#!/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/>.

# This is a helper script that could be used as a shebang interpreter for SPARQL files.

# Just set the SPARQL file executable and add the first line comment.

# The endpoint and relation parameters are optional.

# Parameters must be at the beginning of the file before any empty lines.

# Only the first occurrence of each parameter is relevant.

# This interpreter produces a human-readable table if STDOUT is a terminal

# and machine-readable relational data if STDOUT is not a terminal.

# So the stream can be piped to any relational transformation (e.g. relpipe-tr-sql)

# or output filter (e.g. relpipe-out-csv).

#

# Example:

#

#	#!/usr/bin/env rdf-sparql-interpreter.sh

#	# endpoint: https://query.wikidata.org/sparql

#	# relation: few_statements

#	

#	SELECT * WHERE { ?subject ?predicate ?object } LIMIT 3

# Execute the SPARQL query and send the results to the STDOUT

# either raw or formatted as a table depending on whether STDOUT is a terminal or not.

interpret_sparql() {

	if [ -t 1 ] ; then relpipe-in-sparql "$@" | relpipe-out-tabular;

	else relpipe-in-sparql "$@"; fi

}

# Fetches the parameter $2 from file $1.

# Parameters must be formatted as comments: „# name: value“.

# Only the first occurrence is relevant. And empty line terminates processing.

find_parameter() {

	perl -e 'while(<>) { if (/^\s*$/) { last; } elsif (/^# ($ARGV[0]): (.*)$/) { print "$2"; last; } }' "$1" "$2"

}

# Appends the "${options[@]}" array with option $2 taken from the $1 file

# or from environmental variable RELPIPE_IN_SPARQL_${2^^} (the variable has precedence).

prepare_option() {

	variableName="RELPIPE_IN_SPARQL_${2^^}"

	parameterName="$2"

	[[ -n "${!variableName}" ]] && value="${!variableName}" || value="$(find_parameter "$1" "$parameterName")"

	[[ -n "$value" ]] && options+=("--$parameterName" "$value")

}

options=()

prepare_option "$1" endpoint

prepare_option "$1" relation

cat "$1" | interpret_sparql "${options[@]}"