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Java SDK to be used with a First Data Gateway account. This SDK has been created and packaged to offer the easiest way to integrate your application into the First Data Gateway. This SDK gives you the ability to run transactions such as sales, preauthorizations, postauthorizations, credits, voids, and returns; transaction inquiries; setting up scheduled payments and much more.

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package com.github.GBSEcom.simple;

import com.github.GBSEcom.client.JSON;
import lombok.Getter;
import java.util.Optional;
import java.util.UUID;

interface ClientHeaders {
	String getContentType();
	String getClientRequestId();
	String getApiKey();
	Long getTimestamp();
	String getMessageSignature();
}

class ClientHeadersImpl implements ClientHeaders {

	public static  ClientHeaders from(final MerchantCredentials creds, final T payload) {
		final String uuid = UUID.randomUUID().toString();
		return new ClientHeadersImpl(creds, uuid, payload);
	}

	public static ClientHeaders from(final MerchantCredentials creds) {
		return ClientHeadersImpl.from(creds, null);
	}

	public ClientHeadersImpl(final MerchantCredentials merchCreds, final String uuid, final T payloadObj) {
		creds = merchCreds;
		clientRequestId = uuid;
		payload = payloadObj;
	}

	public ClientHeadersImpl(final MerchantCredentials merchCreds, final String uuid) {
		this(merchCreds, uuid, null);
	}

	@Getter
	private final String contentType = "application/json";

	@Getter
	private final String clientRequestId;

	private final MerchantCredentials creds;

	private T payload;

	private Long timestamp;

	private String messageSignature;

	public String getApiKey() {
		return creds.getApiKey();
	}

	public Long getTimestamp() {
		if (timestamp == null) {
			timestamp = Timestamp.now().toMilliseconds();
		}
		return timestamp;
	}

	public String getMessageSignature() {
		if (messageSignature == null) {
			messageSignature = signMsg(getMsgToSign());
		}
		return messageSignature;
	}

	private String getPayload() {
		return Optional.ofNullable(payload)
			.map(p -> new JSON().serialize(p))
			.orElse("");
	}

	private String signMsg(final String msg) {
		final Hmac hmac = new Hmac(creds.getApiSecret());
		return hmac.sign(msg);
	}

	private String getMsgToSign() {
		return creds.getApiKey() +
			clientRequestId +
			getTimestamp() +
			getPayload();
	}
}