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import { Codec } from "./codec.ts"
export type Metadata<I, O> = Array<  | {    type: "atomic"    name: string    docs?: never    factory?: never    args?: never  }  | {    type: "factory"    name: string    docs?: never    factory: (...args: any) => Codec<I, O>    args: any[]  }>
/** Metadata for an atomic codec */export function metadata<I = any, O = any>(name: string): Metadata<I, O>/** Metadata for a factory-made codec */export function metadata<I, O, A extends unknown[]>(  name: string,  factory: (...args: A) => Codec<I, O>,  ...args: A): Metadata<I, O>/** Concatenate multiple metadata arrays */export function metadata<I, O>(...metadata: Metadata<I, O>[]): Metadata<I, O>export function metadata<I, O>(  ...fullArgs:    | Metadata<I, O>[]    | [      name: string,      factory?: (...args: any) => Codec<I, O>,      ...args: any[],    ]): Metadata<I, O> {  if (typeof fullArgs[0] !== "string") return fullArgs.flat()  const [name, factory, ...args] = fullArgs as [name: string, factory?: (...args: any) => Codec<I, O>, ...args: any[]]  return [    factory      ? {        type: "factory",        name,        factory,        args,      }      : {        type: "atomic",        name,      },  ]}
export class CodecVisitor<R> {  #fallback?: <I, O>(codec: Codec<I, O>) => R  #visitors = new Map<Metadata<any, any>[number] | Function, (codec: Codec<any>, ...args: any[]) => R>()
  add<I, O, A extends unknown[]>(codec: (...args: A) => Codec<I, O>, fn: (codec: Codec<I, O>, ...args: A) => R): this  add<I, O>(codec: Codec<I, O>, fn: (codec: Codec<I, O>) => R): this  add(codec: Codec<any> | Metadata<any, any>[number] | Function, fn: (codec: Codec<any>, ...args: any[]) => R): this {    if (codec instanceof Codec) {      codec = codec._metadata[0]!      if (!codec) throw new Error("Cannot register visitor for metadata-less codec")    }    if (this.#visitors.has(codec)) {      throw new Error("Duplicate visitor")    }    this.#visitors.set(codec, fn)    return this  }
  fallback(fn: <I, O>(codec: Codec<I, O>) => R): this {    if (this.#fallback) {      throw new Error("Duplicate fallback")    }    this.#fallback = fn    return this  }
  /**   * ```ts   * visitor.generic(<T>() =>   *   visitor.add($.array<T>, (codec, $el) => {   *     ...   *   })   * )   * ```   */  generic(fn: (visitor: this) => void): this {    fn(this)    return this  }
  visit<I, O>(codec: Codec<I, O>): R {    for (const metadata of codec._metadata) {      let visitor = this.#visitors.get(metadata)      if (visitor) return visitor(codec)      if (metadata.type !== "factory") continue      visitor = this.#visitors.get(metadata.factory)      if (visitor) return visitor(codec, ...metadata.args)    }    if (this.#fallback) {      return this.#fallback(codec)    }    throw new Error("Unrecognized codec")  }}