#include <string.h>

void add(char a[], char b[], char result[], int maxLength) {
  int carry = 0;
  for(int i = maxLength - 1; i >= 0; i--) {
    int sum = (a[i] - '0') + (b[i] - '0') + carry;
    result[i] = (sum % 10) + '0';
    carry = sum / 10;
  }
  if(carry) {
    Serial.println("Overflow error");
  }
}

void multiply(char a[], char b[], char result[], int maxLength) {
  memset(result, '0', maxLength);
  for(int i = strlen(a) - 1; i >= 0; i--) {
    int carry = 0;
    for(int j = strlen(b) - 1; j >= 0; j--) {
      int product = (a[i] - '0') * (b[j] - '0') + (result[i + j + 1] - '0') + carry;
      result[i + j + 1] = (product % 10) + '0';
      carry = product / 10;
    }
    result[i] += carry;
  }
}

void setup() {
  Serial.begin(9600);

  char a[] = "1234567890987654321";
  char b[] = "9876543210123456789";
  int maxLength = strlen(a) + strlen(b) + 1;
  char result[maxLength];
  multiply(a, b, result, maxLength);

  Serial.print("a * b = ");
  Serial.println(result);
}

void loop() {
  // Do nothing
}