core/http-auth-aws/src/main/java/software/amazon/awssdk/http/auth/aws/crt/internal/signer/AwsChunkedV4aPayloadSigner.java [159:252]:
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    private void setupPreExistingTrailers(SdkHttpRequest.Builder request) {
        for (String header : request.matchingHeaders(X_AMZ_TRAILER)) {
            List<String> values = request.matchingHeaders(header);
            if (values.isEmpty()) {
                throw new IllegalArgumentException(header + " must be present in the request headers to be a valid trailer.");
            }
            preExistingTrailers.add(Pair.of(header, values));
            request.removeHeader(header);
        }
    }

    private long calculateChunksLength(long contentLength, long extensionsLength) {
        long lengthInBytes = 0;
        long chunkHeaderLength = Integer.toHexString(chunkSize).length();
        long numChunks = contentLength / chunkSize;

        // normal chunks
        // x<metadata>\r\n<data>\r\n
        lengthInBytes += numChunks * (chunkHeaderLength + extensionsLength + 2 + chunkSize + 2);

        // remaining chunk
        // x<metadata>\r\n<data>\r\n
        long remainingBytes = contentLength % chunkSize;
        if (remainingBytes > 0) {
            long remainingChunkHeaderLength = Long.toHexString(remainingBytes).length();
            lengthInBytes += remainingChunkHeaderLength + extensionsLength + 2 + remainingBytes + 2;
        }

        // final chunk
        // 0<metadata>\r\n
        lengthInBytes += 1 + extensionsLength + 2;

        return lengthInBytes;
    }

    private long calculateExistingTrailersLength() {
        long lengthInBytes = 0;

        for (Pair<String, List<String>> trailer : preExistingTrailers) {
            // size of trailer
            lengthInBytes += calculateTrailerLength(trailer);
        }

        return lengthInBytes;
    }

    private long calculateTrailerLength(Pair<String, List<String>> trailer) {
        // size of trailer-header and colon
        long lengthInBytes = trailer.left().length() + 1L;

        // size of trailer-values
        for (String value : trailer.right()) {
            lengthInBytes += value.length();
        }

        // size of commas between trailer-values, 1 less comma than # of values
        lengthInBytes += trailer.right().size() - 1;

        // terminating \r\n
        return lengthInBytes + 2;
    }

    private long calculateChecksumTrailerLength(String checksumHeaderName) {
        // size of checksum trailer-header and colon
        long lengthInBytes = checksumHeaderName.length() + 1L;

        // get the base checksum for the algorithm
        SdkChecksum sdkChecksum = fromChecksumAlgorithm(checksumAlgorithm);
        // size of checksum value as encoded-string
        lengthInBytes += BinaryUtils.toBase64(sdkChecksum.getChecksumBytes()).length();

        // terminating \r\n
        return lengthInBytes + 2;
    }

    /**
     * Add the checksum as a trailer to the chunk-encoded stream.
     * <p>
     * If the checksum-algorithm is not present, then nothing is done.
     */
    private void setupChecksumTrailerIfNeeded(ChunkedEncodedInputStream.Builder builder) {
        if (checksumAlgorithm == null) {
            return;
        }
        String checksumHeaderName = checksumHeaderName(checksumAlgorithm);
        SdkChecksum sdkChecksum = fromChecksumAlgorithm(checksumAlgorithm);
        ChecksumInputStream checksumInputStream = new ChecksumInputStream(
            builder.inputStream(),
            Collections.singleton(sdkChecksum)
        );

        TrailerProvider checksumTrailer = new ChecksumTrailerProvider(sdkChecksum, checksumHeaderName);

        builder.inputStream(checksumInputStream).addTrailer(checksumTrailer);
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core/http-auth-aws/src/main/java/software/amazon/awssdk/http/auth/aws/internal/signer/AwsChunkedV4PayloadSigner.java [178:271]:
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    private void setupPreExistingTrailers(SdkHttpRequest.Builder request) {
        for (String header : request.matchingHeaders(X_AMZ_TRAILER)) {
            List<String> values = request.matchingHeaders(header);
            if (values.isEmpty()) {
                throw new IllegalArgumentException(header + " must be present in the request headers to be a valid trailer.");
            }
            preExistingTrailers.add(Pair.of(header, values));
            request.removeHeader(header);
        }
    }

    private long calculateChunksLength(long contentLength, long extensionsLength) {
        long lengthInBytes = 0;
        long chunkHeaderLength = Integer.toHexString(chunkSize).length();
        long numChunks = contentLength / chunkSize;

        // normal chunks
        // x<metadata>\r\n<data>\r\n
        lengthInBytes += numChunks * (chunkHeaderLength + extensionsLength + 2 + chunkSize + 2);

        // remaining chunk
        // x<metadata>\r\n<data>\r\n
        long remainingBytes = contentLength % chunkSize;
        if (remainingBytes > 0) {
            long remainingChunkHeaderLength = Long.toHexString(remainingBytes).length();
            lengthInBytes += remainingChunkHeaderLength + extensionsLength + 2 + remainingBytes + 2;
        }

        // final chunk
        // 0<metadata>\r\n
        lengthInBytes += 1 + extensionsLength + 2;

        return lengthInBytes;
    }

    private long calculateExistingTrailersLength() {
        long lengthInBytes = 0;

        for (Pair<String, List<String>> trailer : preExistingTrailers) {
            // size of trailer
            lengthInBytes += calculateTrailerLength(trailer);
        }

        return lengthInBytes;
    }

    private long calculateTrailerLength(Pair<String, List<String>> trailer) {
        // size of trailer-header and colon
        long lengthInBytes = trailer.left().length() + 1L;

        // size of trailer-values
        for (String value : trailer.right()) {
            lengthInBytes += value.length();
        }

        // size of commas between trailer-values, 1 less comma than # of values
        lengthInBytes += trailer.right().size() - 1;

        // terminating \r\n
        return lengthInBytes + 2;
    }

    private long calculateChecksumTrailerLength(String checksumHeaderName) {
        // size of checksum trailer-header and colon
        long lengthInBytes = checksumHeaderName.length() + 1L;

        // get the base checksum for the algorithm
        SdkChecksum sdkChecksum = fromChecksumAlgorithm(checksumAlgorithm);
        // size of checksum value as encoded-string
        lengthInBytes += BinaryUtils.toBase64(sdkChecksum.getChecksumBytes()).length();

        // terminating \r\n
        return lengthInBytes + 2;
    }

    /**
     * Add the checksum as a trailer to the chunk-encoded stream.
     * <p>
     * If the checksum-algorithm is not present, then nothing is done.
     */
    private void setupChecksumTrailerIfNeeded(ChunkedEncodedInputStream.Builder builder) {
        if (checksumAlgorithm == null) {
            return;
        }
        String checksumHeaderName = checksumHeaderName(checksumAlgorithm);
        SdkChecksum sdkChecksum = fromChecksumAlgorithm(checksumAlgorithm);
        ChecksumInputStream checksumInputStream = new ChecksumInputStream(
            builder.inputStream(),
            Collections.singleton(sdkChecksum)
        );

        TrailerProvider checksumTrailer = new ChecksumTrailerProvider(sdkChecksum, checksumHeaderName);

        builder.inputStream(checksumInputStream).addTrailer(checksumTrailer);
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