September 2017 Update

Task/Huffman-coding/Zkl/huffman-coding-1.zkl

@@ -0,0 +1,23 @@
+fcn buildHuffman(text){  //-->(encode dictionary, decode dictionary)
+   ft:=Dictionary();
+   foreach c in (text){ ft[c]=ft.find(c,0)+1 }  // leafs w/count
+
+   // build the tree, which is a list of lists of ...
+   tree:=ft.pump(List,fcn([(c,cnt)]){  //-->L( (cnt, ((sym,code))), ...)
+      L(cnt, L(L(c,"")))
+   }).copy();	// make it writable
+
+   while(tree.len()>1){  // fake up a [lame] priorty queue
+      tree=tree.sort(fcn(a,b){ a[0]>b[0] }); //prioritize high to low
+      a,b:=tree.pop(-2,2);	        //remove 2 least frequent symbols
+      mc:=fcn(n,c){ n[1] = c + n[1]; };	//(sym,code),"0"|"1"
+      a[1].apply2(mc,"0"); b[1].apply2(mc,"1"); // mc(a[1],"0")
+      tree.append( L(a[0]+b[0],a[1].extend(b[1])) ); //(a,b)-->new node
+   }//-->L(L(39, L( L(" ","000"),L("e","0010"),L("a","0011") ...
+
+   tree=tree[0][1].pump(List,fcn(i){ // flatten rather than traverse
+	if(T.isType(i))return(Void.Recurse,i,self.fcn); i });
+   encodeTable:=tree.toDictionary();  // symbol:Huffman code
+   decodeTable:=encodeTable.pump(Dictionary(),"reverse"); // code:symbol
+   return(encodeTable,decodeTable);
+}