presto-main/src/main/java/io/prestosql/operator/window/WindowPartition.java/WindowPartition/getFrameValue
class WindowPartition:    private long getFrameValue(int channel, String type)
    {
        checkCondition(!pagesIndex.isNull(channel, currentPosition), INVALID_WINDOW_FRAME, "Window frame %s offset must not be null", type);
        long value = pagesIndex.getLong(channel, currentPosition);
        checkCondition(value >= 0, INVALID_WINDOW_FRAME, "Window frame %s offset must not be negative", value);
        return value;
    }