WilliamWhispell commented on a change in pull request #9671: URL: https://github.com/apache/arrow/pull/9671#discussion_r597129309
########## File path: go/parquet/internal/utils/bit_reader.go ########## @@ -0,0 +1,344 @@ +// Licensed to the Apache Software Foundation (ASF) under one +// or more contributor license agreements. See the NOTICE file +// distributed with this work for additional information +// regarding copyright ownership. The ASF licenses this file +// to you under the Apache License, Version 2.0 (the +// "License"); you may not use this file except in compliance +// with the License. You may obtain a copy of the License at +// +// http://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, software +// distributed under the License is distributed on an "AS IS" BASIS, +// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. +// See the License for the specific language governing permissions and +// limitations under the License. + +package utils + +import ( + "encoding/binary" + "errors" + "io" + "math" + "reflect" + "unsafe" + + "github.com/apache/arrow/go/arrow" + "github.com/apache/arrow/go/arrow/bitutil" + "github.com/apache/arrow/go/arrow/memory" +) + +// masks for grabbing the trailing bits based on the number of trailing bits desired +var trailingMask [64]uint64 + +func init() { + // generate the masks at init so we don't have to hard code them. + for i := 0; i < 64; i++ { + trailingMask[i] = (math.MaxUint64 >> (64 - i)) + } +} + +// trailingBits returns a value constructed from the bits trailing bits of +// the value v that is passed in. If bits >= 64, then we just return v. +func trailingBits(v uint64, bits uint) uint64 { + if bits >= 64 { + return v + } + return v & trailingMask[bits] +} + +// reader is a useful interface to define the functionality we need for implementation +type reader interface { + io.Reader + io.ReaderAt + io.Seeker +} + +// default buffer length +const buflen = 1024 + +// BitReader implements functionality for reading bits or bytes buffering up to a uint64 +// at a time from the reader in order to improve efficiency. It also provides +// methods to read multiple bytes in one read such as encoded ints/values. +// +// This BitReader is the basis for the other utility classes like RLE decoding +// and such, providing the necessary functions for interpreting the values. +type BitReader struct { + reader reader + buffer uint64 + byteoffset int64 + bitoffset uint + raw [8]byte + + unpackBuf [buflen]uint32 +} + +// NewBitReader takes in a reader that implements io.Reader, io.ReaderAt and io.Seeker +// interfaces and returns a BitReader for use with various bit level manipulations. +func NewBitReader(r reader) *BitReader { + return &BitReader{reader: r} +} + +// CurOffset returns the current Byte offset into the data that the reader is at. +func (b *BitReader) CurOffset() int64 { + return b.byteoffset + bitutil.BytesForBits(int64(b.bitoffset)) +} + +// Reset allows reusing a BitReader by setting a new reader and resetting the internal +// state back to zeros. +func (b *BitReader) Reset(r reader) { + b.reader = r + b.buffer = 0 + b.byteoffset = 0 + b.bitoffset = 0 +} + +// GetVlqInt reads a Vlq encoded int from the stream. The encoded value must start +// at the beginning of a byte and this returns false if there weren't enough bytes +// in the buffer or reader. This will call `ReadByte` which in turn retrieves byte +// aligned values from the reader +func (b *BitReader) GetVlqInt() (uint64, bool) { + tmp, err := binary.ReadUvarint(b) + if err != nil { + return 0, false + } + return tmp, true +} + +// GetZigZagVlqInt reads a zigzag encoded integer, returning false if there weren't +// enough bytes remaining. +func (b *BitReader) GetZigZagVlqInt() (int64, bool) { + u, ok := b.GetVlqInt() + if !ok { + return 0, false + } + + return int64(u>>1) ^ -int64(u&1), true +} + +// ReadByte reads a single aligned byte from the underlying stream, or populating +// error if there aren't enough bytes left. +func (b *BitReader) ReadByte() (byte, error) { + var tmp byte + if ok := b.GetAligned(1, &tmp); !ok { + return 0, errors.New("failed to read byte") + } + + return tmp, nil +} + +// GetAligned reads nbytes from the underlying stream into the passed interface value. +// Returning false if there aren't enough bytes remaining in the stream or if an invalid +// type is passed. The bytes are read aligned to byte boundaries. +// +// v must be a pointer to a byte or sized uint type (*byte, *uint16, *uint32, *uint64). +// encoded values are assumed to be little endian. +func (b *BitReader) GetAligned(nbytes int, v interface{}) bool { + // figure out the number of bytes to represent v + typBytes := int(reflect.TypeOf(v).Elem().Size()) + if nbytes > typBytes { + return false + } + + bread := bitutil.BytesForBits(int64(b.bitoffset)) + + b.byteoffset += bread + n, err := b.reader.ReadAt(b.raw[:nbytes], b.byteoffset) + if err != nil && err != io.EOF { + return false + } + if n != nbytes { + return false + } + // zero pad the the bytes + memory.Set(b.raw[n:typBytes], 0) + + switch v := v.(type) { + case *byte: + *v = b.raw[0] + case *uint64: + *v = binary.LittleEndian.Uint64(b.raw[:typBytes]) + case *uint32: + *v = binary.LittleEndian.Uint32(b.raw[:typBytes]) + case *uint16: + *v = binary.LittleEndian.Uint16(b.raw[:typBytes]) + default: + return false + } + + b.byteoffset += int64(nbytes) + + b.bitoffset = 0 + b.fillbuffer() + return true +} + +// fillbuffer fills the uint64 buffer with bytes from the underlying stream +func (b *BitReader) fillbuffer() error { + n, err := b.reader.ReadAt(b.raw[:], b.byteoffset) + if err != nil && n == 0 && err != io.EOF { + return err + } + for i := n; i < 8; i++ { + b.raw[i] = 0 + } + b.buffer = binary.LittleEndian.Uint64(b.raw[:]) + return nil +} + +// next reads an integral value from the next bits in the buffer +func (b *BitReader) next(bits uint) (v uint64, err error) { + v = trailingBits(b.buffer, b.bitoffset+bits) >> b.bitoffset + b.bitoffset += bits + // if we need more bits to get what was requested then refill the buffer + if b.bitoffset >= 64 { + b.byteoffset += 8 + b.bitoffset -= 64 + if err = b.fillbuffer(); err != nil { + return 0, err + } + v |= trailingBits(b.buffer, b.bitoffset) << (bits - b.bitoffset) + } + return +} + +// GetBatchIndex is like GetBatch but for IndexType (used for dictionary decoding) +func (b *BitReader) GetBatchIndex(bits uint, out []IndexType) (i int, err error) { + if bits > 32 { + return 0, errors.New("must be 32 bits or less per read") + } + + var val uint64 + + length := len(out) + // if we're not currently byte-aligned, read bits until we are byte-aligned. + for ; i < length && b.bitoffset != 0; i++ { + val, err = b.next(bits) + out[i] = IndexType(val) + if err != nil { + return + } + } + + b.reader.Seek(b.byteoffset, io.SeekStart) + // grab as many 32 byte chunks as possible in one shot + if i < length { // IndexType should be a 32 bit value so we can do quick unpacking right into the output + numUnpacked := unpack32(b.reader, (*(*[]uint32)(unsafe.Pointer(&out)))[i:], int(bits)) + i += numUnpacked + b.byteoffset += int64(numUnpacked * int(bits) / 8) + } + + // re-fill our buffer just in case. + b.fillbuffer() + // grab the remaining values that aren't 32 byte aligned + for ; i < length; i++ { + val, err = b.next(bits) + out[i] = IndexType(val) + if err != nil { + break + } + } + return +} + +// GetBatchBools is like GetBatch but optimized for reading bits as boolean values +func (b *BitReader) GetBatchBools(out []bool) (int, error) { + bits := uint(1) + length := len(out) + + i := 0 + // read until we are byte-aligned + for ; i < length && b.bitoffset != 0; i++ { Review comment: seemed like duplicate code, I'll relook to see if it is -- This is an automated message from the Apache Git Service. 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