1 files changed, 57 insertions, 0 deletions

diff --git a/src/bitio/bitreader.nim b/src/bitio/bitreader.nim
new file mode 100644
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--- /dev/null
+++ b/src/bitio/bitreader.nim
@@ -0,0 +1,57 @@
+# 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))