3 files changed, 85 insertions, 2 deletions
diff --git a/src/math.c b/src/math.c
index 366bd5e..4e9fee0 100644
--- a/src/math.c
+++ b/src/math.c
@@ -1,6 +1,7 @@
 #include "config.h"
 
 #include <assert.h>
+#include <errno.h>
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
@@ -9,8 +10,23 @@
 
 
 int
+add_size(const size_t x, const size_t y, size_t *const out) {
+	int rc = EOVERFLOW;
+
+	const bool overflows = (SIZE_MAX - x) < y;
+	if (overflows) {
+		goto out;
+	}
+
+	*out = x + y;
+	rc = 0;
+out:
+	return rc;
+}
+
+int
 mul_size(const size_t x, const size_t y, size_t *const out) {
-	int rc = -1;
+	int rc = EOVERFLOW;
 
 	assert(x != 0U);
 	assert(y != 0U);
diff --git a/src/math.h b/src/math.h
index 13f1795..4dab4a7 100644
--- a/src/math.h
+++ b/src/math.h
@@ -1,2 +1,5 @@
 int
+add_size(const size_t x, const size_t y, size_t *const out);
+
+int
 mul_size(const size_t x, const size_t y, size_t *const out);
diff --git a/tests/math.c b/tests/math.c
index 34f54ce..d2b64c6 100644
--- a/tests/math.c
+++ b/tests/math.c
@@ -1,9 +1,72 @@
 #include "../src/math.c"
 
+#include <stdlib.h>
+
 #include "../src/testing.h"
 
 
 static void
+test_add_size(void) {
+	test_start("add_size()");
+
+	size_t val;
+	{
+		testing("zero values");
+
+		assert(add_size(0U, 0U, &val) == 0);
+		assert(val == 0U);
+
+		assert(add_size(1U, 0U, &val) == 0);
+		assert(val == 1U);
+
+		assert(add_size(0U, 1U, &val) == 0);
+		assert(val == 1U);
+
+		assert(add_size(0U, 3U, &val) == 0);
+		assert(val == 3U);
+
+		assert(add_size(30U, 0U, &val) == 0);
+		assert(val == 30U);
+
+		test_ok();
+	}
+	{
+		testing("small values");
+
+		assert(add_size(1U, 2U, &val) == 0);
+		assert(val == 3U);
+
+		assert(add_size(34U, 56U, &val) == 0);
+		assert(val == 90U);
+
+		assert(add_size(121U, 122U, &val) == 0);
+		assert(val == 243U);
+
+		test_ok();
+	}
+	{
+		testing("big values");
+
+		assert(add_size(SIZE_MAX, 0U, &val) == 0);
+		assert(val == SIZE_MAX);
+
+		assert(add_size(SIZE_MAX - 10U, 3U, &val) == 0);
+		assert(val == (SIZE_MAX - 7U));
+
+		test_ok();
+	}
+	{
+		testing("overflowing values");
+
+		val = 0U;
+		assert(add_size(SIZE_MAX, 1U, &val) != 0);
+		assert(val == 0U);
+
+		test_ok();
+	}
+}
+
+static void
 test_mul_size(void) {
 	test_start("mul_size()");
 
@@ -62,6 +125,7 @@ test_mul_size(void) {
 
 int
 main(void) {
+	test_add_size();
 	test_mul_size();
-	return 0;
+	return EXIT_SUCCESS;
 }