use fastly::{http::{StatusCode, HeaderName, header::*}, mime, Error, Request, Response};

static ALLOWED_HEADERS: [HeaderName; 1] = [CONTENT_TYPE];

fn handle_request(req: Request) -> Result<(), Error> {
  // Fetch response from backend.
  let mut beresp = req.clone_without_body().send("origin_0")?;

  // Remove unwanted headers from response.
  filter_headers(&mut beresp);

  // If the response is HTML, we can parse it for ESI tags.
  if beresp
    .get_content_type()
    .map(|c| c.subtype() == mime::HTML)
    .unwrap_or(false)
  {
    let processor = esi::Processor::new(
      esi::Configuration::default()
    );

    processor.execute_esi(req, beresp, &|req| {      
      match req.get_path() {
        // Override requests for the content fragment
        "/_fragments/content.html" => Ok(Response::from_body("<p>Content injected at the edge.</p>")),

        // Send all other fragment requests to the main origin
        // with a cache TTL of two minutes (120s)
        _ => Ok(req.with_ttl(120).send("origin_0")?)
      }
    })?;

    Ok(())
  } else {
    // Otherwise, we can just return the response.
    beresp.send_to_client();
    Ok(())
  }
}

fn filter_headers(resp: &mut Response) {
    let to_remove: Vec<_> = resp
        .get_header_names()
        .filter(|header| !ALLOWED_HEADERS.contains(header))
        .cloned()
        .collect();

    for header in to_remove {
        resp.remove_header(header);
    }
}

fn main() {
  env_logger::builder()
    .filter(None, log::LevelFilter::Trace)
    .init();

  if let Err(err) = handle_request(Request::from_client()) {
    println!("returning error response");

    Response::from_status(StatusCode::INTERNAL_SERVER_ERROR)
      .with_body(err.to_string())
      .send_to_client();
  }
}