Implement `btree::iterator::+=` and `-=`.

PiperOrigin-RevId: 735878054
Change-Id: I37e0c89b66f5e31376e007dda8d4420a6dfe5269

absl/container/btree_benchmark.cc

diff --git a/absl/container/btree_benchmark.cc b/absl/container/btree_benchmark.cc
index 0d26fd4..d0dac37 100644
--- a/absl/container/btree_benchmark.cc
+++ b/absl/container/btree_benchmark.cc
@@ -735,7 +735,7 @@
 
 BIG_TYPE_PTR_BENCHMARKS(32);
 
-void BM_BtreeSet_IteratorSubtraction(benchmark::State& state) {
+void BM_BtreeSet_IteratorDifference(benchmark::State& state) {
   absl::InsecureBitGen bitgen;
   std::vector<int> vec;
   // Randomize the set's insertion order so the nodes aren't all full.
@@ -756,6 +756,52 @@
   }
 }
 
+BENCHMARK(BM_BtreeSet_IteratorDifference)->Range(1 << 10, 1 << 20);
+
+void BM_BtreeSet_IteratorAddition(benchmark::State& state) {
+  absl::InsecureBitGen bitgen;
+  std::vector<int> vec;
+  // Randomize the set's insertion order so the nodes aren't all full.
+  vec.reserve(static_cast<size_t>(state.range(0)));
+  for (int i = 0; i < state.range(0); ++i) vec.push_back(i);
+  absl::c_shuffle(vec, bitgen);
+
+  absl::btree_set<int> set;
+  for (int i : vec) set.insert(i);
+
+  size_t distance = absl::Uniform(bitgen, 0u, set.size());
+  while (state.KeepRunningBatch(distance)) {
+    // Let the increment go all the way to the `end` iterator.
+    const size_t begin =
+        absl::Uniform(absl::IntervalClosed, bitgen, 0u, set.size() - distance);
+    auto it = set.find(static_cast<int>(begin));
+    benchmark::DoNotOptimize(it += static_cast<int>(distance));
+    distance = absl::Uniform(bitgen, 0u, set.size());
+  }
+}
+
+BENCHMARK(BM_BtreeSet_IteratorAddition)->Range(1 << 10, 1 << 20);
+
+void BM_BtreeSet_IteratorSubtraction(benchmark::State& state) {
+  absl::InsecureBitGen bitgen;
+  std::vector<int> vec;
+  // Randomize the set's insertion order so the nodes aren't all full.
+  vec.reserve(static_cast<size_t>(state.range(0)));
+  for (int i = 0; i < state.range(0); ++i) vec.push_back(i);
+  absl::c_shuffle(vec, bitgen);
+
+  absl::btree_set<int> set;
+  for (int i : vec) set.insert(i);
+
+  size_t distance = absl::Uniform(bitgen, 0u, set.size());
+  while (state.KeepRunningBatch(distance)) {
+    size_t end = absl::Uniform(bitgen, distance, set.size());
+    auto it = set.find(static_cast<int>(end));
+    benchmark::DoNotOptimize(it -= static_cast<int>(distance));
+    distance = absl::Uniform(bitgen, 0u, set.size());
+  }
+}
+
 BENCHMARK(BM_BtreeSet_IteratorSubtraction)->Range(1 << 10, 1 << 20);
 
 }  // namespace

absl/container/btree_map.h

diff --git a/absl/container/btree_map.h b/absl/container/btree_map.h
index 470de2a..32a82ef 100644
--- a/absl/container/btree_map.h
+++ b/absl/container/btree_map.h
@@ -47,8 +47,10 @@
 // iterator at the current position. Another important difference is that
 // key-types must be copy-constructible.
 //
-// Another API difference is that btree iterators can be subtracted, and this
-// is faster than using std::distance.
+// There are other API differences: first, btree iterators can be subtracted,
+// and this is faster than using `std::distance`. Additionally, btree
+// iterators can be advanced via `operator+=` and `operator-=`, which is faster
+// than using `std::advance`.
 //
 // B-tree maps are not exception-safe.
 

absl/container/btree_set.h

diff --git a/absl/container/btree_set.h b/absl/container/btree_set.h
index e57d6d9..16181de 100644
--- a/absl/container/btree_set.h
+++ b/absl/container/btree_set.h
@@ -46,8 +46,10 @@
 // reason, `insert()`, `erase()`, and `extract_and_get_next()` return a valid
 // iterator at the current position.
 //
-// Another API difference is that btree iterators can be subtracted, and this
-// is faster than using std::distance.
+// There are other API differences: first, btree iterators can be subtracted,
+// and this is faster than using `std::distance`. Additionally, btree
+// iterators can be advanced via `operator+=` and `operator-=`, which is faster
+// than using `std::advance`.
 //
 // B-tree sets are not exception-safe.
 

absl/container/btree_test.cc

diff --git a/absl/container/btree_test.cc b/absl/container/btree_test.cc
index c398922..1d2c2a6 100644
--- a/absl/container/btree_test.cc
+++ b/absl/container/btree_test.cc
@@ -3351,7 +3351,7 @@
   set.insert(0);
 }
 
-TEST(Btree, IteratorSubtraction) {
+TEST(Btree, IteratorDifference) {
   absl::BitGen bitgen;
   std::vector<int> vec;
   // Randomize the set's insertion order so the nodes aren't all full.
@@ -3369,6 +3369,94 @@
   }
 }
 
+TEST(Btree, IteratorAddition) {
+  absl::BitGen bitgen;
+  std::vector<int> vec;
+
+  // Randomize the set's insertion order so the nodes aren't all full.
+  constexpr int kSetSize = 1000000;
+  for (int i = 0; i < kSetSize; ++i) vec.push_back(i);
+  absl::c_shuffle(vec, bitgen);
+
+  absl::btree_set<int> set;
+  for (int i : vec) set.insert(i);
+
+  for (int i = 0; i < 1000; ++i) {
+    int begin = absl::Uniform(bitgen, 0, kSetSize);
+    int end = absl::Uniform(bitgen, begin, kSetSize);
+    ASSERT_LE(begin, end);
+
+    auto it = set.find(begin);
+    it += end - begin;
+    ASSERT_EQ(it, set.find(end)) << end;
+
+    it += begin - end;
+    ASSERT_EQ(it, set.find(begin)) << begin;
+  }
+}
+
+TEST(Btree, IteratorAdditionOutOfBounds) {
+  const absl::btree_set<int> set({5});
+
+  auto it = set.find(5);
+
+  auto forward = it;
+  forward += 1;
+  EXPECT_EQ(forward, set.end());
+
+  auto backward = it;
+  EXPECT_EQ(backward, set.begin());
+
+  if (IsAssertEnabled()) {
+    EXPECT_DEATH(forward += 1, "n == 0");
+    EXPECT_DEATH(backward += -1, "position >= node->start");
+  }
+}
+
+TEST(Btree, IteratorSubtraction) {
+  absl::BitGen bitgen;
+  std::vector<int> vec;
+
+  // Randomize the set's insertion order so the nodes aren't all full.
+  constexpr int kSetSize = 1000000;
+  for (int i = 0; i < kSetSize; ++i) vec.push_back(i);
+  absl::c_shuffle(vec, bitgen);
+
+  absl::btree_set<int> set;
+  for (int i : vec) set.insert(i);
+
+  for (int i = 0; i < 1000; ++i) {
+    int begin = absl::Uniform(bitgen, 0, kSetSize);
+    int end = absl::Uniform(bitgen, begin, kSetSize);
+    ASSERT_LE(begin, end);
+
+    auto it = set.find(end);
+    it -= end - begin;
+    ASSERT_EQ(it, set.find(begin)) << begin;
+
+    it -= begin - end;
+    ASSERT_EQ(it, set.find(end)) << end;
+  }
+}
+
+TEST(Btree, IteratorSubtractionOutOfBounds) {
+  const absl::btree_set<int> set({5});
+
+  auto it = set.find(5);
+
+  auto backward = it;
+  EXPECT_EQ(backward, set.begin());
+
+  auto forward = it;
+  forward -= -1;
+  EXPECT_EQ(forward, set.end());
+
+  if (IsAssertEnabled()) {
+    EXPECT_DEATH(backward -= 1, "position >= node->start");
+    EXPECT_DEATH(forward -= -1, "n == 0");
+  }
+}
+
 TEST(Btree, DereferencingEndIterator) {
   if (!IsAssertEnabled()) GTEST_SKIP() << "Assertions not enabled.";
 

absl/container/internal/btree.h

diff --git a/absl/container/internal/btree.h b/absl/container/internal/btree.h
index 689e71a..a742829 100644
--- a/absl/container/internal/btree.h
+++ b/absl/container/internal/btree.h
@@ -60,6 +60,7 @@
 #include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/macros.h"
+#include "absl/base/optimization.h"
 #include "absl/container/internal/common.h"
 #include "absl/container/internal/common_policy_traits.h"
 #include "absl/container/internal/compressed_tuple.h"
@@ -708,6 +709,8 @@
   }
 
   // Getter for the parent of this node.
+  // TODO(ezb): assert that the child of the returned node at position
+  // `node_->position()` maps to the current node.
   btree_node *parent() const { return *GetField<0>(); }
   // Getter for whether the node is the root of the tree. The parent of the
   // root of the tree is the leftmost node in the tree which is guaranteed to
@@ -1175,6 +1178,26 @@
     return distance_slow(other);
   }
 
+  // Advances the iterator by `n`. Values of `n` must not result in going past
+  // the `end` iterator (for a positive `n`) or before the `begin` iterator (for
+  // a negative `n`).
+  btree_iterator &operator+=(difference_type n) {
+    assert_valid_generation(node_);
+    if (n == 0) return *this;
+    if (n < 0) return decrement_n_slow(-n);
+    return increment_n_slow(n);
+  }
+
+  // Moves the iterator by `n` positions backwards. Values of `n` must not
+  // result in going before the `begin` iterator (for a positive `n`) or past
+  // the `end` iterator (for a negative `n`).
+  btree_iterator &operator-=(difference_type n) {
+    assert_valid_generation(node_);
+    if (n == 0) return *this;
+    if (n < 0) return increment_n_slow(-n);
+    return decrement_n_slow(n);
+  }
+
   // Accessors for the key/value the iterator is pointing at.
   reference operator*() const {
     ABSL_HARDENING_ASSERT(node_ != nullptr);
@@ -1277,6 +1300,7 @@
     increment_slow();
   }
   void increment_slow();
+  btree_iterator &increment_n_slow(difference_type n);
 
   void decrement() {
     assert_valid_generation(node_);
@@ -1286,6 +1310,7 @@
     decrement_slow();
   }
   void decrement_slow();
+  btree_iterator &decrement_n_slow(difference_type n);
 
   const key_type &key() const {
     return node_->key(static_cast<size_type>(position_));
@@ -2172,6 +2197,80 @@
   }
 }
 
+template <typename N, typename R, typename P>
+btree_iterator<N, R, P> &btree_iterator<N, R, P>::increment_n_slow(
+    difference_type n) {
+  N *node = node_;
+  int position = position_;
+  ABSL_ASSUME(n > 0);
+  while (n > 0) {
+    if (node->is_leaf()) {
+      if (position + n < node->finish()) {
+        position += n;
+        break;
+      } else {
+        n -= node->finish() - position;
+        position = node->finish();
+        btree_iterator save = {node, position};
+        while (position == node->finish() && !node->is_root()) {
+          position = node->position();
+          node = node->parent();
+        }
+        if (position == node->finish()) {
+          ABSL_HARDENING_ASSERT(n == 0);
+          return *this = save;
+        }
+      }
+    } else {
+      --n;
+      assert(position < node->finish());
+      node = node->child(static_cast<field_type>(position + 1));
+      while (node->is_internal()) {
+        node = node->start_child();
+      }
+      position = node->start();
+    }
+  }
+  node_ = node;
+  position_ = position;
+  return *this;
+}
+
+template <typename N, typename R, typename P>
+btree_iterator<N, R, P> &btree_iterator<N, R, P>::decrement_n_slow(
+    difference_type n) {
+  N *node = node_;
+  int position = position_;
+  ABSL_ASSUME(n > 0);
+  while (n > 0) {
+    if (node->is_leaf()) {
+      if (position - n >= node->start()) {
+        position -= n;
+        break;
+      } else {
+        n -= 1 + position - node->start();
+        position = node->start() - 1;
+        while (position < node->start() && !node->is_root()) {
+          position = node->position() - 1;
+          node = node->parent();
+        }
+        ABSL_HARDENING_ASSERT(position >= node->start());
+      }
+    } else {
+      --n;
+      assert(position >= node->start());
+      node = node->child(static_cast<field_type>(position));
+      while (node->is_internal()) {
+        node = node->child(node->finish());
+      }
+      position = node->finish() - 1;
+    }
+  }
+  node_ = node;
+  position_ = position;
+  return *this;
+}
+
 ////
 // btree methods
 template <typename P>