Bulk Data $export (Java)

Instructions

FHIR's Bulk Data Access ($export) hands you a whole population of resources as newline-delimited JSON (NDJSON) files. Because assembling those files can take minutes, the operation is asynchronous: a client kicks it off, polls until the server is done, then downloads the files the server lists.

This kata is the offline, deterministic version of Batch-1 Day 19, Assignment 24 (Topic 20.5.2, FHIR Bulk Data $export). You model the three steps of the async request pattern against captured HttpResponse values — no sockets, no clock.

The async $export pattern

1. Kickoff. The client GETs [base]/Patient/$export. A successful kickoff returns 202 Accepted with a Content-Location header whose value is the status (poll) URL. The response has no useful body yet.
2. Poll. The client repeatedly GETs the status URL:
   • 202 Accepted — still in progress; wait and poll again.
   • 200 OK — export complete; the body is the manifest.
3. Complete. The manifest is a JSON object with an output array. Each entry describes one produced file: a type and a url pointing at an NDJSON file. The client downloads every url, in the order listed.

A manifest looks like:

{
  "transactionTime": "2024-01-01T00:00:00Z",
  "request": "https://ex/fhir/Patient/$export",
  "requiresAccessToken": true,
  "output": [
    { "type": "Patient",     "url": "http://ex/Patient.ndjson" },
    { "type": "Observation", "url": "http://ex/Observation.ndjson" }
  ],
  "error": []
}

Your task

Implement BulkExport with:

• static String kickoff(HttpResponse response) — return the Content-Location header value. If the status is not 202, or the header is missing, throw IllegalStateException.
• static List<String> pollUntilComplete(Iterator<HttpResponse> polls) — consume responses in order:
  • 202 → still in progress; move on to the next response.
  • 200 → complete; parse the manifest body and return the url of every object in the output array, in document order.
  • any other status → throw IllegalStateException.
  • iterator exhausted before a 200 arrives → throw IllegalStateException.

The provided nested HttpResponse type is already implemented (constructor, plus status(), header(name) with case-insensitive lookup, and body()); leave it as-is. Do not add a JSON library — extract the urls with java.util.regex, e.g. the pattern "url"\s*:\s*"([^"]*)".