diff --git a/include/tree_sitter/parser.h b/include/tree_sitter/parser.h
index b9d13915..8673b059 100644
--- a/include/tree_sitter/parser.h
+++ b/include/tree_sitter/parser.h
@@ -33,7 +33,7 @@ extern "C" {
 
 #define PARSE_FN() \
 static const ts_tree * \
-ts_parse(void *data, ts_input input)
+ts_parse(void *data, ts_input input, ts_input_edit *edit)
 
 #define LEX_FN() \
 static ts_tree * \
@@ -59,9 +59,7 @@ ts_parser constructor_name() { \
 
 #define START_PARSER() \
 ts_lr_parser *parser = (ts_lr_parser *)data; \
-ts_lr_parser_reset(parser); \
-parser->lexer.input = input; \
-ts_lexer_advance(&parser->lexer); \
+ts_lr_parser_initialize(parser, input, edit); \
 next_state:
 
 #define LOOKAHEAD_SYM() \
@@ -264,11 +262,15 @@ static ts_lr_parser * ts_lr_parser_make() {
     return result;
 }
 
-static void ts_lr_parser_reset(ts_lr_parser *parser) {
+static void ts_lr_parser_initialize(ts_lr_parser *parser, ts_input input, ts_input_edit *edit) {
     ts_stack_shrink(&parser->stack, 0);
     parser->lookahead = NULL;
     parser->next_lookahead = NULL;
+
+    input.seek_fn(input.data, 0);
     parser->lexer = ts_lexer_make();
+    parser->lexer.input = input;
+    ts_lexer_advance(&parser->lexer);
 }
 
 static ts_symbol ts_lr_parser_lookahead_sym(const ts_lr_parser *parser) {
@@ -283,9 +285,8 @@ static void ts_lr_parser_shift(ts_lr_parser *parser, state_id parse_state) {
 }
 
 static void ts_lr_parser_reduce(ts_lr_parser *parser, ts_symbol symbol, int immediate_child_count, const int *collapse_flags) {
-    ts_tree *lookahead = ts_stack_reduce(&parser->stack, symbol, immediate_child_count, collapse_flags);
     parser->next_lookahead = parser->lookahead;
-    parser->lookahead = lookahead;
+    parser->lookahead = ts_stack_reduce(&parser->stack, symbol, immediate_child_count, collapse_flags);
 }
 
 static int ts_lr_parser_handle_error(ts_lr_parser *parser, size_t count, const ts_symbol *expected_symbols) {
diff --git a/include/tree_sitter/runtime.h b/include/tree_sitter/runtime.h
index 21492e52..64f39d80 100644
--- a/include/tree_sitter/runtime.h
+++ b/include/tree_sitter/runtime.h
@@ -48,14 +48,20 @@ typedef struct {
 } ts_input;
 
     typedef struct {
-        const ts_tree * (* parse_fn)(void *data, ts_input input);
-        void (* free_fn)(void *data);
-        const char **symbol_names;
-        void *data;
-    } ts_parser;
+        size_t position;
+        size_t bytes_inserted;
+        size_t bytes_removed;
+    } ts_input_edit;
 
-    const ts_tree * ts_parser_parse(ts_parser *, ts_input);
-    void ts_parser_free(ts_parser *);
+typedef struct {
+    const ts_tree * (* parse_fn)(void *data, ts_input input, ts_input_edit *edit);
+    void (* free_fn)(void *data);
+    const char **symbol_names;
+    void *data;
+} ts_parser;
+
+const ts_tree * ts_parser_parse(ts_parser *, ts_input, ts_input_edit *edit);
+void ts_parser_free(ts_parser *);
 
 typedef struct ts_document ts_document;
 
@@ -64,7 +70,7 @@ void ts_document_free(ts_document *doc);
 void ts_document_set_parser(ts_document *doc, ts_parser parser);
 void ts_document_set_input(ts_document *doc, ts_input input);
 void ts_document_set_input_string(ts_document *doc, const char *text);
-void ts_document_edit(ts_document *doc, size_t position, size_t deleted_bytes, size_t inserted_bytes);
+void ts_document_edit(ts_document *doc, ts_input_edit edit);
 
 const ts_tree * ts_document_tree(const ts_document *doc);
 const char * ts_document_string(const ts_document *doc);
diff --git a/spec/runtime/parser_spec.cc b/spec/runtime/parser_spec.cc
index 3b48bf11..380f7001 100644
--- a/spec/runtime/parser_spec.cc
+++ b/spec/runtime/parser_spec.cc
@@ -48,7 +48,11 @@ describe("parsing", [&]() {
             string inserted_text(", \"key2\": 4 ");
 
             reader->content.insert(position, inserted_text);
-            ts_document_edit(doc, position, 0, inserted_text.length());
+            ts_document_edit(doc, {
+                .position = position,
+                .bytes_removed = 0,
+                .bytes_inserted = inserted_text.length()
+            });
         });
 
         it("updates the parse tree", [&]() {
@@ -64,7 +68,7 @@ describe("parsing", [&]() {
             ));
         });
 
-        it_skip("re-reads only the changed portion of the input", [&]() {
+        it("re-reads only the changed portion of the input", [&]() {
             AssertThat(reader->chunks_read, Equals(vector<string>({
                 ""
             })));
diff --git a/src/runtime/document.c b/src/runtime/document.c
index 46fbcea4..d625180c 100644
--- a/src/runtime/document.c
+++ b/src/runtime/document.c
@@ -30,12 +30,11 @@ const char * ts_document_string(const ts_document *document) {
 
 void ts_document_set_input(ts_document *document, ts_input input) {
     document->input = input;
-    document->tree = ts_parser_parse(&document->parser, input);
+    document->tree = ts_parser_parse(&document->parser, input, NULL);
 }
 
-void ts_document_edit(ts_document *document, size_t position, size_t bytes_removed, size_t bytes_inserted) {
-    document->input.seek_fn(document->input.data, 0);
-    document->tree = ts_parser_parse(&document->parser, document->input);
+void ts_document_edit(ts_document *document, ts_input_edit edit) {
+    document->tree = ts_parser_parse(&document->parser, document->input, &edit);
 }
 
 typedef struct {
diff --git a/src/runtime/parser.c b/src/runtime/parser.c
index c7f3a86a..211e422b 100644
--- a/src/runtime/parser.c
+++ b/src/runtime/parser.c
@@ -1,7 +1,7 @@
 #include "tree_sitter/runtime.h"
 
-const ts_tree * ts_parser_parse(ts_parser *parser, ts_input input) {
-    return parser->parse_fn(parser->data, input);
+const ts_tree * ts_parser_parse(ts_parser *parser, ts_input input, ts_input_edit *edit) {
+    return parser->parse_fn(parser->data, input, edit);
 }
 
 void ts_parser_free(ts_parser *parser) {
diff --git a/src/runtime/stack.c b/src/runtime/stack.c
index 75a2ff1a..2425df14 100644
--- a/src/runtime/stack.c
+++ b/src/runtime/stack.c
@@ -13,6 +13,7 @@ typedef struct {
 } ts_stack;
 
 static int INITIAL_STACK_SIZE = 100;
+static int INITIAL_STATE = 0;
 
 ts_stack ts_stack_make() {
     ts_stack result = {
@@ -23,7 +24,7 @@ ts_stack ts_stack_make() {
 }
 
 state_id ts_stack_top_state(const ts_stack *stack) {
-    if (stack->size == 0) return 0;
+    if (stack->size == 0) return INITIAL_STATE;
     return stack->entries[stack->size - 1].state;
 }