[TWEAK] Performance improvement by dropping the need for a hashmap and using the stack indices directly

Cargo.lock

@@ -7,6 +7,7 @@ name = "advent-of-code-2022-rust"
 version = "0.1.0"
 dependencies = [
  "lazy_static",
+ "nohash-hasher",
  "regex",
  "rustc-hash",
 ]
@@ -32,6 +33,12 @@ version = "2.5.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2dffe52ecf27772e601905b7522cb4ef790d2cc203488bbd0e2fe85fcb74566d"
 
+[[package]]
+name = "nohash-hasher"
+version = "0.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2bf50223579dc7cdcfb3bfcacf7069ff68243f8c363f62ffa99cf000a6b9c451"
+
 [[package]]
 name = "regex"
 version = "1.7.0"

Cargo.toml

@@ -9,4 +9,5 @@ edition = "2021"
 [dependencies]
 regex = "1"
 lazy_static = "1.4.0"
-rustc-hash = "1.1.0"
+rustc-hash = "1.1.0"
+nohash-hasher = "0.2.0"

src/day5.rs

@@ -1,3 +1,5 @@
+use std::collections::HashMap;
+use nohash_hasher::{BuildNoHashHasher, NoHashHasher};
 use rustc_hash::FxHashMap;
 use crate::day_solver::DaySolver;
 
@@ -25,15 +27,14 @@ impl StackInstruction {
 #[derive(Debug, Clone)]
 pub struct Day5 {
     stacks: Vec<Vec<char>>,
-    stack_ids_rev: FxHashMap<u8, usize>,
     instructions: Vec<StackInstruction>
 }
 
 impl Day5 {
     fn execute_instruction_part1(&mut self, instr: &StackInstruction) {
         for _ in 0..instr.amount {
-            let moving_crate = self.stacks[self.stack_ids_rev[&instr.from]].pop().unwrap();
-            self.stacks[self.stack_ids_rev[&instr.to]].push(moving_crate);
+            let moving_crate = self.stacks[instr.from as usize].pop().unwrap();
+            self.stacks[instr.to as usize].push(moving_crate);
         }
     }
 
@@ -48,11 +49,12 @@ impl Day5 {
 
     fn execute_instruction_part2(&mut self, instr: &StackInstruction) {
 
-        let from_stack = &self.stacks[self.stack_ids_rev[&instr.from]];
+        // TODO prevent copying into a Vec here (we don't need a stack allocation...)
+        let from_stack = &self.stacks[instr.from as usize];
         let from_stack_remaining = from_stack.len() - instr.amount as usize;
         let to_move: Vec<char> = from_stack.iter().skip(from_stack_remaining).cloned().collect();
-        self.stacks[self.stack_ids_rev[&instr.to]].extend(to_move);
-        self.stacks[self.stack_ids_rev[&instr.from]].truncate(from_stack_remaining);
+        self.stacks[instr.to as usize].extend(to_move);
+        self.stacks[instr.from as usize].truncate(from_stack_remaining);
     }
 
     fn execute_instructions_part2(&self) -> Day5 {
@@ -87,15 +89,19 @@ impl Day5 {
         }
         let stack_ids: Vec<u8> = stack_id_strs.iter().map(|s| s.parse::<u8>().unwrap()).collect();
 
-        let mut stacks: Vec<Vec<char>> = vec![Vec::with_capacity(stack_lines.len() - 1); stack_id_idxs.len()];
+        let mut stacks: Vec<Vec<char>> = vec![Vec::with_capacity(stack_lines.len() - 1);
+                                              stack_ids.iter().max().unwrap().to_owned() as usize + 1];
         for stack_line in stack_lines.iter().rev().skip(1) {
             stack_id_idxs.iter().enumerate()
-                .for_each(|(i, stack_id_idx)| if let Some(c) = stack_line.chars().nth(stack_id_idx.to_owned()).filter(|c| !c.is_whitespace()) { stacks[i].push(c) })
+                .for_each(|(i, stack_id_idx)| if let Some(c) = stack_line.chars()
+                    .nth(stack_id_idx.to_owned())
+                    .filter(|c| !c.is_whitespace()) {
+                    stacks[stack_ids[i] as usize].push(c)
+                })
         }
 
         return Day5 {
             stacks,
-            stack_ids_rev: FxHashMap::from_iter(stack_ids.iter().enumerate().map(|(k, v)| (v.clone(), k))),
             instructions: instruction_lines.iter().map(|s| StackInstruction::parse(s)).collect()
         };
     }