todo.merge

export function mergeTodos(current: readonly Todo[], incoming: readonly Todo[]): readonly Todo[]

import { mergeTodos } from "#fune/todo.merge@^1";

import { type Todo } from "./todo_item.ts";  ← from todo.item ^1.0.0 · built alongside by fune

/** Manual order: order ascending, ties by position in the array (a stable sort). */
function inManualOrder(todos: readonly Todo[], which: string): Todo[] {
  const seen = new Set<string>();
  for (const todo of todos) {
    if (seen.has(todo.id)) throw new Error(`duplicate id "${todo.id}" in the ${which} list`);
    seen.add(todo.id);
  }
  return [...todos].sort((a, b) => a.order - b.order);
}

/**
 * Merge by id. A todo in both lists is taken from `incoming` (the newer
 * copy: todo.item keeps no edit time to compare), in the place the current
 * one had, because that is where the person put it. Todos only in `incoming`
 * follow, in their own manual order. Todos only in `current` stay. The
 * result is in manual order, renumbered 0 to n-1.
 */
export function mergeTodos(current: readonly Todo[], incoming: readonly Todo[]): readonly Todo[] {
  const mine = inManualOrder(current, "current");
  const theirs = inManualOrder(incoming, "incoming");
  const byId = new Map(theirs.map((todo) => [todo.id, todo]));
  const kept = new Set(mine.map((todo) => todo.id));
  const merged = [...mine.map((todo) => byId.get(todo.id) ?? todo), ...theirs.filter((todo) => !kept.has(todo.id))];
  return merged.map((todo, order) => ({ ...todo, order }));
}

Install

fune build

With that line in your source, in a TypeScript project (language typescript in fune.project), fune build resolves it and its 1 dependency, pins them in fune.lock, downloads only the TypeScript package of each, and builds the code above into your project’s .fune/build, one readable file per capability with a header linking back here. Or pin a range in fune.project and build in one step:

fune add todo.merge

The manifest, vectors and README with only the TypeScript implementation. Install it without the registry with fune add ./todo.merge-1.0.0-typescript.fune, or fetch it from a terminal with fune pull todo.merge@1.0.0:typescript.

The whole function, every language, is one file too: todo.merge-1.0.0.fune, 19,135 bytes, sha256 70fd49a1e279c942df6f45cf86dbc03153376c38c62cd1836180141e1f1f11db. It installs into a project of any language.

// fune: before todo.merge

// fune: after todo.merge

// fune: replace todo.item in todo.merge

// fune: step todo.merge after <n|label>

More from the author

## Which copy wins

The incoming one. A todo (todo.item) records when it was created and completed but not when it was last edited, so there is no honest way to tell which of two copies is newer. Importing a file is the person saying "these are the versions I want", so the file's row is treated as the newer copy: every field comes from it, done state and timestamps included.

## Order

The result is in manual order (todo.sort's `manual`: `order` ascending, ties by array position), renumbered 0 to n-1, as todo.list's `moveTodo` leaves a list:

1. The current todos in their manual order, each replaced by its incoming copy where there is one. A replaced todo keeps its **current** position: the person arranged this list, and the file's `order` is a position in a different list. 2. Then the todos only in `incoming`, in the incoming list's own manual order, so a file's order among its new rows is kept.

So a new todo whose file `order` is 0 still goes after the existing ones, and gaps in the current orders close up.

## Errors

An id that appears twice in either list is an error (`duplicate id "a" in the incoming list`), because it is ambiguous which copy to keep. The todos are not otherwise checked: todo.import-csv has already checked a file's rows with todo.item's `validateTodo`.

Files