#include "stdio.h"
#define NMAX 5
unsigned short int A[]={0x1E26, 0x163C, 0x9A2c, 0x1234, 0xFEDC};
unsigned short int CRC_CCITT(unsigned short int A[], int N, int Debug){
	// 生成多項式 X^16+X^12+X^5+1 (16進数0x11021)
	unsigned long CH=0x11021, LDT=A[0];
	unsigned short int GDT;	if(Debug) printf(" %05X",LDT);
	for(int i=0, j=0; i<(N-1)*16; i++){
		if(i % 16==0) GDT=A[++j];//下位ワード取り出し
		LDT=(LDT<<1) | (GDT>>15);//１ビット左シフト
		if(Debug) printf("\n %05X",LDT);
		if(Debug && (LDT & 0x10000)) printf("→ %05X",LDT ^ CH);
		if(LDT & 0x10000) LDT ^= CH;//X^16部分が1なら多項式とEXOR
		GDT <<= 1;
	}
	return (unsigned short int) LDT;
}
int main(){
	//　CRCを求める
	unsigned short int CRC= CRC_CCITT(A, NMAX, true);
	printf("\n ** Result %05X",CRC);
	getchar();
	
	A[NMAX-1] ^=CRC;//最後のデータにCRCをEXOR
	printf("\n ** Result %05X",CRC_CCITT(A, NMAX, true));
	getchar();
	
	A[NMAX-1] ^=CRC;//最後のデータを元に戻す
	printf("\n ** No Debug Result %05X",CRC_CCITT(A, NMAX,false));
	getchar();
	
	A[NMAX-1] ^=CRC;//最後のデータにCRCをEXOR
	printf("\n ** No Debug Result %05X",CRC_CCITT(A, NMAX, false));
	getchar();
	return 0;
}