mongodb/db/scanandorder.h

// scanandorder.h

#pragma once

/* todo:
   _ handle compound keys with differing directions.  we don't handle this yet: neither here nor in indexes i think!!!
   _ limit amount of data
*/

/* see also IndexDetails::getKeysFromObject, which needs some merging with this. */

class KeyType : boost::noncopyable {
public:
	JSObj pattern; // e.g., { ts : -1 }
public:
	KeyType(JSObj _keyPattern) {
		pattern = _keyPattern;
		assert( !pattern.isEmpty() );
	}

	// returns the key value for o
	JSObj getKeyFromObject(JSObj o) { 
		return o.extractFields(pattern);
	}
};

/* todo:
   _ respect limit
   _ check for excess mem usage
   _ response size limit from runquery; push it up a bit.
*/

inline bool fillQueryResultFromObj(BufBuilder& b, set<string> *filter, JSObj& js) { 
	if( filter ) {
		JSObj x;
		bool ok = x.addFields(js, *filter) > 0;
		if( ok ) 
			b.append((void*) x.objdata(), x.objsize());
		return ok;
	}

	b.append((void*) js.objdata(), js.objsize());
	return true;
}

typedef multimap<JSObj,JSObj> BestMap;
class ScanAndOrder { 
	BestMap best;
	int limit;   // max to send back.
	KeyType order;
	int dir;
	unsigned approxSize;

	void _add(JSObj& k, JSObj o) { 
		best.insert(make_pair(k,o));
	}

	// T may be iterator or reverse_iterator
	void _addIfBetter(JSObj& k, JSObj o, BestMap::iterator i) {
		const JSObj& worstBestKey = i->first;
		if( worstBestKey.woCompare(k) == dir ) { 
			// k is better, 'upgrade'
			best.erase(i);
			_add(k, o);
		}
	}

public:
	ScanAndOrder(int _limit, JSObj _order) : order(_order) {
		limit = _limit > 0 ? _limit : 0x7fffffff;
		approxSize = 0;

		// todo: do order right for compound keys.  this is temp.
		dir = 1;
		Element e = order.pattern.firstElement();
		if( e.type() == Number && e.number() < 0 ) {
			dir = -1;
		}
	}

	void add(JSObj o) { 
		JSObj k = order.getKeyFromObject(o);
		if( (int) best.size() < limit ) {
			approxSize += k.objsize();
			uassert( approxSize < 1 * 1024 * 1024 );
			_add(k, o);
			return;
		}
		BestMap::iterator i;
		if( dir < 0 )
			i = best.begin();
		else { 
			assert( best.end() != best.begin() );
			i = best.end(); i--;
		}
		_addIfBetter(k, o, i);
	}

	template<class T>
	void _fill(BufBuilder& b, set<string> *filter, int& nout, T begin, T end) { 
		int n = 0;
		for( T i = begin; i != end; i++ ) {
			JSObj& o = i->second;
			if( fillQueryResultFromObj(b, filter, o) ) { 
				n++;
				if( n >= limit )
					goto done;
				uassert( b.len() < 4000000 ); // appserver limit
			}
		}
done:
		nout = n;
	}

	/* scanning complete. stick the query result in b for n objects. */
	void fill(BufBuilder& b, set<string> *filter, int& nout) { 
		if( dir > 0 )
			_fill(b, filter, nout, best.begin(), best.end());
		else
			_fill(b, filter, nout, best.rbegin(), best.rend());
	}

};