Copyright (c) 2008, http://www.snakeyaml.org
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.
/**
 * Copyright (c) 2008, http://www.snakeyaml.org
 *
 * 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 Also:
Variable substitution
Variable substitution/**
 * Construct scalar for format ${VARIABLE} replacing the template with the value from environment.
 * @see <a href="https://bitbucket.org/asomov/snakeyaml/wiki/Variable%20substitution">Variable substitution</a>
 * @see <a href="https://docs.docker.com/compose/compose-file/#variable-substitution">Variable substitution</a>
 */
public class EnvScalarConstructor extends Constructor {
    public static final Tag ENV_TAG = new Tag("!ENV");
    public static final Pattern ENV_FORMAT = Pattern.compile("^\\$\\{\\s*((?<name>\\w+)((?<separator>:?(-|\\?))(?<value>\\w+)?)?)\\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
Params:
name –        - variable name in the template
separator –   - separator in the template, can be :-, -, :?, ?
value –       - default value or the error in the template
environment – - the value from environment for the provided variable
Returns:
the value to apply in the template/**
     * 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
Params:
key – - the name of the variable
Returns:
value or null if not set/**
     * 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);
    }
}