"use strict";

/**
 * Pipeline helper for reading data
 * @class PipelineD
 * @namespace mungedb-aggregate.pipeline
 * @module mungedb-aggregate
 * @constructor
 **/
var PipelineD = module.exports = function PipelineD(){
	if(this.constructor == PipelineD) throw new Error("Never create instances of this! Use the static helpers only.");
}, klass = PipelineD, base = Object, proto = klass.prototype = Object.create(base.prototype, {constructor:{value:klass}});

// DEPENDENCIES
var DocumentSource = require('./documentSources/DocumentSource'),
	CursorDocumentSource = require('./documentSources/CursorDocumentSource'),
	Cursor = require('../Cursor');

/**
 * Create a Cursor wrapped in a DocumentSourceCursor, which is suitable to be the first source for a pipeline to begin with.
 * This source will feed the execution of the pipeline.
 *
 * //NOTE: DEVIATION FROM THE MONGO: We don't have special optimized cursors; You could support something similar by overriding `Pipeline#run` to call `DocumentSource#coalesce` on the `inputSource` if you really need it.
 *
 * This method looks for early pipeline stages that can be folded into
 * the underlying cursor, and when a cursor can absorb those, they
 * are removed from the head of the pipeline.  For example, an
 * early match can be removed and replaced with a Cursor that will
 * do an index scan.
 *
 * @param pipeline  {Pipeline}  the logical "this" for this operation
 * @param ctx       {Object}    Context for expressions
 * @returns	{CursorDocumentSource}	the cursor that was created
**/
klass.prepareCursorSource = function prepareCursorSource(pipeline, /*dbName,*/ expCtx){

	var sources = pipeline.sourceVector;

	//NOTE: SKIPPED: look for initial match
	//NOTE: SKIPPED: create a query object

	//Look for an initial simple project; we'll avoid constructing Values for fields that won't make it through the projection
	var projection = {};
	var deps = [];
	var status = DocumentSource.GetDepsReturn.SEE_NEXT;
	for (var i=0; i < sources.length && status != DocumentSource.GetDepsReturn.EXHAUSTIVE; i++) {
		status = sources[i].getDependencies(deps);
	}
	if (status == DocumentSource.GetDepsReturn.EXHAUSTIVE) {
		projection = DocumentSource.depsToProjection(deps);
	}

	//NOTE: SKIPPED: Look for an initial sort
	//NOTE: SKIPPED: Create the sort object

//	//get the full "namespace" name
//	var fullName = dbName + "." + pipeline.collectionName;

	//NOTE: SKIPPED: if(DEV) log messages

	//Create the necessary context to use a Cursor
	//NOTE: SKIPPED: pSortedCursor bit
	//NOTE: SKIPPED: pUnsortedCursor bit
	var cursorWithContext = new CursorDocumentSource.CursorWithContext(/*fullName*/);

	// Now add the Cursor to cursorWithContext
	cursorWithContext._cursor = new Cursor( pipeline.collectionName );

	// wrap the cursor with a DocumentSource and return that
	var source = new CursorDocumentSource( cursorWithContext, expCtx );

//	source.namespace = fullName;

	//NOTE: SKIPPED: Note the query and sort

	if (Object.keys(projection).length) source.setProjection(projection);

	return source;
};