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See Chapter 7.2.1.4 of Knuth "The Art of Computer Programming" Vol. 4 Fascicle 3. On page 38 he gives algorithms to efficiently enumerate all partitions of an integer $n$ (see Algorithm P), or to enumerate all partitions of $n$ into exactly $m$ parts (see Algorithm H). Running Algorithm H repeatedly with $m$ varying from 1 to $r$ should solve your problem in essentially optimal time (i.e. within a constant factor of the size of the output).