string_variable_substitution.c

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

static int do_sub(const char *key, size_t key_len, char *buffer, size_t buffer_len) {
	if (strncmp("HWVERSION", key, key_len) == 0) {
		return snprintf(buffer, buffer_len, "%d.%d.%d", 1, 2, 3);
	} else if (strncmp("xyzzy", key, key_len) == 0) {
		return snprintf(buffer, buffer_len, "%s", "quux");
	} else {
		return 0;
	}
}

// needs error handling
void substitute(const char *tpl, char *buffer, size_t buffer_len) {
	const char *vstart;
	char *wp = buffer;
	char *end = wp + buffer_len;
	int state = 0;
	while (*tpl) {
		switch (state) {
			case 0:
				if (*tpl == '%') {
					vstart = tpl+1;
					state = 1;
				} else {
					*wp++ = *tpl;
				}
				break;
			case 1:
				if (*tpl == '%') {
					int cap = end - wp;
					int written = do_sub(vstart, tpl - vstart, wp, end - wp);
					if (written >= cap) {
						// TODO: handle error
					}
					wp += written;
					state = 0;
				}
				break;
		}
		tpl++;
	}
	*wp++ = 0;
}

int main(int argc, char *argv[]) {
	const char *tpl = "http://myapi.com/%HWVERSION%/blah/%xyzzy%/%ERROR%";
	char buffer[128];
	substitute(tpl, buffer, 128);
	puts(buffer);
	return 0;
}