github.com/whtcorpsinc/MilevaDB-Prod@v0.0.0-20211104133533-f57f4be3b597/soliton/bitmap/concurrent.go (about)

     1  // Copyright 2020 WHTCORPS INC, Inc.
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // See the License for the specific language governing permissions and
    12  // limitations under the License.
    13  
    14  package bitmap
    15  
    16  import (
    17  	"sync/atomic"
    18  	"unsafe"
    19  )
    20  
    21  const (
    22  	segmentWidth             = 32
    23  	segmentWidthPower        = 5
    24  	bitMask           uint32 = 0x80000000
    25  )
    26  
    27  var bytesConcurrentBitmap = int64(int(unsafe.Sizeof(ConcurrentBitmap{})))
    28  
    29  // ConcurrentBitmap is a static-length bitmap which is thread-safe on setting.
    30  // It is implemented using CAS, as atomic bitwise operation is not supported by
    31  // golang yet. (See https://github.com/golang/go/issues/24244)
    32  // CAS operation is narrowed down to uint32 instead of longer types like uint64,
    33  // to reduce probability of racing.
    34  type ConcurrentBitmap struct {
    35  	segments []uint32
    36  	bitLen   int
    37  }
    38  
    39  // NewConcurrentBitmap initializes a ConcurrentBitmap which can causetstore
    40  // bitLen of bits.
    41  func NewConcurrentBitmap(bitLen int) *ConcurrentBitmap {
    42  	segmentLen := (bitLen + segmentWidth - 1) >> segmentWidthPower
    43  	return &ConcurrentBitmap{
    44  		segments: make([]uint32, segmentLen),
    45  		bitLen:   bitLen,
    46  	}
    47  }
    48  
    49  // BytesConsumed returns size of this bitmap in bytes.
    50  func (cb *ConcurrentBitmap) BytesConsumed() int64 {
    51  	return bytesConcurrentBitmap + int64(segmentWidth/8*cap(cb.segments))
    52  }
    53  
    54  // Set sets the bit on bitIndex to be 1 (bitIndex starts from 0).
    55  // isSetter indicates whether the function call this time triggers the bit from 0 to 1.
    56  // bitIndex bigger than bitLen initialized will be ignored.
    57  func (cb *ConcurrentBitmap) Set(bitIndex int) (isSetter bool) {
    58  	if bitIndex < 0 || bitIndex >= cb.bitLen {
    59  		return
    60  	}
    61  
    62  	var oldValue, newValue uint32
    63  	segmentPointer := &(cb.segments[bitIndex>>segmentWidthPower])
    64  	mask := bitMask >> uint32(bitIndex%segmentWidth)
    65  	// Repeatedly observe whether bit is already set, and try to set
    66  	// it based on observation.
    67  	for {
    68  		// Observe.
    69  		oldValue = atomic.LoadUint32(segmentPointer)
    70  		if (oldValue & mask) != 0 {
    71  			return
    72  		}
    73  
    74  		// Set.
    75  		newValue = oldValue | mask
    76  		isSetter = atomic.CompareAndSwapUint32(segmentPointer, oldValue, newValue)
    77  		if isSetter {
    78  			return
    79  		}
    80  	}
    81  }
    82  
    83  // UnsafeIsSet returns if a bit on bitIndex is set (bitIndex starts from 0).
    84  // bitIndex bigger than bitLen initialized will return false.
    85  // This method is not thread-safe as it does not use atomic load.
    86  func (cb *ConcurrentBitmap) UnsafeIsSet(bitIndex int) (isSet bool) {
    87  	if bitIndex < 0 || bitIndex >= cb.bitLen {
    88  		return
    89  	}
    90  
    91  	mask := bitMask >> uint32(bitIndex%segmentWidth)
    92  	isSet = ((cb.segments[bitIndex>>segmentWidthPower] & mask) != 0)
    93  	return
    94  }