使用AWK在shell中生成日历小程序

最近在学习sed和awk时，发现了一本入门级别的好书：《Software Design 中文版 03》。
我们这里的日历程序也是从那里得来，这里强烈推荐希望了解sed和awk的同志们入手本书。

代码段：

#  在shell中生成日历
#  使用方式
#  awk -f thisFile +2017
#  awk -f thisFile +2017.10
#  awk -f thisFile +2017.10.10
#
function whichWeek(y, m, d, s) {
    s = 0
    if (((y%4 == 0) && (y%100 != 0)) || (y%400 == 0)) if (M[2] == 28) M[2]++
    if (y>0) s += --y + int(y/4) - int(y/100) + int(y/400)
    while (m > 1) s += M[--m]
    return (s+d)%7
}

function printCalendar(y, m, d, nowWeek, i){
  printf("%d%s %d%s\n", y, "年", m, "月")
  print "日 一 二 三 四 五 六 "
  printf("%s", repeat(" ", ((nowWeek+1)%7)*3))
  for (i=1; i<=M[m]; i++) {
    if ((nowWeek+i) % 7 != 6) {
      printf("%2d ", i)
    } else {
      printf("%2d \n", i)
    }
  }
  print "\n"
}
function repeat(flag, time, i, str){
  for (i=0; i<time; i++) {
    str = str flag
  }

  return str
}
function option(opt) {
  if (ARGC > 1 && ARGV[1] ~ /^\+.*$/) {
    print ARGC, ARGV[1]
    opt = substr(ARGV[1], 2)
    delete ARGV[1]
  }
  return opt
}
BEGIN {
  M[1] = 31
  M[2] = 28
  M[3] = 31
  M[4] = 30
  M[5] = 31
  M[6] = 30
  M[7] = 31
  M[8] = 31
  M[9] = 30
  M[10] = 31
  M[11] = 30
  M[12] = 31
  W[0] = "日"
  W[1] = "一"
  W[2] = "二"
  W[3] = "三"
  W[4] = "四"
  W[5] = "五"
  W[6] = "六"
  split(option(), date, ".")
  nowWeek = whichWeek(date[1]+0, date[2]+0, date[3]+0)
  if (date[3] != "") {
    print "星期" W[nowWeek]
  } else if (date[2] != "") {
    printCalendar(date[1]+0, date[2]+0, M[date[2]], nowWeek)
  } else {
    for (j=1; j<=12; j++) {
      printCalendar(date[1]+0, j, M[i], whichWeek(date[1]+0, j, 0))
    }
  }
}