Reset lexer correctly when old input was blank

spec/runtime/document_spec.cc

@@ -66,6 +66,20 @@ describe("Document", [&]() {
         "(object (string) (array (null) (number)))"));
       AssertThat(spy_input->strings_read, Equals(vector<string>({" [null, 2", ""})));
     });
+
+    it("reads from the new input correctly when the old input was blank", [&]() {
+      ts_document_set_input_string(doc, "");
+      ts_document_parse(doc);
+      TSNode new_root = ts_document_root_node(doc);
+      AssertThat(ts_node_string(new_root, doc), Equals(
+        "(ERROR (UNEXPECTED <EOF>))"));
+
+      ts_document_set_input_string(doc, "1");
+      ts_document_parse(doc);
+      new_root = ts_document_root_node(doc);
+      AssertThat(ts_node_string(new_root, doc), Equals(
+        "(number)"));
+    });
   });
 
   describe("set_language(language)", [&]() {

src/runtime/lexer.c

@@ -123,10 +123,7 @@ TSLexer ts_lexer_make() {
   return result;
 }
 
-void ts_lexer_reset(TSLexer *self, TSLength position, TSPoint point) {
-  if (ts_length_eq(position, self->current_position))
-    return;
-
+static inline void ts_lexer__reset(TSLexer *self, TSLength position, TSPoint point) {
   self->token_start_position = position;
   self->token_end_position = position;
   self->current_position = position;
@@ -141,3 +138,14 @@ void ts_lexer_reset(TSLexer *self, TSLength position, TSPoint point) {
   self->lookahead_size = 0;
   self->lookahead = 0;
 }
+
+void ts_lexer_set_input(TSLexer *self, TSInput input) {
+  self->input = input;
+  ts_lexer__reset(self, ts_length_zero(), ts_point_zero());
+}
+
+void ts_lexer_reset(TSLexer *self, TSLength position, TSPoint point) {
+  if (!ts_length_eq(position, self->current_position))
+    ts_lexer__reset(self, position, point);
+    return;
+}

src/runtime/lexer.h

@@ -8,6 +8,7 @@ extern "C" {
 #include "tree_sitter/parser.h"
 
 TSLexer ts_lexer_make();
+void ts_lexer_set_input(TSLexer *, TSInput);
 void ts_lexer_reset(TSLexer *, TSLength, TSPoint);
 
 #ifdef __cplusplus

src/runtime/parser.c

@@ -342,8 +342,7 @@ static void ts_parser__start(TSParser *self, TSInput input,
     LOG("new_parse");
   }
 
-  self->lexer.input = input;
-  ts_lexer_reset(&self->lexer, ts_length_zero(), ts_point_zero());
+  ts_lexer_set_input(&self->lexer, input);
   ts_stack_clear(self->stack);
 
   LookaheadState head_state = {