diff --git a/project.gyp b/project.gyp
index 5c0b0c98..ed97e4c5 100644
--- a/project.gyp
+++ b/project.gyp
@@ -105,6 +105,7 @@
         'src/runtime/document.c',
         'src/runtime/lexer.c',
         'src/runtime/node.c',
+        'src/runtime/parse_stack.c',
         'src/runtime/parser.c',
         'src/runtime/stack.c',
         'src/runtime/string_input.c',
diff --git a/spec/runtime/parse_stack_spec.cc b/spec/runtime/parse_stack_spec.cc
new file mode 100644
index 00000000..834a6030
--- /dev/null
+++ b/spec/runtime/parse_stack_spec.cc
@@ -0,0 +1,211 @@
+#include "runtime/runtime_spec_helper.h"
+#include "runtime/parse_stack.h"
+#include "runtime/tree.h"
+#include "runtime/length.h"
+
+enum { stateA, stateB, stateC, stateD, stateE, stateF, stateG, };
+enum { symbol0, symbol1, symbol2, symbol3, symbol4, symbol5, symbol6, };
+const char *names[] = { "zero", "one", "two", "three", "four", "five", "six", };
+
+START_TEST
+
+describe("ParseStack", [&]() {
+  ParseStack *stack;
+  const size_t tree_count = 6;
+  TSTree *trees[tree_count];
+
+  before_each([&]() {
+    stack = ts_parse_stack_new();
+    TSLength len = ts_length_make(2, 2);
+    for (size_t i = 0; i < tree_count; i++)
+      trees[i] = ts_tree_make_leaf(i, len, len, false);
+  });
+
+  after_each([&]() {
+    ts_parse_stack_delete(stack);
+    for (size_t i = 0; i < tree_count; i++)
+      ts_tree_release(trees[i]);
+  });
+
+  it("starts with a single null head", [&]() {
+    AssertThat(ts_parse_stack_head_count(stack), Equals(1));
+    AssertThat(ts_parse_stack_head(stack, 0), Equals<ParseStackNode *>(NULL));
+  });
+
+  describe("shift(head_index, state, tree)", [&]() {
+    it("pushes a node with the given state and tree onto the given head", [&]() {
+      ts_parse_stack_shift(stack, 0, 100, trees[0]);
+      ts_parse_stack_shift(stack, 0, 101, trees[1]);
+      ts_parse_stack_shift(stack, 0, 102, trees[2]);
+
+      ParseStackNode *head = ts_parse_stack_head(stack, 0);
+      AssertThat(head->state, Equals(102));
+      AssertThat(head->tree, Equals(trees[2]));
+      AssertThat(head->successor_count, Equals(1));
+
+      head = head->successors[0];
+      AssertThat(head->state, Equals(101));
+      AssertThat(head->tree, Equals(trees[1]));
+      AssertThat(head->successor_count, Equals(1));
+
+      head = head->successors[0];
+      AssertThat(head->state, Equals(100));
+      AssertThat(head->tree, Equals(trees[0]));
+      AssertThat(head->successor_count, Equals(1));
+
+      head = head->successors[0];
+      AssertThat(head, Equals<ParseStackNode *>(NULL));
+    });
+  });
+
+  describe("reduce(head_index, state, symbol, child_count)", [&]() {
+    before_each([&]() {
+      ts_parse_stack_shift(stack, 0, 100, trees[0]);
+      ts_parse_stack_shift(stack, 0, 101, trees[1]);
+      ts_parse_stack_shift(stack, 0, 102, trees[2]);
+    });
+
+    it("replaces the given number of nodes with a single parent node", [&]() {
+      ts_parse_stack_reduce(stack, 0, 103, symbol4, 2);
+
+      ParseStackNode *head = ts_parse_stack_head(stack, 0);
+      AssertThat(head->state, Equals(103));
+      AssertThat(
+        ts_tree_eq(
+          head->tree,
+          ts_tree_make_node(symbol4, 2, tree_array({ trees[1], trees[2] }), false)
+        ),
+        IsTrue());
+      AssertThat(head->successor_count, Equals(1));
+
+      head = head->successors[0];
+      AssertThat(head->state, Equals(100));
+      AssertThat(head->tree, Equals(trees[0]));
+      AssertThat(head->successor_count, Equals(1));
+    });
+
+    describe("when one of the reduced children is an 'extra' tree", [&]() {
+      before_each([&]() {
+        ts_tree_set_extra(trees[1]);
+      });
+
+      it("does not count that child toward the number of children to replace", [&]() {
+        ts_parse_stack_reduce(stack, 0, 103, symbol4, 2);
+
+        ParseStackNode *head = ts_parse_stack_head(stack, 0);
+        AssertThat(head->state, Equals(103));
+        AssertThat(
+          ts_tree_eq(
+            head->tree,
+            ts_tree_make_node(symbol4, 3, tree_array({ trees[0], trees[1], trees[2] }), false)
+          ),
+          IsTrue());
+        AssertThat(head->successor_count, Equals(1));
+
+        head = head->successors[0];
+        AssertThat(head, Equals<ParseStackNode *>(NULL));
+      });
+    });
+  });
+
+  describe("split(head_index)", [&]() {
+    bool merged;
+
+    before_each([&]() {
+      //  A0__B1__C2
+      ts_parse_stack_shift(stack, 0, stateA, trees[0]);
+      ts_parse_stack_shift(stack, 0, stateB, trees[1]);
+      ts_parse_stack_shift(stack, 0, stateC, trees[2]);
+
+      int new_index = ts_parse_stack_split(stack, 0);
+      AssertThat(new_index, Equals(1));
+      AssertThat(ts_parse_stack_head_count(stack), Equals(2));
+    });
+
+    it("creates a new head pointing to the same node as the given head", [&]() {
+      //  A0__B1__C2__D3
+      //   \__E4__F3
+      ts_parse_stack_shift(stack, 0, stateD, trees[3]);
+      ts_parse_stack_reduce(stack, 1, stateE, symbol4, 2);
+      merged = ts_parse_stack_shift(stack, 1, stateF, trees[3]);
+
+      AssertThat(merged, IsFalse());
+      AssertThat(ts_parse_stack_head_count(stack), Equals(2));
+
+      ParseStackNode *head1 = ts_parse_stack_head(stack, 0);
+      AssertThat(head1->state, Equals(stateD));
+      AssertThat(head1->tree, Equals(trees[3]));
+      AssertThat(head1->successor_count, Equals(1));
+
+      ParseStackNode *head2 = ts_parse_stack_head(stack, 1);
+      AssertThat(head2->state, Equals(stateF));
+      AssertThat(head2->tree, Equals(trees[3]));
+      AssertThat(head2->successor_count, Equals(1));
+    });
+
+    it("re-joins the heads when the same state and tree are shifted onto both heads", [&]() {
+      //  A0__B1__C2__D3
+      //   \____E4____/
+      ts_parse_stack_shift(stack, 0, stateD, trees[3]);
+      ts_parse_stack_reduce(stack, 1, stateE, symbol4, 2);
+      TSTree *tree4 = ts_parse_stack_head(stack, 1)->tree;
+      merged = ts_parse_stack_shift(stack, 1, stateD, trees[3]);
+
+      AssertThat(merged, IsTrue());
+      AssertThat(ts_parse_stack_head_count(stack), Equals(1));
+
+      ParseStackNode *head = ts_parse_stack_head(stack, 0);
+      AssertThat(head->state, Equals(stateD));
+      AssertThat(head->tree, Equals(trees[3]));
+      AssertThat(head->successor_count, Equals(2));
+
+      ParseStackNode *successor1 = head->successors[0];
+      AssertThat(successor1->state, Equals(stateC));
+      AssertThat(successor1->tree, Equals(trees[2]))
+      AssertThat(successor1->successor_count, Equals(1));
+
+      ParseStackNode *successor2 = head->successors[1];
+      AssertThat(successor2->state, Equals(stateE));
+      AssertThat(successor2->tree, Equals(tree4))
+      AssertThat(successor2->successor_count, Equals(1));
+    });
+
+    it("re-joins the heads when the same state and tree are reduced onto both heads", [&]() {
+      //  A0__B1__C2__F4
+      //       \__D3__E4
+      ts_parse_stack_reduce(stack, 0, stateD, symbol3, 1);
+      ts_parse_stack_shift(stack, 0, stateE, trees[4]);
+      ts_parse_stack_shift(stack, 1, stateF, trees[4]);
+
+      AssertThat(ts_parse_stack_head_count(stack), Equals(2));
+      ParseStackNode *head1 = ts_parse_stack_head(stack, 0);
+      AssertThat(head1->state, Equals(stateE));
+      AssertThat(head1->tree, Equals(trees[4]));
+      AssertThat(head1->successor_count, Equals(1));
+
+      ParseStackNode *head2 = ts_parse_stack_head(stack, 1);
+      AssertThat(head2->state, Equals(stateF));
+      AssertThat(head2->tree, Equals(trees[4]));
+      AssertThat(head2->successor_count, Equals(1));
+
+      //  A0__B1__C2__G5
+      //       \__D3__/
+      merged = ts_parse_stack_reduce(stack, 0, stateG, symbol5, 1);
+      AssertThat(merged, IsFalse());
+      merged = ts_parse_stack_reduce(stack, 1, stateG, symbol5, 1);
+      AssertThat(merged, IsTrue());
+
+      AssertThat(ts_parse_stack_head_count(stack), Equals(1));
+      ParseStackNode *head = ts_parse_stack_head(stack, 0);
+      AssertThat(head->state, Equals(stateG));
+      AssertThat(
+        ts_tree_eq(
+          head->tree,
+          ts_tree_make_node(symbol5, 1, tree_array({ trees[4] }), false)),
+        IsTrue());
+      AssertThat(head->successor_count, Equals(2));
+    });
+  });
+});
+
+END_TEST
diff --git a/src/runtime/parse_stack.c b/src/runtime/parse_stack.c
new file mode 100644
index 00000000..6120f7bf
--- /dev/null
+++ b/src/runtime/parse_stack.c
@@ -0,0 +1,177 @@
+#include "tree_sitter/parser.h"
+#include "runtime/tree.h"
+#include "runtime/parse_stack.h"
+#include <assert.h>
+
+static const size_t INITIAL_HEAD_CAPACITY = 3;
+
+struct ParseStack {
+  ParseStackNode **heads;
+  int head_count;
+  int head_capacity;
+};
+
+/*
+ *  Section: Lifecycle
+ */
+
+ParseStack *ts_parse_stack_new() {
+  ParseStack *this = malloc(sizeof(ParseStack));
+  *this = (ParseStack) {
+    .heads = calloc(INITIAL_HEAD_CAPACITY, sizeof(ParseStackNode *)),
+    .head_count = 1,
+    .head_capacity = INITIAL_HEAD_CAPACITY,
+  };
+  return this;
+}
+
+void ts_parse_stack_delete(ParseStack *this) {
+  if (this->heads)
+    free(this->heads);
+  free(this);
+}
+
+/*
+ *  Section: Reading
+ */
+
+ParseStackNode *ts_parse_stack_head(const ParseStack *this, int head_index) {
+  assert(head_index < this->head_count);
+  return this->heads[head_index];
+}
+
+int ts_parse_stack_head_count(const ParseStack *this) {
+  return this->head_count;
+}
+
+/*
+ *  Section: Updating
+ */
+
+static ParseStackNode *stack_node_new(ParseStackNode *, TSStateId, TSTree *);
+static void stack_node_retain(ParseStackNode *);
+static bool stack_node_release(ParseStackNode *);
+static void stack_node_add_successor(ParseStackNode *, ParseStackNode *);
+static void parse_stack_remove_head(ParseStack *, int);
+
+bool ts_parse_stack_shift(ParseStack *this, int head_index, TSStateId state, TSTree *tree) {
+  assert(head_index < this->head_count);
+  for (int i =  0; i < head_index; i++) {
+    ParseStackNode *head = this->heads[i];
+    if (head->state == state && ts_tree_eq(head->tree, tree)) {
+      stack_node_add_successor(head, this->heads[head_index]);
+      parse_stack_remove_head(this, head_index);
+      return true;
+    }
+  }
+  ParseStackNode *new_head = stack_node_new(this->heads[head_index], state, tree);
+  this->heads[head_index] = new_head;
+  return false;
+}
+
+bool ts_parse_stack_reduce(ParseStack *this, int head_index, TSStateId state,
+                           TSSymbol symbol, int child_count) {
+  ParseStackNode *head = this->heads[head_index];
+
+  /*
+   *  Walk down the stack to determine which symbols will be reduced.
+   *  The child node count is known ahead of time, but some children
+   *  may be ubiquitous tokens, which don't count.
+   */
+  ParseStackNode *next_node = head;
+  for (int i = 0; i < child_count; i++) {
+    TSTree *child = next_node->tree;
+    if (ts_tree_is_extra(child))
+      child_count++;
+    next_node = next_node->successors[0];
+    if (!next_node)
+      break;
+  }
+
+  TSTree **children = malloc(child_count * sizeof(TSTree *));
+  next_node = head;
+  for (int i = 0; i < child_count; i++) {
+    children[child_count - i - 1] = next_node->tree;
+    next_node = next_node->successors[0];
+  }
+
+  TSTree *parent = ts_tree_make_node(symbol, child_count, children, false);
+
+  for (int i =  0; i < head_index; i++) {
+    ParseStackNode *head = this->heads[i];
+    if (head->state == state && ts_tree_eq(head->tree, parent)) {
+      stack_node_add_successor(head, this->heads[head_index]);
+      ts_tree_release(parent);
+      parse_stack_remove_head(this, head_index);
+      return true;
+    }
+  }
+
+  stack_node_retain(next_node);
+  stack_node_release(this->heads[head_index]);
+  this->heads[head_index] = stack_node_new(next_node, state, parent);
+  return false;
+}
+
+int ts_parse_stack_split(ParseStack *this, int head_index) {
+  assert(head_index < this->head_count);
+  if (this->head_count == this->head_capacity) {
+    this->head_capacity += 3;
+    this->heads = realloc(this->heads, this->head_capacity * sizeof(ParseStackNode *));
+  }
+  int new_index = this->head_count++;
+  this->heads[new_index] = this->heads[head_index];
+  stack_node_retain(this->heads[new_index]);
+  return new_index;
+}
+
+/*
+ *  Section: Private
+ */
+
+static ParseStackNode *stack_node_new(ParseStackNode *next, TSStateId state, TSTree *tree) {
+  ParseStackNode *this = malloc(sizeof(ParseStackNode));
+  *this = (ParseStackNode) {
+    .ref_count = 1,
+    .successor_count = 1,
+    .successors = {next, NULL, NULL},
+    .state = state,
+    .tree = tree,
+  };
+  return this;
+}
+
+static void stack_node_retain(ParseStackNode *this) {
+  if (!this) return;
+  assert(this->ref_count != 0);
+  this->ref_count++;
+}
+
+static bool stack_node_release(ParseStackNode *this) {
+  if (!this) return false;
+  assert(this->ref_count != 0);
+  this->ref_count--;
+  if (this->ref_count == 0) {
+    for (int i = 0; i < this->successor_count; i++)
+      stack_node_release(this->successors[i]);
+    ts_tree_release(this->tree);
+    free(this);
+    return true;
+  } else {
+    return false;
+  }
+}
+
+static void stack_node_add_successor(ParseStackNode *this, ParseStackNode *successor) {
+  stack_node_retain(successor);
+  this->successors[this->successor_count] = successor;
+  this->successor_count++;
+}
+
+static void parse_stack_remove_head(ParseStack *this, int head_index) {
+  stack_node_release(this->heads[head_index]);
+  for (int i = head_index; i < this->head_count - 1; i++) {
+    this->heads[head_index] = this->heads[head_index + 1];
+  }
+  this->head_count--;
+}
diff --git a/src/runtime/parse_stack.h b/src/runtime/parse_stack.h
new file mode 100644
index 00000000..05d498ed
--- /dev/null
+++ b/src/runtime/parse_stack.h
@@ -0,0 +1,34 @@
+#ifndef RUNTIME_PARSE_STACK_H_
+#define RUNTIME_PARSE_STACK_H_
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include "tree_sitter/parser.h"
+
+typedef struct ParseStack ParseStack;
+
+typedef struct ParseStackNode {
+  TSTree *tree;
+  TSStateId state;
+  struct ParseStackNode *successors[4];
+  short unsigned int successor_count;
+  short unsigned int ref_count;
+} ParseStackNode;
+
+ParseStack *ts_parse_stack_new();
+void ts_parse_stack_delete(ParseStack *);
+
+ParseStackNode *ts_parse_stack_head(const ParseStack *, int);
+int ts_parse_stack_head_count(const ParseStack *);
+
+bool ts_parse_stack_shift(ParseStack *, int, TSStateId, TSTree *);
+bool ts_parse_stack_reduce(ParseStack *, int, TSStateId, TSSymbol, int);
+int ts_parse_stack_split(ParseStack *, int);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif  // RUNTIME_PARSE_STACK_H_