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Facade over tracing concepts
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/**
 * Copyright 2022 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package io.micrometer.tracing;

import io.micrometer.common.lang.Nullable;

/**
 * This API was heavily influenced by Brave. Parts of its documentation were taken
 * directly from Brave.
 * 
 * Decides whether to start a new trace based on request properties such as an HTTP path.
 *
 * @param  type of the input, for example a request or method
 * @author OpenZipkin Brave Authors
 * @author Marcin Grzejszczak
 * @since 1.0.0
 */
public interface SamplerFunction {

    /**
     * Always deferring {@link SamplerFunction}.
     * @param  type of the input, for example a request or method
     * @return decision deferring sampler function
     */
    @SuppressWarnings("unchecked")
    static  SamplerFunction deferDecision() {
        return (SamplerFunction) Constants.DEFER_DECISION;
    }

    /**
     * Never sampling {@link SamplerFunction}.
     * @param  type of the input, for example a request or method
     * @return never sampling sampler function
     */
    @SuppressWarnings("unchecked")
    static  SamplerFunction neverSample() {
        return (SamplerFunction) Constants.NEVER_SAMPLE;
    }

    /**
     * Always sampling {@link SamplerFunction}.
     * @param  type of the input, for example a request or method
     * @return always sampling sampler function
     */
    @SuppressWarnings("unchecked")
    static  SamplerFunction alwaysSample() {
        return (SamplerFunction) Constants.ALWAYS_SAMPLE;
    }

    /**
     * Returns an overriding sampling decision for a new trace.
     * @param arg parameter to evaluate for a sampling decision. {@code null} input
     * results in a {@code null} result
     * @return {@code true} to sample a new trace or {@code false} to deny. {@code null}
     * defers the decision.
     */
    @Nullable
    Boolean trySample(@Nullable T arg);

    /**
     * Constant {@link SamplerFunction}s.
     */
    enum Constants implements SamplerFunction