From 52c08b0282aacb44c2e75cefeae5626a7aa1b21a Mon Sep 17 00:00:00 2001
From: pacien
Date: Fri, 23 Nov 2018 12:31:11 +0100
Subject: Implement bitstream
---
src/bitstream.nim | 112 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
src/integers.nim | 24 ++++++++++++
2 files changed, 136 insertions(+)
create mode 100644 src/bitstream.nim
create mode 100644 src/integers.nim
(limited to 'src')
diff --git a/src/bitstream.nim b/src/bitstream.nim
new file mode 100644
index 0000000..81401ce
--- /dev/null
+++ b/src/bitstream.nim
@@ -0,0 +1,112 @@
+# "à-la-gzip" 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 .
+
+import streams
+import integers
+
+# Stream functions
+
+proc newEIO(msg: string): ref IOError =
+ new(result)
+ result.msg = msg
+
+proc read[T](s: Stream, t: typedesc[T]): T =
+ if readData(s, addr(result), sizeof(T)) != sizeof(T):
+ raise newEIO("cannot read from stream")
+
+proc peek[T](s: Stream, t: typedesc[T]): T =
+ if peekData(s, addr(result), sizeof(T)) != sizeof(T):
+ raise newEIO("cannot read from stream")
+
+# Utils
+
+iterator chunks*(totalBitLength: int, chunkType: typedesc[SomeInteger]): tuple[index: int, chunkBitLength: int] =
+ let chunkBitLength = sizeof(chunkType) * wordBitLength
+ let wordCount = totalBitLength div chunkBitLength
+ for i in 0..<(wordCount): yield (i, chunkBitLength)
+ let remainder = totalBitLength mod chunkBitLength
+ if remainder > 0: yield (wordCount, remainder)
+
+# BitStream
+
+type BitStream* = ref object
+ stream: Stream
+ bitOffset: int
+ writeBuffer: uint8
+
+proc bitStream*(stream: Stream): BitStream =
+ BitStream(stream: stream, bitOffset: 0, writeBuffer: 0)
+
+proc flush*(bitStream: BitStream) =
+ if bitStream.bitOffset == 0: return
+ bitStream.stream.write(bitStream.writeBuffer)
+ bitStream.stream.flush()
+ (bitStream.bitOffset, bitStream.writeBuffer) = (0, 0'u8)
+
+proc atEnd*(bitStream: BitStream): bool =
+ bitStream.stream.atEnd()
+
+proc readBits*[T: SomeUnsignedInt](bitStream: BitStream, bits: int, to: typedesc[T]): T =
+ let targetBitLength = sizeof(T) * wordBitLength
+ if bits < 0 or bits > targetBitLength:
+ raise newException(RangeError, "invalid bit length")
+ elif bits == 0:
+ result = 0
+ elif bits < targetBitLength - bitStream.bitOffset:
+ result = bitStream.stream.peek(T) shl (targetBitLength - bits - bitStream.bitOffset) shr (targetBitLength - bits)
+ elif bits == targetBitLength - bitStream.bitOffset:
+ result = bitStream.stream.read(T) shl (targetBitLength - bits - bitStream.bitOffset) shr (targetBitLength - bits)
+ else:
+ let rightBits = targetBitLength - bitStream.bitOffset
+ let leftBits = bits - rightBits
+ let right = bitStream.stream.read(T) shr bitStream.bitOffset
+ let left = bitStream.stream.peek(T) shl (targetBitLength - leftBits) shr (targetBitLength - bits)
+ result = left or right
+ bitStream.bitOffset = (bitStream.bitOffset + bits) mod wordBitLength
+
+proc readBool*(bitStream: BitStream): bool =
+ bitStream.readBits(1, uint8) != 0
+
+proc readSeq*[T: SomeUnsignedInt](bitStream: BitStream, bitLength: int, to: typedesc[T]): tuple[bitLength: int, data: seq[T]] =
+ result = (0, newSeqOfCap[T](bitLength /^ (sizeof(T) * wordBitLength)))
+ for _, chunkBitLength in chunks(bitLength, T):
+ if bitStream.atEnd(): return
+ result.bitLength += chunkBitLength
+ result.data.add(bitStream.readBits(chunkBitLength, T))
+
+proc writeBits*(bitStream: BitStream, bits: int, value: SomeUnsignedInt) =
+ let valueContainerBitLength = sizeof(value) * wordBitLength
+ if bits < 0 or bits > valueContainerBitLength:
+ raise newException(RangeError, "invalid bit length")
+ var bitsToWrite = bits
+ if bitsToWrite + bitStream.bitOffset >= wordBitLength:
+ bitStream.stream.write(truncateToUint8(value shl bitStream.bitOffset) or bitStream.writeBuffer)
+ bitsToWrite -= wordBitLength - bitStream.bitOffset
+ (bitStream.bitOffset, bitStream.writeBuffer) = (0, 0'u8)
+ while bitsToWrite >= wordBitLength:
+ bitStream.stream.write(truncateToUint8(value shr (bits - bitsToWrite)))
+ bitsToWrite -= wordBitLength
+ if bitsToWrite > 0:
+ let left = truncateToUint8((value shl (valueContainerBitLength - bits)) shr (valueContainerBitLength - bitsToWrite))
+ bitStream.writeBuffer = (left shl bitStream.bitOffset) or bitStream.writeBuffer
+ bitStream.bitOffset = (bitStream.bitOffset + bitsToWrite) mod wordBitLength
+
+proc writeBool*(bitStream: BitStream, value: bool) =
+ bitStream.writeBits(1, value.uint8)
+
+proc writeSeq*[T: SomeUnsignedInt](bitStream: BitStream, bitLength: int, data: seq[T]) =
+ for i, chunkBitLength in chunks(bitLength, T):
+ bitStream.writeBits(chunkBitLength, data[i])
diff --git a/src/integers.nim b/src/integers.nim
new file mode 100644
index 0000000..1b9121c
--- /dev/null
+++ b/src/integers.nim
@@ -0,0 +1,24 @@
+# "à-la-gzip" 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 .
+
+const wordBitLength* = 8
+const wordBitMask* = 0b1111_1111'u8
+
+proc `/^`*[T: Natural](x, y: T): T =
+ (x + y - 1) div y
+
+proc truncateToUint8*(x: SomeUnsignedInt): uint8 =
+ (x and wordBitMask).uint8
--
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