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package org.scijava.script;

import javax.script.Bindings;
import javax.script.ScriptContext;
import javax.script.ScriptEngine;
import javax.script.ScriptException;

/**
 * The contract for script interpreters.
 * 
 * @author Johannes Schindelin
 * @author Curtis Rueden
 */
public interface ScriptInterpreter {

	/**
	 * A special object returned by {@link #interpret(String)} when the
	 * interpreter is expecting additional input before finishing the evaluation.
	 */
	Object MORE_INPUT_PENDING = new Object();

	/**
	 * Reads the persisted history of the current script interpreter.
	 */
	void readHistory();

	/**
	 * Persists the history of the current script interpreter.
	 */
	void writeHistory();

	/**
	 * Obtains the next/previous command in the command history.
	 * 
	 * @param currentCommand the current command (will be stored in the history)
	 * @param forward if true, the next history entry is returned (more recent),
	 *          if false, the previous one
	 * @return the next/previous command
	 */
	String walkHistory(String currentCommand, boolean forward);

	/**
	 * Evaluates a command.
	 * 
	 * @param command the command to evaluate
	 * @return result of the evaluation
	 * @throws ScriptException
	 */
	Object eval(String command) throws ScriptException;

	/**
	 * Interprets the given line of code, which might be part of a multi-line
	 * statement.
	 * 
	 * @param line line of code to interpret
	 * @return value of the line, or {@link #MORE_INPUT_PENDING} if there is still
	 *         pending input
	 * @throws ScriptException in case of an exception
	 */
	Object interpret(String line) throws ScriptException;

	/**
	 * Clears the buffer of not-yet-evaluated lines of code, accumulated from
	 * previous calls to {@link #interpret}. In other words: start over with a new
	 * (potentially multi-line) statement, discarding the current partial one.
	 * 
	 * @see #interpret
	 */
	void reset();

	/**
	 * Returns the associated {@link ScriptLanguage}.
	 */
	ScriptLanguage getLanguage();

	/**
	 * Returns the associated {@link ScriptEngine}.
	 * 
	 * @return the script engine
	 */
	ScriptEngine getEngine();

	/**
	 * Returns the {@link Bindings} of the associated {@link ScriptEngine} at
	 * {@link ScriptContext#ENGINE_SCOPE} scope.
	 */
	Bindings getBindings();

	/**
	 * @return whether the interpreter is ready for a brand new statement.
	 * @see #interpret(String)
	 */
	boolean isReady();

	/**
	 * @return whether the interpreter expects more input. A true value means
	 *         there is definitely more input needed. A false value means no more
	 *         input is needed, but it may not yet be appropriate to evaluate all
	 *         the pending lines. (there's some ambiguity depending on the
	 *         language)
	 * @see #interpret(String)
	 */
	boolean isExpectingMoreInput();

}
