Modify grammar slighly to simplify parse tree

lib/parser.c

@@ -10,6 +10,7 @@
 
 #define CLASS_START_CAPACITY 4
 #define SEQUENCE_START_CAPACITY 8
+#define PARSE_TREE_START_CAPACITY 4
 
 static bool is_special(char c)
 {
@@ -182,23 +183,37 @@ int parse_expr(const char *input, int rem, parse_tree_t *out)
 {
 	int result, used = 0;
 
-	result = parse_sequence(input + used, rem - used, &out->sequence);
+	out->count = 0;
+	out->capacity = PARSE_TREE_START_CAPACITY;
+	out->alternatives = malloc(out->capacity * sizeof(sequence_t));
+	if (NULL == out->alternatives)
+		return -1;
+
+	result = parse_sequence(input + used, rem - used, &out->alternatives[0]);
 	if (result < 0)
 		return -1;
+	++out->count;
 	used += result;
 
-	if (used < rem && '|' == input[used]) {
+	while (used < rem) {
+		if ('|' != input[used])
+			break;
 		++used;
 
-		out->alternative = malloc(sizeof(parse_tree_t));
-		if (NULL == out->alternative)
-			return -1;
-		result = parse_expr(input + used, rem - used, out->alternative);
+		if (out->count >= out->capacity) {
+			out->capacity *= 2;
+			out->alternatives = realloc(
+			    out->alternatives, out->capacity * sizeof(sequence_t));
+			if (NULL == out->alternatives)
+				return -1;
+		}
+
+		result = parse_sequence(
+		    input + used, rem - used, &out->alternatives[out->count]);
 		if (result < 0)
-			return -1;
+			break;
+		++out->count;
 		used += result;
-	} else {
-		out->alternative = NULL;
 	}
 
 	return used;
@@ -232,9 +247,9 @@ static void sequence_free(sequence_t *s)
 
 void parse_tree_free_children(parse_tree_t *t)
 {
-	sequence_free(&t->sequence);
-	if (NULL != t->alternative) {
-		parse_tree_free_children(t->alternative);
-		free(t->alternative);
+	if (NULL != t->alternatives) {
+		for (int i = 0; i < t->count; ++i)
+			sequence_free(&t->alternatives[i]);
+		free(t->alternatives);
 	}
 }

lib/parser.h

@@ -35,8 +35,8 @@ typedef struct {
 } sequence_t;
 
 typedef struct _parse_tree {
-	sequence_t sequence;
-	struct _parse_tree *alternative;
+	int count, capacity;
+	sequence_t *alternatives;
 } parse_tree_t;
 
 typedef struct _term {