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GEARS Runtime Common
There is a newer version: 1.17.5
package com.xlrit.gears.base.util;

import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.function.Function;
import java.util.function.Supplier;
import java.util.stream.Collector;

import com.xlrit.gears.base.function.Operators;
import lombok.Getter;
import org.checkerframework.checker.nullness.qual.Nullable;

import static java.util.stream.Collector.Characteristics.*;

public class ExtraCollectors {
    private ExtraCollectors() {}

    public static  Collector, T> first() {
        return createSingleElementCollector(Operators::first);
    }

    public static  Collector, T> last() {
        return createSingleElementCollector(Operators::last);
    }

    public static  Collector, T> only() {
        return createSingleElementCollector(Operators::only);
    }

    private static  Collector, T> createSingleElementCollector(Function, T> fun) {
        return Collector.of(
            ArrayList::new,
            ArrayList::add,
            (left, right) -> {
                left.addAll(right);
                return left;
            },
            fun,
            UNORDERED
        );
    }

    public static Collector, Long> sumLong() {
        return createSummingCollector(LongSummer::new);
    }

    public static Collector, BigDecimal> sumDecimal() {
        return createSummingCollector(BigDecimalSummer::new);
    }

    private static  Collector,T> createSummingCollector(Supplier> supplier) {
        return Collector.of(
            supplier,
            Summer::add,
            Summer::combine,
            Summer::getValue,
            CONCURRENT, UNORDERED
        );
    }

    public interface Summer {
        void add(T value);
        default Summer combine(Summer other) {
            this.add(other.getValue());
            return this;
        }
        T getValue();
    }

    @Getter
    static class LongSummer implements Summer {
        private Long value;

        public LongSummer() {
            this.value = 0L;
        }

        @Override
        public void add(Long value) {
            if (this.value == null) return;
            if (value == null) this.value = null;
            else this.value += value;
        }
    }

    @Getter
    static class BigDecimalSummer implements Summer {
        private BigDecimal value;

        public BigDecimalSummer() {
            this.value = BigDecimal.ZERO;
        }

        public void add(BigDecimal value) {
            if (this.value == null) return;
            if (value == null) this.value = null;
            else this.value = this.value.add(value);
        }
    }
}