Moved to a package: https://github.com/Protonk/preroll

legis.R

# fast json library for R. 
# drop in replacement for rjson (another library)
# use whatever you prefer
library(RJSONIO)

## hardcoded for this test 

ny.json <- fromJSON("/Users/protonk/dev/R/nomnom/data.json")

unflatten <- function(json) {
  # build vectors of possible names
  # bit magical, but we strip list names, then
  # convert to a flat character vector and extract those
  legis.names <- unique(names(unlist(unname(json))))
  # prefilling the vector outside the loop is a nice 
  # performance gain
  prefill <- vector("numeric", length = length(legis.names))
  sortStretch <- function(orig) {
    # Assign all to NA, so that...
    prefill[] <- NA
    # we replace ONLY those elements whose names occur in
    # this list element's vector with the content of
    # this element's vector
    prefill[match(names(orig), legis.names)] <- orig
    return(prefill)
  }
  square <- lapply(json, sortStretch)
  # pack into a matrix, now that they are the same length
  # then transpose because that's what rollcall() expects
  out.mat <- t(do.call(rbind, square))
  # name (we've already assured that the sorting order will be correct)
  rownames(out.mat) <- legis.names
  return(out.mat)
}
ny.mat <- unflatten(ny.json)