Allow time offsets to be negative, e.g. `at 1530 - 15 minutes`.

This is useful if you have been given some time for some event in some
format and you want your computer to do something to prepare for it.
Without having to do time arithmetic in a shellscript.

The syntax matches what the at(1) usually used on Linux supports.

1 files changed, 49 insertions, 13 deletions

diff --git a/usr.bin/at/parsetime.c b/usr.bin/at/parsetime.c
index 8c83bcc..0604a1c 100644
--- a/usr.bin/at/parsetime.c
+++ b/usr.bin/at/parsetime.c
@@ -64,7 +64,7 @@ enum {	/* symbols */
     MIDNIGHT, NOON, TEATIME,
     PM, AM, TOMORROW, TODAY, NOW,
     MINUTES, HOURS, DAYS, WEEKS, MONTHS, YEARS,
-    NUMBER, PLUS, DOT, SLASH, ID, JUNK,
+    NUMBER, PLUS, MINUS, DOT, SLASH, ID, JUNK,
     JAN, FEB, MAR, APR, MAY, JUN,
     JUL, AUG, SEP, OCT, NOV, DEC,
     SUN, MON, TUE, WED, THU, FRI, SAT
@@ -246,6 +246,8 @@ token(void)
 	    return sc_tokid = DOT;
 	else if (sc_token[0] == '+')
 	    return sc_tokid = PLUS;
+	else if (sc_token[0] == '-')
+	    return sc_tokid = MINUS;
 	else if (sc_token[0] == '/')
 	    return sc_tokid = SLASH;
 	else
@@ -277,22 +279,14 @@ expect(int desired)
 
 
 /*
- * plus() parses a now + time
- *
- *  at [NOW] PLUS NUMBER [MINUTES|HOURS|DAYS|WEEKS|MONTHS|YEARS]
- *
+ * plus_or_minus() holds functionality common to plus() and minus()
  */
-
 static void
-plus(struct tm *tm)
+plus_or_minus(struct tm *tm, int delay)
 {
-    int delay;
     int expectplur;
 
-    expect(NUMBER);
-
-    delay = atoi(sc_token);
-    expectplur = (delay != 1) ? 1 : 0;
+    expectplur = (delay != 1 && delay != -1) ? 1 : 0;
 
     switch (token()) {
     case YEARS:
@@ -323,11 +317,43 @@ plus(struct tm *tm)
     tm->tm_isdst = -1;
     if (mktime(tm) < 0)
 	plonk(sc_tokid);
+} /* plus_or_minus */
+
+
+/*
+ * plus() parses a now + time
+ *
+ *  at [NOW] PLUS NUMBER [MINUTES|HOURS|DAYS|WEEKS|MONTHS|YEARS]
+ *
+ */
+static void
+plus(struct tm *tm)
+{
+    int delay;
+
+    expect(NUMBER);
 
+    delay = atoi(sc_token);
+    plus_or_minus(tm, delay);
 } /* plus */
 
 
 /*
+ * minus() is like plus but can not be used with NOW
+ */
+static void
+minus(struct tm *tm)
+{
+    int delay;
+
+    expect(NUMBER);
+
+    delay = -atoi(sc_token);
+    plus_or_minus(tm, delay);
+} /* minus */
+
+
+/*
  * tod() computes the time of day
  *     [NUMBER [DOT NUMBER] [AM|PM]]
  */
@@ -379,7 +405,8 @@ tod(struct tm *tm)
      * if we've gone past that time - but if we're specifying a time plus
      * a relative offset, it's okay to bump things
      */
-    if ((sc_tokid == EOF || sc_tokid == PLUS) && tm->tm_hour > hour) {
+    if ((sc_tokid == EOF || sc_tokid == PLUS || sc_tokid == MINUS) && 
+	tm->tm_hour > hour) {
 	tm->tm_mday++;
 	tm->tm_wday++;
     }
@@ -456,6 +483,9 @@ month(struct tm *tm)
     case PLUS:
 	    plus(tm);
 	    break;
+    case MINUS:
+	    minus(tm);
+	    break;
 
     case TOMORROW:
 	    /* do something tomorrow */
@@ -588,6 +618,12 @@ parsetime(int argc, char **argv)
 	    plus(&runtime);
 	    break;
 
+	    /* MINUS is different from PLUS in that NOW is not
+	     * an optional prefix for it
+	     */
+    case MINUS:
+	    minus(&runtime);
+	    break;
     case NUMBER:
 	    tod(&runtime);
 	    month(&runtime);