/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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
 *
 *      https://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 org.apache.commons.codec.digest;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.nio.charset.StandardCharsets;
import java.nio.file.DirectoryStream;
import java.nio.file.Files;
import java.nio.file.Path;
import java.security.MessageDigest;
import java.util.HashMap;
import java.util.Map;
import java.util.Objects;
import java.util.Set;
import java.util.TreeSet;
import java.util.function.Supplier;

/**
 * Computes <a href="https://git-scm.com/">Git</a> object identifiers and their generalizations described by the
 * <a href="https://www.swhid.org/swhid-specification/">SWHID specification</a>.
 *
 * <p>When the hash algorithm is SHA-1, the identifiers produced by this class are identical to those used by Git.
 * Other hash algorithms produce generalized identifiers as described by the SWHID specification.</p>
 *
 * <p>This class is immutable and thread-safe. However, the {@link MessageDigest} instances passed to it generally won't be.</p>
 *
 * @see <a href="https://git-scm.com/book/en/v2/Git-Internals-Git-Objects">Git Internals – Git Objects</a>
 * @see <a href="https://www.swhid.org/swhid-specification/">SWHID Specification</a>
 * @since 1.22.0
 */
public class GitIdentifiers {

    /**
     * Represents a single entry in a Git tree object.
     *
     * <p>A Git tree object encodes a directory snapshot. Each entry holds:</p>
     * <ul>
     *   <li>a {@link FileMode} that determines the Unix file mode (e.g. {@code 100644} for a regular file),</li>
     *   <li>the entry name (file or directory name, without a path separator),</li>
     *   <li>the raw object id of the referenced blob or sub-tree.</li>
     * </ul>
     *
     * <p>Entries are ordered by {@link #compareTo} using Git's tree-sort rule: directory names are compared as if they ended with {@code '/'}, so that {@code foo/}
     * sorts after {@code foobar}.</p>
     *
     * @see <a href="https://git-scm.com/book/en/v2/Git-Internals-Git-Objects">Git Internals – Git Objects</a>
     * @see <a href="https://www.swhid.org/swhid-specification/v1.2/5.Core_identifiers/#53-directories">SWHID Directory Identifier</a>
     */
    static class DirectoryEntry implements Comparable<DirectoryEntry> {

        /**
         * The entry name (file or directory name, no path separator).
         */
        private final String name;

        /**
         * The raw object id of the referenced blob or sub-tree.
         */
        private final byte[] rawObjectId;

        /**
         * The key used for ordering entries within a tree object.
         *
         * <p>>Git appends {@code '/'} to directory names before comparing.</p>
         */
        private final String sortKey;

        /**
         * The Git object type, which determines the Unix file-mode prefix.
         */
        private final FileMode type;

        /**
         * Constructs a new entry.
         *
         * @param name The name of the entry, not containing {@code '/'}.
         * @param type The type of the entry, not null.
         * @param rawObjectId The id of the entry, not null.
         */
        DirectoryEntry(final String name, final FileMode type, final byte[] rawObjectId) {
            if (Objects.requireNonNull(name, "name").indexOf('/') >= 0) {
                throw new IllegalArgumentException("Entry name must not contain '/': " + name);
            }
            this.name = name;
            this.type = Objects.requireNonNull(type, "type");
            this.sortKey = type == FileMode.DIRECTORY ? name + "/" : name;
            this.rawObjectId = Objects.requireNonNull(rawObjectId, "rawObjectId");
        }

        @Override
        public int compareTo(final DirectoryEntry o) {
            return sortKey.compareTo(o.sortKey);
        }

        @Override
        public boolean equals(final Object obj) {
            if (obj == this) {
                return true;
            }
            if (!(obj instanceof DirectoryEntry)) {
                return false;
            }
            final DirectoryEntry other = (DirectoryEntry) obj;
            return name.equals(other.name);
        }

        @Override
        public int hashCode() {
            return name.hashCode();
        }

    }

    /**
     * The type of a Git tree entry, which maps to a Unix file-mode string.
     *
     * <p>Git encodes the file type and permission bits as an ASCII octal string that precedes the entry name in the binary tree format. The values defined here
     * cover the four entry types that Git itself produces.</p>
     *
     * @see <a href="https://git-scm.com/book/en/v2/Git-Internals-Git-Objects">Git Internals – Git Objects</a>
     */
    public enum FileMode {

        /**
         * A subdirectory. Subdirectories can only be specified by SHA or through a tree mark set with {@code --import-marks}.
         *
         * @see <a href="https://git-scm.com/docs/git-fast-import">git-fast-import - Backend for fast Git data importers</a>
         */
        DIRECTORY(new byte[] { '4', '0', '0', '0', '0' }),

        /**
         * A regular, but executable, file.
         */
        EXECUTABLE(new byte[] { '1', '0', '0', '7', '5', '5' }),

        /**
         * A gitlink, SHA-1 of the object refers to a commit in another repository. Git links can only be specified either by SHA or through a commit mark. They
         * are used to implement submodules.
         *
         * @see <a href="https://git-scm.com/docs/gitdatamodel">gitdatamodel - Git&#39;s core data model</a>
         * @see <a href="https://git-scm.com/docs/git-fast-import">git-fast-import - Backend for fast Git data importers</a>
         */
        GIT_LINK(new byte[] { '1', '6', '0', '0', '0', '0' }),

        /**
         * A regular (non-executable) file.
         * <p>
         * The majority of files in most projects use this mode. If in doubt, this is what you want.
         * </p>
         */
        REGULAR(new byte[] { '1', '0', '0', '6', '4', '4' }),

        /**
         * A symbolic link. The content of the file will be the link target.
         */
        SYMBOLIC_LINK(new byte[] { '1', '2', '0', '0', '0', '0' });

        private static FileMode get(final Path path) {
            // Symbolic links first
            if (Files.isSymbolicLink(path)) {
                return SYMBOLIC_LINK;
            }
            if (Files.isDirectory(path)) {
                return DIRECTORY;
            }
            if (Files.isExecutable(path)) {
                return EXECUTABLE;
            }
            return REGULAR;
        }

        /**
         * Serialized {@code mode}: since this is mutable, it must remain private.
         */
        private final byte[] modeBytes;

        FileMode(final byte[] modeBytes) {
            // No need for a defensive copy since the array is private and never exposed,
            this.modeBytes = modeBytes;
        }
    }

    /**
     * Builds a Git tree identifier for a virtual directory structure, such as the contents of
     * an archive.
     */
    public static final class TreeIdBuilder implements Supplier<byte[]> {

        /**
         * Supplies a blob identifier that may throw {@link IOException}.
         */
        @FunctionalInterface
        private interface BlobIdSupplier {
            byte[] get() throws IOException;
        }

        private static String requireNoParentTraversal(final String name) {
            if ("..".equals(name)) {
                throw new IllegalArgumentException("Path component not allowed: " + name);
            }
            return name;
        }

        private final Map<String, TreeIdBuilder> dirEntries = new HashMap<>();
        private final Map<String, DirectoryEntry> fileEntries = new HashMap<>();
        private final MessageDigest messageDigest;

        private TreeIdBuilder(final MessageDigest messageDigest) {
            this.messageDigest = Objects.requireNonNull(messageDigest, "messageDigest");
        }

        /**
         * Adds and returns the {@link TreeIdBuilder} for the named subdirectory, creating it if absent.
         *
         * @param name The relative path of the subdirectory in normalized form (may contain {@code '/'}).
         * @return The {@link TreeIdBuilder} for the subdirectory.
         * @throws IllegalArgumentException If any path component is {@code ".."}.
         */
        public TreeIdBuilder addDirectory(final String name) {
            TreeIdBuilder current = this;
            for (final String component : name.split("/", -1)) {
                // Noop segments
                if (component.isEmpty() || ".".equals(component)) {
                    continue;
                }
                current = current.dirEntries.computeIfAbsent(requireNoParentTraversal(component), k -> new TreeIdBuilder(messageDigest));
            }
            return current;
        }

        private void addFile(final FileMode mode, final String name, final BlobIdSupplier blobId) throws IOException {
            final int slash = name.lastIndexOf('/');
            if (slash < 0) {
                fileEntries.put(name, new DirectoryEntry(requireNoParentTraversal(name), mode, blobId.get()));
            } else {
                addDirectory(name.substring(0, slash)).addFile(mode, name.substring(slash + 1), blobId);
            }
        }

        /**
         * Adds a file entry at the given path within this tree.
         *
         * <p>If {@code name} contains {@code '/'}, intermediate subdirectories are created automatically.</p>
         *
         * @param mode The file mode (e.g. {@link FileMode#REGULAR}).
         * @param name The relative path of the entry in normalized form(may contain {@code '/'}).
         * @param data The file content.
         * @throws IOException If an I/O error occurs.
         * @throws IllegalArgumentException If any path component is {@code ".."}.
         */
        public void addFile(final FileMode mode, final String name, final byte[] data) throws IOException {
            addFile(mode, name, () -> blobId(messageDigest, data));
        }

        /**
         * Adds a file entry at the given path within this tree, streaming content without buffering.
         *
         * <p>If {@code name} contains {@code '/'}, intermediate subdirectories are created automatically.</p>
         *
         * <p>The stream is eagerly drained.</p>
         *
         * @param mode     The file mode (e.g. {@link FileMode#REGULAR}).
         * @param name The relative path of the entry in normalized form(may contain {@code '/'}).
         * @param dataSize The exact number of bytes in {@code data}.
         * @param data     The file content.
         * @throws IOException If the stream cannot be read.
         * @throws IllegalArgumentException If any path component is {@code ".."}.
         */
        public void addFile(final FileMode mode, final String name, final long dataSize, final InputStream data) throws IOException {
            addFile(mode, name, () -> blobId(messageDigest, dataSize, data));
        }

        /**
         * Adds a symbolic link entry at the given path within this tree.
         *
         * <p>If {@code name} contains {@code '/'}, intermediate subdirectories are created automatically.</p>
         *
         * @param name The relative path of the entry in normalized form(may contain {@code '/'}).
         * @param target The target of the symbolic link.
         * @throws IOException If an I/O error occurs.
         * @throws IllegalArgumentException If any path component is {@code ".."}.
         */
        public void addSymbolicLink(final String name, final String target) throws IOException {
            addFile(FileMode.SYMBOLIC_LINK, name, target.getBytes(StandardCharsets.UTF_8));
        }

        /**
         * Computes the Git tree identifier for this directory and all its descendants.
         *
         * @return The raw tree identifier bytes.
         */
        @Override
        public byte[] get() {
            final Set<DirectoryEntry> entries = new TreeSet<>(fileEntries.values());
            dirEntries.forEach((k, v) -> entries.add(new DirectoryEntry(k, FileMode.DIRECTORY, v.get())));
            final ByteArrayOutputStream baos = new ByteArrayOutputStream();
            for (final DirectoryEntry entry : entries) {
                baos.write(entry.type.modeBytes, 0, entry.type.modeBytes.length);
                baos.write(' ');
                final byte[] bytes = entry.name.getBytes(StandardCharsets.UTF_8);
                baos.write(bytes, 0, bytes.length);
                baos.write('\0');
                baos.write(entry.rawObjectId, 0, entry.rawObjectId.length);
            }
            messageDigest.reset();
            DigestUtils.updateDigest(messageDigest, getGitTreePrefix(baos.size()));
            return DigestUtils.updateDigest(messageDigest, baos.toByteArray()).digest();
        }

        private TreeIdBuilder populate(final Path directory) throws IOException {
            try (DirectoryStream<Path> files = Files.newDirectoryStream(directory)) {
                for (final Path path : files) {
                    final String name = Objects.toString(path.getFileName());
                    final FileMode mode = FileMode.get(path);
                    if (mode == FileMode.DIRECTORY) {
                        addDirectory(name).populate(path);
                    } else {
                        addFile(mode, name, () -> blobId(messageDigest, path));
                    }
                }
            }
            return this;
        }
    }

    /**
     * Reads through a byte array and returns a generalized Git blob identifier.
     *
     * <p>The identifier is computed in the way described by the
     * <a href="https://www.swhid.org/swhid-specification/v1.2/5.Core_identifiers/#52-contents">SWHID contents identifier</a>, but it can use any hash
     * algorithm.</p>
     *
     * <p>When the hash algorithm is SHA-1, the identifier is identical to Git blob identifier and SWHID contents identifier.</p>
     *
     * @param messageDigest The MessageDigest to use (for example SHA-1).
     * @param data          Data to digest.
     * @return A generalized Git blob identifier.
     */
    public static byte[] blobId(final MessageDigest messageDigest, final byte[] data) {
        messageDigest.reset();
        DigestUtils.updateDigest(messageDigest, getGitBlobPrefix(data.length));
        return DigestUtils.digest(messageDigest, data);
    }

    /**
     * Reads through a stream of known size and returns a generalized Git blob identifier, without buffering.
     *
     * <p>When the size of the content is known in advance, this overload streams {@code data} directly through
     * the digest without buffering the full content in memory.</p>
     *
     * <p>When the hash algorithm is SHA-1, the identifier is identical to Git blob identifier and SWHID contents identifier.</p>
     *
     * @param messageDigest The MessageDigest to use (for example SHA-1).
     * @param dataSize      The exact number of bytes in {@code data}.
     * @param data          Stream to digest.
     * @return A generalized Git blob identifier.
     * @throws IOException On error reading the stream.
     */
    public static byte[] blobId(final MessageDigest messageDigest, final long dataSize, final InputStream data) throws IOException {
        messageDigest.reset();
        DigestUtils.updateDigest(messageDigest, getGitBlobPrefix(dataSize));
        return DigestUtils.updateDigest(messageDigest, data).digest();
    }

    /**
     * Reads through a file and returns a generalized Git blob identifier.
     *
     * <p>The identifier is computed in the way described by the
     * <a href="https://www.swhid.org/swhid-specification/v1.2/5.Core_identifiers/#52-contents">SWHID contents identifier</a>, but it can use any hash
     * algorithm.</p>
     *
     * <p>When the hash algorithm is SHA-1, the identifier is identical to Git blob identifier and SWHID contents identifier.</p>
     *
     * @param messageDigest The MessageDigest to use (for example SHA-1).
     * @param data          Path to the file to digest.
     * @return A generalized Git blob identifier.
     * @throws IOException On error accessing the file.
     */
    public static byte[] blobId(final MessageDigest messageDigest, final Path data) throws IOException {
        if (Files.isSymbolicLink(data)) {
            final byte[] linkTarget = Files.readSymbolicLink(data).toString().getBytes(StandardCharsets.UTF_8);
            return blobId(messageDigest, linkTarget);
        }
        messageDigest.reset();
        DigestUtils.updateDigest(messageDigest, getGitBlobPrefix(Files.size(data)));
        return DigestUtils.updateDigest(messageDigest, data).digest();
    }

    private static byte[] getGitBlobPrefix(final long dataSize) {
        return getGitPrefix("blob", dataSize);
    }

    private static byte[] getGitPrefix(final String type, final long dataSize) {
        return (type + " " + dataSize + "\0").getBytes(StandardCharsets.UTF_8);
    }

    private static byte[] getGitTreePrefix(final long dataSize) {
        return getGitPrefix("tree", dataSize);
    }

    /**
     * Reads through a directory and returns a generalized Git tree identifier.
     *
     * <p>The identifier is computed in the way described by the
     * <a href="https://www.swhid.org/swhid-specification/v1.2/5.Core_identifiers/#53-directories">SWHID directory identifier</a>, but it can use any hash
     * algorithm.</p>
     *
     * <p>When the hash algorithm is SHA-1, the identifier is identical to Git tree identifier and SWHID directory identifier.</p>
     *
     * @param messageDigest The MessageDigest to use (for example SHA-1).
     * @param data          Path to the directory to digest.
     * @return A generalized Git tree identifier.
     * @throws IOException On error accessing the directory or its contents.
     */
    public static byte[] treeId(final MessageDigest messageDigest, final Path data) throws IOException {
        return treeIdBuilder(messageDigest).populate(data).get();
    }

    /**
     * Returns a new {@link TreeIdBuilder} for constructing a generalized Git tree identifier from a virtual directory
     * structure, such as the contents of an archive.
     *
     * <p>The identifier is computed in the way described by the
     * <a href="https://www.swhid.org/swhid-specification/v1.2/5.Core_identifiers/#53-directories">SWHID directory identifier</a>, but it can use any hash
     * algorithm.</p>
     *
     * <p>When the hash algorithm is SHA-1, the identifier is identical to Git tree identifier and SWHID directory identifier.</p>
     *
     * @param messageDigest The MessageDigest to use (for example SHA-1).
     * @return A new {@link TreeIdBuilder}.
     */
    public static TreeIdBuilder treeIdBuilder(final MessageDigest messageDigest) {
        return new TreeIdBuilder(messageDigest);
    }

    private GitIdentifiers() {
        // utility class
    }
}
