/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

package org.mozilla.javascript.commonjs.module;

import java.io.File;
import java.net.URI;
import java.net.URISyntaxException;
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

import org.mozilla.javascript.BaseFunction;
import org.mozilla.javascript.Context;
import org.mozilla.javascript.Script;
import org.mozilla.javascript.ScriptRuntime;
import org.mozilla.javascript.Scriptable;
import org.mozilla.javascript.ScriptableObject;

/**
 * Implements the require() function as defined by
 * <a href="http://wiki.commonjs.org/wiki/Modules/1.1">Common JS modules</a>.
 * <h1>Thread safety</h1>
 * You will ordinarily create one instance of require() for every top-level
 * scope. This ordinarily means one instance per program execution, except if
 * you use shared top-level scopes and installing most objects into them.
 * Module loading is thread safe, so using a single require() in a shared
 * top-level scope is also safe.
 * <h1>Creation</h1>
 * If you need to create many otherwise identical require() functions for
 * different scopes, you might want to use {@link RequireBuilder} for
 * convenience.
 * <h1>Making it available</h1>
 * In order to make the require() function available to your JavaScript
 * program, you need to invoke either {@link #install(Scriptable)} or
 * {@link #requireMain(Context, String)}.
 * @author Attila Szegedi
 * @version $Id: Require.java,v 1.4 2011/04/07 20:26:11 hannes%helma.at Exp $
 */
public class Require extends BaseFunction {
    private static final long serialVersionUID = 1L;
    private final ModuleScriptProvider moduleScriptProvider;
    private final Scriptable nativeScope;
    private final Scriptable paths;
    private final boolean sandboxed;
    private final Script preExec;
    private final Script postExec;
    private String mainModuleId = null;
    private Scriptable mainExports;

    // Modules that completed loading; visible to all threads
    private final Map<String, Scriptable> exportedModuleInterfaces =
        new ConcurrentHashMap<String, Scriptable>();
    private final Object loadLock = new Object();
    // Modules currently being loaded on the thread. Used to resolve circular
    // dependencies while loading.
    private static final ThreadLocal<Map<String, Scriptable>>
        loadingModuleInterfaces = new ThreadLocal<Map<String,Scriptable>>();

    /**
     * Creates a new instance of the require() function. Upon constructing it,
     * you will either want to install it in the global (or some other) scope
     * using {@link #install(Scriptable)}, or alternatively, you can load the
     * program's main module using {@link #requireMain(Context, String)} and
     * then act on the main module's exports.
     * @param cx the current context
     * @param nativeScope a scope that provides the standard native JavaScript
     * objects.
     * @param moduleScriptProvider a provider for module scripts
     * @param preExec an optional script that is executed in every module's
     * scope before its module script is run.
     * @param postExec an optional script that is executed in every module's
     * scope after its module script is run.
     * @param sandboxed if set to true, the require function will be sandboxed.
     * This means that it doesn't have the "paths" property, and also that the
     * modules it loads don't export the "module.uri" property.
     */
    public Require(Context cx, Scriptable nativeScope,
            ModuleScriptProvider moduleScriptProvider, Script preExec,
            Script postExec, boolean sandboxed) {
        this.moduleScriptProvider = moduleScriptProvider;
        this.nativeScope = nativeScope;
        this.sandboxed = sandboxed;
        this.preExec = preExec;
        this.postExec = postExec;
        setPrototype(ScriptableObject.getFunctionPrototype(nativeScope));
        if(!sandboxed) {
            paths = cx.newArray(nativeScope, 0);
            defineReadOnlyProperty(this, "paths", paths);
        }
        else {
            paths = null;
        }
    }

    /**
     * Calling this method establishes a module as being the main module of the
     * program to which this require() instance belongs. The module will be
     * loaded as if require()'d and its "module" property will be set as the
     * "main" property of this require() instance. You have to call this method
     * before the module has been loaded (that is, the call to this method must
     * be the first to require the module and thus trigger its loading). Note
     * that the main module will execute in its own scope and not in the global
     * scope. Since all other modules see the global scope, executing the main
     * module in the global scope would open it for tampering by other modules.
     * @param cx the current context
     * @param mainModuleId the ID of the main module
     * @return the "exports" property of the main module
     * @throws IllegalStateException if the main module is already loaded when
     * required, or if this require() instance already has a different main
     * module set.
     */
    public Scriptable requireMain(Context cx, String mainModuleId) {
        if(this.mainModuleId != null) {
            if (!this.mainModuleId.equals(mainModuleId)) {
                throw new IllegalStateException("Main module already set to " +
                    this.mainModuleId);
            }
            return mainExports;
        }

        ModuleScript moduleScript;
        try {
            // try to get the module script to see if it is on the module path
            moduleScript = moduleScriptProvider.getModuleScript(
                    cx, mainModuleId, null, null, paths);
        } catch (RuntimeException x) {
            throw x;
        } catch (Exception x) {
            throw new RuntimeException(x);
        }

        if (moduleScript != null) {
            mainExports = getExportedModuleInterface(cx, mainModuleId,
                    null, null, true);
        } else if (!sandboxed) {

            URI mainUri = null;

            // try to resolve to an absolute URI or file path
            try {
                mainUri = new URI(mainModuleId);
            } catch (URISyntaxException usx) {
                // fall through
            }

            // if not an absolute uri resolve to a file path
            if (mainUri == null || !mainUri.isAbsolute()) {
                File file = new File(mainModuleId);
                if (!file.isFile()) {
                    throw ScriptRuntime.throwError(cx, nativeScope,
                            "Module \"" + mainModuleId + "\" not found.");
                }
                mainUri = file.toURI();
            }
            mainExports = getExportedModuleInterface(cx, mainUri.toString(),
                    mainUri, null, true);
        }

        this.mainModuleId = mainModuleId;
        return mainExports;
    }

    /**
     * Binds this instance of require() into the specified scope under the
     * property name "require".
     * @param scope the scope where the require() function is to be installed.
     */
    public void install(Scriptable scope) {
        ScriptableObject.putProperty(scope, "require", this);
    }

    @Override
    public Object call(Context cx, Scriptable scope, Scriptable thisObj,
            Object[] args)
    {
        if(args == null || args.length < 1) {
            throw ScriptRuntime.throwError(cx, scope,
                    "require() needs one argument");
        }

        String id = (String)Context.jsToJava(args[0], String.class);
        URI uri = null;
        URI base = null;
        if (id.startsWith("./") || id.startsWith("../")) {
            if (!(thisObj instanceof ModuleScope)) {
                throw ScriptRuntime.throwError(cx, scope,
                        "Can't resolve relative module ID \"" + id +
                                "\" when require() is used outside of a module");
            }

            ModuleScope moduleScope = (ModuleScope) thisObj;
            base = moduleScope.getBase();
            URI current = moduleScope.getUri();
            uri = current.resolve(id);

            if (base == null) {
                // calling module is absolute, resolve to absolute URI
                // (but without file extension)
                id = uri.toString();
            } else {
                // try to convert to a relative URI rooted on base
                id = base.relativize(current).resolve(id).toString();
                if (id.charAt(0) == '.') {
                    // resulting URI is not contained in base,
                    // throw error or make absolute depending on sandbox flag.
                    if (sandboxed) {
                        throw ScriptRuntime.throwError(cx, scope,
                            "Module \"" + id + "\" is not contained in sandbox.");
                    }
                    id = uri.toString();
                }
            }
        }
        return getExportedModuleInterface(cx, id, uri, base, false);
    }

    @Override
    public Scriptable construct(Context cx, Scriptable scope, Object[] args) {
        throw ScriptRuntime.throwError(cx, scope,
                "require() can not be invoked as a constructor");
    }

    private Scriptable getExportedModuleInterface(Context cx, String id,
            URI uri, URI base, boolean isMain)
    {
        // Check if the requested module is already completely loaded
        Scriptable exports = exportedModuleInterfaces.get(id);
        if(exports != null) {
            if(isMain) {
                throw new IllegalStateException(
                        "Attempt to set main module after it was loaded");
            }
            return exports;
        }
        // Check if it is currently being loaded on the current thread
        // (supporting circular dependencies).
        Map<String, Scriptable> threadLoadingModules =
            loadingModuleInterfaces.get();
        if(threadLoadingModules != null) {
            exports = threadLoadingModules.get(id);
            if(exports != null) {
                return exports;
            }
        }
        // The requested module is neither already loaded, nor is it being
        // loaded on the current thread. End of fast path. We must synchronize
        // now, as we have to guarantee that at most one thread can load
        // modules at any one time. Otherwise, two threads could end up
        // attempting to load two circularly dependent modules in opposite
        // order, which would lead to either unacceptable non-determinism or
        // deadlock, depending on whether we underprotected or overprotected it
        // with locks.
        synchronized(loadLock) {
            // Recheck if it is already loaded - other thread might've
            // completed loading it just as we entered the synchronized block.
            exports = exportedModuleInterfaces.get(id);
            if(exports != null) {
                return exports;
            }
            // Nope, still not loaded; we're loading it then.
            final ModuleScript moduleScript = getModule(cx, id, uri, base);
            if (sandboxed && !moduleScript.isSandboxed()) {
                throw ScriptRuntime.throwError(cx, nativeScope, "Module \""
                        + id + "\" is not contained in sandbox.");
            }
            exports = cx.newObject(nativeScope);
            // Are we the outermost locked invocation on this thread?
            final boolean outermostLocked = threadLoadingModules == null;
            if(outermostLocked) {
                threadLoadingModules = new HashMap<String, Scriptable>();
                loadingModuleInterfaces.set(threadLoadingModules);
            }
            // Must make the module exports available immediately on the
            // current thread, to satisfy the CommonJS Modules/1.1 requirement
            // that "If there is a dependency cycle, the foreign module may not
            // have finished executing at the time it is required by one of its
            // transitive dependencies; in this case, the object returned by
            // "require" must contain at least the exports that the foreign
            // module has prepared before the call to require that led to the
            // current module's execution."
            threadLoadingModules.put(id, exports);
            try {
                // Support non-standard Node.js feature to allow modules to
                // replace the exports object by setting module.exports.
                Scriptable newExports = executeModuleScript(cx, id, exports,
                        moduleScript, isMain);
                if (exports != newExports) {
                    threadLoadingModules.put(id, newExports);
                    exports = newExports;
                }
            }
            catch(RuntimeException e) {
                // Throw loaded module away if there was an exception
                threadLoadingModules.remove(id);
                throw e;
            }
            finally {
                if(outermostLocked) {
                    // Make loaded modules visible to other threads only after
                    // the topmost triggering load has completed. This strategy
                    // (compared to the one where we'd make each module
                    // globally available as soon as it loads) prevents other
                    // threads from observing a partially loaded circular
                    // dependency of a module that completed loading.
                    exportedModuleInterfaces.putAll(threadLoadingModules);
                    loadingModuleInterfaces.set(null);
                }
            }
        }
        return exports;
    }

    private Scriptable executeModuleScript(Context cx, String id,
            Scriptable exports, ModuleScript moduleScript, boolean isMain)
    {
        final ScriptableObject moduleObject = (ScriptableObject)cx.newObject(
                nativeScope);
        URI uri = moduleScript.getUri();
        URI base = moduleScript.getBase();
        defineReadOnlyProperty(moduleObject, "id", id);
        if(!sandboxed) {
            defineReadOnlyProperty(moduleObject, "uri", uri.toString());
        }
        final Scriptable executionScope = new ModuleScope(nativeScope, uri, base);
        // Set this so it can access the global JS environment objects.
        // This means we're currently using the "MGN" approach (ModuleScript
        // with Global Natives) as specified here:
        // <http://wiki.commonjs.org/wiki/Modules/ProposalForNativeExtension>
        executionScope.put("exports", executionScope, exports);
        executionScope.put("module", executionScope, moduleObject);
        moduleObject.put("exports", moduleObject, exports);
        install(executionScope);
        if(isMain) {
            defineReadOnlyProperty(this, "main", moduleObject);
        }
        executeOptionalScript(preExec, cx, executionScope);
        moduleScript.getScript().exec(cx, executionScope);
        executeOptionalScript(postExec, cx, executionScope);
        return ScriptRuntime.toObject(cx, nativeScope,
                ScriptableObject.getProperty(moduleObject, "exports"));
    }

    private static void executeOptionalScript(Script script, Context cx,
            Scriptable executionScope)
    {
        if(script != null) {
            script.exec(cx, executionScope);
        }
    }

    private static void defineReadOnlyProperty(ScriptableObject obj,
            String name, Object value) {
        ScriptableObject.putProperty(obj, name, value);
        obj.setAttributes(name, ScriptableObject.READONLY |
                ScriptableObject.PERMANENT);
    }

    private ModuleScript getModule(Context cx, String id, URI uri, URI base) {
        try {
            final ModuleScript moduleScript =
                    moduleScriptProvider.getModuleScript(cx, id, uri, base, paths);
            if (moduleScript == null) {
                throw ScriptRuntime.throwError(cx, nativeScope, "Module \""
                        + id + "\" not found.");
            }
            return moduleScript;
        }
        catch(RuntimeException e) {
            throw e;
        }
        catch(Exception e) {
            throw Context.throwAsScriptRuntimeEx(e);
        }
    }

    @Override
    public String getFunctionName() {
        return "require";
    }

    @Override
    public int getArity() {
        return 1;
    }

    @Override
    public int getLength() {
        return 1;
    }
}