addBignumber

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

void Reverse(char s[]) {
    char temp;
    int i, n = strlen(s);
    for (i = 0; i < (n / 2); i++) {
        temp = s[n - i - 1];
        s[n - i - 1] = s[i];
        s[i] = temp;
    }
}

int checkInput(char s[]) {
    int i;
    if ((s[0] > '9' || s[0] < '0') && s[0] != '-') return 0;
    for (i = 1; i < strlen(s); i++) {
        if (s[i] > '9' || s[i] < '0') return 0;
        if (s[i] == ' ') return 0;
    }
    return 1;
}

void enter(char s[]) {
    int check = 1;
    do {
        gets(s);
        check = checkInput(s);
        if (!check) printf("Please enter again :\n");
    } while (!check);
}

int compareNumber(char num1[], char num2[]) {
    if (strlen(num1) > strlen(num2)) return 1;
    if (strlen(num2) > strlen(num1)) return 2;
    int x = strcmp(num1, num2);
    if (x > 0) return 1;
    else if (x < 0) return 2;
    return 3;
}

void add0(char num1[], char num2[]) {
    Reverse(num1);
    Reverse(num2);
    while (strlen(num1) < strlen(num2)) strcat(num1, "0");
    while (strlen(num1) > strlen(num2)) strcat(num2, "0");
}

void delete0(char s[]) {
    char st[1000];
    int i, j = 0, d;
    for (i = 0; i < strlen(s); i++)
        if (s[i] != '0') {
            d = i;
            break;
        }
    for (i = d; i < strlen(s); i++)
        st[j++] = s[i];
    st[j] = '\0';
    strcpy(s, st);
    if (j == 0) strcpy(s, "0");
}

void addNumber(char num1[], char num2[], char sum[1001]) {
    int i, carry = 0;
    add0(num1, num2);
        for (i = 0; i < strlen(num1); i++) {
        sum[i] = num1[i] + num2[i] - 96 + carry;
        carry = sum[i] / 10;
        sum[i] = sum[i] % 10 + 48;
    }
    sum[strlen(num1)] = '\0';
    Reverse(sum);
}

void SubNumber(char s1[], char s2[], char s[], int minus) {
    add0(s1, s2);
    int i, h, carry = 0;
    for (i = 0; i < strlen(s1); i++) {
        h = s1[i] - s2[i] - carry;
        if (h < 0) {
            carry = 1;
            s[i] = (h + 10) % 10 + 48;
        } else {
            carry = 0;
            s[i] = h + 48;
        }
    }
    s[strlen(s1)] = '\0';
    Reverse(s);
    delete0(s);
    Reverse(s);
    if (minus  == -1) strcat(s, "-");
    Reverse(s);
}

void switchToPositive(char s[]) {
    int i;
    for (i = 0; i < strlen(s) - 1; i++)
        s[i] = s[i + 1];
    s[strlen(s) - 1] = '\0';
}

void printResult(char s1[], char s2[], char resultAdd[]) {
    printf("%s + %s = %s\n", s1, s2, resultAdd);
}

void addTwoPos(char s1[], char s2[], char resultAdd[]) {
    char st1[1000], st2[1000];
    strcpy(st1, s1);
    strcpy(st2, s2);
    addNumber(st1, st2, resultAdd);
}

void addNagPos(char s1[], char s2[], char resultAdd[]) {
    char st1[1000], st2[1000];
    strcpy(st1, s1);
    strcpy(st2, s2);
    switchToPositive(st1);
    int x = compareNumber(st1, st2);
    if (x == 1) {
        SubNumber(st1, st2, resultAdd, -1);
    } else if (x == 2) {
        SubNumber(st2, st1, resultAdd, 1);
    }  else {
        strcpy(resultAdd, "0");
    }
}

void addTowNag(char s1[], char s2[], char resultAdd[]) {
    char st1[1000], st2[1000];
    strcpy(st1, s1);
    strcpy(st2, s2);
    switchToPositive(st1);
    switchToPositive(st2);
    char temp[1000];
    addNumber(st1, st2, temp);
    strcpy(resultAdd, "-");
    strcat(resultAdd, temp);
}

int main() {
    char s1[1000], s2[1000], resultAdd[1000];
    while (1) {
        printf("Enter first number  :  ");
        enter(s1);
        printf("Enter second number :  ");
        enter(s2);
        if (s1[0] != '-' && s2[0] != '-') {
            addTwoPos(s1, s2, resultAdd);
        } else if (s1[0] == '-' && s2[0] != '-') {
            addNagPos(s1, s2, resultAdd);
        } else if (s1[0] != '-' && s2[0] == '-') {
            addNagPos(s2, s1, resultAdd);
        } else {
            addTowNag(s1, s2, resultAdd);
        }
        printResult(s1, s2, resultAdd);
    }
    return 0;
}