Day 3: Mull It Over

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[-] sjmulder@lemmy.sdf.org 5 points 2 weeks ago* (last edited 2 weeks ago)

C

Yay parsers! I've gotten quite comfortable writing these with C. Using out pointers arguments for the cursor that are only updated if the match is successful makes for easy bookkeeping.

Code

#include "common.h"

static int
parse_exact(const char **stringp, const char *expect)
{
	const char *s = *stringp;
	int i;

	for (i=0; s[i] && expect[i] && s[i] == expect[i]; i++)
		;
	if (expect[i])
		return 0;

	*stringp  = &s[i];
	return 1;
}

static int
parse_int(const char **stringp, int *outp)
{
	char *end;
	int val;

	val = (int)strtol(*stringp, &end, 10);
	if (end == *stringp)
		return 0;

	*stringp = end;
	if (outp) *outp = val;
	return 1;
}

static int
parse_mul(const char **stringp, int *ap, int *bp)
{
	const char *cur = *stringp;
	int a,b;

	if (!parse_exact(&cur, "mul(") ||
	    !parse_int(&cur, &a) ||
	    !parse_exact(&cur, ",") ||
	    !parse_int(&cur, &b) ||
	    !parse_exact(&cur, ")"))
		return 0;

	*stringp = cur;
	if (ap) *ap = a;
	if (bp) *bp = b;
	return 1;
}

int
main(int argc, char **argv)
{
	static char buf[32*1024];
	const char *cur;
	size_t nr;
	int p1=0,p2=0, a,b, dont=0;

	if (argc > 1)
		DISCARD(freopen(argv[1], "r", stdin));

	nr = fread(buf, 1, sizeof(buf), stdin);
	assert(!ferror(stdin));
	assert(nr != sizeof(buf));
	buf[nr] = '\0';

	for (cur = buf; *cur; )
		if (parse_exact(&cur, "do()"))
			dont = 0;
		else if (parse_exact(&cur, "don't()"))
			dont = 1;
		else if (parse_mul(&cur, &a, &b)) {
			p1 += a * b;
			if (!dont) p2 += a * b;
		} else
			cur++;

	printf("03: %d %d\n", p1, p2);
}

https://github.com/sjmulder/aoc/blob/master/2024/c/day03.c

[-] sjmulder@lemmy.sdf.org 2 points 2 weeks ago

Got the code a little shorter:

Code

#include "common.h"

static int
parse_mul(const char **stringp, int *ap, int *bp)
{
	const char *cur = *stringp, *end;

	if (strncmp(cur, "mul(", 4)) { return 0; } cur += 4;
	*ap = (int)strtol(cur, (char **)&end, 10);
	if (end == cur)  { return 0; } cur = end;
	if (*cur != ',') { return 0; } cur += 1;
	*bp = (int)strtol(cur, (char **)&end, 10);
	if (end == cur)  { return 0; } cur = end;
	if (*cur != ')') { return 0; } cur += 1;

	*stringp = cur;
	return 1;
}

int
main(int argc, char **argv)
{
	static char buf[32*1024];
	const char *p;
	size_t nr;
	int p1=0,p2=0, a,b, dont=0;

	if (argc > 1)
		DISCARD(freopen(argv[1], "r", stdin));

	nr = fread(buf, 1, sizeof(buf), stdin);
	assert(!ferror(stdin));
	assert(nr != sizeof(buf));
	buf[nr] = '\0';

	for (p = buf; *p; )
		if (parse_mul(&p, &a, &b)) { p1 += a*b; p2 += a*b*!dont; }
		else if (!strncmp(p, "do()", 4))    { dont = 0; p += 4; }
		else if (!strncmp(p, "don't()", 7)) { dont = 1; p += 7; }
		else p++;

	printf("03: %d %d\n", p1, p2);
}

this post was submitted on 03 Dec 2024

20 points (95.5% liked)

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