/**
 * Copyright (c) 2008, SnakeYAML
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.yaml.snakeyaml.env;

import org.yaml.snakeyaml.constructor.AbstractConstruct;
import org.yaml.snakeyaml.constructor.Constructor;
import org.yaml.snakeyaml.error.MissingEnvironmentVariableException;
import org.yaml.snakeyaml.nodes.Node;
import org.yaml.snakeyaml.nodes.ScalarNode;
import org.yaml.snakeyaml.nodes.Tag;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 * Construct scalar for format ${VARIABLE} replacing the template with the value from environment.
 * @see <a href="https://bitbucket.org/snakeyaml/snakeyaml/wiki/Variable%20substitution">Variable substitution</a>
 * @see <a href="https://docs.docker.com/compose/compose-file/#variable-substitution">Variable substitution</a>
 */
public class EnvScalarConstructor extends Constructor {
    public static final Tag ENV_TAG = new Tag("!ENV");
    // name must be a word -> \w+
    // value can be any non-space -> \S+
    public static final Pattern ENV_FORMAT = Pattern.compile("^\\$\\{\\s*((?<name>\\w+)((?<separator>:?(-|\\?))(?<value>\\S+)?)?)\\s*\\}$");

    public EnvScalarConstructor() {
        this.yamlConstructors.put(ENV_TAG, new ConstructEnv());
    }

    private class ConstructEnv extends AbstractConstruct {
        public Object construct(Node node) {
            String val = constructScalar((ScalarNode) node);
            Matcher matcher = ENV_FORMAT.matcher(val);
            matcher.matches();
            String name = matcher.group("name");
            String value = matcher.group("value");
            String separator = matcher.group("separator");
            return apply(name, separator, value != null ? value : "", getEnv(name));
        }
    }

    /**
     * Implement the logic for missing and unset variables
     *
     * @param name        - variable name in the template
     * @param separator   - separator in the template, can be :-, -, :?, ?
     * @param value       - default value or the error in the template
     * @param environment - the value from environment for the provided variable
     * @return the value to apply in the template
     */
    public String apply(String name, String separator, String value, String environment) {
        if (environment != null && !environment.isEmpty()) return environment;
        // variable is either unset or empty
        if (separator != null) {
            //there is a default value or error
            if (separator.equals("?")) {
                if (environment == null)
                    throw new MissingEnvironmentVariableException("Missing mandatory variable " + name + ": " + value);
            }
            if (separator.equals(":?")) {
                if (environment == null)
                    throw new MissingEnvironmentVariableException("Missing mandatory variable " + name + ": " + value);
                if (environment.isEmpty())
                    throw new MissingEnvironmentVariableException("Empty mandatory variable " + name + ": " + value);
            }
            if (separator.startsWith(":")) {
                if (environment == null || environment.isEmpty())
                    return value;
            } else {
                if (environment == null)
                    return value;
            }
        }
        return "";
    }

    /**
     * Get value of the environment variable
     *
     * @param key - the name of the variable
     * @return value or null if not set
     */
    public String getEnv(String key) {
        return System.getenv(key);
    }
}