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# gzip-like LZSS compressor
# Copyright (C) 2018 Pacien TRAN-GIRARD
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU Affero General Public License as
# published by the Free Software Foundation, either version 3 of the
# License, or (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU Affero General Public License for more details.
#
# You should have received a copy of the GNU Affero General Public License
# along with this program. If not, see <https://www.gnu.org/licenses/>.
import streams
import integers
type BitReader* = ref object
stream: Stream
bitOffset: int
overflowBuffer: uint8
proc bitReader*(stream: Stream): BitReader =
BitReader(stream: stream, bitOffset: 0, overflowBuffer: 0)
proc atEnd*(bitReader: BitReader): bool =
bitReader.bitOffset == 0 and bitReader.stream.atEnd()
proc readBits*[T: SomeUnsignedInt](bitReader: BitReader, bits: int, to: typedesc[T]): T =
if bits < 0 or bits > sizeof(T) * wordBitLength: raise newException(RangeError, "invalid bit length")
if bits == 0: return 0
var bitsRead = 0
if bitReader.bitOffset > 0:
let bitsFromBuffer = min(bits, wordBitLength - bitReader.bitOffset)
result = (bitReader.overflowBuffer shr bitReader.bitOffset).leastSignificantBits(bitsFromBuffer)
bitReader.bitOffset = (bitReader.bitOffset + bitsFromBuffer) mod wordBitLength
bitsRead += bitsFromBuffer
while bits - bitsRead >= wordBitLength:
result = result or (bitReader.stream.readUint8().T shl bitsRead)
bitsRead += wordBitLength
if bits - bitsRead > 0:
bitReader.overflowBuffer = bitReader.stream.readUint8()
bitReader.bitOffset = bits - bitsRead
result = result or (bitReader.overflowBuffer.leastSignificantBits(bitReader.bitOffset).T shl bitsRead)
proc readBool*(bitReader: BitReader): bool =
bitReader.readBits(1, uint8) != 0
proc readSeq*[T: SomeUnsignedInt](bitReader: BitReader, maxBitLength: int, to: typedesc[T]): tuple[bitLength: int, data: seq[T]] =
result = (0, newSeqOfCap[T](maxBitLength /^ (sizeof(T) * wordBitLength)))
for _, chunkBitLength in chunks(maxBitLength, T):
if bitReader.atEnd(): return
let bitsToRead = if bitReader.stream.atEnd(): sizeof(T) * wordBitLength - bitReader.bitOffset else: chunkBitLength
result.bitLength += bitsToRead
result.data.add(bitReader.readBits(bitsToRead, T))
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