Slice bit string

slice_bit_string.c

#include<stdio.h>
#include<stdbool.h>

void print_bit_string(unsigned char n) {
	for(int i=7; 0<=i; i--) {
		printf("%d", (n >> i) & 1);
	}
}

unsigned char slice_bit_string(unsigned char data[], int data_len, char slice_unit, int idx) {
	int data_len_bits = data_len * 8;

	unsigned char sliced, mask;
	int shift, data_idx;

	mask = (1 << slice_unit) - 1;
	data_idx = (idx * slice_unit) / 8;
	shift    = (idx * slice_unit) % 8;

	sliced = (data[data_idx] & (mask << shift)) >> shift;

	// mask したいデータが配列の要素を跨っている時
	if (8 < shift + slice_unit && idx * slice_unit <= data_len_bits) {
		// temp_mask: A に対応するマスク
		// ex)      data: 0101 0101
		//          mask: 0011 1000
		//     temp_mask: 0011        // mask >> (8-shift)
		//                  A    B

		int temp_mask = mask >> (8-shift);
		sliced |= (data[data_idx+1] & temp_mask) << (8-shift);
	}

	return sliced;
}

int main() {
	unsigned char data[] = {248, 137};
	int data_len = sizeof(data) / sizeof(unsigned char);

	for (int i = 0; i < data_len; i++) {
		printf("data[%d] = ", i);
		print_bit_string(data[i]);
		printf("\n");
	}

	int slice_unit = 3; // 分割単位
	int loop_cnt = data_len * 8 / slice_unit;

	if (data_len * 8 % slice_unit)
		loop_cnt += 1;

	unsigned char sliced;

	for (int i = 0; i < loop_cnt; i++) {
		sliced = slice_bit_string(data, data_len, 3, i);

		printf("sliced[%d] = %d = ", i, sliced);
		print_bit_string(sliced);
		printf("\n");
	}

	return 0;
}