String Calculator

.gitignore

calc
*.o
*.dSYM

calc.c

#include <stdio.h>
#include <string.h>

#include "calculator.h"

int is_numeric(char *n);

int
main(int argc, char *argv[])
{
    if (argc != 3) {
        fprintf(stderr, "Usage: calc <number> <number>\n");
        return 1;
    }

    if (!is_numeric(argv[1])) {
        fprintf(stderr, "Error: First argument must be a number\n");
        return 1;
    }

    if (!is_numeric(argv[2])) {
        fprintf(stderr, "Error: Second argument must be a number\n");
        return 1;
    }

    if (!add_print(argv[1], argv[2])) {
        fprintf(stderr, "Error: could not add values\n");
        return 1;
    }

    return 0;
}

int
is_numeric(char *n)
{
    int i,
        success = 1,
        length  = strlen(n);

    success = success && (length > 0);

    for (i = 0; i < length; i++) {
        success = success && (n[i] >= '0' && n[i] <= '9');
    }

    return success;
}

calculator.c

#include <stdio.h>
#include <stdlib.h>
#include <strings.h>
#include <errno.h>
#include <stdarg.h>

#include "calculator.h"

#define check_mem(P) if (!P) { goto error; }
#define check(C) if (!(C)) { goto error; }

size_t   max_length(char *first, ...);
int      int_value(char n);
char     add_digits(int a1, int a2, int *c);
char   * pad(char *number, size_t width, char pad);

char *
add(char *n1, char *n2)
{
    int  v1, v2, ri,
         carry = 0,
         done  = 0,
         i1    = strlen(n1) - 1,
         i2    = strlen(n2) - 1;

    size_t max = max_length(n1, n2, NULL);

    char * result = pad(NULL, max + 1, ' ');

    ri = max; // set index to space before terminating NUL

    while (!done && ri > 0) {
        v1 = v2 = 0;

        if (i1 >= 0) {
            v1 = int_value(n1[i1]);
            check(v1 >= 0);
        }

        if (i2 >= 0) {
            v2 = int_value(n2[i2]);
            check(v2 >= 0);
        }

        result[ri] = add_digits(v1, v2, &carry);

        ri--;
        i1--;
        i2--;

        if (i1 < 0 && i2 < 0) { done = 1; }
    }

    if (carry > 0) {
        result[ri] = ('0' + carry);
    }

    return result;
error:
    free(result);

    return NULL;
}

int
add_print(char *n1, char *n2)
{
    char *v1     = NULL,
         *v2     = NULL,
         *div    = NULL,
         *result = add(n1, n2);

    check_mem(result);

    size_t width = max_length(result, n1, n2, NULL);

    v1 = pad(n1, width, ' ');
    check_mem(v1);

    v2 = pad(n2, width, ' ');
    check_mem(v2);

    div = pad(NULL, (width + 1), '-');
    check_mem(div);

    printf(" %s\n+%s\n%s\n %s\n", v1, v2, div, result);

    free(result);
    free(v1);
    free(v2);
    free(div);

    return 1;
error:
    if (result) { free(result); }
    if (v1)     { free(v1); }
    if (v2)     { free(v2); }
    if (div)    { free(div); }

    return 0;
}

size_t
max_length(char *first, ...)
{
    size_t  max = strlen(first);
    va_list ap;
    char    *next;

    va_start(ap, first);
    while ((next = va_arg(ap, char*)) != NULL) {
        if (strlen(next) > max) { max = strlen(next); }
    }
    va_end(ap);

    return max;
}

int
int_value(char n)
{
    int  result;

    errno = 0;

    result = strtol(&n, NULL, 10);

    if (result == 0 && errno != 0) {
        result = -1;
    }

    return result;
}

char
add_digits(int a1, int a2, int *c)
{
    int sum = a1 + a2 + *c,
        r   = sum % 10;

    *c = sum / 10;

    return ('0' + r);
}

char *
pad(char *number, size_t width, char pad)
{
    char *out = calloc(width + 1, sizeof(char));
    int  oi = width - 1,
         ni = -1;

    if (out == NULL) { goto error; }

    memset(out, pad, width);

    if (number) { ni = strlen(number) - 1; }

    while (ni >= 0) {
        out[oi] = number[ni];

        ni--;
        oi--;
    }

    return out;
error:
    return NULL;
}

calculator.h

#ifndef _CALCULATOR_H
#define _CALCULATOR_H

char * add(char *n1, char *n2);
int    add_print(char *n1, char *n2);

#endif

Makefile

CFLAGS=-g -Wall -Wextra

all: calc

calc: calculator.o

clean:
	rm -rf calc *.o *.dSYM