Add ability to specify random number distribution in stochastic rounding

chop.m

@@ -50,6 +50,12 @@
 %      underflow, or subnormal numbers will be produced only if necessary 
 %      for the data type of X.  This option is useful for exploring
 %      low precisions independent of range limitations.
+%   6. If options.randfunc is supplied, then in stochastic rounding (modes
+%      5 and 6) the random numbers used for rounding will be generated
+%      using that function. It should be a function that has a single argument
+%      for the number of random numbers to generate and returns a vector of
+%      the random numbers. If this option is not specified, the default 
+%      function using MATLAB's rand() is used.
 %
 %   On the first call: if options is omitted or only partially specified 
 %   the defaults stated above are used.
@@ -79,6 +85,7 @@
       fpopts.format = 'h'; fpopts.subnormal = 1;
       fpopts.round = 1; fpopts.flip = 0; fpopts.p = 0.5;
       fpopts.explim = 1;
+      fpopts.randfunc = @(n) rand(n, 1);
       reset_format_settings = 1;
 elseif nargin == 2 && ~isempty(options)
     % This is not the first call, but fpopts might have all empty fields.
@@ -116,6 +123,11 @@
     else
        fpopts.explim = 1;
     end
+    if isfield(options,'randfunc') && ~isempty(options.randfunc)
+       fpopts.randfunc = options.randfunc;
+    else
+       fpopts.randfunc = @(n) rand(n, 1);
+    end
     reset_format_settings = 1;
 end
 

roundit.m

@@ -16,13 +16,19 @@
 %   If options.flip = 1 (default 0) then each element of the rounded result 
 %   has, with probability options.p (default 0.5), a randomly chosen bit
 %   (in its binary representation) flipped. 
+%   The random number generator used for stochastic rounding can be
+%   specified by setting options.randfunc to an anonymous function that
+%   takes as an argument the number of random numbers to generate and
+%   returns a vector of the random numbers. By default, this is the MATLAB
+%   rand function.
 
 if nargin < 2 || isempty(options) 
    options.round = 1; options.flip = 0; options.p = 0.5;
 end    
 if ~isfield(options,'round'), options.round = 1; end
 if ~isfield(options,'flip'), options.flip = 0; end
 if ~isfield(options,'p'), options.p = 0.5; end
+if ~isfield(options,'randfunc'), options.randfunc = @(n) rand(n, 1); end
 
 mysign = @(x) sign(x) + (x==0); % mysign(0) = 1;
 
@@ -57,7 +63,7 @@
     if isempty(k)
        y = x; 
     else   
-      rnd = rand(length(k),1);
+      rnd = options.randfunc(length(k));
       vals = frac(k);  vals = vals(:);
 
       switch options.round

test_roundit.m

@@ -112,6 +112,16 @@
 end
 assert_eq(true, abs(j / nsamp - options.p) <= 1e-2)
 
+% Test custom random number generator function
+% Force round up behavior by biasing generator
+options = rmfield(options,'p');
+options = rmfield(options,'flip');
+options.round = 6;
+options.randfunc = @(n) rand(n,1) + 0.6;
+A = [0 -1.1 -1.5; -1.9 -2.4 -0.5];
+y = roundit(A,options);
+assert_eq(y,[0 -1 -1; -1 -2 0])
+
 fprintf('All tests successful!\n')
 
 %%%%%%%%%%%%%%%%%%%%%%%