# 部品
split_gyymmdd <- function (gyymmdd) {
  # "4290121" --> c("4", "29", "01", "21")
  g  = substr (gyymmdd, 1, 1)
  yy = substr (gyymmdd, 2, 3)
  mm = substr (gyymmdd, 4, 5)
  dd = substr (gyymmdd, 6, 7)

  return (c(g, yy, mm, dd))
}

wareki2seireki <- function (gyymmdd) {
  # gyymmdd --> yyyymmdd
  x = split_gyymmdd (gyymmdd)
  g  = x[1]
  yy = as.integer (x[2])
  mm = x[3]
  dd = x[4]

  yyyy_i = switch (g,
                   "1" = yy + 1867,
                   "2" = yy + 1911,
                   "3" = yy + 1925,
                   "4" = yy + 1988
  )
  yyyy = as.character (yyyy_i)

  yyyymmdd = paste (yyyy, mm, dd, sep="")
  return (yyyymmdd)
}

# 値確認
is_7digits <- function (str) {
  # "1234567" --> TRUE
  # "1 3 5 7" --> FALSE
  trimmed_str = gsub ("[[:blank:]]", "", str)
  return (nchar (trimmed_str) == 7)
}

is_valid_g <- function (gyymmdd) {
  # g(元号)が有効な値 --> TRUE
  g = as.integer (split_gyymmdd (gyymmdd)[1])
  return (1 <= g && g <= 4)
}

is_valid_gyy <- function (gyymmdd) {
  # g(元号)に対するyy(年)が有効な値 --> TRUE
  x = split_gyymmdd (gyymmdd)
  g  = x[1]
  yy = as.integer (x[2])

  result = switch (g,
                   "1" = (1 <= yy && yy <= 45),
                   "2" = (1 <= yy && yy <= 15),
                   "3" = (1 <= yy && yy <= 64),
                   "4" = (1 <= yy && yy <= 29)
  )
  return (result)
}

is_valid_date <- function (yyyymmdd) {
  # yyyymmdd(西暦) が日付型として扱える  --> TRUE
  # 2/29のチェック等も可 ex.) "20170229" --> FALSE
  return (!(is.na (as.Date (yyyymmdd, format="%Y%m%d"))))
}