4 files changed, 154 insertions, 0 deletions

diff --git a/src/huffmantree.nim b/src/huffmantree.nim
new file mode 100644
index 0000000..1711879
--- /dev/null
+++ b/src/huffmantree.nim
@@ -0,0 +1,70 @@
+# 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 integers, bitreader, bitwriter
+
+const valueLengthFieldBitLength* = 6  # 64
+
+type HuffmanTreeNodeKind* = enum
+  branch,
+  leaf
+
+type HuffmanTreeNode*[T: SomeUnsignedInt] = ref object
+  case kind: HuffmanTreeNodeKind
+    of branch:
+      left, right: HuffmanTreeNode[T]
+    of leaf:
+      value: T
+
+proc huffmanBranch*[T](left, right: HuffmanTreeNode[T]): HuffmanTreeNode[T] =
+  HuffmanTreeNode[T](kind: branch, left: left, right: right)
+
+proc huffmanLeaf*[T](value: T): HuffmanTreeNode[T] =
+  HuffmanTreeNode[T](kind: leaf, value: value)
+
+proc `==`*[T](a, b: HuffmanTreeNode[T]): bool =
+  if a.kind != b.kind: return false
+  case a.kind:
+    of branch: a.left == b.left and a.right == b.right
+    of leaf: a.value == b.value
+
+proc maxValue*[T](node: HuffmanTreeNode[T]): T =
+  case node.kind:
+    of branch: max(node.left.maxValue(), node.right.maxValue())
+    of leaf: node.value
+
+proc deserialise*[T](bitReader: BitReader, valueType: typedesc[T]): HuffmanTreeNode[T] =
+  let valueBitLength = bitReader.readBits(valueLengthFieldBitLength, uint8).int
+  proc readNode(): HuffmanTreeNode[T] =
+    case bitReader.readBool():
+      of false: huffmanBranch(readNode(), readNode())
+      of true: huffmanLeaf(bitReader.readBits(valueBitLength, valueType))
+  readNode()
+
+proc serialise*[T](tree: HuffmanTreeNode[T], bitWriter: BitWriter) =
+  let maxValue = tree.maxValue()
+  let valueBitLength = maxValue.bitLength()
+  proc writeNode(node: HuffmanTreeNode[T]) =
+    case node.kind:
+      of branch:
+        bitWriter.writeBool(false)
+        writeNode(node.left)
+        writeNode(node.right)
+      of leaf:
+        bitWriter.writeBool(true)
+        bitWriter.writeBits(valueBitLength, node.value)
+  bitWriter.writeBits(valueLengthFieldBitLength, valueBitLength.uint8)
+  writeNode(tree)
diff --git a/src/integers.nim b/src/integers.nim
index 7b0f166..c93c9b8 100644
--- a/src/integers.nim
+++ b/src/integers.nim
@@ -22,6 +22,12 @@ proc `/^`*[T: Natural](x, y: T): T =
 proc truncateToUint8*(x: SomeUnsignedInt): uint8 =
   (x and uint8.high).uint8
 
+proc bitLength*[T: SomeUnsignedInt](x: T): int =
+  var buf = x
+  while buf > 0.T:
+    buf = buf shr 1
+    result += 1
+
 proc leastSignificantBits*[T: SomeUnsignedInt](x: T, bits: int): T =
   let maskOffset = sizeof(T) * wordBitLength - bits
   if maskOffset >= 0: (x shl maskOffset) shr maskOffset else: x
diff --git a/tests/thuffmantree.nim b/tests/thuffmantree.nim
new file mode 100644
index 0000000..ec40bdb
--- /dev/null
+++ b/tests/thuffmantree.nim
@@ -0,0 +1,74 @@
+# 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 unittest, streams
+import bitreader, bitwriter, huffmantree
+
+suite "huffmantree":
+  let tree = huffmanBranch(
+    huffmanLeaf(1'u),
+    huffmanBranch(
+      huffmanLeaf(2'u),
+      huffmanLeaf(3'u)))
+
+  test "equality":
+    check huffmanLeaf(12'u) == huffmanLeaf(12'u)
+    check huffmanLeaf(12'u) != huffmanLeaf(21'u)
+    check huffmanLeaf(12'u) != huffmanBranch(huffmanLeaf(12'u), huffmanLeaf(12'u))
+    check huffmanBranch(huffmanLeaf(12'u), huffmanLeaf(21'u)) == huffmanBranch(huffmanLeaf(12'u), huffmanLeaf(21'u))
+    check huffmanBranch(huffmanLeaf(12'u), huffmanLeaf(21'u)) != huffmanBranch(huffmanLeaf(12'u), huffmanLeaf(1'u))
+    check tree == tree
+
+  test "maxValue":
+    check tree.maxValue() == 3
+
+  test "deserialise":
+    let stream = newStringStream()
+    defer: stream.close()
+    let bitWriter = stream.bitWriter()
+    bitWriter.writeBits(valueLengthFieldBitLength, 2'u8)
+    bitWriter.writeBool(false)   # root
+    bitWriter.writeBool(true)    # 1 leaf
+    bitWriter.writeBits(2, 1'u)
+    bitWriter.writeBool(false)   # right branch
+    bitWriter.writeBool(true)    # 2 leaf
+    bitWriter.writeBits(2, 2'u)
+    bitWriter.writeBool(true)    # 3 leaf
+    bitWriter.writeBits(2, 3'u)
+    bitWriter.flush()
+
+    stream.setPosition(0)
+    let bitReader = stream.bitReader()
+    check huffmantree.deserialise(bitReader, uint) == tree
+
+  test "serialise":
+    let stream = newStringStream()
+    defer: stream.close()
+    let bitWriter = stream.bitWriter()
+    tree.serialise(bitWriter)
+    bitWriter.flush()
+
+    stream.setPosition(0)
+    let bitReader = stream.bitReader()
+    check bitReader.readBits(valueLengthFieldBitLength, uint8) == 2
+    check bitReader.readBool() == false  # root
+    check bitReader.readBool() == true   # 1 leaf
+    check bitReader.readBits(2, uint8) == 1
+    check bitReader.readBool() == false  # right branch
+    check bitReader.readBool() == true   # 2 leaf
+    check bitReader.readBits(2, uint8) == 2
+    check bitReader.readBool() == true   # 3 leaf
+    check bitReader.readBits(2, uint8) == 3
diff --git a/tests/tintegers.nim b/tests/tintegers.nim
index 956e4aa..851e926 100644
--- a/tests/tintegers.nim
+++ b/tests/tintegers.nim
@@ -27,6 +27,10 @@ suite "integers":
     check truncateToUint8(0x00FA'u16) == 0xFA'u8
     check truncateToUint8(0xFFFA'u16) == 0xFA'u8
 
+  test "bitLength":
+    check bitLength(0b1_1111) == 5
+    check bitLength(0b1000_0000) == 8
+
   test "leastSignificantBits":
     check leastSignificantBits(0xFF'u8, 3) == 0b0000_0111'u8
     check leastSignificantBits(0b0001_0101'u8, 3) == 0b0000_0101'u8