
// The purpose of the `Content` object is to abstract away the data conversions
// to and from raw content entities as strings. For example, you want to be able
// to pass in a Javascript object and have it be automatically converted into a
// JSON string if the `content-type` is set to a JSON-based media type.
// Conversely, you want to be able to transparently get back a Javascript object
// in the response if the `content-type` is a JSON-based media-type.

// One limitation of the current implementation is that it [assumes the `charset` is UTF-8](https://github.com/spire-io/shred/issues/5).

// The `Content` constructor takes an options object, which *must* have either a
// `body` or `data` property and *may* have a `type` property indicating the
// media type. If there is no `type` attribute, a default will be inferred.
var Content = function(options) {
  this.body = options.body;
  this.data = options.data;
  this.type = options.type;
};

Content.prototype = {
  // Treat `toString()` as asking for the `content.body`. That is, the raw content entity.
  //
  //     toString: function() { return this.body; }
  //
  // Commented out, but I've forgotten why. :/
};


// `Content` objects have the following attributes:
Object.defineProperties(Content.prototype,{
  
// - **type**. Typically accessed as `content.type`, reflects the `content-type`
//   header associated with the request or response. If not passed as an options
//   to the constructor or set explicitly, it will infer the type the `data`
//   attribute, if possible, and, failing that, will default to `text/plain`.
  type: {
    get: function() {
      if (this._type) {
        return this._type;
      } else {
        if (this._data) {
          switch(typeof this._data) {
            case "string": return "text/plain";
            case "object": return "application/json";
          }
        }
      }
      return "text/plain";
    },
    set: function(value) {
      this._type = value;
      return this;
    },
    enumerable: true
  },

// - **data**. Typically accessed as `content.data`, reflects the content entity
//   converted into Javascript data. This can be a string, if the `type` is, say,
//   `text/plain`, but can also be a Javascript object. The conversion applied is
//   based on the `processor` attribute. The `data` attribute can also be set
//   directly, in which case the conversion will be done the other way, to infer
//   the `body` attribute.
  data: {
    get: function() {
      if (this._body) {
        return this.processor.parser(this._body);
      } else {
        return this._data;
      }
    },
    set: function(data) {
      if (this._body&&data) Errors.setDataWithBody(this);
      this._data = data;
      return this;
    },
    enumerable: true
  },

// - **body**. Typically accessed as `content.body`, reflects the content entity
//   as a UTF-8 string. It is the mirror of the `data` attribute. If you set the
//   `data` attribute, the `body` attribute will be inferred and vice-versa. If
//   you attempt to set both, an exception is raised.
  body: {
    get: function() {
      if (this._data) {
        return this.processor.stringify(this._data);
      } else {
        return this._body.toString();
      }
    },
    set: function(body) {
      if (this._data&&body) Errors.setBodyWithData(this);
      this._body = body;
      return this;
    },
    enumerable: true
  },

// - **processor**. The functions that will be used to convert to/from `data` and
//   `body` attributes. You can add processors. The two that are built-in are for
//   `text/plain`, which is basically an identity transformation and
//   `application/json` and other JSON-based media types (including custom media
//   types with `+json`). You can add your own processors. See below.
  processor: {
    get: function() {
      var processor = Content.processors[this.type];
      if (processor) {
        return processor;
      } else {
        // Return the first processor that matches any part of the
        // content type. ex: application/vnd.foobar.baz+json will match json.
        var main = this.type.split(";")[0];
        var parts = main.split(/\+|\//);
        for (var i=0, l=parts.length; i < l; i++) {
          processor = Content.processors[parts[i]]
        }
        return processor || {parser:identity,stringify:toString};
      }
    },
    enumerable: true
  },

// - **length**. Typically accessed as `content.length`, returns the length in
//   bytes of the raw content entity.
  length: {
    get: function() {
      if (typeof Buffer !== 'undefined') {
        return Buffer.byteLength(this.body);
      }
      return this.body.length;
    }
  }
});

Content.processors = {};

// The `registerProcessor` function allows you to add your own processors to
// convert content entities. Each processor consists of a Javascript object with
// two properties:
// - **parser**. The function used to parse a raw content entity and convert it
//   into a Javascript data type.
// - **stringify**. The function used to convert a Javascript data type into a
//   raw content entity.
Content.registerProcessor = function(types,processor) {
  
// You can pass an array of types that will trigger this processor, or just one.
// We determine the array via duck-typing here.
  if (types.forEach) {
    types.forEach(function(type) {
      Content.processors[type] = processor;
    });
  } else {
    // If you didn't pass an array, we just use what you pass in.
    Content.processors[types] = processor;
  }
};

// Register the identity processor, which is used for text-based media types.
var identity = function(x) { return x; }
  , toString = function(x) { return x.toString(); }
Content.registerProcessor(
  ["text/html","text/plain","text"],
  { parser: identity, stringify: toString });

// Register the JSON processor, which is used for JSON-based media types.
Content.registerProcessor(
  ["application/json; charset=utf-8","application/json","json"],
  {
    parser: function(string) {
      return JSON.parse(string);
    },
    stringify: function(data) {
      return JSON.stringify(data); }});

var qs = require('querystring');
// Register the post processor, which is used for JSON-based media types.
Content.registerProcessor(
  ["application/x-www-form-urlencoded"],
  { parser : qs.parse, stringify : qs.stringify });

// Error functions are defined separately here in an attempt to make the code
// easier to read.
var Errors = {
  setDataWithBody: function(object) {
    throw new Error("Attempt to set data attribute of a content object " +
        "when the body attributes was already set.");
  },
  setBodyWithData: function(object) {
    throw new Error("Attempt to set body attribute of a content object " +
        "when the data attributes was already set.");
  }
}
module.exports = Content;
