Moving average over non-numeric values (correct errors, fill missing values)

Hi,

how can I smooth string values in a column?

I have time series data (timestamp; string) with some errors or gaps in it:

timestamp;string

1521585642;a

1521585643;a

1521585644;

1521585645;a

1521585646;a

1521585647;x

1521585648;a

1521585649;a

1521585650;a

I would like to fill the gap and replace the error ("x") with the values in proximity (lets say we want the most frequent value looking at the last 2 and next 2 values). You could call this moving average with strings. The result in this simple example would be all "a" in the string-column.

I feel like this comes close, but MAXA does not work with strings of course:

Smooth = 
CALCULATE (
    CALCULATE (
        MAXA( 'timeseries'[string] );
        'timeseries'[Datetime]
            >= VALUES ( 'timeseries'[Datetime] ) - 4 ;
        'timeseries'[Datetime] <= VALUES ( 'timeseries'[Datetime] )
    );
    ALLEXCEPT ( 'timeseries'; 'timeseries'[Tag];'timeseries'[Logfile];'timeseries'[Datetime] )
)

Any Ideas would be greatly appreciated. I was not able to find a solution.