import { withResponseObserver } from "@nifrajs/core/response-observer"
import type { Middleware } from "@nifrajs/core/server"

export interface PoweredByOptions {
  /** Header name. Default `"x-powered-by"`. */
  readonly header?: string
  /** Header value. Default `"Nifra"`. */
  readonly value?: string
  /** Do not overwrite an existing header by default. */
  readonly respectExisting?: boolean
}

/**
 * Opt-in `X-Powered-By` style header. Nifra does not emit this by default; use it only when you want a
 * public framework/product marker.
 */
export function poweredBy(options: PoweredByOptions = {}): Middleware {
  const header = options.header ?? "x-powered-by"
  const value = options.value ?? "Nifra"
  const respectExisting = options.respectExisting !== false
  if (header.trim() === "") throw new Error("poweredBy: header is empty")
  if (/[\r\n]/.test(value)) throw new Error("poweredBy: value contains a newline")

  // Respecting an existing value IS the static tier's contract (declared headers are defaults), so
  // the default configuration declares the header instead of writing it per response and leaves the
  // app on its fused/native lanes. Overwriting is a decision about the response, so it stays a hook.
  if (respectExisting) {
    return { name: "powered-by", responseHeaders: { [header]: value } }
  }
  return withResponseObserver({
    name: "powered-by",
    onResponseHeaders(headers) {
      headers.set(header, value)
    },
  })
}