From 5f106571a6b7f15ef13f1661f63f17ee5bb66602 Mon Sep 17 00:00:00 2001
From: Kevin Caffrey <kcaffrey@gmail.com>
Date: Mon, 25 Dec 2023 21:34:18 -0500
Subject: [PATCH] simple optimizations to reduce allocations and use better
 hashing

---
 README.md     |  4 ++--
 src/bin/25.rs | 56 ++++++++++++++++++++++++++++-----------------------
 2 files changed, 33 insertions(+), 27 deletions(-)

diff --git a/README.md b/README.md
index c258e2d..d2aadfe 100644
--- a/README.md
+++ b/README.md
@@ -65,9 +65,9 @@ Solutions for [Advent of Code](https://adventofcode.com/) in [Rust](https://www.
 | [Day 22](./src/bin/22.rs) | `78.3µs` | `194.8µs` |
 | [Day 23](./src/bin/23.rs) | `330.4µs` | `13.3ms` |
 | [Day 24](./src/bin/24.rs) | `257.2µs` | `16.2µs` |
-| [Day 25](./src/bin/25.rs) | `2.1ms` | `-` |
+| [Day 25](./src/bin/25.rs) | `947.0µs` | `-` |
 
-**Total: 32.61ms**
+**Total: 31.46ms**
 <!--- benchmarking table --->
 
 ---
diff --git a/src/bin/25.rs b/src/bin/25.rs
index 744fdd3..a72af6f 100644
--- a/src/bin/25.rs
+++ b/src/bin/25.rs
@@ -1,59 +1,59 @@
-use std::collections::{HashMap, HashSet, VecDeque};
+use std::collections::VecDeque;
+
+use fxhash::{FxHashMap, FxHashSet};
 
 advent_of_code::solution!(25, 1);
 
 pub fn part_one(input: &str) -> Option<usize> {
-    let graph = parse_graph(input);
+    let mut flow: NetworkFlow = parse_graph(input).into();
     let s = 0;
-    (1..graph.vertices).find_map(|t| find_cut_of_size(&graph, s, t, 3))
+    (1..flow.vertices).find_map(|t| find_cut_of_size(&mut flow, s, t, 3))
 }
 
-fn find_cut_of_size(graph: &Graph, s: usize, t: usize, cut: i16) -> Option<usize> {
-    let mut net: NetworkFlow = graph.into();
-    let mut queue = VecDeque::new();
+fn find_cut_of_size(net: &mut NetworkFlow, s: usize, t: usize, cut: i16) -> Option<usize> {
+    net.flow.fill(0);
 
     let mut flow = 0;
-    let mut pred = vec![None; net.vertices];
     while flow <= cut {
         // Find the shortest path from s to t.
-        pred.fill(None);
-        queue.clear();
-        queue.push_back(s);
+        net.pred.fill(None);
+        net.queue.clear();
+        net.queue.push_back(s);
         let mut seen_vertices = 0;
-        while let Some(cur) = queue.pop_front() {
-            if pred[t].is_some() {
+        while let Some(cur) = net.queue.pop_front() {
+            if net.pred[t].is_some() {
                 break;
             }
             seen_vertices += 1;
-            for &next in &graph.adjacency[cur] {
+            for &next in &net.adjacency[cur] {
                 if next != s
-                    && pred[next].is_none()
+                    && net.pred[next].is_none()
                     && net.capacity(cur, next) > net.flow(cur, next)
                 {
-                    pred[next] = Some(cur);
-                    queue.push_back(next);
+                    net.pred[next] = Some(cur);
+                    net.queue.push_back(next);
                 }
             }
         }
 
         // If there was no path, and the cut was the right size,
         // return an answer.
-        if pred[t].is_none() {
-            return (flow <= cut).then_some(seen_vertices * (graph.vertices - seen_vertices));
+        if net.pred[t].is_none() {
+            return (flow <= cut).then_some(seen_vertices * (net.vertices - seen_vertices));
         }
 
         // If we found a path, find the min flow along the path that we will use to update the
         // flow for the residual.
         let mut df = i16::MAX;
         let mut cur = t;
-        while let Some(prev) = pred[cur] {
+        while let Some(prev) = net.pred[cur] {
             df = df.min(net.capacity(prev, cur) - net.flow(prev, cur));
             cur = prev;
         }
 
         // Update the residual flow.
         let mut cur = t;
-        while let Some(prev) = pred[cur] {
+        while let Some(prev) = net.pred[cur] {
             net.add_flow(prev, cur, df);
             net.add_flow(cur, prev, -df);
             cur = prev;
@@ -81,13 +81,13 @@ fn parse_graph(input: &str) -> Graph {
 #[derive(Debug, Default, Clone, PartialEq, Eq)]
 struct Graph {
     vertices: usize,
-    adjacency: Vec<HashSet<usize>>,
+    adjacency: Vec<FxHashSet<usize>>,
 }
 
 #[derive(Debug, Default, Clone, PartialEq, Eq)]
 struct GraphBuilder<'a> {
     graph: Graph,
-    vertex_ids: HashMap<&'a str, usize>,
+    vertex_ids: FxHashMap<&'a str, usize>,
 }
 
 impl<'a> GraphBuilder<'a> {
@@ -115,11 +115,14 @@ impl<'a> GraphBuilder<'a> {
     }
 }
 
-#[derive(Debug, Clone, PartialEq, Eq)]
+#[derive(Debug, Default, Clone, PartialEq, Eq)]
 struct NetworkFlow {
     vertices: usize,
+    adjacency: Vec<FxHashSet<usize>>,
     capacity: Vec<i16>,
     flow: Vec<i16>,
+    pred: Vec<Option<usize>>,
+    queue: VecDeque<usize>,
 }
 
 impl NetworkFlow {
@@ -136,18 +139,21 @@ impl NetworkFlow {
     }
 }
 
-impl From<&Graph> for NetworkFlow {
-    fn from(graph: &Graph) -> Self {
+impl From<Graph> for NetworkFlow {
+    fn from(graph: Graph) -> Self {
         let mut ret = Self {
             vertices: graph.vertices,
             capacity: vec![0; graph.vertices * graph.vertices],
             flow: vec![0; graph.vertices * graph.vertices],
+            pred: vec![None; graph.vertices],
+            ..Default::default()
         };
         for v0 in 0..ret.vertices {
             for &v1 in &graph.adjacency[v0] {
                 ret.capacity[v0 * ret.vertices + v1] = 1;
             }
         }
+        ret.adjacency = graph.adjacency;
         ret
     }
 }