toy-db.ts

import { LevelUp } from 'levelup'
import { Transform } from 'stream'

export function Document(db: LevelUp) {
  return { createCollection }

  function createCollection<T>({ collection }: { collection: string }) {
    return <
      Indexers extends {
        [name: string]: {
          prefix: string
          reducer: (value: T) => string | number | Array<string | number>
        }
      }
    >(
      indexers: Indexers,
    ): Record<keyof Indexers, { createReadStream: LevelUp['createReadStream'] }> & {
      put: (key: string, value: T) => Promise<void>
      get: (key: string) => Promise<T | void>
      del: (key: string) => Promise<void>
    } => {
      return {
        batch: async (
          ops: Array<{ type: 'put'; key: string; value: T } | { type: 'del'; key: string }>,
        ): Promise<void> => {
          let readyToDelete = [] as { type: 'del'; key: string }[]
          const putOps = ops.filter(it => it.type === 'put') as { type: 'put'; key: string; value: T }[]
          const delOps = ops.filter(it => it.type === 'del') as { type: 'del'; key: string }[]

          let items = (await (db as any).getMany(delOps.map(it => it.key))) as T[]
          if (items.length !== delOps.length) {
            for (const delOp of delOps) {
              const item = await db.get(delOp.key)
              typeof item !== undefined ? del(delOp.key, item).forEach(it => readyToDelete.push(it)) : null
            }
          } else {
            items.forEach((it, i) => del(delOps[i]!.key, it).forEach(it => readyToDelete.push(it)))
          }

          return db.batch([...putOps.flatMap(it => add(it.key, it.value)), ...readyToDelete])
        },
        put: async (key: string, value: T): Promise<void> => {
          // delete the old indexes
          return db.batch(add(key, value))
        },
        get: async (key: string): Promise<T | undefined> => {
          return db.get(key)
        },
        del: async (key: string): Promise<void> => {
          const item = (await db.get(key)) as T | undefined
          if (!item) {
            return
          }
          return db.batch(del(key, item))
        },
        ...Object.keys(indexers).reduce((acc, it) => {
          const reducer = indexers[it]!
          acc[it as keyof Indexers] = { createReadStream }
          return acc

          function createReadStream(args: {
            gt?: string
            gte?: string
            lt?: string
            lte?: string
            reverse?: boolean
            limit?: number
          }) {
            const templatedArgs = ['gt', 'gte', 'lt', 'lte'].reduce(
              (acc, it) => {
                if (it in args) {
                  acc[it as keyof typeof acc] = `${collection}!${reducer.prefix}!${
                    args[it as 'gt' | 'gte' | 'lt' | 'lte']
                  }`
                }
                return acc
              },
              {} as {
                gt?: string
                gte?: string
                lt?: string
                lte?: string
              },
            )

            const fetch = new Transform({
              readableObjectMode: true,
              writableObjectMode: true,
              transform(chunk, encoding, callback) {
                db.get(chunk.value, (err, value) => {
                  if (err) return callback(err)
                  callback(null, { key: chunk.value.replace(`${collection}!default!`, ''), value })
                })
              },
            })

            return db
              .createReadStream({
                ...args,
                ...templatedArgs,
              })
              .pipe(fetch)
          }
        }, {} as Record<keyof Indexers, { createReadStream: LevelUp['createReadStream'] }>),
      }

      function add(key: string, value: T): { type: 'put'; key: string; value: unknown }[] {
        return [
          { type: 'put', key: `${collection}!default!${key}`, value },
          ...Object.values(indexers).flatMap(it => {
            const reducedValue = it.reducer(value)
            const reducedValues = Array.isArray(reducedValue) ? reducedValue : [reducedValue]
            return reducedValues.map(value => {
              return {
                type: 'put' as const,
                key: `${collection}!${it.prefix}!${value}`,
                value: `${collection}!default!${key}`,
              }
            })
          }),
        ]
      }
      function del(key: string, item: T): { type: 'del'; key: string }[] {
        return [
          { type: 'del', key: `${collection}!default!${key}` },
          ...Object.values(indexers).flatMap(it => {
            const reducedValue = it.reducer(item!)
            const reducedValues = Array.isArray(reducedValue) ? reducedValue : [reducedValue]
            return reducedValues.map(value => {
              return {
                type: 'del' as const,
                key: `${collection}!${it.prefix}!${value}`,
              }
            })
          }),
        ]
      }
    }
  }
}