Advent-of-Code/2021/day3/solution-part2.c

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int countLines(char filename[]){
	FILE *fp;
	fp=fopen(filename, "r");
	long int lines =0;

	if ( fp == NULL ) {
		return -1;
	}

	while (EOF != (fscanf(fp, "%*[^\n]"), fscanf(fp,"%*c")))
		++lines;

	fclose(fp); // closing file
	return lines;
}


void countPositionBits(int linesize, int noOfLines, char *lines[noOfLines], unsigned int digitCount[]){
	for ( int i = 0; i < noOfLines; i++){
		for ( int j = 0; j < linesize; j++ ){
			if ( lines[i][j] == '1' )
				digitCount[j]++;
		}
	}
}

char* filterO2List(unsigned int linesize, unsigned int noOfLines, char **lines, unsigned int iteration){
	if ( noOfLines == 1 )
		return lines[0];

	unsigned int digitCount[linesize];
	memset( &digitCount, 0, linesize*sizeof(unsigned int) );
	countPositionBits(linesize, noOfLines, lines, digitCount);

	char bitKeep = ( digitCount[iteration] >= ((double)noOfLines) / 2 ) ? '1' : '0';

	unsigned int newLinesIndex=0;
	//Not sure why +4 here but any less and it buffer overflows
	char **newLines = (char**)malloc((digitCount[iteration]+4) * sizeof(char*));

	for ( unsigned int i=0; i < noOfLines; i++ ){
		if (lines[i][iteration] == bitKeep)
			newLines[newLinesIndex++] = lines[i];
	}

	return filterO2List( linesize, newLinesIndex, newLines, ++iteration );

}

char* filterCO2List(unsigned int linesize, unsigned int noOfLines, char **lines, unsigned int iteration){
	if ( noOfLines == 1 )
		return lines[0];

	unsigned int digitCount[linesize];
	memset( &digitCount, 0, linesize*sizeof(unsigned int) );
	countPositionBits(linesize, noOfLines, lines, digitCount);

	char bitKeep = ( digitCount[iteration] >= ((double)noOfLines) / 2 ) ? '0' : '1';

	unsigned int newLinesIndex=0;
	//Not sure why +4 here but any less and it buffer overflows
	char **newLines = (char**)malloc((digitCount[iteration]+4) * sizeof(char*));

	for ( unsigned int i=0; i < noOfLines; i++ ){
		if (lines[i][iteration] == bitKeep)
			newLines[newLinesIndex++] = lines[i];
	}

	return filterCO2List( linesize, newLinesIndex, newLines, ++iteration );

}

int main( int argc, char *argv[] ){
	if( argc == 2 ) {

		unsigned int gamma = 0;
		unsigned int epsilon = 0;
		unsigned int lineLength = 0;
		unsigned int noOfLines = countLines( argv[1] );
		unsigned int iteration=0;

		// Read the values into an array
		FILE *fp=fopen(argv[1], "r");
		char ch = fgetc(fp);
		while ( ch != '\n' ){
			lineLength++;
			ch = fgetc(fp);
		}

		rewind(fp);

		unsigned int digitCount[lineLength];
		memset( &digitCount, 0, lineLength*sizeof(int) );
		
		// Make a big array to hold the lines
		char **lines = (char**)malloc(noOfLines * sizeof(char*));

		// Loop through the file and populate the array
		for ( unsigned int i = 0; !feof(fp); i++){

			char *line = (char*)malloc(lineLength);
			fscanf(fp, "%s\n", line);
			lines[i] = line;
		}

		char *o2Rating = filterO2List( lineLength, noOfLines, lines, 0 );
		char *co2Rating = filterCO2List( lineLength, noOfLines, lines, 0 );

		int o2RatingDec = (int)strtol(o2Rating, NULL, 2);
		int co2RatingDec = (int)strtol(co2Rating, NULL, 2);

		printf("O2 Rating: %s -> %i\n", o2Rating, o2RatingDec);
		printf("CO2 Rating: %s -> %i\n", co2Rating, co2RatingDec);
		printf("Answer: %i\n", o2RatingDec * co2RatingDec);

		return 0;
	} else {
		printf("You need to provide a file\n");
		return 1;
	}
}