diff -r 62eac7ab4cf4 -r d44ed75822e7 src/AttributeFinder.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/AttributeFinder.h	Wed Jan 16 17:23:05 2019 +0100
@@ -0,0 +1,97 @@
+/**
+ * Relational pipes
+ * Copyright © 2019 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, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * 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/>.
+ */
+#pragma once
+
+#include <vector>
+#include <filesystem>
+
+#include <relpipe/writer/typedefs.h>
+#include <relpipe/writer/AttributeMetadata.h>
+#include <relpipe/writer/RelationalWriter.h>
+
+#include "RequestedField.h"
+
+namespace relpipe {
+namespace in {
+namespace filesystem {
+
+namespace fs = std::filesystem;
+using namespace relpipe::writer;
+
+class AttributeFinder {
+public:
+
+	/**
+	 * Single requested fields might generate multiple attributes in the relation.
+	 * But usually it is 1:1.
+	 * @param field requested field from the user (usually from CLI arguments)
+	 * @return attribute metadata to be used in the RelationalWriter.startRelation()
+	 */
+	virtual vector<AttributeMetadata> toMetadata(const RequestedField& field) = 0;
+
+	/**
+	 * Writing of the record for current file is starting.
+	 * Following writeField() calls are related to this file.
+	 * @param file
+	 */
+	virtual void startFile(const fs::path& file) = 0;
+
+	/**
+	 * Writing of the record for current file is finished. All resources linked to this file should be released.
+	 */
+	virtual void endFile() = 0;
+
+	/**
+	 * Writes field attribute(s). The attribute count must match with count of AttributeMetadata returned in toMetadata().
+	 * @param writer
+	 * @param field
+	 */
+	virtual void writeField(RelationalWriter* writer, const RequestedField& field) = 0;
+
+	/**
+	 * Writes empty attribute(s) in case of non-existent file or an error. 
+	 * The attribute count must match with count of AttributeMetadata returned in toMetadata().
+	 * @param writer
+	 * @param field
+	 */
+	virtual void writeEmptyField(RelationalWriter* writer, const RequestedField& field) {
+		// TODO: better handling of null values (when null values are supported by the format specification)
+		for (AttributeMetadata m : toMetadata(field)) {
+			switch (m.typeId) {
+				case TypeId::BOOLEAN:
+					writer->writeAttribute(L"false");
+					break;
+				case TypeId::INTEGER:
+					writer->writeAttribute(L"0");
+					break;
+				case TypeId::STRING:
+					writer->writeAttribute(L"");
+					break;
+				default:
+					throw RelpipeWriterException(L"Unsupported attribute type in writeEmptyField().");
+			}
+		}
+	}
+
+	virtual ~AttributeFinder() {
+	}
+};
+
+}
+}
+}
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