/*
 * Copyright 2024-2025 Revetware LLC.
 *
 * 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
 *
 * http://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 com.soklet.servlet.jakarta;

import com.soklet.core.MarshaledResponse;
import com.soklet.core.Request;
import com.soklet.core.Response;
import com.soklet.core.ResponseCookie;
import jakarta.servlet.ServletOutputStream;
import jakarta.servlet.http.Cookie;
import jakarta.servlet.http.HttpServletResponse;

import javax.annotation.Nonnull;
import javax.annotation.Nullable;
import javax.annotation.concurrent.NotThreadSafe;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.net.MalformedURLException;
import java.net.URL;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.time.Duration;
import java.time.Instant;
import java.time.ZoneId;
import java.time.format.DateTimeFormatter;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.HashSet;
import java.util.List;
import java.util.Locale;
import java.util.Map;
import java.util.Optional;
import java.util.Set;
import java.util.TreeMap;
import java.util.function.Supplier;
import java.util.stream.Collectors;

import static java.lang.String.format;
import static java.util.Objects.requireNonNull;

/**
 * Soklet integration implementation of {@link HttpServletResponse}.
 *
 * @author <a href="https://www.revetkn.com">Mark Allen</a>
 */
@NotThreadSafe
public final class SokletHttpServletResponse implements HttpServletResponse {
        @Nonnull
        private static final Integer DEFAULT_RESPONSE_BUFFER_SIZE_IN_BYTES;
        @Nonnull
        private static final Charset DEFAULT_CHARSET;
        @Nonnull
        private static final DateTimeFormatter DATE_TIME_FORMATTER;

        static {
                DEFAULT_RESPONSE_BUFFER_SIZE_IN_BYTES = 1_024;
                DEFAULT_CHARSET = StandardCharsets.ISO_8859_1; // Per Servlet spec
                DATE_TIME_FORMATTER = DateTimeFormatter.ofPattern("EEE, dd MMM yyyy HH:mm:ss zzz")
                                .withLocale(Locale.US)
                                .withZone(ZoneId.of("GMT"));
        }

        @Nonnull
        private final String requestPath; // e.g. "/test/abc".  Always starts with "/"
        @Nonnull
        private final List<Cookie> cookies;
        @Nonnull
        private final Map<String, List<String>> headers;
        @Nonnull
        private ByteArrayOutputStream responseOutputStream;
        @Nonnull
        private ResponseWriteMethod responseWriteMethod;
        @Nonnull
        private Integer statusCode;
        @Nonnull
        private Boolean responseCommitted;
        @Nonnull
        private Boolean responseFinalized;
        @Nullable
        private Locale locale;
        @Nullable
        private String errorMessage;
        @Nullable
        private String redirectUrl;
        @Nullable
        private Charset charset;
        @Nullable
        private String contentType;
        @Nonnull
        private Integer responseBufferSizeInBytes;
        @Nullable
        private SokletServletOutputStream servletOutputStream;
        @Nullable
        private SokletServletPrintWriter printWriter;

        @Nonnull
        public static SokletHttpServletResponse withRequest(@Nonnull Request request) {
                requireNonNull(request);
                return new SokletHttpServletResponse(request.getPath());
        }

        @Nonnull
        public static SokletHttpServletResponse withRequestPath(@Nonnull String requestPath) {
                requireNonNull(requestPath);
                return new SokletHttpServletResponse(requestPath);
        }

        private SokletHttpServletResponse(@Nonnull String requestPath) {
                requireNonNull(requestPath);

                this.requestPath = requestPath;
                this.statusCode = HttpServletResponse.SC_OK;
                this.responseWriteMethod = ResponseWriteMethod.UNSPECIFIED;
                this.responseBufferSizeInBytes = DEFAULT_RESPONSE_BUFFER_SIZE_IN_BYTES;
                this.responseOutputStream = new ByteArrayOutputStream(DEFAULT_RESPONSE_BUFFER_SIZE_IN_BYTES);
                this.cookies = new ArrayList<>();
                this.headers = new TreeMap<>(String.CASE_INSENSITIVE_ORDER);
                this.responseCommitted = false;
                this.responseFinalized = false;
        }

        @Nonnull
        public Response toResponse() {
                // In the servlet world, there is really no difference between Response and MarshaledResponse
                MarshaledResponse marshaledResponse = toMarshaledResponse();

                return Response.withStatusCode(marshaledResponse.getStatusCode())
                                .body(marshaledResponse.getBody().orElse(null))
                                .headers(marshaledResponse.getHeaders())
                                .cookies(marshaledResponse.getCookies())
                                .build();
        }

        @Nonnull
        public MarshaledResponse toMarshaledResponse() {
                byte[] body = getResponseOutputStream().toByteArray();

                Map<String, Set<String>> headers = getHeaders().entrySet().stream()
                                .collect(Collectors.toMap(entry -> entry.getKey(), entry -> new HashSet<>(entry.getValue())));

                Set<ResponseCookie> cookies = getCookies().stream()
                                .map(cookie -> {
                                        ResponseCookie.Builder builder = ResponseCookie.with(cookie.getName(), cookie.getValue())
                                                        .path(cookie.getPath())
                                                        .secure(cookie.getSecure())
                                                        .httpOnly(cookie.isHttpOnly())
                                                        .domain(cookie.getDomain());

                                        if (cookie.getMaxAge() >= 0)
                                                builder.maxAge(Duration.ofSeconds(cookie.getMaxAge()));

                                        return builder.build();
                                })
                                .collect(Collectors.toSet());

                return MarshaledResponse.withStatusCode(getStatus())
                                .body(body)
                                .headers(headers)
                                .cookies(cookies)
                                .build();
        }

        @Nonnull
        protected String getRequestPath() {
                return this.requestPath;
        }

        @Nonnull
        protected List<Cookie> getCookies() {
                return this.cookies;
        }

        @Nonnull
        protected Map<String, List<String>> getHeaders() {
                return this.headers;
        }

        @Nonnull
        protected Integer getStatusCode() {
                return this.statusCode;
        }

        protected void setStatusCode(@Nonnull Integer statusCode) {
                requireNonNull(statusCode);
                this.statusCode = statusCode;
        }

        @Nonnull
        protected Optional<String> getErrorMessage() {
                return Optional.ofNullable(this.errorMessage);
        }

        protected void setErrorMessage(@Nullable String errorMessage) {
                this.errorMessage = errorMessage;
        }

        @Nonnull
        protected Optional<String> getRedirectUrl() {
                return Optional.ofNullable(this.redirectUrl);
        }

        protected void setRedirectUrl(@Nullable String redirectUrl) {
                this.redirectUrl = redirectUrl;
        }

        @Nonnull
        protected Optional<Charset> getCharset() {
                return Optional.ofNullable(this.charset);
        }

        protected void setCharset(@Nullable Charset charset) {
                this.charset = charset;
        }

        @Nonnull
        protected Boolean getResponseCommitted() {
                return this.responseCommitted;
        }

        protected void setResponseCommitted(@Nonnull Boolean responseCommitted) {
                requireNonNull(responseCommitted);
                this.responseCommitted = responseCommitted;
        }

        @Nonnull
        protected Boolean getResponseFinalized() {
                return this.responseFinalized;
        }

        protected void setResponseFinalized(@Nonnull Boolean responseFinalized) {
                requireNonNull(responseFinalized);
                this.responseFinalized = responseFinalized;
        }

        protected void ensureResponseIsUncommitted() {
                if (getResponseCommitted())
                        throw new IllegalStateException("Response has already been committed.");
        }

        @Nonnull
        protected String dateHeaderRepresentation(@Nonnull Long millisSinceEpoch) {
                requireNonNull(millisSinceEpoch);
                return DATE_TIME_FORMATTER.format(Instant.ofEpochMilli(millisSinceEpoch));
        }

        @Nonnull
        protected Optional<SokletServletOutputStream> getServletOutputStream() {
                return Optional.ofNullable(this.servletOutputStream);
        }

        protected void setServletOutputStream(@Nullable SokletServletOutputStream servletOutputStream) {
                this.servletOutputStream = servletOutputStream;
        }

        @Nonnull
        protected Optional<SokletServletPrintWriter> getPrintWriter() {
                return Optional.ofNullable(this.printWriter);
        }

        public void setPrintWriter(@Nullable SokletServletPrintWriter printWriter) {
                this.printWriter = printWriter;
        }

        @Nonnull
        protected ByteArrayOutputStream getResponseOutputStream() {
                return this.responseOutputStream;
        }

        protected void setResponseOutputStream(@Nonnull ByteArrayOutputStream responseOutputStream) {
                requireNonNull(responseOutputStream);
                this.responseOutputStream = responseOutputStream;
        }

        @Nonnull
        protected Integer getResponseBufferSizeInBytes() {
                return this.responseBufferSizeInBytes;
        }

        protected void setResponseBufferSizeInBytes(@Nonnull Integer responseBufferSizeInBytes) {
                requireNonNull(responseBufferSizeInBytes);
                this.responseBufferSizeInBytes = responseBufferSizeInBytes;
        }

        @Nonnull
        protected ResponseWriteMethod getResponseWriteMethod() {
                return this.responseWriteMethod;
        }

        protected void setResponseWriteMethod(@Nonnull ResponseWriteMethod responseWriteMethod) {
                requireNonNull(responseWriteMethod);
                this.responseWriteMethod = responseWriteMethod;
        }

        protected enum ResponseWriteMethod {
                UNSPECIFIED,
                SERVLET_OUTPUT_STREAM,
                PRINT_WRITER
        }

        // Implementation of HttpServletResponse methods below:

        @Override
        public void addCookie(@Nullable Cookie cookie) {
                ensureResponseIsUncommitted();

                if (cookie != null)
                        getCookies().add(cookie);
        }

        @Override
        public boolean containsHeader(@Nullable String name) {
                return getHeaders().containsKey(name);
        }

        @Override
        @Nullable
        public String encodeURL(@Nullable String url) {
                return url;
        }

        @Override
        @Nullable
        public String encodeRedirectURL(@Nullable String url) {
                return url;
        }

        @Override
        public void sendError(int sc,
                                                                                                @Nullable String msg) throws IOException {
                ensureResponseIsUncommitted();
                setStatus(sc);
                setErrorMessage(msg);
                setResponseCommitted(true);
        }

        @Override
        public void sendError(int sc) throws IOException {
                ensureResponseIsUncommitted();
                setStatus(sc);
                setErrorMessage(null);
                setResponseCommitted(true);
        }

        @Override
        public void sendRedirect(@Nullable String location) throws IOException {
                doSendRedirect(location, HttpServletResponse.SC_FOUND, true);
        }

        @Override
        public void setDateHeader(@Nullable String name,
                                                                                                                long date) {
                ensureResponseIsUncommitted();
                setHeader(name, dateHeaderRepresentation(date));
        }

        @Override
        public void addDateHeader(@Nullable String name,
                                                                                                                long date) {
                ensureResponseIsUncommitted();
                addHeader(name, dateHeaderRepresentation(date));
        }

        @Override
        public void setHeader(@Nullable String name,
                                                                                                @Nullable String value) {
                ensureResponseIsUncommitted();

                if (name != null && !name.isBlank() && value != null) {
                        List<String> values = new ArrayList<>();
                        values.add(value);
                        getHeaders().put(name, values);
                }
        }

        @Override
        public void addHeader(@Nullable String name,
                                                                                                @Nullable String value) {
                ensureResponseIsUncommitted();

                if (name != null && !name.isBlank() && value != null)
                        getHeaders().computeIfAbsent(name, k -> new ArrayList<>()).add(value);
        }

        @Override
        public void setIntHeader(@Nullable String name,
                                                                                                         int value) {
                ensureResponseIsUncommitted();
                setHeader(name, String.valueOf(value));
        }

        @Override
        public void addIntHeader(@Nullable String name,
                                                                                                         int value) {
                ensureResponseIsUncommitted();
                addHeader(name, String.valueOf(value));
        }

        @Override
        public void setStatus(int sc) {
                ensureResponseIsUncommitted();
                this.statusCode = sc;
        }

        @Override
        public int getStatus() {
                return getStatusCode();
        }

        @Override
        @Nullable
        public String getHeader(@Nullable String name) {
                if (name == null)
                        return null;

                List<String> values = getHeaders().get(name);
                return values == null || values.size() == 0 ? null : values.get(0);
        }

        @Override
        @Nonnull
        public Collection<String> getHeaders(@Nullable String name) {
                if (name == null)
                        return List.of();

                List<String> values = getHeaders().get(name);
                return values == null ? List.of() : Collections.unmodifiableList(values);
        }

        @Override
        @Nonnull
        public Collection<String> getHeaderNames() {
                return Collections.unmodifiableSet(getHeaders().keySet());
        }

        @Override
        @Nonnull
        public String getCharacterEncoding() {
                return getCharset().orElse(DEFAULT_CHARSET).name();
        }

        @Override
        @Nullable
        public String getContentType() {
                return this.contentType;
        }

        @Override
        @Nonnull
        public ServletOutputStream getOutputStream() throws IOException {
                // Returns a ServletOutputStream suitable for writing binary data in the response.
                // The servlet container does not encode the binary data.
                // Calling flush() on the ServletOutputStream commits the response.
                // Either this method or getWriter() may be called to write the body, not both, except when reset() has been called.
                ResponseWriteMethod currentResponseWriteMethod = getResponseWriteMethod();

                if (currentResponseWriteMethod == ResponseWriteMethod.UNSPECIFIED) {
                        setResponseWriteMethod(ResponseWriteMethod.SERVLET_OUTPUT_STREAM);
                        this.servletOutputStream = SokletServletOutputStream.builderWithOutputStream(getResponseOutputStream())
                                        .writeOccurredCallback((ignored) -> {
                                                // Flip to "committed" if any write occurs
                                                setResponseCommitted(true);
                                        }).writeFinalizedCallback((ignored) -> {
                                                setResponseFinalized(true);
                                        }).build();
                        return getServletOutputStream().get();
                } else if (currentResponseWriteMethod == ResponseWriteMethod.SERVLET_OUTPUT_STREAM) {
                        return getServletOutputStream().get();
                } else {
                        throw new IllegalStateException(format("Cannot use %s for writing response; already using %s",
                                        ServletOutputStream.class.getSimpleName(), PrintWriter.class.getSimpleName()));
                }
        }

        @Nonnull
        protected Boolean writerObtained() {
                return getResponseWriteMethod() == ResponseWriteMethod.PRINT_WRITER;
        }

        @Nonnull
        protected Optional<String> extractCharsetFromContentType(@Nullable String type) {
                if (type == null)
                        return Optional.empty();

                String[] parts = type.split(";");

                for (int i = 1; i < parts.length; i++) {
                        String p = parts[i].trim();
                        if (p.toLowerCase(Locale.ROOT).startsWith("charset=")) {
                                String cs = p.substring("charset=".length()).trim();

                                if (cs.startsWith("\"") && cs.endsWith("\"") && cs.length() >= 2)
                                        cs = cs.substring(1, cs.length() - 1);

                                return Optional.of(cs);
                        }
                }

                return Optional.empty();
        }

        // Helper: remove any charset=... from Content-Type (preserve other params)
        @Nonnull
        protected Optional<String> stripCharsetParam(@Nullable String type) {
                if (type == null)
                        return Optional.empty();

                String[] parts = type.split(";");
                String base = parts[0].trim();
                List<String> kept = new ArrayList<>();

                for (int i = 1; i < parts.length; i++) {
                        String p = parts[i].trim();

                        if (!p.toLowerCase(Locale.ROOT).startsWith("charset=") && !p.isEmpty())
                                kept.add(p);
                }

                return Optional.ofNullable(kept.isEmpty() ? base : base + "; " + String.join("; ", kept));
        }

        // Helper: ensure Content-Type includes the given charset (replacing any existing one)
        @Nonnull
        protected Optional<String> withCharset(@Nullable String type,
                                                                                                                                                                 @Nonnull String charsetName) {
                requireNonNull(charsetName);

                if (type == null)
                        return Optional.empty();

                String baseNoCs = stripCharsetParam(type).orElse("text/plain");
                return Optional.of(baseNoCs + "; charset=" + charsetName);
        }

        @Override
        public PrintWriter getWriter() throws IOException {
                // Returns a PrintWriter object that can send character text to the client.
                // The PrintWriter uses the character encoding returned by getCharacterEncoding().
                // If the response's character encoding has not been specified as described in getCharacterEncoding
                // (i.e., the method just returns the default value ISO-8859-1), getWriter updates it to ISO-8859-1.
                // Calling flush() on the PrintWriter commits the response.
                //
                // Either this method or getOutputStream() may be called to write the body, not both, except when reset() has been called.
                // Returns a PrintWriter that uses the character encoding returned by getCharacterEncoding().
                // If not specified yet, calling getWriter() fixes the encoding to ISO-8859-1 per spec.
                ResponseWriteMethod currentResponseWriteMethod = getResponseWriteMethod();

                if (currentResponseWriteMethod == ResponseWriteMethod.UNSPECIFIED) {
                        // Freeze encoding now
                        Charset enc = getCharset().orElse(DEFAULT_CHARSET);
                        setCharset(enc); // record the chosen encoding explicitly

                        // If a content type is already present and lacks charset, append the frozen charset to header
                        if (this.contentType != null) {
                                Optional<String> csInHeader = extractCharsetFromContentType(this.contentType);
                                if (csInHeader.isEmpty() || !csInHeader.get().equalsIgnoreCase(enc.name())) {
                                        String updated = withCharset(this.contentType, enc.name()).orElse(null);

                                        if (updated != null) {
                                                this.contentType = updated;
                                                setHeader("Content-Type", updated);
                                        } else {
                                                setHeader("Content-Type", this.contentType);
                                        }
                                }
                        }

                        setResponseWriteMethod(ResponseWriteMethod.PRINT_WRITER);

                        this.printWriter =
                                        SokletServletPrintWriter.builderWithWriter(
                                                                        new OutputStreamWriter(getResponseOutputStream(), enc))
                                                        .writeOccurredCallback((ignored) -> setResponseCommitted(true))   // commit on first write
                                                        .writeFinalizedCallback((ignored) -> setResponseFinalized(true))
                                                        .build();

                        return getPrintWriter().get();
                } else if (currentResponseWriteMethod == ResponseWriteMethod.PRINT_WRITER) {
                        return getPrintWriter().get();
                } else {
                        throw new IllegalStateException(format("Cannot use %s for writing response; already using %s",
                                        PrintWriter.class.getSimpleName(), ServletOutputStream.class.getSimpleName()));
                }
        }

        @Override
        public void setCharacterEncoding(@Nullable String charset) {
                ensureResponseIsUncommitted();

                // Spec: no effect after getWriter() or after commit
                if (writerObtained())
                        return;

                if (charset == null || charset.isBlank()) {
                        // Clear explicit charset; default will be chosen at writer time if needed
                        setCharset(null);

                        // If a Content-Type is set, remove its charset=... parameter
                        if (this.contentType != null) {
                                String updated = stripCharsetParam(this.contentType).orElse(null);
                                this.contentType = updated;
                                if (updated == null || updated.isBlank()) {
                                        getHeaders().remove("Content-Type");
                                } else {
                                        setHeader("Content-Type", updated);
                                }
                        }

                        return;
                }

                Charset cs = Charset.forName(charset);
                setCharset(cs);

                // If a Content-Type is set, reflect/replace the charset=... in the header
                if (this.contentType != null) {
                        String updated = withCharset(this.contentType, cs.name()).orElse(null);

                        if (updated != null) {
                                this.contentType = updated;
                                setHeader("Content-Type", updated);
                        } else {
                                setHeader("Content-Type", this.contentType);
                        }
                }
        }

        @Override
        public void setContentLength(int len) {
                ensureResponseIsUncommitted();
                setHeader("Content-Length", String.valueOf(len));
        }

        @Override
        public void setContentLengthLong(long len) {
                ensureResponseIsUncommitted();
                setHeader("Content-Length", String.valueOf(len));
        }

        @Override
        public void setContentType(@Nullable String type) {
                // This method may be called repeatedly to change content type and character encoding.
                // This method has no effect if called after the response has been committed.
                // It does not set the response's character encoding if it is called after getWriter has been called
                // or after the response has been committed.
                if (isCommitted())
                        return;

                if (!writerObtained()) {
                        // Before writer: charset can still be established/overridden
                        this.contentType = type;

                        if (type == null || type.isBlank()) {
                                getHeaders().remove("Content-Type");
                                return;
                        }

                        // If caller specified charset=..., adopt it as the current explicit charset
                        Optional<String> cs = extractCharsetFromContentType(type);
                        if (cs.isPresent()) {
                                setCharset(Charset.forName(cs.get()));
                                setHeader("Content-Type", type);
                        } else {
                                // No charset in type. If an explicit charset already exists (via setCharacterEncoding),
                                // reflect it in the header; otherwise just set the type as-is.
                                if (getCharset().isPresent()) {
                                        String updated = withCharset(type, getCharset().get().name()).orElse(null);

                                        if (updated != null) {
                                                this.contentType = updated;
                                                setHeader("Content-Type", updated);
                                        } else {
                                                setHeader("Content-Type", type);
                                        }
                                } else {
                                        setHeader("Content-Type", type);
                                }
                        }
                } else {
                        // After writer: charset is frozen. We can change the MIME type, but we must NOT change encoding.
                        // If caller supplies a charset, normalize the header back to the locked encoding.
                        this.contentType = type;

                        if (type == null || type.isBlank()) {
                                // Allowed: clear header; does not change actual encoding used by writer
                                getHeaders().remove("Content-Type");
                                return;
                        }

                        String locked = getCharacterEncoding(); // the frozen encoding name
                        String normalized = withCharset(type, locked).orElse(null);

                        if (normalized != null) {
                                this.contentType = normalized;
                                setHeader("Content-Type", normalized);
                        } else {
                                this.contentType = type;
                                setHeader("Content-Type", type);
                        }
                }
        }

        @Override
        public void setBufferSize(int size) {
                ensureResponseIsUncommitted();

                // Per Servlet spec, setBufferSize must be called before any content is written
                if (writerObtained() || getServletOutputStream().isPresent() || getResponseOutputStream().size() > 0)
                        throw new IllegalStateException("setBufferSize must be called before any content is written");

                setResponseBufferSizeInBytes(size);
                setResponseOutputStream(new ByteArrayOutputStream(getResponseBufferSizeInBytes()));
        }

        @Override
        public int getBufferSize() {
                return getResponseBufferSizeInBytes();
        }

        @Override
        public void flushBuffer() throws IOException {
                ensureResponseIsUncommitted();
                setResponseCommitted(true);
                getResponseOutputStream().flush();
        }

        @Override
        public void resetBuffer() {
                ensureResponseIsUncommitted();
                getResponseOutputStream().reset();
        }

        @Override
        public boolean isCommitted() {
                return getResponseCommitted();
        }

        @Override
        public void reset() {
                // Clears any data that exists in the buffer as well as the status code, headers.
                // The state of calling getWriter() or getOutputStream() is also cleared.
                // It is legal, for instance, to call getWriter(), reset() and then getOutputStream().
                // If getWriter() or getOutputStream() have been called before this method, then the corresponding returned
                // Writer or OutputStream will be staled and the behavior of using the stale object is undefined.
                // If the response has been committed, this method throws an IllegalStateException.

                ensureResponseIsUncommitted();

                setStatusCode(HttpServletResponse.SC_OK);
                setServletOutputStream(null);
                setPrintWriter(null);
                setResponseWriteMethod(ResponseWriteMethod.UNSPECIFIED);
                setResponseOutputStream(new ByteArrayOutputStream(getResponseBufferSizeInBytes()));
                getHeaders().clear();
                getCookies().clear();

                // Clear content-type/charset & locale to a pristine state
                this.contentType = null;
                setCharset(null);
                this.locale = null;
        }

        @Override
        public void setLocale(@Nullable Locale locale) {
                ensureResponseIsUncommitted();
                this.locale = locale;
        }

        @Override
        public Locale getLocale() {
                return this.locale;
        }

        // *** Jakarta-specific below

        @Nullable
        private Supplier<Map<String, String>> trailerFieldsSupplier;

        @Nonnull
        protected String resolveRedirectLocation(@Nonnull String location) {
                requireNonNull(location);

                // This accepts relative URLs and converts them to a location String suitable for the Location header.
                String finalLocation;

                if (location.startsWith("/")) {
                        finalLocation = location;
                } else {
                        try {
                                new URL(location); // absolute
                                finalLocation = location;
                        } catch (MalformedURLException ignored) {
                                String base = getRequestPath();
                                int idx = base.lastIndexOf('/');
                                String parent = (idx <= 0) ? "/" : base.substring(0, idx);
                                finalLocation = parent.endsWith("/") ? parent + location : parent + "/" + location;
                        }
                }

                return finalLocation;
        }

        protected void doSendRedirect(@Nonnull String location,
                                                                                                                                int statusCode,
                                                                                                                                boolean clearBuffer) throws IOException {
                requireNonNull(location);

                ensureResponseIsUncommitted();
                setStatus(statusCode);

                String finalLocation = resolveRedirectLocation(location);

                setRedirectUrl(finalLocation);
                setHeader("Location", finalLocation);

                if (clearBuffer)
                        resetBuffer();

                flushBuffer();
                setResponseCommitted(true);
        }

        @Override
        public void sendRedirect(@Nonnull String location,
                                                                                                         int statusCode) throws IOException {
                requireNonNull(location);
                doSendRedirect(location, statusCode, true);
        }

        @Override
        public void sendRedirect(@Nonnull String location,
                                                                                                         boolean clearBuffer) throws IOException {
                requireNonNull(location);
                doSendRedirect(location, HttpServletResponse.SC_FOUND, clearBuffer);
        }

        @Override
        public void sendRedirect(@Nonnull String location,
                                                                                                         int statusCode,
                                                                                                         boolean clearBuffer) throws IOException {
                requireNonNull(location);
                doSendRedirect(location, statusCode, clearBuffer);
        }

        @Override
        public void setTrailerFields(@Nullable Supplier<Map<String, String>> supplier) {
                // Store the supplier; Soklet does not currently write HTTP trailers when sending the response body.
                this.trailerFieldsSupplier = supplier;
        }

        @Override
        @Nullable
        public Supplier<Map<String, String>> getTrailerFields() {
                return this.trailerFieldsSupplier;
        }

        @Override
        public void setCharacterEncoding(@Nullable Charset charset) {
                if (charset == null)
                        setCharacterEncoding((String) null);
                else
                        setCharacterEncoding(charset.name());
        }
}