Task: S-expressions

C: #include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>

enum { S_NONE, S_LIST, S_STRING, S_SYMBOL };

typedef struct {
	int type;
	size_t len;
	void *buf;
} s_expr, *expr;

void whine(const char *s)
{
	fprintf(stderr, "parse error before ==>%.10s\n", s);
}

expr parse_string(const char *s, char **e)
{
	expr ex = calloc(sizeof(s_expr), 1);
	char buf[256] = {0};
	int i = 0;

	while (*s) {
		if (i >= 256) {
			fprintf(stderr, "string too long:\n");
			whine(s);
			goto fail;
		}
		switch (*s) {
		case '\\':
			switch (*++s) {
			case '\\':
			case '"':	buf[i++] = *s++;
					continue;

			default:	whine(s);
					goto fail;
			}
		case '"':	goto success;
		default:	buf[i++] = *s++;
		}
	}
fail:
	free(ex);
	return 0;

success:
	*(const char **)e = s + 1;
	ex->type = S_STRING;
	ex->buf = strdup(buf);
	ex->len = strlen(buf);
	return ex;
}

expr parse_symbol(const char *s, char **e)
{
	expr ex = calloc(sizeof(s_expr), 1);
	char buf[256] = {0};
	int i = 0;

	while (*s) {
		if (i >= 256) {
			fprintf(stderr, "symbol too long:\n");
			whine(s);
			goto fail;
		}
		if (isspace(*s)) goto success;
		if (*s == ')' || *s == '(') {
			s--;
			goto success;
		}

		switch (*s) {
		case '\\':
			switch (*++s) {
			case '\\': case '"': case '(': case ')':
					buf[i++] = *s++;
					continue;
			default:	whine(s);
					goto fail;
			}
		case '"':	whine(s);
				goto success;
		default:	buf[i++] = *s++;
		}
	}
fail:
	free(ex);
	return 0;

success:
	*(const char **)e = s + 1;
	ex->type = S_SYMBOL;
	ex->buf = strdup(buf);
	ex->len = strlen(buf);
	return ex;
}

void append(expr list, expr ele)
{
	list->buf = realloc(list->buf, sizeof(expr) * ++list->len);
	((expr*)(list->buf))[list->len - 1] = ele;
}

expr parse_list(const char *s, char **e)
{
	expr ex = calloc(sizeof(s_expr), 1), chld;
	char *next;

	ex->len = 0;

	while (*s) {
		if (isspace(*s)) {
			s++;
			continue;
		}

		switch (*s) {
		case '"':
			chld = parse_string(s+1, &next);
			if (!chld) goto fail;
			append(ex, chld);
			s = next;
			continue;
		case '(':
			chld = parse_list(s+1, &next);
			if (!chld) goto fail;
			append(ex, chld);
			s = next;
			continue;
		case ')':
			goto success;

		default:
			chld = parse_symbol(s, &next);
			if (!chld) goto fail;
			append(ex, chld);
			s = next;
			continue;
		}
	}

fail:
	whine(s);
	free(ex);
	return 0;

success:
	*(const char **)e = s+1;
	ex->type = S_LIST;
	return ex;
}

expr parse_term(const char *s, char **e)
{
	while (*s) {
		if (isspace(*s)) {
			s++;
			continue;
		}
		switch(*s) {
		case '(':
			return parse_list(s+1, e);
		case '"':
			return parse_string(s+1, e);
		default:
			return parse_symbol(s+1, e);
		}
	}
	return 0;
}

void print_expr(expr e, int depth)
{
#define sep() for(i = 0; i < depth; i++) printf("    ")
	int i;
	if (!e) return;


	switch(e->type) {
	case S_LIST:
		sep();
		puts("(");
		for (i = 0; i < e->len; i++)
			print_expr(((expr*)e->buf)[i], depth + 1);
		sep();
		puts(")");
		return;
	case S_SYMBOL:
	case S_STRING:
		sep();
		if (e->type == S_STRING) putchar('"');
		for (i = 0; i < e->len; i++) {
			switch(((char*)e->buf)[i]) {
			case '"':
			case '\\':
				putchar('\\');
				break;
			case ')': case '(':
				if (e->type == S_SYMBOL)
					putchar('\\');
			}

			putchar(((char*)e->buf)[i]);
		}
		if (e->type == S_STRING) putchar('"');
		putchar('\n');
		return;
	}
}

int main()
{
	char *next;
	const char *in = "((data da\\(\\)ta \"quot\\\\ed data\" 123 4.5)\n"
			" (\"data\" (!@# (4.5) \"(mo\\\"re\" \"data)\")))";

	expr x = parse_term(in, &next);

	printf("input is:\n%s\n", in);
	printf("parsed as:\n");
	print_expr(x, 0);
	return 0;
}