Advanced Exercism • c

Clock

Challenge Overview

Implement a clock that handles times without dates.

Visit the clock exercise on Exercism to read the full instructions and download the exercise files.

Dig Deeper

Minimize duplicate code

clock.h

#include <stdbool.h>

#define MAX_STR_LEN sizeof("##:##")

typedef struct
{
   char text[MAX_STR_LEN];
} clock_t;

clock_t clock_create(int hour, int minute);
clock_t clock_add(clock_t clock, int minute_add);
clock_t clock_subtract(clock_t clock, int minute_subtract);
bool clock_is_equal(clock_t a, clock_t b);

#endif

clock.c

#include "clock.h"
#include <stdio.h>
#include <string.h>
const int DAY_MINUTES = 1440;
const int HOUR_MINUTES = 60;

static int normalize_minutes(int minutes)
{
   return (minutes < 0) ? DAY_MINUTES + (minutes % DAY_MINUTES)
                        : minutes % DAY_MINUTES;
}

static void set_text(clock_t *clock, int all_minutes)
{
   int hours = all_minutes / 60;
   int minutes = all_minutes % 60;
   sprintf(clock->text, "%02d:%02d", hours, minutes);
}

static int minutes_from_text(char text[static 5])
{
   int hour, minute;
   sscanf(text, "%d:%d", &hour, &minute);
   return (hour * HOUR_MINUTES) + minute;
}

clock_t clock_create(int hour, int minute)
{
   clock_t clock;
   set_text(&clock, normalize_minutes((hour * 60) + minute));
   return clock;
}

clock_t clock_add(clock_t clock, int minute_add)
{
   return clock_create(0, minutes_from_text(clock.text) + minute_add);
}

clock_t clock_subtract(clock_t clock, int minute_subtract)
{
   return clock_create(0, minutes_from_text(clock.text) - minute_subtract);
}

bool clock_is_equal(clock_t a, clock_t b)
{
   return (!strcmp(a.text, b.text));
}

The header defines the maximum length of the Clock struct text to be 14. In practice, six bytes would suffice for 2 hour digits, a colon, 2 minute digits and the null terminating character, however, when setting the text, some compilers realize that int values could be large enough so that the null terminating character could be written past the end of the text, unless it is made to be at least fourteen bytes.

This approach starts by defining some const values for the amount of minutes in a day and the amount of minutes in an hour. The const values are given meaningful names instead of using the integer literals as magic numbers.

const int DAY_MINUTES = 1440;
const int HOUR_MINUTES = 60;

static int normalize_minutes(int minutes)
{
   return (minutes < 0) ? DAY_MINUTES + (minutes % DAY_MINUTES)
                        : minutes % DAY_MINUTES;
}

The normalize_minutes function uses a ternary operator to handle rollover past midnight for either positive or negative values. If the minutes are ngative, the statement uses the modulo operator (%) to return the result of handling negative minutes.

For example, if the minutes is -1500, which rolls over past midnight in the negative direction, DAY_MINUTES (1440) added by the remainder of -1500 divided by DAY_MINUTES (1500 % 1440 = -60) is 1380.

Otherwise the statement uses the modulo operator (%) to return the result of handling positive minutes.

For example, if the minutes is 1500, which rolls over past midnight in the positive direction, the remainder of 1500 divided by DAY_MINUTES (1440) is 60.

The set_text function takes a pointer to a Clock and the minutes. The sprintf function is used to set the Clock text from the minutes.

static void set_text(clock_t *clock, int all_minutes)
{
   int hours = all_minutes / 60;
   int minutes = all_minutes % 60;
   sprintf(clock->text, "%02d:%02d", hours, minutes);
}

static int minutes_from_text(char text[static 5])
{
   int hour, minute;
   sscanf(text, "%d:%d", &hour, &minute);
   return (hour * HOUR_MINUTES) + minute;
}

The minutes_from_text function takes the char array of the Clock text. It uses [static 5] to constrain the text array to having no more than five elements. The sscanf function is used to parse out the hour and minute values from the text into variables. The function returns the amount of total minutes from the hour and minute variables.

The clock_create function takes the hour and minute values and uses the set_text function to set the text field of a new Clock struct from the normalized total minutes from the hour and minute. After creating and setting the Clock struct, it returns the struct.

clock_t clock_create(int hour, int minute)
{
   clock_t clock;
   set_text(&clock, normalize_minutes((hour * 60) + minute));
   return clock;
}

clock_t clock_add(clock_t clock, int minute_add)
{
   return clock_create(0, minutes_from_text(clock.text) + minute_add);
}

clock_t clock_subtract(clock_t clock, int minute_subtract)
{
   return clock_create(0, minutes_from_text(clock.text) - minute_subtract);
}

The clock_add and clock_subtract functions both take in a Clock struct and a minute value and return the result of calling the clock_create function.

The clock_add function passes in 0 for the hour and adds the minute value to the value returned from passing in the Clock struct to the minutes_from_text function.
The clock_subtract function passes in 0 for the hour and subtracts the minute value from the value returned from passing in the Clock struct to the minutes_from_text function.

Having clock_add and clock_subtract use the clock_create function is a way to keep the code DRY.

bool clock_is_equal(clock_t a, clock_t b)
{
   return (!strcmp(a.text, b.text));
}

The clock_is_equal function uses the strcmp function to return the result of comparing the text fields of the two passed-in Clock structs. It uses the falsiness of 0 and the logical NOT operator (!) to return true if strcmp returns 0, meaning there is no difference between the two strings.

Source: Exercism c/clock