escaped_\uCODE_->to_\xBY\xTE\xCO\xDE, C (clang)

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//clang 3.8.0

#include  <stdio.h>

static int x0_int(char string[]){
	//function return by char, a 1-st hexadecimal value as int, or -1 if not hexadecimal

int code;										//decimal value of hex character if this is hex
	const int o = 48; 								//start offset for charcodes
	int j = 0; //first position is null
		
	switch (string[j]) {//compare charcode
		case o+0 ... o+9:  return code = string[j]-o; //break;			//0-9
		case o+17 ... o+22: return code = string[j]-o-7; //break;		//A-F
		case o+49 ... o+54: return code = string[j]-o-39; //break;		//a-f
		default: return code = -1; //break;								//else -1
	}
}

char * get_x_string(char u_code[]){ //u_code - is escaped code of unicode symbol, like "\\u1ed8"
	//function return string with x-bytes, like "\xe1\xbb\x98", by code of charcode
	int prefix1 = 14;   														//1110 	binary
	int prefix2 = 2;															//10	binary
	unsigned int temp;															//temp integer
	char result[256];															//result string
	char tempstring[256];														//temp string to concatenate
	
	if(u_code[0]=='\\' && u_code[1]=='u'){										//if "\u" at beginning
/*
	In "\u1ed8"
	1ed8 -> 0001'1110'1101'1000 (go to 3-bytes UTF-8 sequence with 12-16 bits):
	Leading UTF-8 byte: 1110 (prefix) 0001 (first four bits).
	Continuation byte: 10 (prefix) 1110 11 (again 6 bits).
	Continuation byte: 10 (префикс) 01 1000 (again 6 bits).
	Result: 11100001, 10111011, 10011000 -> e1, bb, 98.
*/
		temp = prefix1<<4 | x0_int(&u_code[2]);									//first byte as int
		if(temp==-1){printf("not hex char"); return 0;}							//or not
		printf("первый байт: %x\n", temp);										//print this
		sprintf(tempstring, "\\x%x", temp);										//write 1-st \x byte
		strcpy(result, tempstring);												//copy to result
		
		temp = prefix2<<6 | x0_int(&u_code[3])<<2 | x0_int(&u_code[4])>>2;		//second byte as int
		if(temp==-1){printf("not hex char"); return 0;}							//or not
		printf("второй байт: %x\n", temp);										//print this (IF THIS STRING WILL BE COMMENTED, RETURNED STRING CANNOT PRINT FULLY)
		sprintf(tempstring, "\\x%x", temp);										//write 2-nd \x byte
		strcat(result, tempstring);												//add to result
		
		temp = prefix2<<6 | x0_int(&u_code[4])%4<<4 | x0_int(&u_code[5]);		//third byte as int
		if(temp==-1){printf("not hex char"); return 0;}							//or not
		printf("третий байт: %x\n", temp);										//print this
		sprintf(tempstring, "\\x%x", temp);										//write 3-rd \x byte
		strcat(result, tempstring);												//add to result
		
		printf("return this result: %s\n", result);								//print result
		return result;															//return this
	}
	else{printf("Not \\u string. return 0;\n"); return 0;}						//if not unicode char
}

int main(void)
{
	char *test = "\\u1ed8";
    printf("%s", test);
	printf("\n");
	//for(int i=0;i<strlen(test);i++){printf("%x ", test[i]);}
	printf("\n");
	char *x_string;
	x_string = get_x_string(test);
    printf("\n");
	printf("x_string: %s", x_string);
	
}

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Compilation time: 0.22 sec, absolute running time: 0.1 sec, cpu time: 0.04 sec, memory peak: 3 Mb, absolute service time: 0,41 sec