add huffman tree structure and serialisation

2 files changed, 76 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