Rework gigasecond to be slightly more challenging (#925)

* Rework gigasecond to be slightly more challenging

* clang-format

* review suggestions

config.json

@@ -446,7 +446,7 @@
         "uuid": "000340f6-e30d-4d49-a255-016237d6fe60",
         "practices": [],
         "prerequisites": [],
-        "difficulty": 1,
+        "difficulty": 3,
         "topics": [
           "dates"
         ]

exercises/practice/gigasecond/.docs/instructions.append.md

@@ -0,0 +1,7 @@
+## Use UTC
+
+To solve this problem, you'll need to convert a `time_t` value into a `struct tm` value.
+The [`time.h`][time.h] library gives you two functions to do this.
+Make sure you use the one that results in a calendar time expressed in **UTC**.
+
+[time.h]: https://en.cppreference.com/w/c/chrono

exercises/practice/gigasecond/.meta/example.c

@@ -1,6 +1,10 @@
 #include "gigasecond.h"
 
-time_t gigasecond_after(time_t start)
+#define GIGASECOND 1000000000
+
+void gigasecond(time_t start, char *buffer, size_t size)
 {
-   return start + 1000000000;
+   time_t after = start + GIGASECOND;
+   struct tm *t = gmtime(&after);
+   strftime(buffer, size, "%Y-%m-%d %H:%M:%S", t);
 }

exercises/practice/gigasecond/.meta/example.h

@@ -3,6 +3,6 @@
 
 #include <time.h>
 
-time_t gigasecond_after(time_t);
+void gigasecond(time_t input, char *output, size_t size);
 
 #endif

exercises/practice/gigasecond/gigasecond.h

@@ -3,6 +3,6 @@
 
 #include <time.h>
 
-time_t gigasecond_after(time_t);
+void gigasecond(time_t input, char *output, size_t size);
 
 #endif

exercises/practice/gigasecond/test_gigasecond.c

@@ -1,6 +1,8 @@
 #include "test-framework/unity.h"
 #include "gigasecond.h"
 
+#define BUFFER_SIZE sizeof("YYYY-mm-dd HH:MM:SS")
+
 static inline size_t is_leap_year(int year)
 {
    if (year % 400 == 0)
@@ -49,49 +51,55 @@ void tearDown(void)
 
 static void test_date(void)
 {
-   time_t expected = construct_date(2043, 1, 1, 1, 46, 40);
-   time_t actual = gigasecond_after(construct_date(2011, 4, 25, 0, 0, 0));
-   TEST_ASSERT(expected == actual);
+   time_t before = construct_date(2011, 4, 25, 0, 0, 0);
+   char after[BUFFER_SIZE] = { 0 };
+   gigasecond(before, after, BUFFER_SIZE);
+   TEST_ASSERT_EQUAL_STRING("2043-01-01 01:46:40", after);
 }
 
 static void test_another_date(void)
 {
    TEST_IGNORE();   // delete this line to run test
-   time_t expected = construct_date(2009, 2, 19, 1, 46, 40);
-   time_t actual = gigasecond_after(construct_date(1977, 6, 13, 0, 0, 0));
-   TEST_ASSERT(expected == actual);
+   time_t before = construct_date(1977, 6, 13, 0, 0, 0);
+   char after[BUFFER_SIZE] = { 0 };
+   gigasecond(before, after, BUFFER_SIZE);
+   TEST_ASSERT_EQUAL_STRING("2009-02-19 01:46:40", after);
 }
 
 static void test_third_date(void)
 {
    TEST_IGNORE();
-   time_t expected = construct_date(1991, 3, 27, 1, 46, 40);
-   time_t actual = gigasecond_after(construct_date(1959, 7, 19, 0, 0, 0));
-   TEST_ASSERT(expected == actual);
+   time_t before = construct_date(1959, 7, 19, 0, 0, 0);
+   char after[BUFFER_SIZE] = { 0 };
+   gigasecond(before, after, BUFFER_SIZE);
+   TEST_ASSERT_EQUAL_STRING("1991-03-27 01:46:40", after);
 }
 
 static void test_date_and_time(void)
 {
    TEST_IGNORE();
-   time_t expected = construct_date(2046, 10, 2, 23, 46, 40);
-   time_t actual = gigasecond_after(construct_date(2015, 1, 24, 22, 0, 0));
-   TEST_ASSERT(expected == actual);
+   time_t before = construct_date(2015, 1, 24, 22, 0, 0);
+   char after[BUFFER_SIZE] = { 0 };
+   gigasecond(before, after, BUFFER_SIZE);
+   TEST_ASSERT_EQUAL_STRING("2046-10-02 23:46:40", after);
 }
 
 static void test_date_and_time_with_day_rollover(void)
 {
    TEST_IGNORE();
-   time_t expected = construct_date(2046, 10, 3, 1, 46, 39);
-   time_t actual = gigasecond_after(construct_date(2015, 1, 24, 23, 59, 59));
-   TEST_ASSERT(expected == actual);
+   time_t before = construct_date(2015, 1, 24, 23, 59, 59);
+   char after[BUFFER_SIZE] = { 0 };
+   gigasecond(before, after, BUFFER_SIZE);
+   TEST_ASSERT_EQUAL_STRING("2046-10-03 01:46:39", after);
 }
 
 /*
 static void test_your_birthday(void)
 {
    time_t birthday = construct_date(1989, 1, 1, 1, 1, 1);
-   time_t gigday = gigasecond_after(birthday);
-   printf("%s", ctime(&gigday));
+   char gigday[BUFFER_SIZE] = {0};
+   gigasecond(birthday, gigday, BUFFER_SIZE);
+   printf("%s", gigday);
 }
 */