Implement class for calculating commit offsets

app/src/lib/dragging/reorderDropzoneIndexer.ts

@@ -0,0 +1,73 @@
+import type { Commit } from '$lib/vbranches/types';
+
+/**
+ * This class is used to determine how far a commit has been drag and dropped.
+ *
+ * We expect the dropzones to be in the following order:
+ *
+ * ```
+ * const indexer = new ReorderDropzoneIndexer(commits);
+ *
+ * <ReorderDropzone index={indexer.topDropzoneIndex} />
+ * <Commit id={commits[0].id} />
+ * <ReorderDropzone index={indexer.dropzoneIndexBelowCommit(commits[0].id)} />
+ * <Commit id={commits[1].id} />
+ * <ReorderDropzone index={indexer.dropzoneIndexBelowCommit(commits[1].id)} />
+ * ```
+ */
+export class ReorderDropzoneIndexer {
+	private dropzoneIndexes = new Map<string, number>();
+	private commitIndexes = new Map<string, number>();
+
+	constructor(commits: Commit[]) {
+		this.dropzoneIndexes.set('top', 0);
+
+		commits.forEach((commit, index) => {
+			this.dropzoneIndexes.set(commit.id, index + 1);
+			this.commitIndexes.set(commit.id, index);
+		});
+	}
+
+	get topDropzoneIndex() {
+		return this.dropzoneIndexes.get('top') ?? 0;
+	}
+
+	dropzoneIndexBelowCommit(commitId: string) {
+		const index = this.dropzoneIndexes.get(commitId);
+
+		if (index == undefined) {
+			throw new Error(`Commit ${commitId} not found in dropzoneIndexes`);
+		}
+
+		return index;
+	}
+
+	commitIndex(commitId: string) {
+		const index = this.commitIndexes.get(commitId);
+
+		if (index == undefined) {
+			throw new Error(`Commit ${commitId} not found in commitIndexes`);
+		}
+
+		return index;
+	}
+
+	/**
+	 * A negative offset means the commit has been dragged up, and a positive offset means the commit has been dragged down.
+	 */
+	dropzoneCommitOffset(dropzoneIndex: number, commitId: string) {
+		const commitIndex = this.commitIndexes.get(commitId);
+
+		if (commitIndex == undefined) {
+			throw new Error(`Commit ${commitId} not found in commitIndexes`);
+		}
+
+		const offset = dropzoneIndex - commitIndex;
+
+		if (offset > 0) {
+			return offset - 1;
+		} else {
+			return offset;
+		}
+	}
+}