> こういうフラットな配列から木構造を作るのってどうやったらいいんだろう？

Of https://twitter.com/suin/status/1390626681547751429

interface Chapter {
    level: number;
    text: string;
}
interface ChapterNode extends Chapter {
    children: ChapterNode[];
}
const input = [
    { level: 1, text: "1章" },
    { level: 1, text: "2章" },
    { level: 2, text: "2.1" },
    { level: 2, text: "2.2" },
    { level: 1, text: "第3章" },
    { level: 2, text: "3.1" },
    { level: 2, text: "3.2" },
    { level: 3, text: "3.2.1" },
    { level: 3, text: "3.2.2" },
];
const treefy = (queue: Chapter[], level = 1): ChapterNode[] =>
    queue.reduce(
        (acc, cur, idx, array) =>
            cur.level === level
                ? acc.concat({
                    ...cur,
                    children: treefy(
                        takeWhile((c) => c.level >= level + 1, array.slice(idx + 1)),
                        level + 1
                    ),
                })
                : acc,
        [] as ChapterNode[]
    );
const takeWhile = <T>(func: (head: T) => boolean, accum: T[]): T[] =>
    accum.length && func(accum[0])
        ? [accum[0], ...takeWhile(func, accum.slice(1))]
        : ([] as T[]);
console.log(JSON.stringify(treefy(input), null, 2));

$ deno run treefy.ts
[
  {
    "level": 1,
    "text": "1章",
    "children": []
  },
  {
    "level": 1,
    "text": "2章",
    "children": [
      {
        "level": 2,
        "text": "2.1",
        "children": []
      },
      {
        "level": 2,
        "text": "2.2",
        "children": []
      }
    ]
  },
  {
    "level": 1,
    "text": "第3章",
    "children": [
      {
        "level": 2,
        "text": "3.1",
        "children": []
      },
      {
        "level": 2,
        "text": "3.2",
        "children": [
          {
            "level": 3,
            "text": "3.2.1",
            "children": []
          },
          {
            "level": 3,
            "text": "3.2.2",
            "children": []
          }
        ]
      }
    ]
  }
]

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