src/java.base/share/classes/jdk/internal/lang/stable/StableFunction.java

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package jdk.internal.lang.stable;

import jdk.internal.vm.annotation.ForceInline;

import java.util.Map;
import java.util.Set;
import java.util.function.Function;
import java.util.function.Supplier;

// Note: It would be possible to just use `StableMap::get` with some additional logic
// instead of this class but explicitly providing a class like this provides better
// debug capability, exception handling, and may provide better performance.
/**
 * Implementation of a stable Function.
 *
 * @implNote This implementation can be used early in the boot sequence as it does not
 *           rely on reflection, MethodHandles, Streams etc.
 *
 * @param values   a delegate map of inputs to StableValue mappings
 * @param original the original Function
 * @param <T>      the type of the input to the function
 * @param <R>      the type of the result of the function
 */
public record StableFunction<T, R>(Map<? extends T, StableValueImpl<R>> values,
                                   Function<? super T, ? extends R> original) implements Function<T, R> {

    @ForceInline
    @Override
    public R apply(T value) {
        final StableValueImpl<R> stable = values.get(value);
        if (stable == null) {
            throw new IllegalArgumentException("Input not allowed: " + value);
        }
        return stable.orElseSet(new Supplier<R>() {
            @Override  public R get() { return original.apply(value); }});
    }

    @Override
    public int hashCode() {
        return System.identityHashCode(this);
    }

    @Override
    public boolean equals(Object obj) {
        return obj == this;
    }

    @Override
    public String toString() {
        return StableUtil.renderMappings(this, "StableFunction", values.entrySet(), true);
    }

    public static <T, R> StableFunction<T, R> of(Set<? extends T> inputs,
                                                 Function<? super T, ? extends R> original) {
        return new StableFunction<>(StableUtil.map(inputs), original);
    }

}