Reduce complexity of day 15. Runtime 4s -> 0.01s

src/day15.rs

@@ -1,4 +1,4 @@
-use itertools::Itertools;
+use std::collections::HashMap;
 use std::io;
 
 fn hash(s: &str) -> u64 {
@@ -19,20 +19,11 @@ pub fn hash_sequence() -> u64 {
   buffer.trim().split(',').map(hash).sum()
 }
 
-#[derive(Eq, PartialEq, Copy, Clone, Hash)]
 enum Op<'a> {
   Add(&'a str, u64),
   Rem(&'a str),
 }
 
-fn same_operand(a: &Op, b: &Op) -> bool {
-  match a {
-    Op::Add(s, _) | Op::Rem(s) => match b {
-      Op::Add(t, _) | Op::Rem(t) => s == t,
-    },
-  }
-}
-
 fn parseop<'a>(op: &'a str) -> Op<'a> {
   if op.ends_with('-') {
     Op::Rem(&op[0..op.len() - 1])
@@ -47,24 +38,35 @@ fn parse<'a>(ops: &Vec<&'a str>) -> Vec<Op<'a>> {
 }
 
 fn condense<'a>(ops: &Vec<Op<'a>>) -> Vec<Op<'a>> {
+  let mut last_delete_map = HashMap::new();
+  ops.iter().enumerate().for_each(|(i, op)| {
+    if let Op::Rem(opstr) = op {
+      last_delete_map.insert(opstr, i);
+    }
+  });
   let remove_deletes = ops.iter().enumerate().filter(|(i, op)| match op {
     Op::Rem(_) => false,
-    Op::Add(_, _) => !ops
-      .iter()
-      .rposition(|lop| same_operand(op, lop) && matches!(*lop, Op::Rem(_)))
-      .is_some_and(|j| j > *i),
+    Op::Add(opstr, _) => !last_delete_map.get(opstr).is_some_and(|j| j > i),
   });
-  let remove_deletes_clone = remove_deletes.clone();
 
-  let map_to_last = remove_deletes.map(|(_, op)| {
-    *remove_deletes_clone
-      .clone()
-      .rfind(|(_, rop)| same_operand(op, rop))
-      .unwrap()
-      .1
+  let mut last_value_map = HashMap::new();
+  remove_deletes.clone().for_each(|(_, op)| {
+    if let Op::Add(s, v) = op {
+      last_value_map.insert(s, v);
+    }
   });
-  let map_unique = map_to_last.into_iter().unique();
-  map_unique.collect()
+
+  let map_to_last = remove_deletes.filter_map(|(_, op)| {
+    if let Op::Add(opstr, _) = op {
+      match last_value_map.remove(opstr) {
+        Some(v) => Some(Op::Add(opstr, *v)),
+        None => None,
+      }
+    } else {
+      None
+    }
+  });
+  map_to_last.collect()
 }
 
 fn perform_ops(ops: &Vec<&str>) -> u64 {