UNITY (programming Language)

UNITY is a programming language constructed by K. Mani Chandy and Jayadev Misra for their book ''Parallel Program Design: A Foundation''. It is a theoretical language which focuses on ''what'', instead of ''where'', ''when'' or ''how''. The language contains no method of flow control, and program statements run in a nondeterministic way until statements cease to cause changes during execution. This allows for programs to run indefinitely, such as auto-pilot or power plant safety systems, as well as programs that would normally terminate (which here converge to a fixed point).

Description

All statements are assignments, and are separated by #. A statement can consist of multiple assignments, of 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 $\Theta(n)$ expected time, $\Theta(n)$ processors and $\Theta(n^2)$ expected work. The reason you only have $\Theta(n)$ ''expected'' time, is that k is always chosen randomly from $\{0,1\}$. This can be fixed by flipping k manually.

Program bubblesort
declare
n: integer,
A: array ..n-1of integer
initially
n = 20 #
<, , i : 0 <= i and i < n :: A = rand() % 100 >
assign
<# k : 0 <= k < 2 ::
<, , i : i % 2 = k and 0 <= i < n - 1 ::
A A +1:= A +1 A
if A > A +1> >
end

Rank-sort

You can sort in $\Theta(\log n)$ time with rank-sort. You need $\Theta(n^2)$ processors, and do $\Theta(n^2)$ work.

Program ranksort
declare
n: integer,
A,R: array ..n-1of integer
initially
n = 15 #
<, , i : 0 <= i < n ::
A R = rand() % 100, i >
assign
<, , i : 0 <= i < n ::
R := <+ j : 0 <= j < n and (A < A or (A = A and j < i)) :: 1 > >
#
<, , i : 0 <= i < n ::
A [i := A >
end

Floyd–Warshall algorithm

Using the [i.html" ;"title="[i">[i := A >
end

Floyd–Warshall algorithm

Using the Floyd–Warshall algorithm all pairs Shortest path problem">shortest path algorithm, we include intermediate nodes iteratively, and get $\Theta(n)$ time, using $\Theta(n^2)$ processors and $\Theta(n^3)$ work.

Program shortestpath
declare
n,k: integer,
D: array ..n-1, 0..n-1of integer
initially
n = 10 #
k = 0 #
<, , i,j : 0 <= i < n and 0 <= j < n ::
D ,j= rand() % 100 >
assign
<, , i,j : 0 <= i < n and 0 <= j < n ::
D ,j:= min(D ,j D ,k+ D ,j > , ,
k := k + 1 if k < n - 1
end

We can do this even faster. The following programs computes all pairs shortest path in $\Theta(\log^2 n)$ time, using $\Theta(n^3)$ processors and $\Theta(n^3 \log n)$ work.

Program shortestpath2
declare
n: integer,
D: array ..n-1, 0..n-1of integer
initially
n = 10 #
<, , i,j : 0 <= i < n and 0 <= j < n ::
D ,j= rand() % 10 >
assign
<, , i,j : 0 <= i < n and 0 <= j < n ::
D ,j:= min(D ,j ,k+ D ,j>) >
end

After round $r$, D ,j/code> contains the length of the shortest path from $i$ to $j$ of length $0 \dots r$. In the next round, of length $0 \dots 2r$, and so on.


References

* K. Mani Chandy and Jayadev Misra (1988) ''Parallel Program Design: A Foundation''.

Experimental programming languages