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With inspiration from other libraries

There is a newer version: 2.12.15

package io.virtdata.libbasics.shared.stateful;

import io.virtdata.annotations.Categories;
import io.virtdata.annotations.Category;
import io.virtdata.annotations.Example;
import io.virtdata.annotations.ThreadSafeMapper;
import io.virtdata.libbasics.core.threadstate.SharedState;

import java.util.HashMap;
import java.util.function.DoubleUnaryOperator;
import java.util.function.Function;

/**
 * Save a value to a named thread-local variable, where the variable
 * name is fixed or a generated variable name from a provided function.
 * Note that the input type is not that suitable for constructing names,
 * so this is more likely to be used in an indirect naming pattern like
 * SaveDouble(Load('id'))
 */
@Categories(Category.state)
@ThreadSafeMapper
public class SaveDouble implements DoubleUnaryOperator {

    private final String name;
    private final Function nameFunc;

    @Example({"Save('foo')","save the current double value to the name 'foo' in this thread"})
    public SaveDouble(String name) {
        this.name = name;
        this.nameFunc=null;
    }

    @Example({"Save(NumberNameToString())","save a double value to a named variable in the current thread" +
            ", where the variable name is provided by a function."})
    public SaveDouble(Function nameFunc) {
        this.name=null;
        this.nameFunc = nameFunc;
    }

    @Override
    public double applyAsDouble(double operand) {
        HashMap map = SharedState.tl_ObjectMap.get();
        String varname=(nameFunc!=null) ? String.valueOf(nameFunc.apply(operand)) : name;
        map.put(varname,operand);
        return operand;
    }
}