: [0; 4] }
+    }
+
+    fn value_a(&self) -> usize {
+        let address = Self::get_address('a');
+        self.registers[address]
+    }
+}
+
+fn part1(program: &[Instruction]) {
+    let mut registers = Registers::new();
+
+    run_program(program, &mut registers);
+    println!("{}", registers.value_a());
+}
+
+fn part2(program: &[Instruction]) {
+    let mut registers = Registers::new();
+
+    registers.set('c', &Source::Value(1));
+
+    run_program(program, &mut registers);
+    println!("{}", registers.value_a());
+}
+
+fn run_program(program: &[Instruction], registers: &mut Registers) {
+    let mut pointer = 0;
+
+    while let Some(instruction) = program.get(pointer) {
+        match instruction {
+            Instruction::Cpy(source, register) => {
+                registers.set(*register, source);
+                pointer += 1;
+            }
+            Instruction::Inc(register) => {
+                registers.inc(*register);
+                pointer += 1;
+            }
+            Instruction::Dec(register) => {
+                registers.dec(*register);
+                pointer += 1;
+            }
+            Instruction::Jnz(register, offset) => {
+                if registers.is_nonzero(register) {
+                    pointer = (pointer as isize + offset) as usize;
+                } else {
+                    pointer += 1;
+                }
+            }
+        }
+    }
+}
+
+#[derive(Debug)]
+enum Instruction {
+    Cpy(Source, char),
+    Inc(char),
+    Dec(char),
+    Jnz(Source, isize),
+}
+
+#[derive(Debug)]
+enum Source {
+    Register(char),
+    Value(usize),
+}
+
+impl Source {
+    fn parse(s: &str) -> Self {
+        let first_char = s.chars().next().unwrap();
+        if first_char.is_ascii_digit() {
+            Self::Value(s.parse().unwrap())
+        } else {
+            Self::Register(first_char)
+        }
+    }
+}
+
+impl Instruction {
+    fn parse(s: &str) -> Self {
+        let words: Vec<_> = s.split_whitespace().collect();
+        dbg!(&words);
+        match words[0] {
+            "cpy" => Self::Cpy(Source::parse(words[1]), words[2].chars().next().unwrap()),
+            "inc" => Self::Inc(words[1].chars().next().unwrap()),
+            "dec" => Self::Dec(words[1].chars().next().unwrap()),
+            "jnz" => Self::Jnz(Source::parse(words[1]), words[2].parse().unwrap()),
+            _ => panic!("unknown pattern: {s}"),
+        }
+    }
+}