Add iso8601_difference()

src/libiso8601/400_manip.c

@@ -98,6 +98,38 @@ void iso8601_subtract_elapsed(struct iso8601_date* date, const struct iso8601_el
 
 
 
+void iso8601_difference(const struct iso8601_date* d1, const struct iso8601_date* d2,
+    struct iso8601_elapsed* per, int* sign)
+{
+    const struct iso8601_date* e1, * e2;
+
+    /* compute sign */
+    if(iso8601_lt(d1, d2)) {
+        if(sign) *sign = -1;
+        e1 = d2;
+        e2 = d1;
+    } else {
+        if(sign) *sign = 1;
+        e1 = d1;
+        e2 = d2;
+    }
+
+    if(!per) return;
+
+    /* compute difference, knowing that e1 >= e2 */
+    per->sec = (e1->day - e2->day) * 86400
+        + iso8601_leap_elapsed(e2, e1 /* start, end */)
+        + (e1->sec - e2->sec);
+    if(e1->nsec < e2->nsec) {
+        --per->sec;
+        per->nsec = BILLION + e1->nsec - e2->nsec;
+    } else {
+        per->nsec = e1->nsec - e2->nsec;
+    }
+}
+
+
+
 /* options for text editors
 kate: replace-trailing-space-save true; space-indent true; tab-width 4;
 vim: expandtab:ts=4:sw=4

src/libiso8601/400_manip.h

@@ -9,8 +9,8 @@
 
 /*! \defgroup manip Manipulation routines.
 
-This set of functions is useful for converting dates and times into formats understood by humans,
-and vice versa.
+This set of functions is useful for performing arithmetic etc. on dates. It uses 
+struct iso8601_elapsed to represent the magnitude of time differences.
 
 */
 /*!@{*/
@@ -73,6 +73,23 @@ void iso8601_subtract_elapsed(struct iso8601_date* date, const struct iso8601_el
 
 
 
+/*! \brief Find difference between dates.
+
+\param d1 First date.
+\param d2 Second date.
+\param[out] per Magnitude of period elapsed between two dates. Pointer may be 0.
+\param[out] sign Set to sign of difference (-1 or +1). Pointer may be 0.
+
+This function will perform the calculation <code>|d1 - d2|</code>, storing the result in \a per (if
+it is not a null pointer). The sign of the result is stored in \a sign (if it is not a null 
+pointer), i.e. -1 if \a d2 &gt; \a d1 or +1 if \a d2 &lt;= \a d1.
+
+*/
+void iso8601_difference(const struct iso8601_date* d1, const struct iso8601_date* d2,
+    struct iso8601_elapsed* per, int* sign);
+
+
+
 /*!@}*/
 /* options for text editors
 kate: replace-trailing-space-save true; space-indent true; tab-width 4;

src/tests/manip.c

@@ -159,6 +159,7 @@ void usage(void)
         "Operators:\n"
         "   +   (date, period)      Advance date by elapsed time, push date.\n"
         "   -   (date, period)      Regress date by elapsed time, push date.\n"
+        "   dp  (date, date)        Compute difference, print elapsed time.\n"
         "   <   (date, date)        Prints 1 if first date less than second.\n"
         "   <=  (date, date)        Prints 1 if first date less than or equal to second.\n"
         "   ==  (date, date)        Prints 1 if dates equal.\n"
@@ -177,8 +178,7 @@ int parse_command(char* cmd)
     struct iso8601_elapsed period;
     double period_d;
     char date_str[40];
-
-    int ret = 0;
+    int ret = 0, sign;
 
     cmd = strtok(cmd, " \n");
     while(cmd) {
@@ -209,6 +209,19 @@ int parse_command(char* cmd)
 
             }
 
+        } else if(!strcmp(cmd, "dp")) {
+            if(date_stack_pop_date(&date, &details) || date_stack_pop_date(&date2, &details2)) {
+                ret = 1;
+
+            } else {
+                iso8601_difference(&date2, &date, &period, &sign);
+                printf("Difference: %c %lu.%09lu\n",
+                    (sign < 0) ? '-' : '+',
+                    (unsigned long)period.sec,
+                    (unsigned long)period.nsec);
+
+            }
+
         } else if(!strcmp(cmd, "<")) {
             if(date_stack_pop_date(&date, &details) || date_stack_pop_date(&date2, &details2)) {
                 ret = 1;