1 files changed, 59 insertions, 0 deletions

diff --git a/src/bitwriter.nim b/src/bitwriter.nim
new file mode 100644
index 0000000..aac96f9
--- /dev/null
+++ b/src/bitwriter.nim
@@ -0,0 +1,59 @@
+# "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 <https://www.gnu.org/licenses/>.
+
+import streams
+import integers
+
+type BitWriter* = ref object
+  stream: Stream
+  bitOffset: int
+  writeBuffer: uint8
+
+proc bitWriter*(stream: Stream): BitWriter =
+  BitWriter(stream: stream, bitOffset: 0, writeBuffer: 0)
+
+proc flush*(bitWriter: BitWriter) =
+  if bitWriter.bitOffset == 0: return
+  bitWriter.stream.write(bitWriter.writeBuffer)
+  bitWriter.stream.flush()
+  (bitWriter.bitOffset, bitWriter.writeBuffer) = (0, 0'u8)
+
+proc atEnd*(bitWriter: BitWriter): bool =
+  bitWriter.stream.atEnd()
+
+proc writeBits*(bitWriter: BitWriter, 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 + bitWriter.bitOffset >= wordBitLength:
+    bitWriter.stream.write(truncateToUint8(value shl bitWriter.bitOffset) or bitWriter.writeBuffer)
+    bitsToWrite -= wordBitLength - bitWriter.bitOffset
+    (bitWriter.bitOffset, bitWriter.writeBuffer) = (0, 0'u8)
+  while bitsToWrite >= wordBitLength:
+    bitWriter.stream.write(truncateToUint8(value shr (bits - bitsToWrite)))
+    bitsToWrite -= wordBitLength
+  if bitsToWrite > 0:
+    let left = truncateToUint8((value shl (valueContainerBitLength - bits)) shr (valueContainerBitLength - bitsToWrite))
+    bitWriter.writeBuffer = (left shl bitWriter.bitOffset) or bitWriter.writeBuffer
+    bitWriter.bitOffset = (bitWriter.bitOffset + bitsToWrite) mod wordBitLength
+
+proc writeBool*(bitWriter: BitWriter, value: bool) =
+  bitWriter.writeBits(1, value.uint8)
+
+proc writeSeq*[T: SomeUnsignedInt](bitWriter: BitWriter, bitLength: int, data: seq[T]) =
+  for i, chunkBitLength in chunks(bitLength, T):
+    bitWriter.writeBits(chunkBitLength, data[i])