export interface ScanOptions {
  gte?: string
  lt?: string
  limit?: number
  reverse?: boolean
  /** When false/omitted, yielded values are `undefined`. */
  values?: boolean
}

export type BatchOp = { type: 'set'; key: string; value: unknown } | { type: 'delete'; key: string }

/**
 * A flat, string-keyed backend. Values are JSON-serialisable; byte-oriented backends serialise
 * internally with a {@link Codec}. All path, index and typing logic lives above this interface.
 */
export interface KVDriver {
  /** Same order as `keys`; `undefined` for missing keys. */
  getMany(keys: string[]): Promise<(unknown | undefined)[]>
  setMany(entries: [key: string, value: unknown][]): Promise<void>
  deleteMany(keys: string[]): Promise<void>
  /** MUST yield in key order (descending when `reverse`). `gte` is inclusive, `lt` exclusive. */
  scan(opts: ScanOptions): AsyncIterable<[key: string, value: unknown]>

  // Optional capabilities. Stores feature-detect them and fall back where possible.
  /** Writes each entry only if its key is absent. `true` = written. */
  setIfAbsent?(entries: [string, unknown][]): Promise<boolean[]>
  /** Applies all ops atomically. */
  batch?(ops: BatchOp[]): Promise<void>
  deleteRange?(gte: string, lt: string): Promise<void>
}

/** Serialises values for byte/string-oriented backends. */
export interface Codec {
  encode(value: unknown): string
  decode(encoded: string): unknown
}

export const JsonCodec: Codec = {
  encode: (value) => JSON.stringify(value),
  decode: (encoded) => JSON.parse(encoded),
}

/**
 * Base class for drivers: implement the four required methods and get `deleteRange`
 * (scan + deleteMany in chunks) and single-key helpers for free.
 */
export abstract class BaseKVDriver implements KVDriver {
  abstract getMany(keys: string[]): Promise<(unknown | undefined)[]>
  abstract setMany(entries: [string, unknown][]): Promise<void>
  abstract deleteMany(keys: string[]): Promise<void>
  abstract scan(opts: ScanOptions): AsyncIterable<[string, unknown]>

  /** Chunk size used by the default `deleteRange`. */
  protected deleteRangeChunk = 1000

  async deleteRange(gte: string, lt: string): Promise<void> {
    for (;;) {
      const keys: string[] = []
      for await (const [key] of this.scan({ gte, lt, limit: this.deleteRangeChunk })) keys.push(key)
      if (keys.length === 0) return
      await this.deleteMany(keys)
      if (keys.length < this.deleteRangeChunk) return
    }
  }

  async get(key: string): Promise<unknown | undefined> {
    return (await this.getMany([key]))[0]
  }

  set(key: string, value: unknown): Promise<void> {
    return this.setMany([[key, value]])
  }

  delete(key: string): Promise<void> {
    return this.deleteMany([key])
  }
}