timeadd: support time difference

1 files changed, 61 insertions, 6 deletions

diff --git a/timeadd.c b/timeadd.c
index 061a0ae..d2f0159 100644
--- a/timeadd.c
+++ b/timeadd.c
@@ -8,22 +8,77 @@
 #include <time.h>
 #include <string.h>
 #include <stdlib.h>
+#include <unistd.h>
+#include <stdbool.h>
 
-int main() {
+static void print_usage(const char *program_name) {
+	fprintf(stderr, "Usage: %s [-d]\n", program_name);
+	fputs("Options:\n"
+		" -d   Display time difference instead of a monotonic clock\n",
+		stderr);
+}
+
+struct timespec timediff(struct timespec begin, struct timespec end) {
+	struct timespec diff;
+
+	diff.tv_sec = end.tv_sec - begin.tv_sec;
+	diff.tv_nsec = end.tv_nsec - begin.tv_nsec;
+
+	if (end.tv_nsec < begin.tv_nsec) {
+		/* alternative case: 3.2 -> 4.1, diff is 0.9 */
+		diff.tv_sec--;
+		diff.tv_nsec += 1000000000;
+	}
+
+	return diff;
+}
+
+int main(int argc, char **argv) {
+	const char *out_format;
 	char buf[4096];
+	struct timespec tp[2] = { { 0 } };
+	int tp_i = 0;
+	int opt;
+	bool diff = false;
+
+	while ((opt = getopt(argc, argv, "d")) != -1) {
+		switch (opt) {
+		case 'd':
+			diff = true;
+			break;
+		default:
+			print_usage(argv[0]);
+			return 1;
+		}
+	}
+
+	if (diff) {
+		out_format = "[%+6lld.%06lu] %s\n";
+	} else {
+		out_format = "[%6llu.%06lu] %s\n";
+	}
 
 	while (fgets(buf, sizeof(buf), stdin) != NULL) {
 		char *newline = strrchr(buf, '\n');
-		struct timespec tp;
 
-		clock_gettime(CLOCK_MONOTONIC, &tp);
+		clock_gettime(CLOCK_MONOTONIC, &tp[tp_i]);
 
 		if (newline)
 			*newline = '\0';
 
-		printf("[%5llu.%06lu] %s\n",
-			(long long) tp.tv_sec,
-			tp.tv_nsec / 1000,
+		if (diff) {
+			/* new time is available in "tp_i". Old time (and print
+			 * target) is the inverse of tp_i (modulo 2). */
+			tp[tp_i ^ 1] = timediff(tp[tp_i ^ 1], tp[tp_i]);
+			/* flip such that "tp_i" refers to difference time and
+			 * inverse "tp_i" points to the new time (in the next
+			 * iteration it becomes the old time) */
+			tp_i ^= 1;
+		}
+
+		printf(out_format,
+			(long long) tp[tp_i].tv_sec,
+			tp[tp_i].tv_nsec / 1000,
 			buf);
 	}