/*
 * Copyright (C) 2020 The Android Open Source Project
 *
 * 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.android.libcore.timezone.tzlookup;

import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.text.ParseException;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

/**
 * A class that knows about the structure of the backward file.
 */
final class BackwardFile {

    private final Map<String, String> links = new HashMap<>();

    private BackwardFile() {}

    static BackwardFile parse(String backwardFile) throws IOException, ParseException {
        BackwardFile backward = new BackwardFile();

        List<String> lines = Files
                .readAllLines(Paths.get(backwardFile), StandardCharsets.US_ASCII);

        // Remove comments
        List<String> linkLines =
                lines.stream()
                        .filter(s -> !(s.startsWith("#") || s.isEmpty()))
                        .collect(Collectors.toList());

        for (String linkLine : linkLines) {
            String[] fields = linkLine.split("\t+");
            if (fields.length < 3 || !fields[0].equals("Link")) {
                throw new ParseException("Line is malformed: " + linkLine, 0);
            }
            backward.addLink(fields[1], fields[2]);
        }
        return backward;
    }

    /**
     * Add a link entry.
     *
     * @param target the new tz ID
     * @param linkName the old tz ID
     */
    private void addLink(String target, String linkName) {
        String oldValue = links.put(linkName, target);
        if (oldValue != null) {
            throw new IllegalStateException("Duplicate link from " + linkName);
        }
    }

    /** Returns a mapping from linkName (old tz ID) to target (new tz ID). */
    Map<String, String> getDirectLinks() {
        // Validate links for cycles and collapse the links if there are links to links. There's a
        // simple check to confirm that no chain is longer than a fixed length, to guard against
        // cycles.
        final int maxChainLength = 2;
        Map<String, String> collapsedLinks = new HashMap<>();
        for (String fromId : links.keySet()) {
            int chainLength = 0;
            String currentId = fromId;
            String lastId = null;
            while ((currentId = links.get(currentId)) != null) {
                chainLength++;
                lastId = currentId;
                if (chainLength >= maxChainLength) {
                    throw new IllegalStateException(
                            "Chain from " + fromId + " is longer than " + maxChainLength);
                }
            }
            if (chainLength == 0) {
                throw new IllegalStateException("Null Link targetId for " + fromId);
            }
            collapsedLinks.put(fromId, lastId);
        }
        return Collections.unmodifiableMap(collapsedLinks);
    }
}