From 637bec5f9859585f89fce930809537d2b2cfab07 Mon Sep 17 00:00:00 2001
From: "Brady J. Garvin" <bgarvin@cse.unl.edu>
Date: Sat, 23 Sep 2023 10:54:31 -0500
Subject: [PATCH] Recorded work from Friday.
---
graph-search/notes.md | 27 +++++++-----
graph-search/src/features/search/search.js | 44 +++++++++++++++++++-
graph-search/src/features/search/solution.js | 2 +-
3 files changed, 60 insertions(+), 13 deletions(-)
diff --git a/graph-search/notes.md b/graph-search/notes.md
index 8c0b31a..e0d0f6a 100644
--- a/graph-search/notes.md
+++ b/graph-search/notes.md
@@ -103,24 +103,31 @@ How many moves does it take to reverse the string `123`?
## Best-First Search
-* Worklist: …
-* Priority: …
+* Worklist: Priority Queue
+* Priority: Distance traveled from the source plus estimated distance until the destination
* 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, e, 17) → 18 d → (e, d, 15)
+ (b, a, 12) → 14 ✓
+ (e, d, 15) → 15 ✓
+ (e, f, 14) → 16
Reversed Path
----
- …
+ d ← e ← c ← a
## A*
-* Worklist: …
-* Priority: …
+* Worklist: Priority Queue
+* Priority: Distance traveled from the source plus estimated distance until the destination (estimated by an *admissible* heuristic)
* Guaranteed to find a path with least weight
* Can have good best- and average-case time efficiency
@@ -128,6 +135,6 @@ How many moves does it take to reverse the string `123`?
* Worklist: …
- Activation Frames Backpointers Returned
- ----------------------------------- ------------ ----------------
+ Activation Frames Backpointers Returned
+ ----------------------------------- ------------ ------------------
edge = (⊥, a), incidence = …
diff --git a/graph-search/src/features/search/search.js b/graph-search/src/features/search/search.js
index 91566ee..0d36cb7 100644
--- a/graph-search/src/features/search/search.js
+++ b/graph-search/src/features/search/search.js
@@ -97,11 +97,51 @@ export function dijkstras(graph, source, destination) {
}
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),
+ 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 = workitem.to;
+ 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 a64ab47..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 = dijkstras;
+ const search = bestFirst;
const firstSolution = search(firstExample, 'a', 'd');
const secondSolution = search(secondExample, '123', '321');
return (
--
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