DeltaTree.cpp

//===- DeltaTree.cpp - B-Tree for Rewrite Delta tracking ------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements the DeltaTree and related classes.
//
//===----------------------------------------------------------------------===//

#include "clang/Rewrite/Core/DeltaTree.h"
#include "clang/Basic/LLVM.h"
#include "llvm/Support/Casting.h"
#include <cassert>
#include <cstring>

using namespace clang;

/// The DeltaTree class is a multiway search tree (BTree) structure with some
/// fancy features.  B-Trees are generally more memory and cache efficient
/// than binary trees, because they store multiple keys/values in each node.
///
/// DeltaTree implements a key/value mapping from FileIndex to Delta, allowing
/// fast lookup by FileIndex.  However, an added (important) bonus is that it
/// can also efficiently tell us the full accumulated delta for a specific
/// file offset as well, without traversing the whole tree.
///
/// The nodes of the tree are made up of instances of two classes:
/// DeltaTreeNode and DeltaTreeInteriorNode.  The later subclasses the
/// former and adds children pointers.  Each node knows the full delta of all
/// entries (recursively) contained inside of it, which allows us to get the
/// full delta implied by a whole subtree in constant time.

namespace {

  /// SourceDelta - As code in the original input buffer is added and deleted,
  /// SourceDelta records are used to keep track of how the input SourceLocation
  /// object is mapped into the output buffer.
  struct SourceDelta {
    unsigned FileLoc;
    int Delta;

    static SourceDelta get(unsigned Loc, int D) {
      SourceDelta Delta;
      Delta.FileLoc = Loc;
      Delta.Delta = D;
      return Delta;
    }
  };

  /// DeltaTreeNode - The common part of all nodes.
  ///
  class DeltaTreeNode {
  public:
    struct InsertResult {
      DeltaTreeNode *LHS, *RHS;
      SourceDelta Split;
    };

  private:
    friend class DeltaTreeInteriorNode;

    /// WidthFactor - This controls the number of K/V slots held in the BTree:
    /// how wide it is.  Each level of the BTree is guaranteed to have at least
    /// WidthFactor-1 K/V pairs (except the root) and may have at most
    /// 2*WidthFactor-1 K/V pairs.
    enum { WidthFactor = 8 };

    /// Values - This tracks the SourceDelta's currently in this node.
    SourceDelta Values[2*WidthFactor-1];

    /// NumValuesUsed - This tracks the number of values this node currently
    /// holds.
    unsigned char NumValuesUsed = 0;

    /// IsLeaf - This is true if this is a leaf of the btree.  If false, this is
    /// an interior node, and is actually an instance of DeltaTreeInteriorNode.
    bool IsLeaf;

    /// FullDelta - This is the full delta of all the values in this node and
    /// all children nodes.
    int FullDelta = 0;

  public:
    DeltaTreeNode(bool isLeaf = true) : IsLeaf(isLeaf) {}

    bool isLeaf() const { return IsLeaf; }
    int getFullDelta() const { return FullDelta; }
    bool isFull() const { return NumValuesUsed == 2*WidthFactor-1; }

    unsigned getNumValuesUsed() const { return NumValuesUsed; }

    const SourceDelta &getValue(unsigned i) const {
      assert(i < NumValuesUsed && "Invalid value #");
      return Values[i];
    }

    SourceDelta &getValue(unsigned i) {
      assert(i < NumValuesUsed && "Invalid value #");
      return Values[i];
    }

    /// DoInsertion - Do an insertion of the specified FileIndex/Delta pair into
    /// this node.  If insertion is easy, do it and return false.  Otherwise,
    /// split the node, populate InsertRes with info about the split, and return
    /// true.
    bool DoInsertion(unsigned FileIndex, int Delta, InsertResult *InsertRes);

    void DoSplit(InsertResult &InsertRes);


    /// RecomputeFullDeltaLocally - Recompute the FullDelta field by doing a
    /// local walk over our contained deltas.
    void RecomputeFullDeltaLocally();

    void Destroy();
  };

  /// DeltaTreeInteriorNode - When isLeaf = false, a node has child pointers.
  /// This class tracks them.
  class DeltaTreeInteriorNode : public DeltaTreeNode {
    friend class DeltaTreeNode;

    DeltaTreeNode *Children[2*WidthFactor];

    ~DeltaTreeInteriorNode() {
      for (unsigned i = 0, e = NumValuesUsed+1; i != e; ++i)
        Children[i]->Destroy();
    }

  public:
    DeltaTreeInteriorNode() : DeltaTreeNode(false /*nonleaf*/) {}

    DeltaTreeInteriorNode(const InsertResult &IR)
        : DeltaTreeNode(false /*nonleaf*/) {
      Children[0] = IR.LHS;
      Children[1] = IR.RHS;
      Values[0] = IR.Split;
      FullDelta = IR.LHS->getFullDelta()+IR.RHS->getFullDelta()+IR.Split.Delta;
      NumValuesUsed = 1;
    }

    const DeltaTreeNode *getChild(unsigned i) const {
      assert(i < getNumValuesUsed()+1 && "Invalid child");
      return Children[i];
    }

    DeltaTreeNode *getChild(unsigned i) {
      assert(i < getNumValuesUsed()+1 && "Invalid child");
      return Children[i];
    }

    static bool classof(const DeltaTreeNode *N) { return !N->isLeaf(); }
  };

} // namespace

/// Destroy - A 'virtual' destructor.
void DeltaTreeNode::Destroy() {
  if (isLeaf())
    delete this;
  else
    delete cast<DeltaTreeInteriorNode>(this);
}

/// RecomputeFullDeltaLocally - Recompute the FullDelta field by doing a
/// local walk over our contained deltas.
void DeltaTreeNode::RecomputeFullDeltaLocally() {
  int NewFullDelta = 0;
  for (unsigned i = 0, e = getNumValuesUsed(); i != e; ++i)
    NewFullDelta += Values[i].Delta;
  if (auto *IN = dyn_cast<DeltaTreeInteriorNode>(this))
    for (unsigned i = 0, e = getNumValuesUsed()+1; i != e; ++i)
      NewFullDelta += IN->getChild(i)->getFullDelta();
  FullDelta = NewFullDelta;
}

/// DoInsertion - Do an insertion of the specified FileIndex/Delta pair into
/// this node.  If insertion is easy, do it and return false.  Otherwise,
/// split the node, populate InsertRes with info about the split, and return
/// true.
bool DeltaTreeNode::DoInsertion(unsigned FileIndex, int Delta,
                                InsertResult *InsertRes) {
  // Maintain full delta for this node.
  FullDelta += Delta;

  // Find the insertion point, the first delta whose index is >= FileIndex.
  unsigned i = 0, e = getNumValuesUsed();
  while (i != e && FileIndex > getValue(i).FileLoc)
    ++i;

  // If we found an a record for exactly this file index, just merge this
  // value into the pre-existing record and finish early.
  if (i != e && getValue(i).FileLoc == FileIndex) {
    // NOTE: Delta could drop to zero here.  This means that the delta entry is
    // useless and could be removed.  Supporting erases is more complex than
    // leaving an entry with Delta=0, so we just leave an entry with Delta=0 in
    // the tree.
    Values[i].Delta += Delta;
    return false;
  }

  // Otherwise, we found an insertion point, and we know that the value at the
  // specified index is > FileIndex.  Handle the leaf case first.
  if (isLeaf()) {
    if (!isFull()) {
      // For an insertion into a non-full leaf node, just insert the value in
      // its sorted position.  This requires moving later values over.
      if (i != e)
        memmove(&Values[i+1], &Values[i], sizeof(Values[0])*(e-i));
      Values[i] = SourceDelta::get(FileIndex, Delta);
      ++NumValuesUsed;
      return false;
    }

    // Otherwise, if this is leaf is full, split the node at its median, insert
    // the value into one of the children, and return the result.
    assert(InsertRes && "No result location specified");
    DoSplit(*InsertRes);

    if (InsertRes->Split.FileLoc > FileIndex)
      InsertRes->LHS->DoInsertion(FileIndex, Delta, nullptr /*can't fail*/);
    else
      InsertRes->RHS->DoInsertion(FileIndex, Delta, nullptr /*can't fail*/);
    return true;
  }

  // Otherwise, this is an interior node.  Send the request down the tree.
  auto *IN = cast<DeltaTreeInteriorNode>(this);
  if (!IN->Children[i]->DoInsertion(FileIndex, Delta, InsertRes))
    return false; // If there was space in the child, just return.

  // Okay, this split the subtree, producing a new value and two children to
  // insert here.  If this node is non-full, we can just insert it directly.
  if (!isFull()) {
    // Now that we have two nodes and a new element, insert the perclated value
    // into ourself by moving all the later values/children down, then inserting
    // the new one.
    if (i != e)
      memmove(&IN->Children[i+2], &IN->Children[i+1],
              (e-i)*sizeof(IN->Children[0]));
    IN->Children[i] = InsertRes->LHS;
    IN->Children[i+1] = InsertRes->RHS;

    if (e != i)
      memmove(&Values[i+1], &Values[i], (e-i)*sizeof(Values[0]));
    Values[i] = InsertRes->Split;
    ++NumValuesUsed;
    return false;
  }

  // Finally, if this interior node was full and a node is percolated up, split
  // ourself and return that up the chain.  Start by saving all our info to
  // avoid having the split clobber it.
  IN->Children[i] = InsertRes->LHS;
  DeltaTreeNode *SubRHS = InsertRes->RHS;
  SourceDelta SubSplit = InsertRes->Split;

  // Do the split.
  DoSplit(*InsertRes);

  // Figure out where to insert SubRHS/NewSplit.
  DeltaTreeInteriorNode *InsertSide;
  if (SubSplit.FileLoc < InsertRes->Split.FileLoc)
    InsertSide = cast<DeltaTreeInteriorNode>(InsertRes->LHS);
  else
    InsertSide = cast<DeltaTreeInteriorNode>(InsertRes->RHS);

  // We now have a non-empty interior node 'InsertSide' to insert
  // SubRHS/SubSplit into.  Find out where to insert SubSplit.

  // Find the insertion point, the first delta whose index is >SubSplit.FileLoc.
  i = 0; e = InsertSide->getNumValuesUsed();
  while (i != e && SubSplit.FileLoc > InsertSide->getValue(i).FileLoc)
    ++i;

  // Now we know that i is the place to insert the split value into.  Insert it
  // and the child right after it.
  if (i != e)
    memmove(&InsertSide->Children[i+2], &InsertSide->Children[i+1],
            (e-i)*sizeof(IN->Children[0]));
  InsertSide->Children[i+1] = SubRHS;

  if (e != i)
    memmove(&InsertSide->Values[i+1], &InsertSide->Values[i],
            (e-i)*sizeof(Values[0]));
  InsertSide->Values[i] = SubSplit;
  ++InsertSide->NumValuesUsed;
  InsertSide->FullDelta += SubSplit.Delta + SubRHS->getFullDelta();
  return true;
}

/// DoSplit - Split the currently full node (which has 2*WidthFactor-1 values)
/// into two subtrees each with "WidthFactor-1" values and a pivot value.
/// Return the pieces in InsertRes.
void DeltaTreeNode::DoSplit(InsertResult &InsertRes) {
  assert(isFull() && "Why split a non-full node?");

  // Since this node is full, it contains 2*WidthFactor-1 values.  We move
  // the first 'WidthFactor-1' values to the LHS child (which we leave in this
  // node), propagate one value up, and move the last 'WidthFactor-1' values
  // into the RHS child.

  // Create the new child node.
  DeltaTreeNode *NewNode;
  if (auto *IN = dyn_cast<DeltaTreeInteriorNode>(this)) {
    // If this is an interior node, also move over 'WidthFactor' children
    // into the new node.
    DeltaTreeInteriorNode *New = new DeltaTreeInteriorNode();
    memcpy(&New->Children[0], &IN->Children[WidthFactor],
           WidthFactor*sizeof(IN->Children[0]));
    NewNode = New;
  } else {
    // Just create the new leaf node.
    NewNode = new DeltaTreeNode();
  }

  // Move over the last 'WidthFactor-1' values from here to NewNode.
  memcpy(&NewNode->Values[0], &Values[WidthFactor],
         (WidthFactor-1)*sizeof(Values[0]));

  // Decrease the number of values in the two nodes.
  NewNode->NumValuesUsed = NumValuesUsed = WidthFactor-1;

  // Recompute the two nodes' full delta.
  NewNode->RecomputeFullDeltaLocally();
  RecomputeFullDeltaLocally();

  InsertRes.LHS = this;
  InsertRes.RHS = NewNode;
  InsertRes.Split = Values[WidthFactor-1];
}

//===----------------------------------------------------------------------===//
//                        DeltaTree Implementation
//===----------------------------------------------------------------------===//

//#define VERIFY_TREE

#ifdef VERIFY_TREE
/// VerifyTree - Walk the btree performing assertions on various properties to
/// verify consistency.  This is useful for debugging new changes to the tree.
static void VerifyTree(const DeltaTreeNode *N) {
  const auto *IN = dyn_cast<DeltaTreeInteriorNode>(N);
  if (IN == 0) {
    // Verify leaves, just ensure that FullDelta matches up and the elements
    // are in proper order.
    int FullDelta = 0;
    for (unsigned i = 0, e = N->getNumValuesUsed(); i != e; ++i) {
      if (i)
        assert(N->getValue(i-1).FileLoc < N->getValue(i).FileLoc);
      FullDelta += N->getValue(i).Delta;
    }
    assert(FullDelta == N->getFullDelta());
    return;
  }

  // Verify interior nodes: Ensure that FullDelta matches up and the
  // elements are in proper order and the children are in proper order.
  int FullDelta = 0;
  for (unsigned i = 0, e = IN->getNumValuesUsed(); i != e; ++i) {
    const SourceDelta &IVal = N->getValue(i);
    const DeltaTreeNode *IChild = IN->getChild(i);
    if (i)
      assert(IN->getValue(i-1).FileLoc < IVal.FileLoc);
    FullDelta += IVal.Delta;
    FullDelta += IChild->getFullDelta();

    // The largest value in child #i should be smaller than FileLoc.
    assert(IChild->getValue(IChild->getNumValuesUsed()-1).FileLoc <
           IVal.FileLoc);

    // The smallest value in child #i+1 should be larger than FileLoc.
    assert(IN->getChild(i+1)->getValue(0).FileLoc > IVal.FileLoc);
    VerifyTree(IChild);
  }

  FullDelta += IN->getChild(IN->getNumValuesUsed())->getFullDelta();

  assert(FullDelta == N->getFullDelta());
}
#endif  // VERIFY_TREE

static DeltaTreeNode *getRoot(void *Root) {
  return (DeltaTreeNode*)Root;
}

DeltaTree::DeltaTree() {
  Root = new DeltaTreeNode();
}

DeltaTree::DeltaTree(const DeltaTree &RHS) {
  // Currently we only support copying when the RHS is empty.
  assert(getRoot(RHS.Root)->getNumValuesUsed() == 0 &&
         "Can only copy empty tree");
  Root = new DeltaTreeNode();
}

DeltaTree::~DeltaTree() {
  getRoot(Root)->Destroy();
}

/// getDeltaAt - Return the accumulated delta at the specified file offset.
/// This includes all insertions or delections that occurred *before* the
/// specified file index.
int DeltaTree::getDeltaAt(unsigned FileIndex) const {
  const DeltaTreeNode *Node = getRoot(Root);

  int Result = 0;

  // Walk down the tree.
  while (true) {
    // For all nodes, include any local deltas before the specified file
    // index by summing them up directly.  Keep track of how many were
    // included.
    unsigned NumValsGreater = 0;
    for (unsigned e = Node->getNumValuesUsed(); NumValsGreater != e;
         ++NumValsGreater) {
      const SourceDelta &Val = Node->getValue(NumValsGreater);

      if (Val.FileLoc >= FileIndex)
        break;
      Result += Val.Delta;
    }

    // If we have an interior node, include information about children and
    // recurse.  Otherwise, if we have a leaf, we're done.
    const auto *IN = dyn_cast<DeltaTreeInteriorNode>(Node);
    if (!IN) return Result;

    // Include any children to the left of the values we skipped, all of
    // their deltas should be included as well.
    for (unsigned i = 0; i != NumValsGreater; ++i)
      Result += IN->getChild(i)->getFullDelta();

    // If we found exactly the value we were looking for, break off the
    // search early.  There is no need to search the RHS of the value for
    // partial results.
    if (NumValsGreater != Node->getNumValuesUsed() &&
        Node->getValue(NumValsGreater).FileLoc == FileIndex)
      return Result+IN->getChild(NumValsGreater)->getFullDelta();

    // Otherwise, traverse down the tree.  The selected subtree may be
    // partially included in the range.
    Node = IN->getChild(NumValsGreater);
  }
  // NOT REACHED.
}

/// AddDelta - When a change is made that shifts around the text buffer,
/// this method is used to record that info.  It inserts a delta of 'Delta'
/// into the current DeltaTree at offset FileIndex.
void DeltaTree::AddDelta(unsigned FileIndex, int Delta) {
  assert(Delta && "Adding a noop?");
  DeltaTreeNode *MyRoot = getRoot(Root);

  DeltaTreeNode::InsertResult InsertRes;
  if (MyRoot->DoInsertion(FileIndex, Delta, &InsertRes)) {
    Root = new DeltaTreeInteriorNode(InsertRes);
#ifdef VERIFY_TREE
    MyRoot = Root;
#endif
  }

#ifdef VERIFY_TREE
  VerifyTree(MyRoot);
#endif
}