From 71dc5aef5950352d0fb7d8fff44937234c86b570 Mon Sep 17 00:00:00 2001
From: LongHairedHacker <sebastian@sebastians-site.de>
Date: Fri, 18 Dec 2020 15:33:43 +0100
Subject: [PATCH] Day 18 part 1 works

---
 inputs/day18.txt | 378 +++++++++++++++++++++++++++++++++++++++++++++++
 src/bin/day18.rs | 113 ++++++++++++++
 2 files changed, 491 insertions(+)
 create mode 100644 inputs/day18.txt
 create mode 100644 src/bin/day18.rs

diff --git a/inputs/day18.txt b/inputs/day18.txt
new file mode 100644
index 0000000..07a3ba6
--- /dev/null
+++ b/inputs/day18.txt
@@ -0,0 +1,378 @@
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diff --git a/src/bin/day18.rs b/src/bin/day18.rs
new file mode 100644
index 0000000..c229950
--- /dev/null
+++ b/src/bin/day18.rs
@@ -0,0 +1,113 @@
+use std::error::Error;
+use std::fs::File;
+use std::io::{self, BufRead};
+use std::vec::Vec;
+
+fn extract_paren_expression(input: &str) -> String {
+    let mut res: String = "".to_string();
+    let mut open_count = 1;
+    let mut close_count = 0;
+
+    for c in input[1..].chars() {
+        match c {
+            '(' => open_count += 1,
+            ')' => {
+                close_count += 1;
+                if open_count == close_count {
+                    return res;
+                }
+            }
+            _ => {}
+        }
+        res += &c.to_string();
+    }
+
+    panic!(
+        "Encountered unablanced parentheses {} vs. {}",
+        open_count, close_count
+    );
+}
+
+fn extract_number(input: &str) -> String {
+    input.chars().take_while(|c| c.is_ascii_digit()).collect()
+}
+
+fn eval_expression(input: &str) -> u64 {
+    let mut clean: String = input.chars().filter(|c| *c != ' ').collect();
+    let mut res = 0;
+    let mut op = '^';
+
+    while clean != "" {
+        let c = clean.chars().nth(0).unwrap();
+        if c.is_ascii_digit() {
+            let substr = extract_number(&clean);
+            clean = clean[substr.len()..].to_string();
+            let num: u64 = substr.parse().unwrap();
+            match op {
+                '^' => res = num,
+                '+' => res += num,
+                '*' => res *= num,
+                _ => panic!("Unsupported Op: {}", op),
+            }
+        } else if c == '+' || c == '*' {
+            op = c;
+            clean = clean[1..].to_string();
+        } else if c == '(' {
+            let substr = extract_paren_expression(&clean);
+            clean = clean[substr.len() + 2..].to_string();
+            let num = eval_expression(&substr);
+            match op {
+                '^' => res = num,
+                '+' => res += num,
+                '*' => res *= num,
+                _ => panic!("Unsupported Op: {}", op),
+            }
+        }
+    }
+
+    res
+}
+
+fn main() -> Result<(), Box<dyn Error>> {
+    let file = File::open("inputs/day18.txt")?;
+    let lines = io::BufReader::new(file).lines().map(|l| l.unwrap());
+
+    let res = lines.map(|l| eval_expression(&l)).fold(0, |x, y| x + y);
+    println!("Answer: {}", res);
+
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::*;
+
+    #[test]
+    fn test_extractor() {
+        assert_eq!(
+            extract_paren_expression("((2 + 4 * 9) * (6 + 9 * 8 + 6) + 6) + 2 + 4 * 2"),
+            "(2 + 4 * 9) * (6 + 9 * 8 + 6) + 6"
+        );
+    }
+
+    #[test]
+    fn test_basics() {
+        assert_eq!(eval_expression("23"), 23);
+        assert_eq!(eval_expression("2 + 3 * 4"), 20);
+        assert_eq!(eval_expression("2 + (3 * 4)"), 14);
+    }
+
+    #[test]
+    fn test_problems() {
+        assert_eq!(eval_expression("2 * 3 + (4 * 5)"), 26);
+        assert_eq!(eval_expression("5 + (8 * 3 + 9 + 3 * 4 * 3)"), 437);
+        assert_eq!(
+            eval_expression("5 * 9 * (7 * 3 * 3 + 9 * 3 + (8 + 6 * 4))"),
+            12240
+        );
+        assert_eq!(
+            eval_expression("((2 + 4 * 9) * (6 + 9 * 8 + 6) + 6) + 2 + 4 * 2"),
+            13632
+        );
+    }
+}