pub(crate) async fn fetch_all(

    datasets: Vec<ServerConfigDataset>,

    datasetsdir: &Path,

) -> Result<(), MyError> {

    let mut results = Vec::new();

    for dataset in &datasets {

        let datasetdir = datasetsdir.join(&dataset.name);

        results.push(fetch(dataset, datasetdir).await?);

    Ok(())

}

async fn fetch_member(dir: &Path, member: String) -> Result<IndexEntry, MyError> {

    let resp = reqwest::get(&member).await?.text().await?;

    let base: Iri<String> = Iri::new(member.clone())?;

    let _graph = parse_graph(&resp, Some(base))?;

    let filename = blob_to_sha512(resp.as_bytes());

    let filepath = dir.join(&filename);

    tokio::fs::write(&filepath, &resp)

        .await

        .map_err(|e| MyError::io_error(filepath, e))?;

    Ok(IndexEntry {

        name: filename,

        base_uri: Some(member),

    })

}

async fn fetch(

    dataset_config: &ServerConfigDataset,

    datasetdir: PathBuf,

) -> Result<Vec<IndexEntry>, MyError> {

    let resp = reqwest::get(&dataset_config.url).await?.text().await?;

    let base: Iri<String> = Iri::new(dataset_config.url.clone())?;

    let graph = parse_graph(&resp, Some(base.clone()))?;

    let members = list_members(&graph, &base)?;

    let tempdir =

        tempfile::tempdir_in(&datasetdir).map_err(|e| MyError::io_error(datasetdir.clone(), e))?;

    let tempdirpath = tempdir.path();

    let mut result = Vec::new();

    for member in members {

        if let Ok(index_entry) = fetch_member(tempdirpath, member).await {

            result.push(index_entry);

        }

    result.sort_unstable();

    if dataset_has_changed(&datasetdir, &result).await? {

        save_dataset(datasetdir, tempdir, &result).await?;

    }

    Ok(result)

}

async fn get_newest_dir(datasetdir: &Path) -> Result<Option<IndexPath>, Box<dyn Error>> {

    let mut read = tokio::fs::read_dir(datasetdir).await?;

    let mut newest: Option<IndexPath> = None;

    while let Ok(Some(entry)) = read.next_entry().await {

        let entry = if let Ok(entry) = IndexPath::try_from(entry.path().to_path_buf()) {

            entry

            continue;

        if let Some(n) = newest {

            if n.dir_name() > entry.dir_name() {

                newest = Some(n);

            } else {

                newest = Some(entry);

            }

        } else {

            newest = Some(entry);

        }

    Ok(newest)

}

async fn dataset_has_changed(

    datasetdir: &Path,

    new_dataset: &[IndexEntry],

) -> Result<bool, MyError> {

    if let Ok(Some(newest_index_path)) = get_newest_dir(datasetdir).await {

        let prev_index = read_index(&newest_index_path)?;

        return Ok(new_dataset != &prev_index[..]);

    }

    // There is no directory with a dataset yet, so the dataset has changed

    // from not existing to existing.

    Ok(true)

}

async fn save_dataset(

    datasetdir: PathBuf,

    tempdir: TempDir,

    dataset: &[IndexEntry],

) -> Result<(), Box<dyn Error>> {

    // keep the temporary directory if retrieval went well

    let saved_dir = tempdir.into_path();

    let date = format!("{}", Utc::now().format("%+"));

    let dir = datasetdir.join(date);

    tokio::fs::rename(saved_dir, &dir).await?;

    write_index(&dir, dataset)?;

    Ok(())

}

fn list_members(graph: &LightGraph, base: &Iri<String>) -> Result<Vec<String>, Box<dyn Error>> {

    let ldp = Namespace::new("http://www.w3.org/ns/ldp#")?;

    let ldp_contains = ldp.get("contains")?;

    let mut members = Vec::new();

    for t in graph.triples_matching([base], [ldp_contains], Any) {

        if let Some(iri) = t?.o().iri() {

            members.push(iri.as_str().to_string());

        }

    Ok(members)

}