Không hiểu về cách xử lí bignum

#include "stdafx.h"
#include<stdio.h>
#include<iostream>
#include<fstream>
#include<string>
#define MAXDIGITS 100
#define PLUS 1
#define MINUS -1
using namespace std;
typedef struct
{
	char digits[MAXDIGITS];
	int signbit;
	int lastdigit;
} bignum;

// khai bao
void main();
void print_bignum(bignum n);
int kiemtra(char s[]);
void scan_bignum(bignum &n);
void zero_justify(bignum n);
void add_bignum(bignum a, bignum b, bignum &c);
void subtract_bignum(bignum a, bignum b, bignum &c);
int compare_bignum(bignum a, bignum b);
// cai 
void main()
{
	bignum a, b, c;
	cout << "nhap vao so lon a:";
	scan_bignum(a);
	cout << "nhap vao so lon b:";
	scan_bignum(b);
	add_bignum(a, b, c);
	cout << "a+b=";
	print_bignum(c);
	subtract_bignum(a, b, c);
	cout << "a-b=";
	print_bignum(c);
	cout << endl;
}
int max(int a, int b)
{
	return (a>b) ? a : b;
}
void print_bignum(bignum n)
{
	int i;
	if (n.signbit == MINUS) cout << "-";
	for (i = n.lastdigit; i >= 0; i--)
	{
		cout << int(n.digits[i]);
	}
	cout << "
";
}
int kiemtra(char s[])
{
	//ktra ký tự dau
	if (s[0] != '-' && !isdigit(s[0]))
		return 0;
	//ktra các ký t? ti?p theo
	int len = strlen(s);
	for (int i = 1; i<len; i++)
	if (!isdigit(s[i])) return 0;
	//không phát hi?n vi ph?m
	return 1;
}
void scan_bignum(bignum &n)
{
	//B1: Nhập chuỗi
	char temp[256];
	cin.getline(temp, 256);
	//B2: Kiểm tra
	if (!kiemtra(temp))
	{	//không hợp lệ
		//gán bignum = 0
		n.signbit = PLUS;
		n.lastdigit = 0;
		n.digits[0] = 0;
	}
	else
	{		//hợp lệ. B3: chuyển thành số bignum
		//xét 2 tru?ng h?p
		if (temp[0] != '-')
		{	//s? duong
			n.signbit = PLUS;
			n.lastdigit = strlen(temp) - 1;
			for (int i = 0; i <= n.lastdigit; i++)
				n.digits[i] = temp[n.lastdigit - i] - 48;
		}
		else
		{				//s? âm
			n.signbit = MINUS;
			n.lastdigit = strlen(temp) - 2;
			for (int i = 0; i <= n.lastdigit; i++)
				n.digits[i] = temp[n.lastdigit + 1 - i] - 48;
		}
	}
}
void zero_justify(bignum n)
{
	while ((n.lastdigit > 0) && (n.digits[n.lastdigit] == 0))
		n.lastdigit--;
	if ((n.lastdigit == 0) && (n.digits[0] == 0))
		n.signbit = PLUS; /* hack to avoid -0 */
}
void add_bignum(bignum a, bignum b, bignum &c)
{
	int carry; /* carry digit */
	int i; /* counter */
	//initialize_bignum(c); //khoi gan chu so = 0;
	if (a.signbit == b.signbit) c.signbit = a.signbit;
	else
	{
		if (a.signbit == MINUS)
		{
			a.signbit = PLUS;
			subtract_bignum(b, a, c);
			a.signbit = MINUS;
		}
		else
		{
			b.signbit = PLUS;
			subtract_bignum(a, b, c);
			b.signbit = MINUS;
		}
		return;
	}
	c.lastdigit = max(a.lastdigit, b.lastdigit) + 1;
	//dieu chinh a,b
	for (i = a.lastdigit + 1; i <= c.lastdigit; i++)
		a.digits[i] = 0;
	for (i = b.lastdigit + 1; i <= c.lastdigit; i++)
		b.digits[i] = 0;
	//tinh
	carry = 0;
	for (i = 0; i <= (c.lastdigit); i++)
	{
		c.digits[i] = (char)
			(carry + a.digits[i] + b.digits[i]) % 10;
		carry = (carry + a.digits[i] + b.digits[i]) / 10;
	}

	zero_justify(c);
}
void subtract_bignum(bignum a, bignum b, bignum &c)
{
	int borrow; /* anything borrowed? */
	int v; /* placeholder digit */
	int i; /* counter */
	if ((a.signbit == MINUS) || (b.signbit == MINUS)) {
		b.signbit = -1 * b.signbit;
		add_bignum(a, b, c);
		b.signbit = -1 * b.signbit;
		return;
	}
	if (compare_bignum(a, b) == PLUS) {
		subtract_bignum(b, a, c);
		c.signbit = MINUS;
		return;
	}
	c.signbit = PLUS;
	c.lastdigit = max(a.lastdigit, b.lastdigit);
	//dieu chinh a,b
	for (i = a.lastdigit + 1; i <= c.lastdigit; i++)
		a.digits[i] = 0;
	for (i = b.lastdigit + 1; i <= c.lastdigit; i++)
		b.digits[i] = 0;
	borrow = 0;
	//5.3. High-Precision Arithmetic 107
	for (i = 0; i <= (c.lastdigit); i++) {
		v = (a.digits[i] - borrow - b.digits[i]);
		if (a.digits[i] > 0)
			borrow = 0;
		if (v < 0) {
			v = v + 10;
			borrow = 1;
		}
		c.digits[i] = (char)v % 10;
	}
	zero_justify(c);
}

int compare_bignum(bignum a, bignum b)
{
	int i; /* counter */
	if ((a.signbit == MINUS) && (b.signbit == PLUS)) return(PLUS);
	if ((a.signbit == PLUS) && (b.signbit == MINUS)) return(MINUS);
	if (b.lastdigit > a.lastdigit) return (PLUS * a.signbit);
	if (a.lastdigit > b.lastdigit) return (MINUS * a.signbit);
	for (i = a.lastdigit; i >= 0; i--) {    // so sanh so chu so bang nhau
		if (a.digits[i] > b.digits[i])
			return(MINUS * a.signbit);
		if (b.digits[i] > a.digits[i])
			return(PLUS * a.signbit);
	}
	return(0);
}