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Kiwi is a utility library. We really like Google's Guava, and also use Apache Commons. But if they don't have something we need, and we think it is useful, this is where we put it.

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package org.kiwiproject.jackson.ser;

import static com.google.common.base.Preconditions.checkState;
import static com.google.common.base.Verify.verify;
import static java.util.Objects.nonNull;
import static java.util.stream.Collectors.joining;

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.core.TreeNode;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.deser.std.StdDeserializer;
import com.fasterxml.jackson.databind.node.TextNode;

import java.io.IOException;
import java.util.stream.IntStream;

/**
 * Custom Jackson {@link JsonDeserializer} to examine a node and take different action based on whether it is
 * a {@link TextNode} or a container node. If it is a TextNode, return the node's text, otherwise collect the
 * values in the container and join them into a CSV string.
 * 
 * This implementation does not perform extensive error checking; it assumes the element being deserialized
 * contains either a TextNode or is a container of TextNode. If a TextNode, it assumes the content is CSV but does
 * not verify. For example, as a TextNode in YAML:
 *
 * 
 * someProperty: value1,value2,value3
 * 
 * 
 * And as a container node:
 *
 * 
 * someProperty:
 *   - value1
 *   - value2
 *   - value3
 * 
 * 
 * Both of the above YAML configurations will result in the value "value1,value2,value3".
 *
 * @implNote The deserialization requires {@link JsonParser#getCodec()} to return a non-null codec and will throw an
 * {@link IllegalStateException} if the returned codec is null.
 */
public class ListToCsvStringDeserializer extends StdDeserializer {

    @SuppressWarnings("WeakerAccess")
    public ListToCsvStringDeserializer() {
        super(String.class);
    }

    @Override
    public String deserialize(JsonParser parser, DeserializationContext context) throws IOException {
        var codec = parser.getCodec();
        checkState(nonNull(codec),
                "There is no codec associated with the parser; a codec is required to read the content as a tree");

        var treeNode = codec.readTree(parser);

        if (treeNode.isContainerNode()) {
            return containerNodeToCsvString(treeNode);
        }

        return verifyIsTextNode(treeNode).asText();
    }

    private static String containerNodeToCsvString(TreeNode treeNode) {
        return IntStream.range(0, treeNode.size())
                .mapToObj(index -> textNodeToString(treeNode, index))
                .collect(joining(","));
    }

    private static String textNodeToString(TreeNode treeNode, int index) {
        var node = treeNode.get(index);
        return verifyIsTextNode(node).asText();
    }

    private static TextNode verifyIsTextNode(TreeNode node) {
        verify(node instanceof TextNode, "expected node to be TextNode but was: %s", node.getClass().getName());
        return (TextNode) node;
    }
}