June 2018 Update

Task/Huffman-coding/Lua/huffman-coding.lua

@@ -0,0 +1,73 @@
+local build_freqtable = function (data)
+  local freq = { }
+
+  for i = 1, #data do
+    local cur = string.sub (data, i, i)
+    local count = freq [cur] or 0
+    freq [cur] = count + 1
+  end
+
+  local nodes = { }
+  for w, f in next, freq do
+    nodes [#nodes + 1] = { word = w, freq = f }
+  end
+
+  table.sort (nodes, function (a, b) return a.freq > b.freq end) --- reverse order!
+
+  return nodes
+end
+
+local build_hufftree = function (nodes)
+  while true do
+    local n = #nodes
+    local left = nodes [n]
+    nodes [n] = nil
+
+    local right = nodes [n - 1]
+    nodes [n - 1] = nil
+
+    local new = { freq = left.freq + right.freq, left = left, right = right }
+
+    if n == 2 then return new end
+
+    --- insert new node at correct priority
+    local prio = 1
+    while prio < #nodes and nodes [prio].freq > new.freq do
+      prio = prio + 1
+    end
+    table.insert (nodes, prio, new)
+  end
+end
+
+local print_huffcodes do
+  local rec_build_huffcodes
+  rec_build_huffcodes = function (node, bits, acc)
+    if node.word == nil then
+      rec_build_huffcodes (node.left,  bits .. "0", acc)
+      rec_build_huffcodes (node.right, bits .. "1", acc)
+      return acc
+    else --- leaf
+      acc [#acc + 1] = { node.freq, node.word, bits }
+    end
+    return acc
+  end
+
+  print_huffcodes = function (root)
+    local codes = rec_build_huffcodes (root, "", { })
+    table.sort (codes, function (a, b) return a [1] < b [1] end)
+    print ("frequency\tword\thuffman code")
+    for i = 1, #codes do
+      print (string.format ("%9d\t‘%s’\t“%s”", table.unpack (codes [i])))
+    end
+  end
+end
+
+
+local huffcode = function (data)
+  local nodes = build_freqtable (data)
+  local huff = build_hufftree (nodes)
+  print_huffcodes (huff)
+  return 0
+end
+
+return huffcode "this is an example for huffman encoding"