UNITY (programming language)
The UNITY programming language was constructed by K. Mani Chandy and Jayadev Misra for their book Parallel Program Design: A Foundation. It is a rather theoretical language, which tries to focus on what, instead of where, when or how. The peculiar thing about the language is that it has no flow control. The statements in the program run in a random order, until none of the statements causes change if run. A correct program converges into a fixed point.
Description
All statements are assignments, and are separated by #
. A statement can consist of multiple assignments, on the form a,b,c := x,y,z
, or a := x || b := y || c := z
. You can also have a quantified statement list, <# x,y : expression :: statement>
, where x and y are chosen randomly among the values that satisfy expression. A quantified assignment is similar. In <|| x,y : expression :: statement >
, statement is executed simultaneously for all pairs of
x
and y
that satisfy expression.
Examples
Bubble sort
Bubble sort the array by comparing adjacent numbers, and swapping them if they are in the wrong order. Using expected time, processors and expected work. The reason you only have expected time, is that k
is always chosen randomly from . This can be fixed by flipping k
manually.
Program bubblesort
declare
n: integer,
A: array [0..n-1] of integer
initially
n = 20 #
<# i : 0 <= i and i < n :: A[i] = rand() % 100 >
assign
<# k : 0 <= k < 2 ::
<|| i : i % 2 = k and 0 <= i < n - 1 ::
A[i], A[i+1] := A[i+1], A[i]
if A[i] > A[i+1] > >
end
Rank-sort
You can sort in time with rank-sort. You need processors, and do work.
Program ranksort
declare
n: integer,
A,R: array [0..n-1] of integer
initially
n = 15 #
<|| i : 0 <= i < n ::
A[i], R[i] = rand() % 100, i >
assign
<|| i : 0 <= i < n ::
R[i] := <+ j : 0 <= j < n and (A[j] < A[i] or (A[j] = A[i] and j < i)) :: 1 > >
#
<|| i : 0 <= i < n ::
A[R[i]] := A[i] >
end
Floyd-Warshall
Using the Floyd-Warshall all pairs shortest path algorithm, we include intermediate nodes iteratively, and get time, using processors and work.
Program shortestpath
declare
n,k: integer,
D: array [0..n-1, 0..n-1] of integer
initially
n = 10 #
k = 0 #
<|| i,j : 0 <= i < n and 0 <= j < n ::
D[i,j] = rand() % 100 >
assign
<|| i,j : 0 <= i < n and 0 <= j < n ::
D[i,j] := min(D[i,j], D[i,k] + D[k,j]) > ||
k := k + 1 if k < n - 1
end
We can do this even faster. The following programs computes all pairs shortest path in time, using processors and work.
Program shortestpath2
declare
n: integer,
D: array [0..n-1, 0..n-1] of integer
initially
n = 10 #
<|| i,j : 0 <= i < n and 0 <= j < n ::
D[i,j] = rand() % 10 >
assign
<|| i,j : 0 <= i < n and 0 <= j < n ::
D[i,j] := min(D[i,j], <min k : 0 <= k < n :: D[i,k] + D[k,j] >) >
end
After round , D[i,j]
contains the length of the shortest path from to of length . In the next round, of length , and so on.
References
- K. Mani Chandy and Jayadev Misra (1988) Parallel Program Design: A Foundation.