/*
 * Copyright (C) 2018, Thomas Wolf <thomas.wolf@paranor.ch> and others
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Distribution License v. 1.0 which is available at
 * https://www.eclipse.org/org/documents/edl-v10.php.
 *
 * SPDX-License-Identifier: BSD-3-Clause
 */
package org.eclipse.jgit.internal.transport.sshd.proxy;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

/**
 * A basic parser for HTTP response headers. Handles status lines and
 * authentication headers (WWW-Authenticate, Proxy-Authenticate).
 *
 * @see <a href="https://tools.ietf.org/html/rfc7230">RFC 7230</a>
 * @see <a href="https://tools.ietf.org/html/rfc7235">RFC 7235</a>
 */
public final class HttpParser {

	/**
	 * An exception indicating some problem parsing HTPP headers.
	 */
	public static class ParseException extends Exception {

		private static final long serialVersionUID = -1634090143702048640L;

	}

	private HttpParser() {
		// No instantiation
	}

	/**
	 * Parse a HTTP response status line.
	 *
	 * @param line
	 *            to parse
	 * @return the {@link StatusLine}
	 * @throws ParseException
	 *             if the line cannot be parsed or has the wrong HTTP version
	 */
	public static StatusLine parseStatusLine(String line)
			throws ParseException {
		// Format is HTTP/<version> Code Reason
		int firstBlank = line.indexOf(' ');
		if (firstBlank < 0) {
			throw new ParseException();
		}
		int secondBlank = line.indexOf(' ', firstBlank + 1);
		if (secondBlank < 0) {
			// Accept the line even if the (according to RFC 2616 mandatory)
			// reason is missing.
			secondBlank = line.length();
		}
		int resultCode;
		try {
			resultCode = Integer.parseUnsignedInt(
					line.substring(firstBlank + 1, secondBlank));
		} catch (NumberFormatException e) {
			throw new ParseException();
		}
		// Again, accept even if the reason is missing
		String reason = ""; //$NON-NLS-1$
		if (secondBlank < line.length()) {
			reason = line.substring(secondBlank + 1);
		}
		return new StatusLine(line.substring(0, firstBlank), resultCode,
				reason);
	}

	/**
	 * Extract the authentication headers from the header lines. It is assumed
	 * that the first element in {@code reply} is the raw status line as
	 * received from the server. It is skipped. Line processing stops on the
	 * first empty line thereafter.
	 *
	 * @param reply
	 *            The complete (header) lines of the HTTP response
	 * @param authenticationHeader
	 *            to look for (including the terminating ':'!)
	 * @return a list of {@link AuthenticationChallenge}s found.
	 */
	public static List<AuthenticationChallenge> getAuthenticationHeaders(
			List<String> reply, String authenticationHeader) {
		List<AuthenticationChallenge> challenges = new ArrayList<>();
		Iterator<String> lines = reply.iterator();
		// We know we have at least one line. Skip the response line.
		lines.next();
		StringBuilder value = null;
		while (lines.hasNext()) {
			String line = lines.next();
			if (line.isEmpty()) {
				break;
			}
			if (Character.isWhitespace(line.charAt(0))) {
				// Continuation line.
				if (value == null) {
					// Skip if we have no current value
					continue;
				}
				// Skip leading whitespace
				int i = skipWhiteSpace(line, 1);
				value.append(' ').append(line, i, line.length());
				continue;
			}
			if (value != null) {
				parseChallenges(challenges, value.toString());
				value = null;
			}
			int firstColon = line.indexOf(':');
			if (firstColon > 0 && authenticationHeader
					.equalsIgnoreCase(line.substring(0, firstColon + 1))) {
				value = new StringBuilder(line.substring(firstColon + 1));
			}
		}
		if (value != null) {
			parseChallenges(challenges, value.toString());
		}
		return challenges;
	}

	private static void parseChallenges(
			List<AuthenticationChallenge> challenges,
			String header) {
		// Comma-separated list of challenges, each itself a scheme name
		// followed optionally by either: a comma-separated list of key=value
		// pairs, where the value may be a quoted string with backslash escapes,
		// or a single token value, which itself may end in zero or more '='
		// characters. Ugh.
		int length = header.length();
		for (int i = 0; i < length;) {
			int start = skipWhiteSpace(header, i);
			int end = scanToken(header, start);
			if (end <= start) {
				break;
			}
			AuthenticationChallenge challenge = new AuthenticationChallenge(
					header.substring(start, end));
			challenges.add(challenge);
			i = parseChallenge(challenge, header, end);
		}
	}

	private static int parseChallenge(AuthenticationChallenge challenge,
			String header, int from) {
		int length = header.length();
		boolean first = true;
		for (int start = from; start <= length; first = false) {
			// Now we have either a single token, which may end in zero or more
			// equal signs, or a comma-separated list of key=value pairs (with
			// optional legacy whitespace around the equals sign), where the
			// value can be either a token or a quoted string.
			start = skipWhiteSpace(header, start);
			int end = scanToken(header, start);
			if (end == start) {
				// Nothing found. Either at end or on a comma.
				if (start < header.length() && header.charAt(start) == ',') {
					return start + 1;
				}
				return start;
			}
			int next = skipWhiteSpace(header, end);
			// Comma, or equals sign, or end of string
			if (next >= length || header.charAt(next) != '=') {
				if (first) {
					// It must be a token
					challenge.setToken(header.substring(start, end));
					if (next < length && header.charAt(next) == ',') {
						next++;
					}
					return next;
				}
				// This token must be the name of the next authentication
				// scheme.
				return start;
			}
			int nextStart = skipWhiteSpace(header, next + 1);
			if (nextStart >= length) {
				if (next == end) {
					// '=' immediately after the key, no value: key must be the
					// token, and the equals sign is part of the token
					challenge.setToken(header.substring(start, end + 1));
				} else {
					// Key without value...
					challenge.addArgument(header.substring(start, end), null);
				}
				return nextStart;
			}
			if (nextStart == end + 1 && header.charAt(nextStart) == '=') {
				// More than one equals sign: must be the single token.
				end = nextStart + 1;
				while (end < length && header.charAt(end) == '=') {
					end++;
				}
				challenge.setToken(header.substring(start, end));
				end = skipWhiteSpace(header, end);
				if (end < length && header.charAt(end) == ',') {
					end++;
				}
				return end;
			}
			if (header.charAt(nextStart) == ',') {
				if (next == end) {
					// '=' immediately after the key, no value: key must be the
					// token, and the equals sign is part of the token
					challenge.setToken(header.substring(start, end + 1));
					return nextStart + 1;
				}
				// Key without value...
				challenge.addArgument(header.substring(start, end), null);
				start = nextStart + 1;
			} else {
				if (header.charAt(nextStart) == '"') {
					int[] nextEnd = { nextStart + 1 };
					String value = scanQuotedString(header, nextStart + 1,
							nextEnd);
					challenge.addArgument(header.substring(start, end), value);
					start = nextEnd[0];
				} else {
					int nextEnd = scanToken(header, nextStart);
					challenge.addArgument(header.substring(start, end),
							header.substring(nextStart, nextEnd));
					start = nextEnd;
				}
				start = skipWhiteSpace(header, start);
				if (start < length && header.charAt(start) == ',') {
					start++;
				}
			}
		}
		return length;
	}

	private static int skipWhiteSpace(String header, int i) {
		int length = header.length();
		while (i < length && Character.isWhitespace(header.charAt(i))) {
			i++;
		}
		return i;
	}

	private static int scanToken(String header, int from) {
		int length = header.length();
		int i = from;
		while (i < length) {
			char c = header.charAt(i);
			switch (c) {
			case '!':
			case '#':
			case '$':
			case '%':
			case '&':
			case '\'':
			case '*':
			case '+':
			case '-':
			case '.':
			case '^':
			case '_':
			case '`':
			case '|':
			case '0':
			case '1':
			case '2':
			case '3':
			case '4':
			case '5':
			case '6':
			case '7':
			case '8':
			case '9':
				i++;
				break;
			default:
				if (c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z') {
					i++;
					break;
				}
				return i;
			}
		}
		return i;
	}

	private static String scanQuotedString(String header, int from, int[] to) {
		StringBuilder result = new StringBuilder();
		int length = header.length();
		boolean quoted = false;
		int i = from;
		while (i < length) {
			char c = header.charAt(i++);
			if (quoted) {
				result.append(c);
				quoted = false;
			} else if (c == '\\') {
				quoted = true;
			} else if (c == '"') {
				break;
			} else {
				result.append(c);
			}
		}
		to[0] = i;
		return result.toString();
	}
}