fast data.table string detection merge

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

# 產生資料
numDigits <- 6
numRows <- 1000
DT2 <- data.table(str = rollapply(sample(9, numRows*numDigits, TRUE),
                                  numDigits, function(x) str_c(x, collapse = ""),
                                  by = numDigits), value = NA_character_)

# 產生mapping table
allPatterns <- substring(DT2$str,1,3) %>>% unique %>>% sort
DT <- data.table(pattern = str_c("^", allPatterns, ".*"),
                 value = sprintf("A%03i", 1:length(allPatterns)))

# mapping開始
st <- proc.time()
for (i in 1:nrow(DT))
  set(DT2, which(str_detect(DT2$str, DT$pattern[i])),
      which(names(DT2) == "value"), DT$value[i])
proc.time() - st
#    user  system elapsed
#    0.11    0.00    0.11

print(DT2)
#          str value
#    1: 588847  A297
#    2: 472447  A225
#    3: 181823  A048
#    4: 928228  A495
#    5: 331838  A139
#   ---
#  996: 172326  A042
#  997: 522373  A253
#  998: 828978  A437
#  999: 617415  A311
# 1000: 877184  A470