Create a fixture for `@perma/map` from a CAR file containing a HAMT sharded directory

create-perma-map-fixture.js

import { CarIndexedReader } from '@ipld/car'
import * as pb from '@ipld/dag-pb'
import { UnixFS } from 'ipfs-unixfs'

// usage: node create-perma-map-fixture sharded-dir.car

/** @typedef {{ bitfield: number[], links: Array<Link|Element>, prefix?: string }} Link */
/** @typedef {{ key: string, value: string, prefix: string }} Element */

async function main () {
  const file = process.argv[2]
  if (!file) throw new Error('missing CAR file path')

  const reader = await CarIndexedReader.fromFile(file)
  const roots = await reader.getRoots()
  if (!roots.length || roots.length > 1) throw new Error('invalid number of roots in CAR')

  const fixture = await buildFixture(reader, roots[0])
  console.log(JSON.stringify(fixture, null, 2))
}

/**
 * @param {import('@ipld/car/api').BlockReader} reader
 * @param {import('multiformats').CID} link
 * @param {string} [prefix]
 */
async function buildFixture (reader, link, prefix) {
  const block = await reader.get(link)
  if (!block) throw new Error(`missing block: ${link}`)

  const node = pb.decode(block.bytes)
  if (!node.Data) throw new Error('missing node Data')

  /** @type {Array<Link|Element>} */
  const links = []
  for (const l of node.Links) {
    if (!l.Name) throw new Error('link with no name')
    const prefix = l.Name.slice(0, 2)
    const key = l.Name.slice(2)

    if (key) {
      links.push({ prefix, key, value: `value(${key})` })
      continue
    }

    links.push(await buildFixture(reader, l.Hash, prefix))
  }

  const data = UnixFS.unmarshal(node.Data)
  if (data.type !== 'hamt-sharded-directory') throw new Error('expected hamt-sharded-directory type')
  if (data.data == null) throw new Error('missing bitfield')
  const bitfield = Array.from(data.data)

  return { prefix, bitfield, links }
}

main()