1 files changed, 63 insertions, 0 deletions

diff --git a/2022/21/b.rs b/2022/21/b.rs
new file mode 100644
index 0000000..e674dd4
--- /dev/null
+++ b/2022/21/b.rs
@@ -0,0 +1,63 @@
+mod common;
+use common::*;
+
+fn solve_for_humn(name: &str, n1: i64, monkeys: &Vec<Monkey>) -> i64 {
+    if name == "humn" { return n1; }
+
+    let i = monkeys.iter().position(|m| m.name == name).unwrap();
+    if let MonkeyType::Op(op) = &monkeys[i].kind {
+        if let Some(n2) = result_or_unknown(&op.m1, &monkeys) {
+            // n1 = n2 [op] [unknown]
+            match op.op {
+                '+' => solve_for_humn(&op.m2, n1-n2, &monkeys),
+                '-' => solve_for_humn(&op.m2, n2-n1, &monkeys),
+                '*' => solve_for_humn(&op.m2, n1/n2, &monkeys),
+                '/' => solve_for_humn(&op.m2, n2/n1, &monkeys),
+                _ => panic!("invalid operator")
+            }
+        } else {
+            if let Some(n2) = result_or_unknown(&op.m2, &monkeys) {
+                // n1 = [unknown] [op] n2
+                match op.op {
+                    '+' => solve_for_humn(&op.m1, n1-n2, &monkeys),
+                    '-' => solve_for_humn(&op.m1, n1+n2, &monkeys),
+                    '*' => solve_for_humn(&op.m1, n1/n2, &monkeys),
+                    '/' => solve_for_humn(&op.m1, n1*n2, &monkeys),
+                    _ => panic!("invalid operator")
+                }
+            } else { panic!("both sides undetermined"); }
+        }
+    } else { panic!("found number when trying to solve for humn"); }
+}
+
+fn result_or_unknown(name: &str, monkeys: &Vec<Monkey>) -> Option<i64> {
+    if name == "humn" { return None; }
+
+    let i = monkeys.iter().position(|m| m.name == name).unwrap();
+    match &monkeys[i].kind {
+        MonkeyType::Num(n) => Some(*n),
+        MonkeyType::Op(op) => {
+            let n1 = result_or_unknown(&op.m1, &monkeys)?;
+            let n2 = result_or_unknown(&op.m2, &monkeys)?;
+            Some(apply_op(n1, n2, op))
+        }
+    }
+}
+
+pub fn solve(monkeys: &Vec<Monkey>) -> i64 {
+    let i = monkeys.iter().position(|m| m.name == "root").unwrap();
+    if let MonkeyType::Op(op) = &monkeys[i].kind {
+        if let Some(n) = result_or_unknown(&op.m1, &monkeys) {
+            return solve_for_humn(&op.m2, n, monkeys);
+        } else {
+            if let Some(n) = result_or_unknown(&op.m2, &monkeys) {
+                return solve_for_humn(&op.m1, n, &monkeys);
+            } else { panic!("humn not found"); }
+        }
+    } else { panic!("root monkey is of type number"); }
+}
+
+fn main() {
+    let monkeys = read_monkeys_from_stdin();
+    println!("{}", solve(&monkeys));
+}