// The purpose of this function is to return a list of blocks (~180KB) to
	// download. To avoid a prematurely stop of the downloading, all blocks that 
	// are requested from the same source must be located within the same 
	// chunk (=> part ~9MB).
	//  
	// The selection of the chunk to download is one of the CRITICAL parts of the 
	// edonkey network. The selection algorithm must insure the best spreading
	// of files.
	//  
	// The selection is based on 4 criteria:
	//  1.  Frequency of the chunk (availability), very rare chunks must be downloaded 
	//      as quickly as possible to become a new available source.
	//  2.  Parts used for preview (first + last chunk), preview or check a 
	//      file (e.g. movie, mp3)
	//  3.  Request state (downloading in process), try to ask each source for another 
	//      chunk. Spread the requests between all sources.
	//  4.  Completion (shortest-to-complete), partially retrieved chunks should be 
	//      completed before starting to download other one.
	//  
	// The frequency criterion defines three zones: very rare (<10%), rare (<50%)
	// and common (>30%). Inside each zone, the criteria have a specific weight, used 
	// to calculate the priority of chunks. The chunk(s) with the highest 
	// priority (highest=0, lowest=0xffff) is/are selected first.
	//  
	//          very rare   (preview)       rare                      common
	//    0% <---- +0 pt ----> 10% <----- +10000 pt -----> 50% <---- +20000 pt ----> 100%
	// 1.  <------- frequency: +25*frequency pt ----------->
	// 2.  <- preview: +1 pt --><-------------- preview: set to 10000 pt ------------->
	// 3.                       <------ request: download in progress +20000 pt ------>
	// 4a. <- completion: 0% +100, 25% +75 .. 100% +0 pt --><-- !req => completion --->
	// 4b.                                                  <--- req => !completion -->
	//  
	// Unrolled, the priority scale is:
	//  
	// 0..xxxx       unrequested and requested very rare chunks
	// 10000..1xxxx  unrequested rare chunks + unrequested preview chunks
	// 20000..2xxxx  unrequested common chunks (priority to the most complete)
	// 30000..3xxxx  requested rare chunks + requested preview chunks
	// 40000..4xxxx  requested common chunks (priority to the least complete)
	//
	// This algorithm usually selects first the rarest chunk(s). However, partially
	// complete chunk(s) that is/are close to completion may overtake the priority 
	// (priority inversion).
	// For the common chuncks, the algorithm tries to spread the dowload between
	// the sources
	//