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The Amazon Kinesis Client Library for Java enables Java developers to easily consume and process data from Amazon Kinesis.
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package software.amazon.kinesis.retrieval;

import java.time.Instant;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

import software.amazon.awssdk.services.kinesis.model.GetShardIteratorRequest;
import software.amazon.awssdk.services.kinesis.model.ShardIteratorType;
import software.amazon.awssdk.services.kinesis.model.StartingPosition;
import software.amazon.awssdk.services.kinesis.model.SubscribeToShardRequest;
import software.amazon.kinesis.annotations.KinesisClientInternalApi;
import software.amazon.kinesis.checkpoint.SentinelCheckpoint;
import software.amazon.kinesis.common.InitialPositionInStreamExtended;

@KinesisClientInternalApi
public class IteratorBuilder {

    public static SubscribeToShardRequest.Builder request(SubscribeToShardRequest.Builder builder,
            String sequenceNumber, InitialPositionInStreamExtended initialPosition) {
        return builder.startingPosition(request(StartingPosition.builder(), sequenceNumber, initialPosition).build());
    }

    public static SubscribeToShardRequest.Builder reconnectRequest(SubscribeToShardRequest.Builder builder,
            String sequenceNumber, InitialPositionInStreamExtended initialPosition) {
        return builder.startingPosition(
                reconnectRequest(StartingPosition.builder(), sequenceNumber, initialPosition).build());
    }

    public static StartingPosition.Builder request(StartingPosition.Builder builder, String sequenceNumber,
            InitialPositionInStreamExtended initialPosition) {
        return apply(builder, StartingPosition.Builder::type, StartingPosition.Builder::timestamp,
                StartingPosition.Builder::sequenceNumber, initialPosition, sequenceNumber,
                ShardIteratorType.AT_SEQUENCE_NUMBER);
    }

    public static StartingPosition.Builder reconnectRequest(StartingPosition.Builder builder, String sequenceNumber,
            InitialPositionInStreamExtended initialPosition) {
        return apply(builder, StartingPosition.Builder::type, StartingPosition.Builder::timestamp,
                StartingPosition.Builder::sequenceNumber, initialPosition, sequenceNumber,
                ShardIteratorType.AFTER_SEQUENCE_NUMBER);
    }

    public static GetShardIteratorRequest.Builder request(GetShardIteratorRequest.Builder builder,
            String sequenceNumber, InitialPositionInStreamExtended initialPosition) {
        return apply(builder, GetShardIteratorRequest.Builder::shardIteratorType, GetShardIteratorRequest.Builder::timestamp,
                GetShardIteratorRequest.Builder::startingSequenceNumber, initialPosition, sequenceNumber,
                ShardIteratorType.AT_SEQUENCE_NUMBER);
    }

    private final static Map SHARD_ITERATOR_MAPPING;

    static {
        Map map = new HashMap<>();
        map.put(SentinelCheckpoint.LATEST.name(), ShardIteratorType.LATEST);
        map.put(SentinelCheckpoint.TRIM_HORIZON.name(), ShardIteratorType.TRIM_HORIZON);
        map.put(SentinelCheckpoint.AT_TIMESTAMP.name(), ShardIteratorType.AT_TIMESTAMP);

        SHARD_ITERATOR_MAPPING = Collections.unmodifiableMap(map);
    }

    @FunctionalInterface
    private interface UpdatingFunction {
        R apply(R updated, T value);
    }

    private static  R apply(R initial, UpdatingFunction shardIterFunc,
            UpdatingFunction dateFunc, UpdatingFunction sequenceFunction,
            InitialPositionInStreamExtended initialPositionInStreamExtended, String sequenceNumber,
            ShardIteratorType defaultIteratorType) {
        ShardIteratorType iteratorType = SHARD_ITERATOR_MAPPING.getOrDefault(
                sequenceNumber, defaultIteratorType);
        R result = shardIterFunc.apply(initial, iteratorType);
        switch (iteratorType) {
        case AT_TIMESTAMP:
            return dateFunc.apply(result, initialPositionInStreamExtended.getTimestamp().toInstant());
        case AT_SEQUENCE_NUMBER:
        case AFTER_SEQUENCE_NUMBER:
            return sequenceFunction.apply(result, sequenceNumber);
        default:
            return result;
        }
    }

}