database - MapReduce: How to join 2 tables: R(a,b) x S(c,d) where b<c -

given in each record, have

table_name(r/s) | attribute_1(a/c) | attribute_2(b/d) . . . 

for example can content of input file:

r|$a_1$|$b_1$ r|$a_2$|$b_2$ s|$c_1$|$d_1$ r|$a_3$|$b_3$ s|$c_2$|$d_2$ 

an output lines of:

$a_i$|$b_i$|$c_j$|$d_j$ 

,where $b_i < c_j$

(this exercise in book "mining of massive datasets". on page 22 of link: http://infolab.stanford.edu/~ullman/mmds/ch2.pdf (exercise 2.3.5). book freely available)

i've spend half day , on internet , still have no clue how solve it...