def replace_oovs(source_in, target_in, vocabulary, source_out, target_out):
    """Replaces out-of-vocabulary words in source and target text with <unk-N>,
    where N in is the position of the word in the source sequence.
    """

    def format_unk(pos):
        return "<unk-{}>".format(pos)

    if target_in is None:
        target_in = []

    for seq_num, (source_seq, target_seq) in enumerate(
        zip_longest(source_in, target_in)
    ):
        source_seq_out = []
        target_seq_out = []

        word_to_pos = dict()
        for position, token in enumerate(source_seq.strip().split()):
            if token in vocabulary:
                token_out = token
            else:
                if token in word_to_pos:
                    oov_pos = word_to_pos[token]
                else:
                    word_to_pos[token] = position
                    oov_pos = position
                token_out = format_unk(oov_pos)
            source_seq_out.append(token_out)
        source_out.write(" ".join(source_seq_out) + "\n")

        if target_seq is not None:
            for token in target_seq.strip().split():
                if token in word_to_pos:
                    token_out = format_unk(word_to_pos[token])
                else:
                    token_out = token
                target_seq_out.append(token_out)
        if target_out is not None:
            target_out.write(" ".join(target_seq_out) + "\n")
Replaces out-of-vocabulary words in source and target text with <unk-N>,
where N in is the position of the word in the source sequence.
examples/pointer_generator/preprocess.py