langs a-z

Task/LZW-compression/PureBasic/lzw-compression.purebasic

@@ -0,0 +1,97 @@
+Procedure compress(uncompressed.s, List result.u())
+  ;Compress a string to a list of output symbols
+
+  ;Build the dictionary.
+  Protected  dict_size = 255, i
+  newmap dict.u()
+  For i = 0 To 254
+    dict(Chr(i + 1)) = i
+  Next
+
+  Protected w.s, wc.s, *c.Character = @uncompressed
+  w = ""
+  LastElement(result())
+  While *c\c <> #Null
+    wc = w + Chr(*c\c)
+    If FindMapElement(dict(), wc)
+      w = wc
+    Else
+      AddElement(result())
+      result() = dict(w)
+      ;Add wc to the dictionary
+      dict(wc) = dict_size
+      dict_size + 1 ;no check is performed for overfilling the dictionary.
+      w = Chr(*c\c)
+    EndIf
+    *c + 1
+  Wend
+
+  ;Output the code for w
+  If w
+    AddElement(result())
+    result() = dict(w)
+  EndIf
+EndProcedure
+
+Procedure.s decompress(List compressed.u())
+  ;Decompress a list of encoded values to a string
+  If ListSize(compressed()) = 0: ProcedureReturn "": EndIf
+
+  ;Build the dictionary.
+  Protected  dict_size = 255, i
+
+  Dim dict.s(255)
+  For i = 1 To 255
+    dict(i - 1) = Chr(i)
+  Next
+
+  Protected w.s, entry.s, result.s
+  FirstElement(compressed())
+  w = dict(compressed())
+  result = w
+
+  i = 0
+  While NextElement(compressed())
+    i + 1
+    If compressed() < dict_size
+    entry = dict(compressed())
+    ElseIf i = dict_size
+      entry = w + Left(w, 1)
+    Else
+      MessageRequester("Error","Bad compression at [" + Str(i) + "]")
+      ProcedureReturn result;abort
+    EndIf
+    result + entry
+    ;Add w + Left(entry, 1) to the dictionary
+    If ArraySize(dict()) <= dict_size
+      Redim dict(dict_size + 256)
+    EndIf
+    dict(dict_size) = w + Left(entry, 1)
+    dict_size + 1 ;no check is performed for overfilling the dictionary.
+
+    w = entry
+  Wend
+  ProcedureReturn result
+EndProcedure
+
+If OpenConsole()
+  ;How to use:
+
+  Define initial.s, decompressed.s
+
+  Print("Type something: ")
+  initial = Input()
+  NewList compressed.u()
+  compress(initial, compressed())
+  ForEach compressed()
+    Print(Str(compressed()) + " ")
+  Next
+  PrintN("")
+
+  decompressed = decompress(compressed())
+  PrintN(decompressed)
+
+  Print(#CRLF$ + #CRLF$ + "Press ENTER to exit")
+  Input()
+  CloseConsole()
+EndIf