From 06d8916b365d6567dca21d36a0458386ca636e87 Mon Sep 17 00:00:00 2001
From: "Brady J. Garvin" <bgarvin@cse.unl.edu>
Date: Thu, 6 Oct 2022 16:24:30 -0500
Subject: [PATCH] Designed, traced, and coded other iterative graph searches.
---
graph-search/notes.md | 53 ++++++++----
graph-search/src/features/search/search.js | 87 +++++++++++++++++++-
graph-search/src/features/search/solution.js | 2 +-
3 files changed, 122 insertions(+), 20 deletions(-)
diff --git a/graph-search/notes.md b/graph-search/notes.md
index 9414791..7d29ac8 100644
--- a/graph-search/notes.md
+++ b/graph-search/notes.md
@@ -61,51 +61,74 @@ In the weighted directed graph on the whiteboard, what is the shortest path from
## Breadth-First Search (BFS)
-* Worklist: …
+* Worklist: Queue
* Guaranteed to find a path with the fewest edges
Worklist Backpointers
-------- ------------
- (⊥, a)
+ (⊥, a) ✓ a → (⊥, a)
+ (a, c) ✓ c → (a, c)
+ (c, e) ✓ e → (c, e)
+ (c, b) ✓ b → (c, b)
+ (e, d) ✓ d → (e, d)
+ (e, f)
+ (b, e)
+ (b, a)
Reversed Path
----
- …
+ d ← e ← c ← a
## Dijkstra's Algorithm
-* Worklist: …
-* Priority: …
+* Worklist: Priority Queue
+* Priority: Distance travelled from the source
* Guaranteed to find a path with least weight
Worklist Backpointers
------------ --------------
- (⊥, a, 0)
+ (⊥, a, 0) ✓ a → (⊥, a, 0)
+ (a, c, 7) ✓ c → (a, c, 7)
+ (c, e, 12) ✓ b → (c, b, 8)
+ (c, b, 8) ✓ e → (c, e, 12)
+ (b, e, 17) f → (e, f, 14)
+ (b, a, 12) ✓ d → (f, d, 14)
+ (e, d, 15)
+ (e, f, 14) ✓
+ (f, c, 22)
+ (f, d, 14) ✓
Reversed Path
----
- …
+ d ← f ← e ← c ← a
## Best-First Search
-* Worklist: …
-* Priority: …
+* Worklist: Priority Queue
+* Priority: Distance travelled from the source plus estimated distance still to travel
* Guaranteed to find a path if one exists
* Not guaranteed to find a good path (but often does anyway)
* Can have good best- and average-case time efficiency
- Worklist Backpointers
- ------------ --------------
- (⊥, a, 0)
+ Worklist Backpointers
+ ----------------- --------------
+ (⊥, a, 0) → 2 ✓ a → (⊥, a, 0)
+ (a, c, 7) → 9 ✓ c → (a, c, 7)
+ (c, e, 12) → 13 ✓ b → (c, b, 8)
+ (c, b, 8) → 9 ✓ e → (c, e, 12)
+ (b, a, 12) → 14 ✓ d → (e, d, 15)
+ (b, e, 17) → 18
+ (e, d, 15) → 15 ✓
+ (e, f, 14) → 16
Reversed Path
----
- …
+ d ← e ← c ← a
## A*
-* Worklist: …
-* Priority: …
+* Worklist: Priority Queue
+* Priority: Distance travelled from the source plus estimated distance still to travel (estimates computed with an *admissible* heuristic)
* Guaranteed to find a path with least weight
* Can have good best- and average-case time efficiency
diff --git a/graph-search/src/features/search/search.js b/graph-search/src/features/search/search.js
index a30fcf5..b60729c 100644
--- a/graph-search/src/features/search/search.js
+++ b/graph-search/src/features/search/search.js
@@ -37,19 +37,98 @@ export function dfs(graph, source, destination) {
}
export function bfs(graph, source, destination) {
- return undefined; // TODO: stub
+ const backpointers = new Map();
+ const worklist = new Queue();
+ worklist.insert(new Edge(undefined, source));
+ while (worklist.size > 0) {
+ const workitem = worklist.remove();
+ if (backpointers.has(workitem.to)) {
+ continue;
+ }
+ backpointers.set(workitem.to, workitem);
+ if (workitem.to === destination) {
+ const reversedPath = [];
+ for (let current = destination;
+ current !== undefined;
+ current = backpointers.get(current).from) {
+ reversedPath.push(current);
+ }
+ return reversedPath.reverse();
+ }
+ for (const incidence of graph.getIncidences(workitem.to)) {
+ worklist.insert(new Edge(workitem.to, incidence.destination));
+ }
+ }
+ return undefined;
}
export function dijkstras(graph, source, destination) {
- return undefined; // TODO: stub
+ const backpointers = new Map();
+ const worklist = new PriorityQueue();
+ worklist.insert(new Edge(undefined, source, 0), 0);
+ while (worklist.size > 0) {
+ const workitem = worklist.remove();
+ if (backpointers.has(workitem.to)) {
+ continue;
+ }
+ backpointers.set(workitem.to, workitem);
+ if (workitem.to === destination) {
+ const reversedPath = [];
+ for (let current = destination;
+ current !== undefined;
+ current = backpointers.get(current).from) {
+ reversedPath.push(current);
+ }
+ return reversedPath.reverse();
+ }
+ for (const incidence of graph.getIncidences(workitem.to)) {
+ worklist.insert(
+ new Edge(workitem.to, incidence.destination, workitem.distance + incidence.weight),
+ workitem.distance + incidence.weight,
+ );
+ }
+ }
+ return undefined;
}
function heuristic(vertex, destination) {
- return 0; // TODO: stub
+ if ('acf'.includes(vertex)) {
+ return 2;
+ }
+ if ('be'.includes(vertex)) {
+ return 1;
+ }
+ return 0;
}
export function bestFirst(graph, source, destination) {
- return undefined; // TODO: stub
+ const backpointers = new Map();
+ const worklist = new PriorityQueue();
+ worklist.insert(new Edge(undefined, source, 0), heuristic(source, destination));
+ while (worklist.size > 0) {
+ const workitem = worklist.remove();
+ if (backpointers.has(workitem.to) &&
+ backpointers.get(workitem.to).distance <= workitem.distance) {
+ continue;
+ }
+ backpointers.set(workitem.to, workitem);
+ if (workitem.to === destination) {
+ const reversedPath = [];
+ for (let current = destination;
+ current !== undefined;
+ current = backpointers.get(current).from) {
+ reversedPath.push(current);
+ }
+ return reversedPath.reverse();
+ }
+ for (const incidence of graph.getIncidences(workitem.to)) {
+ worklist.insert(
+ new Edge(workitem.to, incidence.destination, workitem.distance + incidence.weight),
+ workitem.distance + incidence.weight + heuristic(incidence.destination, destination),
+ );
+ }
+ }
+ return undefined;
}
export function recursiveDFS(graph, source, destination) {
diff --git a/graph-search/src/features/search/solution.js b/graph-search/src/features/search/solution.js
index 69644f0..da0ee23 100644
--- a/graph-search/src/features/search/solution.js
+++ b/graph-search/src/features/search/solution.js
@@ -29,7 +29,7 @@ function formatSolution(solution) {
}
export function Solution(props) {
- const search = dfs;
+ const search = bestFirst;
const firstSolution = search(firstExample, 'a', 'd');
const secondSolution = search(secondExample, '123', '321');
return (
--
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