SORTING NETWORKS

SORTING NETWORKS

Overview Sorting Network Components Comparison Networks Comparator Zero-one Principle Bitonic Sorting Network Half-Cleaner Bitonic Sorter Merging Network Sorting Network

Comparison Networks • Sorting networks are Comparison Networks that always sort their inputs • Comparison Network composed of: • wires • comparators • Wires: transmits values from place to place • Comparators: two inputs  two outputs

Comparison Networks • Comparators: • inputs: x and y • outputs: x’ and y’ • x’ = min(x, y) • y’ = max(x, y) input output x   x’ = min(x, y) y  y’ = max(x, y) x ------------------------------ x’ = min(x, y) y ------------------------------ y’ = max(x, y) comparator

Comparison Networks • Comparators • n input wires: a1, a2, …, an • input sequence: < a1, a2, …, an> • n output wires: b1, b2, …, bn • output sequence: <b1, b2, …, bn> • Properties: • requirement: graph is acyclic • output produced only when input is available • comparators process in parallel if input is available

Comparison Networks comparators a1 9 ----------------------------------------- b1 a2 5 ----------------------------------------- b2 a3 2 ----------------------------------------- b3 a4 6 ----------------------------------------- b4 values A C E D B

Comparison Networks outputs a1 9 ----------------------------------------- 2 b1 a2 5 ----------------------------------------- 5 b2 a3 2 ----------------------------------------- 6 b3 a4 6 ----------------------------------------- 9 b4 depth: 1 1 2 2 3 depth starts at 0 input wires depth: dx and dy output wires depth: max(dx, dy) + 1 5 2 A C 9 6 E D 2 5 B 6 9

Comparison Networks • Sorting Network is a Comparison Network where the output is sorted • output sequence is monotonically increasing • (b1 <= b2 …<= bn) for every input sequence • Not all Comparison Networks are Sorting Networks • Comparison Network is like a procedure in that it specifies how comparisons are to occur

Zero-One Principle • If a Sorting Network works correctly when each input is drawn from the set {0, 1}, then it works correctly on arbitrary input numbers • (integers, reals, or values from any linearly sorted set)

Zero-One Principle • Lemma 27.1 If a Comparison Network transforms the input sequence a = <a1, a2, …, an> into the output sequence b = <b1, b2, …, bn>, then for any monotonically increasing function f, the network transforms the input sequence: f(a) = <f(a1), f(a2), …, f(an)> into the output sequence: f(b) = <f(b1), f(b2), …, f(bn)>

Zero-One Principle • Proof: • prove claim: if f is a monotonically increasing function, then a single comparator with inputs f(x) and f(y) produces output f(min(x, y)) and f(max(x, y)) f(x) ------------------------------ min(f(x), f(y)) f(y) ------------------------------ max(f(x), f(y)) min(f(x), f(y)) = f(min(x, y)) max(f(x), f(y)) = f(max(x, y))

Zero-One Principle outputs f(x) = ceiling(x/2) a1 9 ----------------------------------------- 1 b1 a2 5 ----------------------------------------- 3 b2 a3 2 ----------------------------------------- 3 b3 a4 6 ----------------------------------------- 5 b4 3 1 A C 5 3 E D 1 3 B 3 5

Zero-One Principle • Theorem 27.2 (Zero-One Principle) If a Comparison Network with n inputs sorts all 2n possible sequences of 0’s and 1’s correctly, then it sorts all sequences of arbitrary numbers correctly.

Zero-One Principle • Proof: By contradiction: Suppose that the network sorts all zero-one sequences, but there exists a sequence of arbitrary numbers that the network does not sort: <a1, a2, …, an> contains elements ai and aj where ai < aj but the network places aj before ai in the output sequence

Zero-One Principle • Define monotonically increasing function f: f(x) = { 0 if x <= ai, 1 if x > ai. aj is placed before ai in the output sequence f(aj) placed before f(ai) when input is <a1, a2, …, an> since f(aj) = 1 and f(ai) = 0, from above we have a contradiction

Bitonic Sorting Network • Bitonic Sequence: a sequence that monotonically increases and then decreases, or can be circularly shifted to become monotonically increasing and then monotonically decreasing examples: <1, 4, 6, 8, 3, 2>, < 6, 9, 4, 2, 3, 5>, and <9, 8, 3, 2, 4, 6> zero-one: 0i1j0k or 1i0j1k when i,j,k >= 0

Bitonic Sorting Network • Half-Cleaner • a bitonic sorter is composed of several stages, each of which is called a half-cleaner • half-cleaner is a Comparison Network of depth 1 • line i compared with i + n/2 for i = 1, 2,…, n/2 • assume that n is even

Bitonic Sorting Network • Example: Half-Cleaner[8] all 1’s or 0’s 0 ---------------------------------- 0 0 ---------------------------------- 0 bitonic 1 ---------------------------------- 0 clean bitonic 1 ---------------------------------- 0 ________ 1 ---------------------------------- 1 0 ---------------------------------- 0 bitonic 0 ---------------------------------- 1 0 ---------------------------------- 1

Bitonic Sorting Network • Lemma 27.3 If the input to a half-cleaner is a bitonic sequence of 0’s and 1’s, then the output satisfies the following properties: • both the top half and the bottom half are bitonic • every element in the top half is at least as small as every element in the bottom half • at least one half is clean (either all 0’s or 1’s)

Bitonic Sorting Network • Proof: • the Comparison Network Half-Cleaner[n] compares inputs i and i + n/2 for i = 1, 2, … n/2 • suppose input is of the form: 00…011…100…0 • there are 3 possible cases for the n/2 midpoint to fall • and the situation where the midpoint falls in a block of 1’s is separated into 2 cases • So 4 cases in total

Bitonic Sorting Network • Cases: bitonic n/2 bitonic n/2 0 0 top top bitonic clean 0 0 0 0 1 1 bitonic 0 0 1 1 bottom bottom 0 0 0 0 top top 1 bitonic clean 0 0 0 0 0 1 1 bitonic 0 0 1 bottom bottom 0

Bitonic Sorting Network • Cases: bitonic n/2 bitonic n/2 0 0 top top 1 bitonic 0 1 0 1 0 1 1 1 bitonic clean 1 0 0 bottom bottom 0 0 0 top top bitonic clean 0 1 0 1 1 0 0 1 bitonic 1 1 0 0 bottom bottom 1

Bitonic Sorting Network • Bitonic Sorter by recursively combining half-cleaners, we can build a bitonic sorter, a network that sorts bitonic sequences • first stage: Half-Cleaner[n] • subsequent sorts with Bitonic-Sorter[n/2] • Depth D(n) of Bitonic-Sorter[n]: D(n) = { 0 if n =1, D(n/2) + 1 if n = 2k and k>= 1

Bitonic Sorting Network • Example: Half-Cleaner[n] Bitonic- Sorter[n/2] Bitonic- Sorter[n/2]

Bitonic Sorting Network • Example: sorted bitonic 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 1 1 1 0 1 0 0 1 0 1 1 1 0 1 1 1 0

Bitonic Sorting Network • Bitonic-Sorter can be used to sort a zero-one sequence • By the Zero-One principle it follows that any bitonic sequence of arbitrary numbers can be sorted using this network

A Merging Network • Networks that can merge two sorted input sequences into one sorted output sequence • The merging network is based on the following • Given two sorted sequences, if we reverse the second sequence and then concatenate the two, the resulting sequence is bitonic. • ie. given : X = 00001111 and Y = 000011111 YR = 111110000 X● YR = 00001111111110000

A Merging Network • We can construct MERGER[n] by modifying the first half cleaner of BITONIC-SORTER[n]. • The key is to perform the reversal of the second half of the inputs implicitly. • Given two sorted sequences <a1, a2, …, an/2> and <an/2+1, an/2+2, …, an> • Want the effect of bitonically sorting the sequence < a1, a2, …, an/2, an, an-1, an-2, ..., an/2+1>

A Merging Network • BITONIC-SORTER[n] compares inputs i and n/2+i for i= 1,2,3,…,n/2. a1 0 ---------------------------------- 0 b1 a2 0 ---------------------------------- 0 b2bitonic a3 1 ---------------------------------- 0 b3 clean bitonic a 4 1 ---------------------------------- 0 b4 a8 1 ---------------------------------- 1 b8 a7 0 ---------------------------------- 0 b7bitonic a6 0 ---------------------------------- 1 b6 a5 0 ---------------------------------- 1 b5

A Merging Network • First stage of the merging network compares inputs i and n/2 + i for i= 1,2,3,…,n/2. a1 0 ---------------------------------- 0 b1 sorted a2 0 ---------------------------------- 0 b2bitonic a3 1 ---------------------------------- 0 b3 clean a4 1 ---------------------------------- 0 b4 a5 0 ---------------------------------- 1 b5 sorted a6 0 ---------------------------------- 1 b6bitonic a7 0 ---------------------------------- 0 b7 a8 1 ---------------------------------- 1 b8

A Merging Network • Since the reversal of a bitonic sequence is bitonic sequence. • Both top and bottom outputs from the first stage of the merging network satisfy the properties of Lemma 27.3 • both the top half and the bottom half are bitonic • every element in the top half is at least as small as every element in the bottom half • at least one half is clean (either all 0’s or 1’s)

A Merging Network • The top and bottom outputs can be bitonically sorted in parallel to produce the sorted output of the merging network.

A Merging Network Bitonic- Sorter[n/2] Bitonic- Sorter[n/2]

A Merging Network 0 0 0 0 0 0 sorted 0 0 1 1 1 0 1 0 0 1 sorted 0 1 1 1 1 sorted 0 1 1 1 1 1 1 1 1 1 1

A Merging Network • First stage is different • Consequently the depth is the same as BITONIC-SORTER[n] • Depth D(n) of MERGER[n]: D(n) = { 0 if n =1, D(n/2) + 1 if n = 2k and k>= 1 • D(n) = lg n

A Sorting Network • Now we have all the necessary tools to construct a network that can sort any input sequence • We are going to use the a fore mentioned merging network to implement a parallel version of merge sort, called SORTER[n]

A Sorting Network Sorter[n/2] Merger[n] Sorter[n/2]

A Sorting Network Merger[4] Merger[2] Merger[8] Merger[2] Merger[2] Merger[4] Merger[2]

A Sorting Network 1 0 0 0 0 0 1 0 1 0 0 1 0 1 0 1 1 0 0 0 0 1 1 0 0 0 1 0 1 0 0 1 1 2 2 3 4 4 4 4 5 5 6 depth

A Sorting Network

Conclusion • Sorting Network Components • Comparison Networks • Zero-one Principle • Bitonic Sorting Network • Half-Cleaner • Bitonic Sorter • Merging Network • Sorting Networks

SORTING NETWORKS