From f553dbf7db9f78667eeb6e84fe17e0977e501e92 Mon Sep 17 00:00:00 2001
From: Aaron Cook <aarongregorycook@gmail.com>
Date: Sat, 13 May 2023 14:09:16 -0500
Subject: [PATCH] #11 Add helpers for doing math with timespec structs

---
 src/timer_math.c | 71 ++++++++++++++++++++++++++++++++++++++++++++++++
 src/timer_math.h | 27 ++++++++++++++++++
 2 files changed, 98 insertions(+)
 create mode 100644 src/timer_math.c
 create mode 100644 src/timer_math.h

diff --git a/src/timer_math.c b/src/timer_math.c
new file mode 100644
index 0000000..bce2efd
--- /dev/null
+++ b/src/timer_math.c
@@ -0,0 +1,71 @@
+#include "timer_math.h"
+
+int
+timer_cmp(const struct timespec *a, const struct timespec *b)
+{
+    if (a->tv_sec < b->tv_sec) {
+        return -1;
+    }
+    else if (a->tv_sec > b->tv_sec) {
+        return 1;
+    }
+    else if (a->tv_nsec < b->tv_nsec) {
+        return -1;
+    }
+    else if (a->tv_nsec > b->tv_nsec) {
+        return 1;
+    }
+    else {
+        return 0;
+    }
+}
+
+void
+timer_add(struct timespec *a, const struct timespec *b)
+{
+    a->tv_sec += b->tv_sec;
+    a->tv_nsec += b->tv_nsec;
+
+    if (a->tv_nsec > 1000000000) {
+        a->tv_sec += 1;
+        a->tv_nsec -= 1000000000;
+    }
+}
+
+void
+timer_add_ms(struct timespec *ts, uint32_t ms)
+{
+    struct timespec tmp = {
+        .tv_sec = ms / 1000,
+        .tv_nsec = (ms % 1000) * 1000000
+    };
+    timer_add(ts, &tmp);
+}
+
+void
+timer_sub(struct timespec *a, const struct timespec *b)
+{
+    if (timer_cmp(a, b) <= 0) {
+        a->tv_sec = 0;
+        a->tv_nsec = 0;
+        return;
+    }
+
+    a->tv_sec -= b->tv_sec;
+    a->tv_nsec -= b->tv_nsec;
+
+    if (a->tv_nsec < 0) {
+        a->tv_nsec += 1000000000;
+        a->tv_sec -= 1;
+    }
+}
+
+void
+timer_sub_ms(struct timespec *ts, uint32_t ms)
+{
+    struct timespec tmp = {
+        .tv_sec = ms / 1000,
+        .tv_nsec = (ms % 1000) * 1000000
+    };
+    timer_sub(ts, &tmp);
+}
diff --git a/src/timer_math.h b/src/timer_math.h
new file mode 100644
index 0000000..99cdcd9
--- /dev/null
+++ b/src/timer_math.h
@@ -0,0 +1,27 @@
+#ifndef _TIMER_MATH_H_
+#define _TIMER_MATH_H_
+
+#include <stdint.h>
+#include <time.h>
+
+/* NOTE
+ * All math here is assuming that the second and nanosecond values of the
+ * timespecs are all non-negative, and nanoseconds is in the range [0, 999999999] */
+
+/* Compare the two timespecs
+ * Return -1 if a < b, 0 if a = b, and 1 if a > b */
+int timer_cmp(const struct timespec *a, const struct timespec *b);
+
+/* Add the timespec value of b to a */
+void timer_add(struct timespec *a, const struct timespec *b);
+
+/* Add ms milliseconds to the timespec*/
+void timer_add_ms(struct timespec *ts, uint32_t ms);
+
+/* Subtract time b from a. If b > a, then a is set to 0 */
+void timer_sub(struct timespec *a, const struct timespec *b);
+
+/* Subtract ms milliseconds from ts. If ts would become negative, it is set to 0 */
+void timer_sub_ms(struct timespec *ts, uint32_t ms);
+
+#endif /* _TIMER_MATH_H_ */