mysed

mysed.c

/**
 * @file mysed.c
 *
 * @brief An oversimplified implementation of sed.
 *
 * @author Ondřej Hošek <[email protected]>
 */

#include <assert.h>
#include <errno.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

/** Size of line buffer, in characters. */
#define LINE_BUF_SIZE 1024

/**
 * Name of the program.
 *
 * Since this is only written once and independent of threads et al.,
 * it has been placed into module scope.
 */
static const char *progname = "(null)";

/**
 * A structure storing the substitutions to be made.
 */
typedef struct sub_s
{
	/** The string which should be replaced by the new string. */
	char *oldstr;

	/** The string which should replace the old string. */
	char *newstr;
} sub_t;

/**
 * Prints a message on how to execute this program and exits with a
 * failure code.
 *
 * @warning This function does not return.
 */
static void usage(void)
{
	(void)fprintf(stderr, "Usage: %s [-e script]... [file]...\n", progname);
	exit(EXIT_FAILURE);
}

/**
 * Frees the substitutions in the given array, and then the array itself.
 *
 * @param subs The array to free.
 * @param nsubs Number of elements stored in the subs array.
 */
static void free_subs(sub_t *subs, size_t nsubs)
{
	size_t i;

	if (subs == NULL)
	{
		return;
	}

	for (i = 0; i < nsubs; ++i)
	{
		free(subs[i].oldstr);
		free(subs[i].newstr);
	}
	free(subs);
}

/**
 * Returns whether the given script is valid.
 *
 * @param scrstr Script string to validate.
 * @return Whether the given script is valid.
 */
static bool script_valid(const char *scrstr)
{
	/* format: s/oldstr/newstr/g */

	size_t i;
	size_t scrlen = strlen(scrstr);
	size_t slashcount = 0;

	/* check if it is at least 6 chars long ("s/x//g") */
	if (scrlen < 6)
	{
		return false;
	}

	/* check if begins with s/ and oldstr is at least one character long */
	if (scrstr[0] != 's' || scrstr[1] != '/' || scrstr[2] == '/')
	{
		return false;
	}

	/* check if ends with /g */
	if (scrstr[scrlen-2] != '/' || scrstr[scrlen-1] != 'g')
	{
		return false;
	}

	/* check if contains exactly three slashes */
	for (i = 0; i < scrlen; ++i)
	{
		if (scrstr[i] == '/')
		{
			++slashcount;
		}
	}

	return (slashcount == 3);
}

/**
 * Prints a standard-format error message to standard error.
 *
 * @param funcname Name of the function that failed.
 */
static void print_failure(const char *funcname)
{
	(void)fprintf(stderr, "%s: %s: %s\n", progname, funcname, strerror(errno));
}

/**
 * Counts the number of occurrences of a string in another.
 *
 * @param haystack The string in which to count the occurrences.
 * @param needle The string whose occurrences to count.
 * @return Number of occurrences of needle in haystack.
 */
static size_t count_occs(const char *haystack, const char *needle)
{
	size_t ret = 0;
	char *occ;

	while ((occ = strstr(haystack, needle)) != NULL)
	{
		++ret;
		haystack = occ + strlen(needle);
	}

	return ret;
}

/**
 * Perform the given substitutions in order on the given file.
 *
 * @warning If a line is longer than LINE_BUF_SIZE-2 characters, the
 * behavior is undefined.
 *
 * @param f File on which to perform substitutions.
 * @param subs The substitutions to perform, in order.
 * @param nsubs The number of substitutions to perform.
 */
static void perform_subs(FILE *f, sub_t *subs, size_t nsubs)
{
	char *oldbuf, *newbuf;
	char *oldwalker, *newwalker;
	char *occ;
	size_t i, subcount, newbuflen, copylen;

	/* initial buffer allocation */
	oldbuf = malloc(LINE_BUF_SIZE);
	if (oldbuf == NULL)
	{
		print_failure("malloc");
		free_subs(subs, nsubs);
		exit(EXIT_FAILURE);
	}

	/* read lines */
	while (fgets(oldbuf, LINE_BUF_SIZE, f) != NULL)
	{
		/* perform substitutions */
		for (i = 0; i < nsubs; ++i)
		{
			/* calculate size of new buffer */
			subcount = count_occs(oldbuf, subs[i].oldstr);
			newbuflen = strlen(oldbuf) + subcount*strlen(subs[i].newstr) - subcount*strlen(subs[i].oldstr);

			/* allocate new buffer */
			newbuf = malloc(newbuflen+1);
			if (newbuf == NULL)
			{
				print_failure("malloc");
				free_subs(subs, nsubs);
				exit(EXIT_FAILURE);
			}

			/* prepare walkers */
			oldwalker = &oldbuf[0];
			newwalker = &newbuf[0];

			/* while there still is an occurrence */
			while ((occ = strstr(oldwalker, subs[i].oldstr)) != NULL)
			{
				/* calculate number of chars to copy */
				copylen = occ - oldwalker;

				/* copy text up and until this occurrence */
				(void)strncpy(newwalker, oldwalker, copylen);

				/* advance walkers */
				oldwalker += copylen;
				newwalker += copylen;

				/* copy new string instead of the old one */
				(void)strncpy(newwalker, subs[i].newstr, strlen(subs[i].newstr));

				/* advance walkers (differently!) */
				newwalker += strlen(subs[i].newstr);
				oldwalker += strlen(subs[i].oldstr);
			}

			/* copy the rest */
			(void)strncpy(newwalker, oldwalker, strlen(oldwalker));
			newwalker += strlen(oldwalker);

			/* NUL-terminate -.- */
			newwalker[0] = '\0';

			/* swizzle buffers */
			free(oldbuf);
			oldbuf = newbuf;
		}

		/* output the completely replaced line */
		(void)fputs(oldbuf, stdout);

		/* renew the buffer */
		free(oldbuf);
		oldbuf = malloc(LINE_BUF_SIZE);
		if (oldbuf == NULL)
		{
			print_failure("malloc");
			free_subs(subs, nsubs);
			exit(EXIT_FAILURE);
		}
	}

	if (ferror(f))
	{
		(void)fprintf(stderr, "%s: I/O error\n", progname);
		free(oldbuf);
		free_subs(subs, nsubs);
		exit(EXIT_FAILURE);
	}

	/* the final step */
	free(oldbuf);
}

/**
 * The main entry point of the program.
 *
 * @param argc Number of command-line arguments.
 * @param argv The command-line arguments themselves.
 * @return Exit code of the program -- zero if successful,
 * nonzero otherwise.
 */
int main(int argc, char **argv)
{
	int opt;
	sub_t *subs = NULL;
	size_t i, subcount = 0;

	/* set program name */
	if (argc > 0)
	{
		progname = argv[0];
	}

	/* parse args */
	while ((opt = getopt(argc, argv, "e:")) != -1)
	{
		switch (opt)
		{
			case 'e':
			{
				sub_t *newsubs;
				sub_t *thenewsub;
				size_t oldlen, newlen;

				/* validate */
				if (!script_valid(optarg))
				{
					(void)fprintf(stderr, "%s: Invalid script '%s'\n", progname, optarg);
					free_subs(subs, subcount);
					exit(EXIT_FAILURE);
				}

				/* calculate string lengths */
				oldlen = strcspn(optarg+2, "/");         /* skip s/ */
				newlen = strcspn(optarg+oldlen+3, "/");  /* skip s/<oldstr>/ */

				/* skip identity substitutions */
				if (oldlen == newlen && strncmp(optarg+2, optarg+oldlen+3, oldlen) == 0)
				{
					continue;
				}

				/* resize array */
				++subcount;
				newsubs = realloc(subs, subcount*sizeof(*newsubs));
				if (newsubs == NULL)
				{
					print_failure("realloc");
					free_subs(subs, subcount);
					exit(EXIT_FAILURE);
				}
				subs = newsubs;

				/* store a "shortcut" */
				thenewsub = &subs[subcount-1];

				/* allocate space for the strings */
				thenewsub->oldstr = malloc(oldlen+1);
				if (thenewsub->oldstr == NULL)
				{
					print_failure("malloc");
					thenewsub->newstr = NULL; /* so that free_subs works */
					free_subs(subs, subcount);
					exit(EXIT_FAILURE);
				}
				thenewsub->newstr = malloc(newlen+1);
				if (thenewsub->newstr == NULL)
				{
					print_failure("malloc");
					free_subs(subs, subcount);
					exit(EXIT_FAILURE);
				}

				/* copy the strings */
				(void)strncpy(thenewsub->oldstr, optarg+2, oldlen);
				(void)strncpy(thenewsub->newstr, optarg+oldlen+3, newlen);

				/* NUL-terminate because strncpy was specified by monkeys on opium */
				thenewsub->oldstr[oldlen] = '\0';
				thenewsub->newstr[newlen] = '\0';
				break;
			}
			case '?':
				free_subs(subs, subcount);
				usage();
				break;
			default:
				assert(0 && "default case of getopt switch");
		}
	}

	/* calculate number of files */
	if (optind < argc)
	{
		/* there are files */
		for (i = optind; i < argc; ++i)
		{
			FILE *f = fopen(argv[i], "r");
			if (f == NULL)
			{
				print_failure("fopen");
				free_subs(subs, subcount);
				exit(EXIT_FAILURE);
			}

			perform_subs(f, subs, subcount);

			(void)fclose(f);
		}
	}
	else
	{
		/* no files; read stdin */
		perform_subs(stdin, subs, subcount);
	}

	free_subs(subs, subcount);

	return 0;
}