c++高精度求差

#include <string>
//求差 a-b a和b都大于零 但是返回值可能有负号 有bug欢迎评论qwq
std::string maths_subtraction(std::string a, std::string b){
	size_t lena = a.length();
	size_t lenb = b.length();
	bool tuiwei = false;
	if (lena > lenb) {
		while (lenb != -1) {
			a[lena] -=b[lenb] + tuiwei - '0';
			if (a[lena] < '0') {
				a[lena] += 10;
				tuiwei = 1;
			}
			else tuiwei = 0;
			lena--; lenb--;
		}
		if (tuiwei) {
			while (lena != -1) {
				if (a[lena] != '0') {
					a[lena] -= 1;
					if (lena == 0) { 
						for (; a[lena++] == '0';);
						return a.substr(lena-1);
					}
					return a;
				}
				a[lena--] = '9';
			}
		}
		return a;
	}
	else if (lena == lenb) {
		if (a >= b) {
			while (lenb != -1) {
				a[lenb] -= b[lenb] + tuiwei - '0';
				if (a[lenb] < '0') {
					a[lenb] += 10;
					tuiwei = 1;
				}
				else tuiwei = 0;
				lenb--;
			}
			for (; a[++lenb] == '0'&&lenb<lena;);
			if (lenb == lena)return "0";
			return a.substr(lena);
		}
		else {
			while (lena != -1) {
				b[lenb] -= a[lena] + tuiwei - '0';
				if (b[lenb] < '0') {
					b[lenb] += 10;
					tuiwei = 1;
				}
				else tuiwei = 0;
				lena--; lenb--;
			}
			for (; b[++lenb] == '0';);
			return b.substr(lenb);
		}
	}
	else {
		while (lena != -1) {
			b[lenb] -= a[lena] + tuiwei - '0';
			if (b[lenb] < '0') {
				b[lenb] += 10;
				tuiwei = 1;
			}
			else tuiwei = 0;
			lena--; lenb--;
		}
		if (tuiwei) {
			while (lenb != -1) {
				if (b[lenb] != '0') {
					b[lenb] -= 1;
					if (lenb == 0) {
						for (; b[lenb++] == '0';);
						return b.substr(lenb-1).insert(0,"-");
					}
					return b.insert(0, "-");
				}
				b[lenb] = '9';
			}
		}
		return b.insert(0, "-");
	}
}

顺便求求有没有更好的实现方式qwq