random <- function(){return(runif(1,0,1))}
dice <- function(n=6){return(1+trunc(n*runif(1,0,1)))}
even <- function(n){return(0 == n %% 2)}

GilbertNet <-
# generates a random undirected graph of Gilbert type
# with edge probability  p  or average degree  adeg  and
# stores it on the file  fnet  in Pajek's format
#
# Example:
#   GilbertNet('testGi.net',100, adeg=3.5)
# -------------------------------------------------------
# by Vladimir Batagelj, R version: Ljubljana, 16. Dec 2004
# based on ALG.1 from: V. Batagelj, U. Brandes:
#   Efficient generation of large random networks
function(fnet,n,p=0,adeg=0){
  net <- file(fnet,"w"); cat("*vertices",n,"\n",file=net)
  if (adeg > 0) {p <- adeg/(n-1)} else {adeg <- p*(n-1)}
  cat('% random Gilbert undirected graph G(n,p) / p = ',
    p,' av_deg = ', adeg, '\n', file=net)
#  for (i in 1:n) cat(i," \"v",i,"\"\n",sep="",file=net)
  cat("*edges\n",file=net);
  v <- 0; m <- 0; u <- -1; lq <- log(1-p)
  while (v < n){
    u <- u + 1 + trunc( log(1 - random()) / lq )
    while (u >= v && v < n) {u <- u - v; v <- v + 1}
    if (v < n) {m <- m + 1; cat(u+1,v+1,'\n',file=net)}
  }
  close(net)
}

ErdosRenyiNet <-
# generates a random undirected graph of Erdos-Renyi type
# with edge probability  p  or average degree  adeg  and
# stores it on the file  fnet  in Pajek's format
#
# Example:
#   ErdosRenyiNet('testER.net',100,175)
# -------------------------------------------------------
# by Vladimir Batagelj, R version: Ljubljana, 20. Dec 2004
# based on ALG.2 from: V. Batagelj, U. Brandes:
#   Efficient generation of large random networks
function(fnet,n,m){
  net <- file(fnet,"w"); cat("*vertices",n,"\n",file=net)
  cat('% random Erdos-Renyi undirected graph G(n,m) / m = ',
    m,'\n',file=net)
#  for (i in 1:n) cat(i," \"v",i,"\"\n",sep="",file=net)
  cat("*edges\n",file=net); L <- new.env(TRUE,NULL)
  for (i in 1:m){
    repeat { u <- dice(n); v <- dice(n)
      if (u!=v) {
        edge <- if (u<v) paste(u,v) else paste(v,u)
        if (!exists(edge,env=L,inherits=FALSE)) break }
    }
    assign(edge,0,env=L); cat(edge,'\n',file=net)
  }
  close(net)
}

ScaleFreeBrstNet <-
# generates a random directed scale free graph
# on n vertices with d attachments to existing vertices
# and stores it on the file  fnet  in Pajek's format
#
# Example:
#   ScaleFreeBrstNet('testSF.net',100, 3)
# -------------------------------------------------------
# by Vladimir Batagelj, R version: Ljubljana, 18. Dec 2004
# based on ALG.5 from: V. Batagelj, U. Brandes:
#   Efficient generation of large random networks
function(fnet,n,d){
  net <- file(fnet,"w"); cat("*vertices",n,"\n",file=net)
  k <- 0; m <- n*d; L <- rep(0,2*m)
  cat('% random scale free graph / n = ',
    n,' d = ', d, '\n', file=net)
#  for (i in 1:n) cat(i," \"v",i,"\"\n",sep="",file=net)
  for(v in 1:n) for (i in 1:d) {
    k <- k+1; L[[k]] <- v; r <- dice(k);
    k <- k+1; L[[k]] <- L[[r]]
  }
  cat("*arcs\n",file=net)
  for (i in 1:m) cat(L[[2*i-1]],L[[2*i]],'\n',file=net)
  close(net)
}

ScaleFreeBipNet <-
# generates a random scale free bipartite graph
# on n+n vertices with d attachments to existing vertices
# and stores it on the file  fnet  in Pajek's format
#
# Example:
#   ScaleFreeBipNet('testSFbi.net',100, 3)
# -------------------------------------------------------
# by Vladimir Batagelj, R version: Ljubljana, 19. Dec 2004
# based on ALG.6 from: V. Batagelj, U. Brandes:
#   Efficient generation of large random networks
function(fnet,n,d){
  net <- file(fnet,"w")
  k <- 0; m <- n*d; L <- rep(0,2*m); M <- rep(0,2*m)
  cat("*vertices",2*n,n,"\n",file=net)
  cat('% random scale free bipartite graph / n = ',
    n,' d = ', d, '\n', file=net)
  for (i in 1:n) cat(i," \"v",i,"\" ",i/(n+1)," 0.1\n",sep="",file=net)
  for (i in 1:n) cat(n+i," \"v",n+i,"\" ",i/(n+1)," 0.9\n",sep="",file=net)
  for(v in 1:n) for (i in 1:d) {
    k <- k+1; L[[k]] <- v; M[[k]] <- n+v
    r <- dice(k); s <- dice(k); k <- k+1
    L[[k]] <- if (even(r)) L[[r]] else M[[r]]
    M[[k]] <- if (even(s)) M[[s]] else L[[s]]
  }
  cat("*edges\n",file=net)
  for (i in 1:m) cat(L[[2*i-1]]," ",L[[2*i]],'\n',
    M[[2*i]]," ",M[[2*i-1]],'\n',sep="",file=net)
  close(net)
}

# GilbertNet('testGi.net',100, adeg=3.5)
# ErdosRenyiNet('testER.net',100,175)
# ScaleFreeBrstNet('testSF.net',100, 3)
# ScaleFreeBipNet('testSFbi.net',100, 3)