text: //===--- StringMap.cpp - String Hash table map implementation -------------===//
  //
  //                     The LLVM Compiler Infrastructure
  //
  // This file is distributed under the University of Illinois Open Source
  // License. See LICENSE.TXT for details.
  //
  //===----------------------------------------------------------------------===//
  //
  // This file implements the StringMap class.
  //
  //===----------------------------------------------------------------------===//
  
  #include "llvm/ADT/StringMap.h"
  #include "llvm/ADT/StringExtras.h"
  #include "llvm/Support/Compiler.h"
  #include "llvm/Support/MathExtras.h"
  #include <cassert>
  
  using namespace llvm;
  
  /// Returns the number of buckets to allocate to ensure that the DenseMap can
  /// accommodate \p NumEntries without need to grow().
  static unsigned getMinBucketToReserveForEntries(unsigned NumEntries) {
    // Ensure that "NumEntries * 4 < NumBuckets * 3"
    if (NumEntries == 0)
      return 0;
    // +1 is required because of the strict equality.
    // For example if NumEntries is 48, we need to return 401.
    return NextPowerOf2(NumEntries * 4 / 3 + 1);
  }
  
  StringMapImpl::StringMapImpl(unsigned InitSize, unsigned itemSize) {
    ItemSize = itemSize;
    
    // If a size is specified, initialize the table with that many buckets.
    if (InitSize) {
      // The table will grow when the number of entries reach 3/4 of the number of
      // buckets. To guarantee that "InitSize" number of entries can be inserted
      // in the table without growing, we allocate just what is needed here.
      init(getMinBucketToReserveForEntries(InitSize));
      return;
    }
    
    // Otherwise, initialize it with zero buckets to avoid the allocation.
    TheTable = nullptr;
    NumBuckets = 0;
    NumItems = 0;
    NumTombstones = 0;
  }
  
  void StringMapImpl::init(unsigned InitSize) {
    assert((InitSize & (InitSize-1)) == 0 &&
           "Init Size must be a power of 2 or zero!");
    NumBuckets = InitSize ? InitSize : 16;
    NumItems = 0;
    NumTombstones = 0;
    
    TheTable = (StringMapEntryBase **)calloc(NumBuckets+1,
                                             sizeof(StringMapEntryBase **) +
                                             sizeof(unsigned));
  
    // Allocate one extra bucket, set it to look filled so the iterators stop at
    // end.
    TheTable[NumBuckets] = (StringMapEntryBase*)2;
  }
  
  /// LookupBucketFor - Look up the bucket that the specified string should end
  /// up in.  If it already exists as a key in the map, the Item pointer for the
  /// specified bucket will be non-null.  Otherwise, it will be null.  In either
  /// case, the FullHashValue field of the bucket will be set to the hash value
  /// of the string.
  unsigned StringMapImpl::LookupBucketFor(StringRef Name) {
    unsigned HTSize = NumBuckets;
    if (HTSize == 0) {  // Hash table unallocated so far?
      init(16);
      HTSize = NumBuckets;
    }
    unsigned FullHashValue = HashString(Name);
    unsigned BucketNo = FullHashValue & (HTSize-1);
    unsigned *HashTable = (unsigned *)(TheTable + NumBuckets + 1);
  
    unsigned ProbeAmt = 1;
    int FirstTombstone = -1;
    while (true) {
      StringMapEntryBase *BucketItem = TheTable[BucketNo];
      // If we found an empty bucket, this key isn't in the table yet, return it.
      if (LLVM_LIKELY(!BucketItem)) {
        // If we found a tombstone, we want to reuse the tombstone instead of an
        // empty bucket.  This reduces probing.
        if (FirstTombstone != -1) {
          HashTable[FirstTombstone] = FullHashValue;
          return FirstTombstone;
        }
        
        HashTable[BucketNo] = FullHashValue;
        return BucketNo;
      }
      
      if (BucketItem == getTombstoneVal()) {
        // Skip over tombstones.  However, remember the first one we see.
        if (FirstTombstone == -1) FirstTombstone = BucketNo;
      } else if (LLVM_LIKELY(HashTable[BucketNo] == FullHashValue)) {
        // If the full hash value matches, check deeply for a match.  The common
        // case here is that we are only looking at the buckets (for item info
        // being non-null and for the full hash value) not at the items.  This
        // is important for cache locality.
        
        // Do the comparison like this because Name isn't necessarily
        // null-terminated!
        char *ItemStr = (char*)BucketItem+ItemSize;
        if (Name == StringRef(ItemStr, BucketItem->getKeyLength())) {
          // We found a match!
          return BucketNo;
        }
      }
      
      // Okay, we didn't find the item.  Probe to the next bucket.
      BucketNo = (BucketNo+ProbeAmt) & (HTSize-1);
      
      // Use quadratic probing, it has fewer clumping artifacts than linear
      // probing and has good cache behavior in the common case.
      ++ProbeAmt;
    }
  }
  
  /// FindKey - Look up the bucket that contains the specified key. If it exists
  /// in the map, return the bucket number of the key.  Otherwise return -1.
  /// This does not modify the map.
  int StringMapImpl::FindKey(StringRef Key) const {
    unsigned HTSize = NumBuckets;
    if (HTSize == 0) return -1;  // Really empty table?
    unsigned FullHashValue = HashString(Key);
    unsigned BucketNo = FullHashValue & (HTSize-1);
    unsigned *HashTable = (unsigned *)(TheTable + NumBuckets + 1);
  
    unsigned ProbeAmt = 1;
    while (true) {
      StringMapEntryBase *BucketItem = TheTable[BucketNo];
      // If we found an empty bucket, this key isn't in the table yet, return.
      if (LLVM_LIKELY(!BucketItem))
        return -1;
      
      if (BucketItem == getTombstoneVal()) {
        // Ignore tombstones.
      } else if (LLVM_LIKELY(HashTable[BucketNo] == FullHashValue)) {
        // If the full hash value matches, check deeply for a match.  The common
        // case here is that we are only looking at the buckets (for item info
        // being non-null and for the full hash value) not at the items.  This
        // is important for cache locality.
        
        // Do the comparison like this because NameStart isn't necessarily
        // null-terminated!
        char *ItemStr = (char*)BucketItem+ItemSize;
        if (Key == StringRef(ItemStr, BucketItem->getKeyLength())) {
          // We found a match!
          return BucketNo;
        }
      }
      
      // Okay, we didn't find the item.  Probe to the next bucket.
      BucketNo = (BucketNo+ProbeAmt) & (HTSize-1);
      
      // Use quadratic probing, it has fewer clumping artifacts than linear
      // probing and has good cache behavior in the common case.
      ++ProbeAmt;
    }
  }
  
  /// RemoveKey - Remove the specified StringMapEntry from the table, but do not
  /// delete it.  This aborts if the value isn't in the table.
  void StringMapImpl::RemoveKey(StringMapEntryBase *V) {
    const char *VStr = (char*)V + ItemSize;
    StringMapEntryBase *V2 = RemoveKey(StringRef(VStr, V->getKeyLength()));
    (void)V2;
    assert(V == V2 && "Didn't find key?");
  }
  
  /// RemoveKey - Remove the StringMapEntry for the specified key from the
  /// table, returning it.  If the key is not in the table, this returns null.
  StringMapEntryBase *StringMapImpl::RemoveKey(StringRef Key) {
    int Bucket = FindKey(Key);
    if (Bucket == -1) return nullptr;
    
    StringMapEntryBase *Result = TheTable[Bucket];
    TheTable[Bucket] = getTombstoneVal();
    --NumItems;
    ++NumTombstones;
    assert(NumItems + NumTombstones <= NumBuckets);
  
    return Result;
  }
  
  /// RehashTable - Grow the table, redistributing values into the buckets with
  /// the appropriate mod-of-hashtable-size.
  unsigned StringMapImpl::RehashTable(unsigned BucketNo) {
    unsigned NewSize;
    unsigned *HashTable = (unsigned *)(TheTable + NumBuckets + 1);
  
    // If the hash table is now more than 3/4 full, or if fewer than 1/8 of
    // the buckets are empty (meaning that many are filled with tombstones),
    // grow/rehash the table.
    if (LLVM_UNLIKELY(NumItems * 4 > NumBuckets * 3)) {
      NewSize = NumBuckets*2;
    } else if (LLVM_UNLIKELY(NumBuckets - (NumItems + NumTombstones) <=
                             NumBuckets / 8)) {
      NewSize = NumBuckets;
    } else {
      return BucketNo;
    }
  
    unsigned NewBucketNo = BucketNo;
    // Allocate one extra bucket which will always be non-empty.  This allows the
    // iterators to stop at end.
    StringMapEntryBase **NewTableArray =
      (StringMapEntryBase **)calloc(NewSize+1, sizeof(StringMapEntryBase *) +
                                               sizeof(unsigned));
    unsigned *NewHashArray = (unsigned *)(NewTableArray + NewSize + 1);
    NewTableArray[NewSize] = (StringMapEntryBase*)2;
  
    // Rehash all the items into their new buckets.  Luckily :) we already have
    // the hash values available, so we don't have to rehash any strings.
    for (unsigned I = 0, E = NumBuckets; I != E; ++I) {
      StringMapEntryBase *Bucket = TheTable[I];
      if (Bucket && Bucket != getTombstoneVal()) {
        // Fast case, bucket available.
        unsigned FullHash = HashTable[I];
        unsigned NewBucket = FullHash & (NewSize-1);
        if (!NewTableArray[NewBucket]) {
          NewTableArray[FullHash & (NewSize-1)] = Bucket;
          NewHashArray[FullHash & (NewSize-1)] = FullHash;
          if (I == BucketNo)
            NewBucketNo = NewBucket;
          continue;
        }
        
        // Otherwise probe for a spot.
        unsigned ProbeSize = 1;
        do {
          NewBucket = (NewBucket + ProbeSize++) & (NewSize-1);
        } while (NewTableArray[NewBucket]);
        
        // Finally found a slot.  Fill it in.
        NewTableArray[NewBucket] = Bucket;
        NewHashArray[NewBucket] = FullHash;
        if (I == BucketNo)
          NewBucketNo = NewBucket;
      }
    }
    
    free(TheTable);
    
    TheTable = NewTableArray;
    NumBuckets = NewSize;
    NumTombstones = 0;
    return NewBucketNo;
  }
  