A fast method of getting the two-mode projection of an edgelist network.

fastTwoModeProject.r

twoModeProject <- function(w, m=1, ...) {
  if(!require(parallel)) {
    stop("Requires the parallel package.")
  }
  if(!require(data.table)) {
    stop("Requires the data.table package.")
  }
  
  if(!("data.table" %in% class(w))) {
    w <- data.table(w)
  }
  
  uid <- as.character(unlist(unique(w[,.SD,.SDcols=m])))
  mc.name <- names(w)[m] # mode choice name
  mo.name <- names(w)[ifelse(m == 1, 2, 1)] # mode off-choice name
  
  # This is a worker function for the parallel processing
  # the parameter, q, is a mode.choice id
  getModeOverlap <- function(q) {
    q <- as.character(q)
    m2 <- w[eval(parse(text=sprintf("%s == q", mc.name))),
            eval(parse(text=mo.name))]
    x <- w[eval(parse(text=paste(mo.name,"%in% m2",sep=""))), list(overlap = .N), by=mc.name]
    x$x1 <- q
    setnames(x, mc.name, "x2")
    setcolorder(x, c("x1", "x2", "overlap"))
    return(x)
  }
  
  getModeOverlap(uid[1])
  
  pp <- do.call('rbind', mclapply(uid, function(x) { getModeOverlap(x) }, ...))
  
  return(pp)  
}