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* @fds-uif/core - Merge Utilities
*
* Deep merge utilities for combining UIF foundation and system definitions.
*
* Merge semantics:
* - Scalars: System replaces foundation
* - Objects: Deep merge recursively
* - Identified arrays: Merge by identifier; matching IDs merge, new IDs append
* - Primitive arrays: Replace wholesale
* - `$remove` marker: Explicitly delete inherited item
* - `$before` marker: Insert new item before a named sibling
* - `$after` marker: Insert new item after a named sibling
*/
import { UifDuplicateIdentifierError, UifMergeError } from './errors.js';
import {
IDENTIFIED_ARRAYS,
MARKER_AFTER,
MARKER_BEFORE,
MARKER_REMOVE,
MERGE_MARKERS,
PROP_EXTENDS,
} from './constants.js';
import type {
AfterMarkerItem,
BeforeMarkerItem,
MergeOptions,
PlainObject,
RemoveMarkerItem,
WarningHandler,
} from './types.js';
// ============================================================================
// Type Guards
// ============================================================================
/** Check if a value is a plain object (not null, not array, not class instance). */
function isPlainObject(value: unknown): value is PlainObject {
if (typeof value !== 'object' || value === null || Array.isArray(value)) {
return false;
}
const proto = Object.getPrototypeOf(value);
return proto === Object.prototype || proto === null;
}
/** Check if an item has the $remove marker. */
function hasRemoveMarker(item: unknown): item is RemoveMarkerItem {
return isPlainObject(item) && item[MARKER_REMOVE] === true;
}
/** Check if an item has the $before marker. */
function hasBeforeMarker(item: unknown): item is BeforeMarkerItem {
return isPlainObject(item) && typeof item[MARKER_BEFORE] === 'string';
}
/** Check if an item has the $after marker. */
function hasAfterMarker(item: unknown): item is AfterMarkerItem {
return isPlainObject(item) && typeof item[MARKER_AFTER] === 'string';
}
// ============================================================================
// Utility Functions
// ============================================================================
/**
* Get the identifier field for an array at a given path.
* @returns The field name if it's an identified array, undefined otherwise.
*/
function getIdentifierField(path: string): string | undefined {
const segments = path.split('.');
const lastSegment = segments[segments.length - 1] ?? '';
// Handle indexed paths like "children[0].children"
const cleanSegment = lastSegment.replace(/\[\d+\]$/, '');
return IDENTIFIED_ARRAYS[cleanSegment];
}
/** Get the identifier value from an array item. */
function getIdentifier(item: unknown, identifierField: string): string | undefined {
if (isPlainObject(item) && typeof item[identifierField] === 'string') {
return item[identifierField] as string;
}
return undefined;
}
/** Strip merge markers from an item, returning a new object. */
function stripMarkers(item: PlainObject): PlainObject {
const { [MARKER_REMOVE]: _r, [MARKER_BEFORE]: _b, [MARKER_AFTER]: _a, ...rest } = item;
return rest;
}
// ============================================================================
// Array Index Building
// ============================================================================
interface IndexedItem {
item: unknown;
index: number;
}
/**
* Build an index of base array items by their identifier.
* @throws {UifDuplicateIdentifierError} If duplicates are found.
*/
function buildBaseIndex(
base: unknown[],
identifierField: string,
path: string,
): { index: Map<string, IndexedItem>; items: unknown[] } {
const index = new Map<string, IndexedItem>();
const items: unknown[] = [];
for (let i = 0; i < base.length; i++) {
const item = base[i];
const id = getIdentifier(item, identifierField);
if (id !== undefined) {
if (index.has(id)) {
throw new UifDuplicateIdentifierError(path, identifierField, id);
}
index.set(id, { item, index: i });
}
items.push(item);
}
return { index, items };
}
// ============================================================================
// Extension Item Categorization
// ============================================================================
interface PositionalInsert {
item: PlainObject;
marker: typeof MARKER_BEFORE | typeof MARKER_AFTER;
reference: string;
}
interface CategorizedItems {
removedIds: Set<string>;
positionalInserts: PositionalInsert[];
mergeItems: Array<{ id: string; item: PlainObject }>;
appendItems: unknown[];
}
/** Categorize extension items by their merge behavior. */
function categorizeExtensionItems(
extension: unknown[],
identifierField: string,
baseIndex: Map<string, IndexedItem>,
): CategorizedItems {
const removedIds = new Set<string>();
const positionalInserts: PositionalInsert[] = [];
const mergeItems: Array<{ id: string; item: PlainObject }> = [];
const appendItems: unknown[] = [];
for (const extItem of extension) {
// Non-object items just get appended
if (!isPlainObject(extItem)) {
appendItems.push(extItem);
continue;
}
const id = getIdentifier(extItem, identifierField);
// Handle $remove marker
if (hasRemoveMarker(extItem)) {
if (id !== undefined) {
removedIds.add(id);
}
continue;
}
// Handle positional markers
if (hasBeforeMarker(extItem)) {
positionalInserts.push({
item: stripMarkers(extItem),
marker: MARKER_BEFORE,
reference: extItem[MARKER_BEFORE],
});
continue;
}
if (hasAfterMarker(extItem)) {
positionalInserts.push({
item: stripMarkers(extItem),
marker: MARKER_AFTER,
reference: extItem[MARKER_AFTER],
});
continue;
}
// If item exists in base, schedule for merge
if (id !== undefined && baseIndex.has(id)) {
mergeItems.push({ id, item: extItem });
continue;
}
// New item without markers - append at end
appendItems.push(extItem);
}
return { removedIds, positionalInserts, mergeItems, appendItems };
}
// ============================================================================
// Positional Insertion
// ============================================================================
interface InsertPosition {
position: number;
isBefore: boolean;
item: PlainObject;
}
/**
* Process positional inserts, handling missing references.
*/
function processPositionalInserts(
inserts: PositionalInsert[],
siblingIndex: Map<string, number>,
result: unknown[],
identifierField: string,
path: string,
onWarning?: WarningHandler,
): InsertPosition[] {
const insertPositions: InsertPosition[] = [];
for (const { item, marker, reference } of inserts) {
const refIndex = siblingIndex.get(reference);
if (refIndex === undefined) {
// Reference not found - warn and append at end
onWarning?.(`${marker} reference "${reference}" not found at ${path}`, {
type: 'positional-marker-not-found',
path,
details: {
marker,
reference,
available: Array.from(siblingIndex.keys()),
},
});
// Append at end as fallback
result.push(item);
const newId = getIdentifier(item, identifierField);
if (newId !== undefined) {
siblingIndex.set(newId, result.length - 1);
}
} else {
insertPositions.push({
position: refIndex,
isBefore: marker === MARKER_BEFORE,
item,
});
}
}
return insertPositions;
}
/**
* Apply positional inserts to an array.
* Sorts by position descending to maintain correct indices during insertion.
*/
function applyPositionalInserts(result: unknown[], positions: InsertPosition[]): void {
// Sort by position descending so we can insert without affecting indices
positions.sort((a, b) => {
if (a.position !== b.position) {
return b.position - a.position;
}
// For same position, $after comes before $before (in reverse order)
return a.isBefore ? 1 : -1;
});
for (const { position, isBefore, item } of positions) {
const insertAt = isBefore ? position : position + 1;
result.splice(insertAt, 0, item);
}
}
// ============================================================================
// Array Merging
// ============================================================================
/**
* Merge two identified arrays using identifier-based matching.
*/
function mergeIdentifiedArrays(
base: unknown[],
extension: unknown[],
identifierField: string,
path: string,
onWarning?: WarningHandler,
): unknown[] {
// Build index of base items
const { index: baseIndex, items: result } = buildBaseIndex(base, identifierField, path);
// Categorize extension items
const { removedIds, positionalInserts, mergeItems, appendItems } = categorizeExtensionItems(
extension,
identifierField,
baseIndex,
);
// Apply merges to existing items
for (const { id, item } of mergeItems) {
const existing = baseIndex.get(id);
if (existing) {
const merged = deepMerge(existing.item as PlainObject, item, `${path}[${id}]`, onWarning);
result[existing.index] = merged;
}
}
// Remove items marked for removal
const filtered = result.filter((item) => {
const id = getIdentifier(item, identifierField);
return id === undefined || !removedIds.has(id);
});
// Build sibling index for positional inserts
const siblingIndex = new Map<string, number>();
for (let i = 0; i < filtered.length; i++) {
const id = getIdentifier(filtered[i], identifierField);
if (id !== undefined) {
siblingIndex.set(id, i);
}
}
// Process and apply positional inserts
const insertPositions = processPositionalInserts(
positionalInserts,
siblingIndex,
filtered,
identifierField,
path,
onWarning,
);
applyPositionalInserts(filtered, insertPositions);
// Append remaining items
filtered.push(...appendItems);
return filtered;
}
// ============================================================================
// Deep Merge
// ============================================================================
/**
* Deep merge two objects with UIF-specific semantics.
*/
function deepMerge<T extends PlainObject>(
base: T,
extension: PlainObject,
path: string = '',
onWarning?: WarningHandler,
): T {
const result: PlainObject = { ...base };
for (const [key, extValue] of Object.entries(extension)) {
// Skip internal markers
if (key.startsWith('$')) {
continue;
}
const baseValue = base[key];
const currentPath = path ? `${path}.${key}` : key;
// Skip undefined/null extension values
if (extValue === undefined || extValue === null) {
continue;
}
// New key - just assign
if (baseValue === undefined) {
result[key] = extValue;
continue;
}
// Both are arrays
if (Array.isArray(baseValue) && Array.isArray(extValue)) {
const identifierField = getIdentifierField(currentPath);
result[key] = identifierField
? mergeIdentifiedArrays(baseValue, extValue, identifierField, currentPath, onWarning)
: extValue; // Primitive array - replace wholesale
continue;
}
// Both are objects
if (isPlainObject(baseValue) && isPlainObject(extValue)) {
result[key] = deepMerge(baseValue, extValue, currentPath, onWarning);
continue;
}
// Type mismatch
if (typeof baseValue !== typeof extValue) {
const baseType = Array.isArray(baseValue) ? 'array' : typeof baseValue;
const extType = Array.isArray(extValue) ? 'array' : typeof extValue;
// Allow scalar to overwrite anything
if (!isPlainObject(extValue) && !Array.isArray(extValue)) {
result[key] = extValue;
continue;
}
throw new UifMergeError(currentPath, baseType, extType, baseValue, extValue);
}
// Same type scalars - extension wins
result[key] = extValue;
}
return result as T;
}
// ============================================================================
// Public API
// ============================================================================
/**
* Merge a UIF system onto a foundation, producing a combined definition.
*
* This function implements the UIF merge semantics:
* - Scalars: System value replaces foundation value
* - Objects: Deep merge recursively
* - Identified arrays (states, children, etc.): Merge by identifier
* - Primitive arrays (restrict): Replace wholesale
* - `$remove` marker: Delete inherited item
* - `$before`/`$after` markers: Position new items relative to siblings
*
* @param base - The base UIF definition (typically a foundation)
* @param extension - The extending UIF definition (typically a system)
* @param options - Merge options including warning handler
* @returns The merged UIF definition
*
* @example
* ```ts
* import { merge } from '@fds-uif/core';
*
* // Basic usage
* const merged = merge(foundation, system);
*
* // With warning handler for debugging
* const merged = merge(foundation, system, {
* onWarning: (msg, ctx) => console.warn(msg, ctx),
* });
* ```
*/
export function merge<T extends PlainObject>(base: T, extension: PlainObject, options: MergeOptions = {}): T {
return deepMerge(base, extension, '', options.onWarning);
}
/**
* Merge and clean - removes 'extends' property from the result.
* Use this when producing final resolved UIFs.
*/
export function mergeAndClean<T extends PlainObject>(
base: T,
extension: PlainObject,
options: MergeOptions = {},
): T {
const merged = merge(base, extension, options);
if (PROP_EXTENDS in merged) {
const { [PROP_EXTENDS]: _, ...rest } = merged;
return rest as T;
}
return merged;
}
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