vocab.jl

"""
    Vocabulary(word,unk_cutoff =1 ,unk_label = "<unk>") 

Stores language model vocabulary.
Satisfies two common language modeling requirements for a vocabulary:
- When checking membership and calculating its size, filters items
by comparing their counts to a cutoff value.
Adds a special "unknown" token which unseen words are mapped to.

# Example
```julia-repl
julia> words = ["a", "c", "-", "d", "c", "a", "b", "r", "a", "c", "d"]
julia> vocabulary = Vocabulary(words, 2) 
  Vocabulary(Dict("<unk>"=>1,"c"=>3,"a"=>3,"d"=>2), 2, "<unk>") 

julia> vocabulary.vocab
  Dict{String,Int64} with 4 entries:
   "<unk>" => 1
   "c"     => 3
   "a"     => 3
   "d"     => 2

Tokens with counts greater than or equal to the cutoff value will
be considered part of the vocabulary.
julia> vocabulary.vocab["c"]
 3

julia> "c" in keys(vocabulary.vocab)
 true

julia> vocabulary.vocab["d"]
 2

julia> "d" in keys(vocabulary.vocab)
 true

Tokens with frequency counts less than the cutoff value will be considered not
part of the vocabulary even though their entries in the count dictionary are
preserved.
julia> "b" in keys(vocabulary.vocab)
 false

julia> "<unk>" in keys(vocabulary.vocab)
 true

We can look up words in a vocabulary using its `lookup` method.
"Unseen" words (with counts less than cutoff) are looked up as the unknown label.
If given one word (a string) as an input, this method will return a string.
julia> lookup("a")
 'a'

julia> word = ["a", "-", "d", "c", "a"]

julia> lookup(vocabulary ,word)
 5-element Array{Any,1}:
  "a"    
  "<unk>"
  "d"    
  "c"    
  "a"

If given a sequence, it will return an Array{Any,1} of the looked up words as shown above.
   
It's possible to update the counts after the vocabulary has been created.
julia> update(vocabulary,["b","c","c"])
 1

julia> vocabulary.vocab["b"]
 1
```
"""
mutable struct Vocabulary
    vocab::Dict{String,Int64}
    unk_cutoff::Int
    unk_label::String
    allword::Vector{String}
end

"""
$(TYPEDSIGNATURES)
"""
function Vocabulary(word::Vector{T}, unk_cutoff=1, unk_label="<unk>") where {T<:AbstractString}
    if unk_label in word
        error("unk_label is in vocab")
    else
        word = push!(word, unk_label)
    end
    vocab = countmap(word)
    for value in vocab
        if value[2] < unk_cutoff && value[1] != unk_label
            delete!(vocab, value[1])
        end
    end
    Vocabulary(vocab, unk_cutoff, unk_label, word)
end

"""
$(TYPEDSIGNATURES)

See [`Vocabulary`](@ref)
"""
function update(vocab::Vocabulary, words)
    vocab.allword = append!(vocab.allword, words)
    vocab.vocab = addcounts!(vocab.vocab, words)
end

"""
$(TYPEDSIGNATURES)

lookup a sequence or words in the vocabulary

Return an Array of String

See [`Vocabulary`](@ref)
"""
function lookup(voc::Vocabulary, word::AbstractVector{T})::Vector{T} where {T<:AbstractString}
    return map(word) do w
        if w in keys(voc.vocab)
            w
        else
            voc.unk_label
        end
    end
end