"""

A forward fill operation that fills missing values with the last valid value.

The `should_fill_fn` function is used to determine if a value should be filled

with the last valid value.

# Arguments

- `should_fill_fn` A function that returns `true` if a value should be filled.

- `init=zero(T)`: The initial value to use as the last valid value.

"""

mutable struct ForwardFill{T,F<:Function} <: StreamOperation

const should_fill_fn::F

called::Bool

last_valid::T

# constructor for String type with default fill function

ForwardFill{T}(

should_fill_fn::F=(x) -> (ismissing(x) || x == "")

;

init=""

) where {T<:String,F<:Function} = new{T,F}(should_fill_fn, false, init)

# default constructor

ForwardFill{T}(

should_fill_fn::F=(x) -> (ismissing(x) || isnan(x))

;

init=zero(T)

) where {T,F<:Function} = new{T,F}(should_fill_fn, false, init)

end

@inline function (op::ForwardFill{T,F})(executor, value) where {T,F}

if !op.should_fill_fn(value)

# don't fill, just update last_valid

op.last_valid = value

end

op.called = true

nothing

end

@inline is_valid(op::ForwardFill) = op.called

@inline get_state(op::ForwardFill{T}) where {T} = op.last_valid