ㄋ

library(pipeR)
library(stringr)
library(data.table)
library(plyr)

x <- replicate(10000L, letters %>>% sample(sample(5:15)) %>>%
                 str_c(collapse = ""))

system.time({
  DT <- lapply(x, function(s){
    str_sub(s, 1L:(max(nchar(s))-1L), 2L:max(nchar(s))) %>>%
      table %>>% as.matrix %>>% t %>>% data.table
  }) %>>%
    rbindlist(fill = TRUE) %>>%
    `[`(j = lapply(.SD, function(x) mapvalues(x, NA_integer_, 0L)))
  pairs <- letters %>>% outer(., str_c) %>>% as.vector
  DT[ , eval(setdiff(pairs, names(DT))) := 0L]
})