Collections — Collection<T, K>

Operational guidance for normalized entity state. Full API reference and examples: @themislib/themis/docs/COLLECTIONS.md. Public API: @themislib/themis/utils/collections/collection-utils.

Use when

• State contains entities with a unique string id and needs O(1) lookup plus stable ordering.
• Reducers need immutable add/update/remove/upsert operations.
• Multiple features reference shared entities and need reference-count cleanup.

Shape and imports

• A collection is { idField, ids, map, refsCount }; ids preserves order and map stores id → item.
• Import helpers from @themislib/themis/utils/collections/collection-utils.
• Use selectors, not components, to read collection contents.

Do

• Initialize with createCollection<T, K>(idField, items?).
• Use helpers (addItem, addItems, upsertItem, updateItem, replaceItem, removeItem, filterCollection) instead of mutating .ids, .map, or .refsCount.
• Preserve parent reducer reference equality when a helper returns the same collection reference.
• Use getItem for O(1) reads and getItems for ordered materialization.
• Use store.createSelector(...) plus getItem/getItems for selector-facing reads.

Reference counting cues

• Use addItemAndCountRef / upsertItemAndCountRef when adding shared entities and incrementing ownership together.
• Use increaseRefsCount and decreaseRefsCount for manual ownership changes.
• decreaseRefsCount removes the item from ids, map, and refsCount when the count reaches <= 0.
• getRefsCount returns 1 for an existing item with no explicit count and 0 for a missing item.

Don't

• Do not store entities as Item[] when id lookup is needed.
• Do not store Map/Set in Redux state; use collections or plain records.
• Do not add derived flags to item records when a separate relationship map or selector can represent the view.
• Do not call getItems(collection).find(...) for id lookup; use getItem(collection, id).
• Do not import collection utilities directly into components; expose app reads through selectors.

Examples

Serializable initial collection state

import { type Collection, createCollection } from "@themislib/themis/utils/collections/collection-utils";

type Todo = { id: string; title: string; completed: boolean };
type TodosState = { todos: Collection<Todo, "id"> };

export const initialState: TodosState = {
  todos: createCollection<Todo, "id">("id"),
};

Add, update, upsert, and remove in reducers

import { addItem, createCollection, removeItem, updateItem, upsertItem } from "@themislib/themis/utils/collections/collection-utils";

type Todo = { id: string; title: string; completed: boolean };
const state = { todos: createCollection<Todo, "id">("id") };

const todos = addItem(state.todos, { id: "todo-1", title: "Draft", completed: false });
const renamed = updateItem(todos, { id: "todo-1", title: "Ship docs" });
const synced = upsertItem(renamed, { id: "todo-2", title: "Review", completed: false });
const withoutDraft = removeItem(synced, "todo-1");

const nextState = withoutDraft === state.todos ? state : { ...state, todos: withoutDraft };

Reference-count shared entities

import { addItemAndCountRef, createCollection, decreaseRefsCount, getRefsCount } from "@themislib/themis/utils/collections/collection-utils";

type Todo = { id: string; title: string; completed: boolean };
const state = { todos: createCollection<Todo, "id">("id") };

const withOwner = addItemAndCountRef(state.todos, { id: "todo-1", title: "Draft", completed: false });
const ownerCount = getRefsCount(withOwner, "todo-1");
const afterRelease = decreaseRefsCount(withOwner, "todo-1");

ownerCount satisfies number;
afterRelease.ids satisfies string[];

Collection selectors expose item and ordered-list reads

import { Store } from "<selected Store family package>";
import { type Collection, createCollection, getItem, getItems } from "@themislib/themis/utils/collections/collection-utils";
import { createReducer } from "@themislib/themis/utils/store/create-reducer";

type Todo = { id: string; title: string; completed: boolean };
type TodosState = { todos: Collection<Todo, "id"> };
const initialState: TodosState = { todos: createCollection<Todo, "id">("id") };
const todosReducer = createReducer(initialState);
const store = new Store({ todos: todosReducer });

export const selectTodo = store.createSelector((state, id: string) => getItem(state.todos.todos, id));
export const selectTodos = store.createSelector((state) => getItems(state.todos.todos));

Plain arrays are fine for tiny ordered facts without id lookup

type BreadcrumbState = {
  labels: string[];
};

const breadcrumbs: BreadcrumbState = {
  labels: ["Projects", "Settings"],
};

❌ Bad: mutating internals corrupts normalized identity

import { type Collection, createCollection } from "@themislib/themis/utils/collections/collection-utils";
import { createAction } from "@themislib/themis/utils/store/create-action";
import { createReducer } from "@themislib/themis/utils/store/create-reducer";

type Todo = { id: string; title: string; completed: boolean };
type TodosState = { todos: Collection<Todo, "id"> };
const initialState: TodosState = { todos: createCollection<Todo, "id">("id") };
const renameTodo = createAction("todos/rename", (id: string, title: string) => ({ id, title }));

// BAD: direct ids/map writes can create duplicates or dangling ids and bypass helper no-op behavior.
export const todosReducer = createReducer(initialState).with(renameTodo, (state, { payload }) => {
  state.todos.map[payload.id] = { ...state.todos.map[payload.id], title: payload.title };
  state.todos.ids.push(payload.id);
  return state;
});

Verification cues

• Reducer tests cover add/update/remove/upsert branches and no-op same-reference behavior.
• Selector tests cover item lookup and list materialization using .select(mockState, ...).
• Manual review checks that detailed helper examples remain in @themislib/themis/docs/COLLECTIONS.md; this skill keeps only implementation cues.

See also

• @themislib/themis/docs/COLLECTIONS.md — human reference for helper signatures and examples.
• core/reducers/SKILL.md — pure immutable reducer updates.
• Selected Store family selector skill — collection selector helpers.
• core/state-serialization/SKILL.md — collections as serializable alternatives to Map/Set.