Answer for https://gist.github.com/2227226/

remove_multiple_spaces.c

#include "remove_multiple_spaces.h"

#include <assert.h>

size_t
remove_multiple_spaces(char *str) {
	assert(str != 0);
	if (str[0] == '\0') {
		return 0;
	}

	char *next, *iter;
	int prevc, iterc;

	prevc = *str;
	next = str+1;
	iter = str+1;

	for (iterc = *iter; (iterc = *iter) != '\0'; iter++) {
		if (iterc == ' ' && prevc == iterc) {
			continue;
		}
		*next++ = iterc;
		prevc = iterc;
	}
	*next = '\0';
	return next-str;
}

remove_multiple_spaces.h

#ifndef __REMOVE_MULTIPLE_SPACES_H
#define __REMOVE_MULTIPLE_SPACES_H

#include <stddef.h>

// Combine continuous spaces into one, return result string length.
// assert(str != NULL);
//
// e.g.:
//
// ""		==> ""
// "    "	==> " "
// " ab   c d  e  "	==> " ab c d e "
size_t remove_multiple_spaces(char *str);
#endif

remove_multiple_spaces_test.c

#include "remove_multiple_spaces.h"

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

static char *
dupstr(const char *str) {
	size_t len = strlen(str);
	char *ret = malloc(len+1);
	strcpy(ret, str);
	return ret;
}

static void
delstr(char *dup) {
	free(dup);
}

struct pair {
	char *origin;
	char *expect;
};

static const struct pair pairs[] = {
	{"", ""},
	{" ", " "},
	{"    ", " "},
	{"   a", " a"},
	{"a   ", "a "},
	{"   a    ", " a "},
	{"   abcdef    ", " abcdef "},
	{"   abcdef    fe  ", " abcdef fe "},
	{"    abc    def  ghi  opq", " abc def ghi opq"},
	{"   \n    \nabc  cde  ", " \n \nabc cde "},
	{NULL, NULL},
};

static void
ForeachPairs(const struct pair *tests, void (*each)(const struct pair *)) {
	const struct pair *iter;
	for (iter = tests; iter->origin != NULL; iter++) {
		each(iter);
	}
}

// TODO Escape special characters, e.g., \n, \t.
static void
CheckReturn(const struct pair *pi) {
	char *str = dupstr(pi->origin);
	size_t len = remove_multiple_spaces(str);
	if (len != strlen(str)) {
		fprintf(stderr,
			"remove_multiple_spaces(%s) got {{%s}}, length %zu, return %zu.\n",
			pi->origin, str, strlen(str), len);
		abort();
	}
	delstr(str);
}

static void
CheckExpect(const struct pair *pi) {
	char *str = dupstr(pi->origin);
	remove_multiple_spaces(str);
	if (strcmp(str, pi->expect) != 0) {
		fprintf(stderr,
			"remove_multiple_spaces(%s) got {{%s}}, expect {{%s}}.\n",
			pi->origin, str, pi->expect);
		abort();
	}
	delstr(str);
}

static void
TestReturn(const struct pair *tests) {
	ForeachPairs(tests, CheckReturn);
}

static void
TestExpect(const struct pair *tests) {
	ForeachPairs(tests, CheckExpect);
}

int
main(void) {
	setvbuf(stderr, 0, _IONBF, 0);
	setvbuf(stdout, 0, _IONBF, 0);

	TestReturn(pairs);
	TestExpect(pairs);

	fprintf(stdout, "remove_multiple_spaces PASSED\n");

	return 0;
}