Scala huffman decoding - scala

Question

I am doing the scala coursera course and trying to write an implementation for the 'decode' method of the huffman algorithm. I am new to scala.
Following is my code(so far).
def decode(tree: CodeTree, bits: List[Bit]): List[Char] = {
def innerDecode(subTree: CodeTree, subBits: List[Bit]): List[Char] =
subBits match {
case head :: rest => {
subTree match {
case Leaf(c, w) => c :: innerDecode(tree, rest)
case Fork(left, right, _, _) => {
if ( head == 0) innerDecode(left, rest)
else innerDecode(right, rest)
}
}
}
case Nil => List[Char]()
}
innerDecode(tree, bits)
}
When writing a test e.g. below:
val decoded = decode(t1, List(0, 1))
assert(decoded === List('a','b'))// decoded is returned as List('a') instead of List('a','b')
Gives t1 as: Fork(Leaf('a',2), Leaf('b',3), List('a','b'), 5)
Can someone please advise why the implementation will return just List('a')
The encode for aabbb I assume is:
a -> 0
b -> 1

Answer 1

In Huffman decoding you take one bit of subBits each time you fork, go right or left. In your implementation you do this when you fork and when you reach a Leaf. Each time you reach a letter you throw one additional bit away.
You could see that with few more tests, in your case:
decode(List(0,1)) === List(a)
decode(List(0,0,1)) === List(a,b)
decode(List(0,0,0)) === List(a,a)
decode(List(0,1,0)) === List(a,a)
Or you could use debugger and go trough execution of your code step by step.