'use strict';
  
  var assert      = require('assert');
  var wrapEmitter = require('emitter-listener');
  
  /*
   *
   * CONSTANTS
   *
   */
  var CONTEXTS_SYMBOL = 'cls@contexts';
  var ERROR_SYMBOL = 'error@context';
  
  // load polyfill if native support is unavailable
  if (!process.addAsyncListener) require('async-listener');
  
  function Namespace(name) {
    this.name   = name;
    // changed in 2.7: no default context
    this.active = null;
    this._set   = [];
    this.id     = null;
  }
  
  Namespace.prototype.set = function (key, value) {
    if (!this.active) {
      throw new Error("No context available. ns.run() or ns.bind() must be called first.");
    }
  
    this.active[key] = value;
    return value;
  };
  
  Namespace.prototype.get = function (key) {
    if (!this.active) return undefined;
  
    return this.active[key];
  };
  
  Namespace.prototype.createContext = function () {
    return Object.create(this.active);
  };
  
  Namespace.prototype.run = function (fn) {
    var context = this.createContext();
    this.enter(context);
    try {
      fn(context);
      return context;
    }
    catch (exception) {
      if (exception) {
        exception[ERROR_SYMBOL] = context;
      }
      throw exception;
    }
    finally {
      this.exit(context);
    }
  };
  
  Namespace.prototype.runAndReturn = function (fn) {
    var value;
    this.run(function (context) {
      value = fn(context);
    });
    return value;
  };
  
  Namespace.prototype.bind = function (fn, context) {
    if (!context) {
      if (!this.active) {
        context = Object.create(this.active);
      }
      else {
        context = this.active;
      }
    }
  
    var self = this;
    return function () {
      self.enter(context);
      try {
        return fn.apply(this, arguments);
      }
      catch (exception) {
        if (exception) {
          exception[ERROR_SYMBOL] = context;
        }
        throw exception;
      }
      finally {
        self.exit(context);
      }
    };
  };
  
  Namespace.prototype.enter = function (context) {
    assert.ok(context, "context must be provided for entering");
  
    this._set.push(this.active);
    this.active = context;
  };
  
  Namespace.prototype.exit = function (context) {
    assert.ok(context, "context must be provided for exiting");
  
    // Fast path for most exits that are at the top of the stack
    if (this.active === context) {
      assert.ok(this._set.length, "can't remove top context");
      this.active = this._set.pop();
      return;
    }
  
    // Fast search in the stack using lastIndexOf
    var index = this._set.lastIndexOf(context);
  
    assert.ok(index >= 0, "context not currently entered; can't exit");
    assert.ok(index,      "can't remove top context");
  
    this._set.splice(index, 1);
  };
  
  Namespace.prototype.bindEmitter = function (emitter) {
    assert.ok(emitter.on && emitter.addListener && emitter.emit, "can only bind real EEs");
  
    var namespace  = this;
    var thisSymbol = 'context@' + this.name;
  
    // Capture the context active at the time the emitter is bound.
    function attach(listener) {
      if (!listener) return;
      if (!listener[CONTEXTS_SYMBOL]) listener[CONTEXTS_SYMBOL] = Object.create(null);
  
      listener[CONTEXTS_SYMBOL][thisSymbol] = {
        namespace : namespace,
        context   : namespace.active
      };
    }
  
    // At emit time, bind the listener within the correct context.
    function bind(unwrapped) {
      if (!(unwrapped && unwrapped[CONTEXTS_SYMBOL])) return unwrapped;
  
      var wrapped  = unwrapped;
      var contexts = unwrapped[CONTEXTS_SYMBOL];
      Object.keys(contexts).forEach(function (name) {
        var thunk = contexts[name];
        wrapped = thunk.namespace.bind(wrapped, thunk.context);
      });
      return wrapped;
    }
  
    wrapEmitter(emitter, attach, bind);
  };
  
  /**
   * If an error comes out of a namespace, it will have a context attached to it.
   * This function knows how to find it.
   *
   * @param {Error} exception Possibly annotated error.
   */
  Namespace.prototype.fromException = function (exception) {
    return exception[ERROR_SYMBOL];
  };
  
  function get(name) {
    return process.namespaces[name];
  }
  
  function create(name) {
    assert.ok(name, "namespace must be given a name!");
  
    var namespace = new Namespace(name);
    namespace.id = process.addAsyncListener({
      create : function () { return namespace.active; },
      before : function (context, storage) { if (storage) namespace.enter(storage); },
      after  : function (context, storage) { if (storage) namespace.exit(storage); },
      error  : function (storage) { if (storage) namespace.exit(storage); }
    });
  
    process.namespaces[name] = namespace;
    return namespace;
  }
  
  function destroy(name) {
    var namespace = get(name);
  
    assert.ok(namespace,    "can't delete nonexistent namespace!");
    assert.ok(namespace.id, "don't assign to process.namespaces directly!");
  
    process.removeAsyncListener(namespace.id);
    process.namespaces[name] = null;
  }
  
  function reset() {
    // must unregister async listeners
    if (process.namespaces) {
      Object.keys(process.namespaces).forEach(function (name) {
        destroy(name);
      });
    }
    process.namespaces = Object.create(null);
  }
  if (!process.namespaces) reset(); // call immediately to set up
  
  module.exports = {
    getNamespace     : get,
    createNamespace  : create,
    destroyNamespace : destroy,
    reset            : reset
  };