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wp-content/plugins/canvas/components/layout-blocks/block-column/utils.jsx

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+const { findIndex } = window.lodash;
+
+/**
+ * Returns the considered adjacent to that of the specified `clientId` for
+ * resizing consideration. Adjacent blocks are those occurring after, except
+ * when the given block is the last block in the set. For the last block, the
+ * behavior is reversed.
+ *
+ * @param {WPBlock[]} blocks   Block objects.
+ * @param {string}    clientId Client ID to consider for adjacent blocks.
+ *
+ * @return {WPBlock[]} Adjacent block objects.
+ */
+export function getAdjacentBlocks( blocks, clientId ) {
+	const index = findIndex( blocks, { clientId } );
+	const isLastBlock = index === blocks.length - 1;
+
+	return isLastBlock ? blocks.slice( 0, index ) : blocks.slice( index + 1 );
+}