#!/bin/bash

# Relational pipes

# Copyright © 2020 František Kučera (Frantovo.cz, GlobalCode.info)

# 

# This program is free software: you can redistribute it and/or modify

# it under the terms of the GNU General Public License as published by

# the Free Software Foundation, version 3 of the License.

# 

# This program is distributed in the hope that it will be useful,

# but WITHOUT ANY WARRANTY; without even the implied warranty of

# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the

# GNU General Public License for more details.

# 

# You should have received a copy of the GNU General Public License

# along with this program. If not, see <http://www.gnu.org/licenses/>.

relpipe_in_sparql_help() {

	cat <<-EOF

		SPARQL query is expected on the STDIN. Exapmple:

		  echo 'SELECT * WHERE { ?subject ?predicate ?object . } LIMIT 3' \

| relpipe-in-sparql | relpipe-out-tabular

		Namespace prefixes are part of the query.

		But because they are usually constant, they can be set as the 

		RELPIPE_IN_SPARQL_PREFIXES environmental variable.

		Then they are prepended to the query.

		SPARQL endpoint can be set ad-hoc by the --endpoint option.

		To configure default SPARQL endpoint, add something like this to your environment:

		  export RELPIPE_IN_SPARQL_ENDPOINT="https://query.wikidata.org/sparql"

		  export RELPIPE_IN_SPARQL_ENDPOINT="https://dbpedia.org/sparql"

		  export RELPIPE_IN_SPARQL_ENDPOINT="https://data.gov.cz/sparql"

		  export RELPIPE_IN_SPARQL_ENDPOINT="https://data.cssz.cz/sparql"

		The relation name defaults to "rdf". Custom name can be set using the --relation option.

EOF

}

relation="rdf";

endpoint="${RELPIPE_IN_SPARQL_ENDPOINT:-https://dbpedia.org/sparql}";

while [[ $# -gt 0 ]]; do

	argument="$1";

	case "$argument" in

		"--relation") relation="$2"; shift; shift; ;;

		"--endpoint") endpoint="$2"; shift; shift; ;;

		"--help") relpipe_in_sparql_help; exit; ;;

	esac

done

[[ -n "$RELPIPE_IN_SPARQL_PREFIXES" ]] && query="$RELPIPE_IN_SPARQL_PREFIXES"; query+=$'\n\n';

query+="$(</dev/stdin)";

# Simple implementation that utilizes the CSV output of the SPARQL endpoint.

relpipe_in_sparql_implementation_csv() {

	curl \

		--header "Accept: text/csv" \

		--data-urlencode query="$query" \

		--fail \

		--silent \

		--show-error \

		"$endpoint" \

}

# More powerful implementation based on XML.

# Can be customized through XSLT.

# But: has more dependencies and avoids streaming.

relpipe_in_sparql_implementation_xml() {

	DIR="$(dirname $(realpath "$0"))";

	curl \

		--header "Accept: application/sparql-results+xml" \

		--data-urlencode query="$query" \

		--fail \

		--silent \

		--show-error \

		"$endpoint" \

		| xsltproc --stringparam "relation" "$relation" "$DIR/relpipe-in-sparql.xsl" - \

		| relpipe-in-xml

}

relpipe_in_sparql_implementation_${RELPIPE_IN_SPARQL_IMPLEMENTATION:-csv}