millions of comparisons, speed wanted

Dear all,

I have a 10 columns matrix which has 2^10=1024 unique rows, 
with values 0 and 
1.
What I would like to do is a little complicated; in a simple 
statement, for a 
subset (say 1000 rows) I want to perform pairwise comparisons 
between all 
rows, find out which rows differ by only one  value, replace 
that value by 
"x", get rid of the comparisons that differ by more than one 
value and repeat 
the algorithm until no further minimization is possible. Any 
row that hasn't 
been minimized is kept for the next iteration.

For 1,000 rows, there are almost 500,000 pairs, but in the 
next iterations I 
could get some 5,000 rows which generates something like 12.5 
millions pairs, 
and that takes a _very_ long time.

The code I have created (10 lines, below) is super fast 
(using vectorization) 
but only for a reasonable number of rows. I am searching for:
- ways to improve my code (if possible)
- ideas: create a C code for the slow parts of the code? use 
MySQL? other 
ways?

As a toy example, having an input matrix called "input", my 
algorithm looks 
like this:

## code start
ncolumns <- 6
input <- bincombinations(ncolumns) # from package e1071
# subset, let's say 97% of rows
input <- input[sample(2^ncolumns, round(2^ncolumns*0.97, 0), ]
minimized <- 1

while (sum(minimized) > 0) {

   minimized <- logical(nrow(input))

   to.be.compared <- combn2(1:nrow(input)) # from package combinat
   
   # the following line takes _a lot_ of time, for millions 
of comparisons
   logical.result <- apply(to.be.compared, 1, function(idx) 
input[idx[1], ] == 
input[idx[2], ])

   compare.minimized <- which(colSums(!logical.result) == 1)

   logical.result <- logical.result[, compare.minimized]

   result <- sapply(compare.minimized, function(idx) 
input[to.be.compared[idx, 
1], ])

   result[!logical.result] <- "x"

   
minimized[unique(as.vector(to.be.compared[compare.minimized, 
]))] <- TRUE

   if (sum(minimized) > 0) {
      input <- rbind(input[!minimized, ], unique(t(result)))
   }
}
## code end

Any suggestion is welcomed, thank you very much in advance.
Adrian

-- 
Adrian DUSA
Romanian Social Data Archive
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050025 Bucharest sector 5
Romania
Tel./Fax: +40 21 3126618 \
          +40 21 3120210 / int.101