sparql.rs
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	//! # SPARQL client

	use std::collections::HashMap;
	use std::io::BufRead;

	use oxrdf::Term;
	use reqwest::header::{HeaderMap, HeaderValue};
	use reqwest::Url;
	use sparesults::*;

	use crate::services::http_client::Client as HttpClient;

	type SparqlSolution = HashMap<String, Term>;

	/// SPARQL client
	pub struct Client {
	pub endpoint: String,
	http_client: HttpClient,
	}

	/// Represent results for a SPARQL query
	/// A query can return a collection of solutions or a boolean.
	pub enum SparqlResults {
	/// Results for SELECT queries
	Solutions(Vec<SparqlSolution>),

	/// Results for INSERT DATA, UPDATE DATA, etc. queries
	Boolean(bool),
	}

	impl Client {
	pub fn new (endpoint: &str) -> Self {
	let mut headers = HeaderMap::new();
	headers.insert("Accept", HeaderValue::from_static("Accept: application/sparql-results+xml"));

	Self {
	endpoint: String::from(endpoint),
	http_client: HttpClient::new(Some(headers)),
	}
	}

	pub async fn query (&self, query: &str) -> SparqlResults {
	let url = Url::parse_with_params(&self.endpoint, &[("query", query)])
	.expect("Can't parse endpoint as absolute URL.");

	let query_results = self.http_client.get(url).await
	.expect("Can't query endpoint")
	.text().await
	.expect("End-point didn't return a reply.");

	parse_sparql_results(&query_results)
	}
	}

	pub fn parse_sparql_results (query_results: &str) -> SparqlResults {
	let results_reader = get_query_results_xml_reader(query_results.as_bytes());

	SparqlResults::read(results_reader)
	}

	impl SparqlResults {
	pub fn read<T>(reader: QueryResultsReader<T>) -> Self
	where T: BufRead
	{
	match reader {
	QueryResultsReader::Solutions(solutions) => {
	Self::Solutions(parse_sparql_solutions(solutions))
	},
	QueryResultsReader::Boolean(bool) => Self::Boolean(bool),
	}
	}

	pub fn into_solutions (self) -> Option<Vec<SparqlSolution>> {
	match self {
	SparqlResults::Solutions(solutions) => Some(solutions),
	SparqlResults::Boolean(_) => None,
	}
	}
	}

	fn get_query_results_xml_reader<T>(reader: T) -> QueryResultsReader<T>
	where T: BufRead
	{
	QueryResultsParser::from_format(QueryResultsFormat::Xml)
	.read_results(reader)
	.expect("Can't read SPARQL results")
	}

	fn parse_sparql_solutions<T> (solutions: SolutionsReader<T>) -> Vec<SparqlSolution>
	where T: BufRead
	{
	solutions
	.map(\|solution\| {
	parse_sparql_result(
	solution.expect("Can't read solution")
	)
	})
	.collect()
	}

	pub fn parse_sparql_result (solution: QuerySolution) -> SparqlSolution {
	solution
	.iter()
	.map(\|(variable, term)\| (
	variable.as_str().to_string(),
	term.clone(),
	))
	.collect()
	}

	pub fn parse_term_uri (term: &Term) -> Option<String> {
	if let Term::NamedNode(node) = term {
	Some(node.as_str().to_string())
	} else {
	None
	}
	}

	pub fn parse_literal (term: &Term) -> Option<String> {
	if let Term::Literal(literal) = term {
	Some(literal.value().to_string())
	} else {
	None
	}
	}

	pub fn is_term_empty(term: &Term) -> bool {
	match term {
	Term::NamedNode(node) => {
	// Special values IRI are considered as empty values.
	node.as_str().contains("/.well-known/genid/")
	}
	Term::BlankNode(_) => true,
	Term::Literal(_) => false,
	}
	}

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