27.1 Comparison networks

Sorting networks are comparison networks that always sort their inputs, so it makes sense to begin our discussion with comparison networks and their characteristics. A comparison network is composed solely of wires and comparators. A comparator, shown in Figure 27.1(a), is a device with two inputs, x and y, and two outputs, x' and y', that performs the following function:

Because the pictorial representation of a comparator in Figure 27.1(a) is too bulky for our purposes, we shall adopt the convention of drawing comparators as single vertical lines, as shown in Figure 27.1(b). Inputs appear on the left and outputs on the right, with the smaller input value appearing on the top output and the larger input value appearing on the bottom output. We can thus think of a comparator as sorting its two inputs.

We shall assume that each comparator operates in O(1) time. In other words, we assume that the time between the appearance of the input values x and y and the production of the output values x' and y' is a constant.

A wire transmits a value from place to place. Wires can connect the output of one comparator to the input of another, but otherwise they are either network input wires or network output wires. Throughout this chapter, we shall assume that a comparison network contains n input wires a₁, a₂,...,a_n, through which the values to be sorted enter the network, and n output wires b₁, b₂,...,b_n, which produce the results computed by the network. Also, we shall speak of the input sequence 〈a₁, a₂,...,a_n〉 and the output sequence 〈b₁, b₂,...,b_n〉, referring to the values on the input and output wires. That is, we use the same name for both a wire and the value it carries. Our intention will always be clear from the context.

Figure 27.2 shows a comparison network, which is a set of comparators interconnected by wires. We draw a comparison network on n inputs as a collection of n horizontal lines with comparators stretched vertically. Note that a line does not represent a single wire, but rather a sequence of distinct wires connecting various comparators. The top line in Figure 27.2, for example, represents three wires: input wire a₁, which connects to an input of comparator A; a wire connecting the top output of comparator A to an input of comparator C; and output wire b₁, which comes from the top output of comparator C. Each comparator input is connected to a wire that is either one of the network's n input wires a₁, a₂,...,a_n or is connected to the output of another comparator. Similarly, each comparator output is connected to a wire that is either one of the network's n output wires b₁, b₂,...,b_n or is connected to the input of another comparator. The main requirement for interconnecting comparators is that the graph of interconnections must be acyclic: if we trace a path from the output of a given comparator to the input of another to an output to an input, etc., the path we trace must never cycle back on itself and go through the same comparator twice. Thus, as in Figure 27.2, we can draw a comparison network with network inputs on the left and network outputs on the right; data move through the network from left to right.

Each comparator produces its output values only when both of its input values are available to it. In Figure 27.2(a), for example, suppose that the sequence 〈9, 5, 2, 6〉 appears on the input wires at time 0. At time 0, then, only comparators A and B have all their input values available. Assuming that each comparator requires one time unit to compute its output values, comparators A and B produce their outputs at time 1; the resulting values are shown in Figure 27.2(b). Note that comparators A and B produce their values at the same time, or "in parallel." Now, at time 1, comparators C and D, but not E, have all their input values available. One time unit later, at time 2, they produce their outputs, as shown in Figure 27.2(c). Comparators C and D operate in parallel as well. The top output of comparator C and the bottom output of comparator D connect to output wires b₁ and b₄, respectively, of the comparison network, and these network output wires therefore carry their final values at time 2. Meanwhile, at time 2, comparator E has its inputs available, and Figure 27.2(d) shows that it produces its output values at time 3. These values are carried on network output wires b₂ and b₃, and the output sequence 〈2, 5, 6, 9〉 is now complete.

Under the assumption that each comparator takes unit time, we can define the "running time" of a comparison network, that is, the time it takes for all the output wires to receive their values once the input wires receive theirs. Informally, this time is the largest number of comparators that any input element can pass through as it travels from an input wire to an output wire. More formally, we define the depth of a wire as follows. An input wire of a comparison network has depth 0. Now, if a comparator has two input wires with depths d_x and d_y, then its output wires have depth max(d_x, d_y) + 1. Because there are no cycles of comparators in a comparison network, the depth of a wire is well defined, and we define the depth of a comparator to be the depth of its output wires. Figure 27.2 shows comparator depths. The depth of a comparison network is the maximum depth of an output wire or, equivalently, the maximum depth of a comparator. The comparison network of Figure 27.2, for example, has depth 3 because comparator E has depth 3. If each comparator takes one time unit to produce its output value, and if network inputs appear at time 0, a comparator at depth d produces its outputs at time d; the depth of the network therefore equals the time for the network to produce values at all of its output wires.

A sorting network is a comparison network for which the output sequence is monotonically increasing (that is, b₁ ≤ b₂ ≤ ··· ≤ b_n) for every input sequence. Of course, not every comparison network is a sorting network, but the network of Figure 27.2 is. To see why, observe that after time 1, the minimum of the four input values has been produced by either the top output of comparator A or the top output of comparator B. After time 2, therefore, it must be on the top output of comparator C. A symmetrical argument shows that after time 2, the maximum of the four input values has been produced by the bottom output of comparator D. All that remains is for comparator E to ensure that the middle two values occupy their correct output positions, which happens at time 3.

A comparison network is like a procedure in that it specifies how comparisons are to occur, but it is unlike a procedure in that its size-the number of comparators that it contains-depends on the number of inputs and outputs. Therefore, we shall actually be describing "families" of comparison networks. For example, the goal of this chapter is to develop a family SORTER of efficient sorting networks. We specify a given network within a family by the family name and the number of inputs (which equals the number of outputs). For example, the n-input, n-output sorting network in the family SORTER is named SORTER[n].