// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using System.Diagnostics;
using System.Diagnostics.Contracts;
using System.Text;

namespace System.IO
{
    // Provides methods for processing file system strings in a cross-platform manner.
    // Most of the methods don't do a complete parsing (such as examining a UNC hostname), 
    // but they will handle most string operations.
    public static partial class Path
    {
        // Public static readonly variant of the separators. The Path implementation itself is using
        // internal const variant of the separators for better performance.
        public static readonly char DirectorySeparatorChar = PathInternal.DirectorySeparatorChar;
        public static readonly char AltDirectorySeparatorChar = PathInternal.AltDirectorySeparatorChar;
        public static readonly char VolumeSeparatorChar = PathInternal.VolumeSeparatorChar;
        public static readonly char PathSeparator = PathInternal.PathSeparator;

        // For generating random file names
        // 8 random bytes provides 12 chars in our encoding for the 8.3 name.
        const int KeyLength = 8;

        [Obsolete("Please use GetInvalidPathChars or GetInvalidFileNameChars instead.")]
        public static readonly char[] InvalidPathChars = GetInvalidPathChars();

        // Changes the extension of a file path. The path parameter
        // specifies a file path, and the extension parameter
        // specifies a file extension (with a leading period, such as
        // ".exe" or ".cs").
        //
        // The function returns a file path with the same root, directory, and base
        // name parts as path, but with the file extension changed to
        // the specified extension. If path is null, the function
        // returns null. If path does not contain a file extension,
        // the new file extension is appended to the path. If extension
        // is null, any existing extension is removed from path.
        public static string ChangeExtension(string path, string extension)
        {
            if (path != null)
            {
                PathInternal.CheckInvalidPathChars(path);

                string s = path;
                for (int i = path.Length - 1; i >= 0; i--)
                {
                    char ch = path[i];
                    if (ch == '.')
                    {
                        s = path.Substring(0, i);
                        break;
                    }
                    if (PathInternal.IsDirectoryOrVolumeSeparator(ch)) break;
                }

                if (extension != null && path.Length != 0)
                {
                    s = (extension.Length == 0 || extension[0] != '.') ?
                        s + "." + extension :
                        s + extension;
                }

                return s;
            }
            return null;
        }

        // Returns the directory path of a file path. This method effectively
        // removes the last element of the given file path, i.e. it returns a
        // string consisting of all characters up to but not including the last
        // backslash ("\") in the file path. The returned value is null if the file
        // path is null or if the file path denotes a root (such as "\", "C:", or
        // "\\server\share").
        public static string GetDirectoryName(string path)
        {
            if (path != null)
            {
                PathInternal.CheckInvalidPathChars(path);
                path = PathInternal.NormalizeDirectorySeparators(path);
                int root = PathInternal.GetRootLength(path);

                int i = path.Length;
                if (i > root)
                {
                    while (i > root && !PathInternal.IsDirectorySeparator(path[--i])) ;
                    return path.Substring(0, i);
                }
            }
            return null;
        }

        // Returns the extension of the given path. The returned value includes the
        // period (".") character of the extension except when you have a terminal period when you get string.Empty, such as ".exe" or
        // ".cpp". The returned value is null if the given path is
        // null or if the given path does not include an extension.
        [Pure]
        public static string GetExtension(string path)
        {
            if (path == null)
                return null;

            PathInternal.CheckInvalidPathChars(path);
            int length = path.Length;
            for (int i = length - 1; i >= 0; i--)
            {
                char ch = path[i];
                if (ch == '.')
                {
                    if (i != length - 1)
                        return path.Substring(i, length - i);
                    else
                        return string.Empty;
                }
                if (PathInternal.IsDirectoryOrVolumeSeparator(ch))
                    break;
            }
            return string.Empty;
        }

        // Returns the name and extension parts of the given path. The resulting
        // string contains the characters of path that follow the last
        // separator in path. The resulting string is null if path is null.
        [Pure]
        public static string GetFileName(string path)
        {
            if (path == null)
                return null;
            
            int offset = PathInternal.FindFileNameIndex(path);
            int count = path.Length - offset;
            return path.Substring(offset, count);
        }

        [Pure]
        public static string GetFileNameWithoutExtension(string path)
        {
            if (path == null)
                return null;

            int length = path.Length;
            int offset = PathInternal.FindFileNameIndex(path);
            
            int end = path.LastIndexOf('.', length - 1, length - offset);
            return end == -1 ?
                path.Substring(offset) : // No extension was found
                path.Substring(offset, end - offset);
        }

        // Returns a cryptographically strong random 8.3 string that can be 
        // used as either a folder name or a file name.
        public static unsafe string GetRandomFileName()
        {

            byte* pKey = stackalloc byte[KeyLength];
            GetCryptoRandomBytes(pKey, KeyLength);

            const int RandomFileNameLength = 12;
            char* pRandomFileName = stackalloc char[RandomFileNameLength];
            Populate83FileNameFromRandomBytes(pKey, KeyLength, pRandomFileName, RandomFileNameLength);
            return new string(pRandomFileName, 0, RandomFileNameLength);
        }

        // Tests if a path includes a file extension. The result is
        // true if the characters that follow the last directory
        // separator ('\\' or '/') or volume separator (':') in the path include 
        // a period (".") other than a terminal period. The result is false otherwise.
        [Pure]
        public static bool HasExtension(string path)
        {
            if (path != null)
            {
                PathInternal.CheckInvalidPathChars(path);

                for (int i = path.Length - 1; i >= 0; i--)
                {
                    char ch = path[i];
                    if (ch == '.')
                    {
                        return i != path.Length - 1;
                    }
                    if (PathInternal.IsDirectoryOrVolumeSeparator(ch)) break;
                }
            }
            return false;
        }

        public static string Combine(string path1, string path2)
        {
            if (path1 == null || path2 == null)
                throw new ArgumentNullException((path1 == null) ? nameof(path1): nameof(path2));
            Contract.EndContractBlock();

            PathInternal.CheckInvalidPathChars(path1);
            PathInternal.CheckInvalidPathChars(path2);

            return CombineNoChecks(path1, path2);
        }

        public static string Combine(string path1, string path2, string path3)
        {
            if (path1 == null || path2 == null || path3 == null)
                throw new ArgumentNullException((path1 == null) ? nameof(path1): (path2 == null) ? nameof(path2): nameof(path3));
            Contract.EndContractBlock();

            PathInternal.CheckInvalidPathChars(path1);
            PathInternal.CheckInvalidPathChars(path2);
            PathInternal.CheckInvalidPathChars(path3);

            return CombineNoChecks(path1, path2, path3);
        }

        public static string Combine(string path1, string path2, string path3, string path4)
        {
            if (path1 == null || path2 == null || path3 == null || path4 == null)
                throw new ArgumentNullException((path1 == null) ? nameof(path1): (path2 == null) ? nameof(path2): (path3 == null) ? nameof(path3): nameof(path4));
            Contract.EndContractBlock();

            PathInternal.CheckInvalidPathChars(path1);
            PathInternal.CheckInvalidPathChars(path2);
            PathInternal.CheckInvalidPathChars(path3);
            PathInternal.CheckInvalidPathChars(path4);

            return CombineNoChecks(path1, path2, path3, path4);
        }

        public static string Combine(params string[] paths)
        {
            if (paths == null)
            {
                throw new ArgumentNullException(nameof(paths));
            }
            Contract.EndContractBlock();

            int finalSize = 0;
            int firstComponent = 0;

            // We have two passes, the first calculates how large a buffer to allocate and does some precondition
            // checks on the paths passed in.  The second actually does the combination.

            for (int i = 0; i < paths.Length; i++)
            {
                if (paths[i] == null)
                {
                    throw new ArgumentNullException(nameof(paths));
                }

                if (paths[i].Length == 0)
                {
                    continue;
                }

                PathInternal.CheckInvalidPathChars(paths[i]);

                if (IsPathRooted(paths[i]))
                {
                    firstComponent = i;
                    finalSize = paths[i].Length;
                }
                else
                {
                    finalSize += paths[i].Length;
                }

                char ch = paths[i][paths[i].Length - 1];
                if (!PathInternal.IsDirectoryOrVolumeSeparator(ch))
                    finalSize++;
            }

            StringBuilder finalPath = StringBuilderCache.Acquire(finalSize);

            for (int i = firstComponent; i < paths.Length; i++)
            {
                if (paths[i].Length == 0)
                {
                    continue;
                }

                if (finalPath.Length == 0)
                {
                    finalPath.Append(paths[i]);
                }
                else
                {
                    char ch = finalPath[finalPath.Length - 1];
                    if (!PathInternal.IsDirectoryOrVolumeSeparator(ch))
                    {
                        finalPath.Append(PathInternal.DirectorySeparatorChar);
                    }

                    finalPath.Append(paths[i]);
                }
            }

            return StringBuilderCache.GetStringAndRelease(finalPath);
        }

        private static string CombineNoChecks(string path1, string path2)
        {
            if (path2.Length == 0)
                return path1;

            if (path1.Length == 0)
                return path2;

            if (IsPathRooted(path2))
                return path2;

            char ch = path1[path1.Length - 1];
            return PathInternal.IsDirectoryOrVolumeSeparator(ch) ?
                path1 + path2 :
                path1 + PathInternal.DirectorySeparatorCharAsString + path2;
        }

        private static string CombineNoChecks(string path1, string path2, string path3)
        {
            if (path1.Length == 0)
                return CombineNoChecks(path2, path3);
            if (path2.Length == 0)
                return CombineNoChecks(path1, path3);
            if (path3.Length == 0)
                return CombineNoChecks(path1, path2);

            if (IsPathRooted(path3))
                return path3;
            if (IsPathRooted(path2))
                return CombineNoChecks(path2, path3);

            bool hasSep1 = PathInternal.IsDirectoryOrVolumeSeparator(path1[path1.Length - 1]);
            bool hasSep2 = PathInternal.IsDirectoryOrVolumeSeparator(path2[path2.Length - 1]);

            if (hasSep1 && hasSep2)
            {
                return path1 + path2 + path3;
            }
            else if (hasSep1)
            {
                return path1 + path2 + PathInternal.DirectorySeparatorCharAsString + path3;
            }
            else if (hasSep2)
            {
                return path1 + PathInternal.DirectorySeparatorCharAsString + path2 + path3;
            }
            else
            {
                // string.Concat only has string-based overloads up to four arguments; after that requires allocating
                // a params string[].  Instead, try to use a cached StringBuilder.
                StringBuilder sb = StringBuilderCache.Acquire(path1.Length + path2.Length + path3.Length + 2);
                sb.Append(path1)
                  .Append(PathInternal.DirectorySeparatorChar)
                  .Append(path2)
                  .Append(PathInternal.DirectorySeparatorChar)
                  .Append(path3);
                return StringBuilderCache.GetStringAndRelease(sb);
            }
        }

        private static string CombineNoChecks(string path1, string path2, string path3, string path4)
        {
            if (path1.Length == 0)
                return CombineNoChecks(path2, path3, path4);
            if (path2.Length == 0)
                return CombineNoChecks(path1, path3, path4);
            if (path3.Length == 0)
                return CombineNoChecks(path1, path2, path4);
            if (path4.Length == 0)
                return CombineNoChecks(path1, path2, path3);

            if (IsPathRooted(path4))
                return path4;
            if (IsPathRooted(path3))
                return CombineNoChecks(path3, path4);
            if (IsPathRooted(path2))
                return CombineNoChecks(path2, path3, path4);

            bool hasSep1 = PathInternal.IsDirectoryOrVolumeSeparator(path1[path1.Length - 1]);
            bool hasSep2 = PathInternal.IsDirectoryOrVolumeSeparator(path2[path2.Length - 1]);
            bool hasSep3 = PathInternal.IsDirectoryOrVolumeSeparator(path3[path3.Length - 1]);

            if (hasSep1 && hasSep2 && hasSep3)
            {
                // Use string.Concat overload that takes four strings
                return path1 + path2 + path3 + path4;
            }
            else
            {
                // string.Concat only has string-based overloads up to four arguments; after that requires allocating
                // a params string[].  Instead, try to use a cached StringBuilder.
                StringBuilder sb = StringBuilderCache.Acquire(path1.Length + path2.Length + path3.Length + path4.Length + 3);

                sb.Append(path1);
                if (!hasSep1)
                {
                    sb.Append(PathInternal.DirectorySeparatorChar);
                }

                sb.Append(path2);
                if (!hasSep2)
                {
                    sb.Append(PathInternal.DirectorySeparatorChar);
                }

                sb.Append(path3);
                if (!hasSep3)
                {
                    sb.Append(PathInternal.DirectorySeparatorChar);
                }

                sb.Append(path4);

                return StringBuilderCache.GetStringAndRelease(sb);
            }
        }

        private static readonly char[] s_base32Char = {
                'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h',
                'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p',
                'q', 'r', 's', 't', 'u', 'v', 'w', 'x',
                'y', 'z', '0', '1', '2', '3', '4', '5'};

        private static unsafe void Populate83FileNameFromRandomBytes(byte* bytes, int byteCount, char* chars, int charCount)
        {
            Debug.Assert(bytes != null);
            Debug.Assert(chars != null);

            // This method requires bytes of length 8 and chars of length 12.
            Debug.Assert(byteCount == 8, $"Unexpected {nameof(byteCount)}");
            Debug.Assert(charCount == 12, $"Unexpected {nameof(charCount)}");

            byte b0 = bytes[0];
            byte b1 = bytes[1];
            byte b2 = bytes[2];
            byte b3 = bytes[3];
            byte b4 = bytes[4];

            // Consume the 5 Least significant bits of the first 5 bytes
            chars[0] = s_base32Char[b0 & 0x1F];
            chars[1] = s_base32Char[b1 & 0x1F];
            chars[2] = s_base32Char[b2 & 0x1F];
            chars[3] = s_base32Char[b3 & 0x1F];
            chars[4] = s_base32Char[b4 & 0x1F];

            // Consume 3 MSB of b0, b1, MSB bits 6, 7 of b3, b4
            chars[5] = s_base32Char[(
                    ((b0 & 0xE0) >> 5) |
                    ((b3 & 0x60) >> 2))];

            chars[6] = s_base32Char[(
                    ((b1 & 0xE0) >> 5) |
                    ((b4 & 0x60) >> 2))];

            // Consume 3 MSB bits of b2, 1 MSB bit of b3, b4
            b2 >>= 5;

            Debug.Assert(((b2 & 0xF8) == 0), "Unexpected set bits");

            if ((b3 & 0x80) != 0)
                b2 |= 0x08;
            if ((b4 & 0x80) != 0)
                b2 |= 0x10;

            chars[7] = s_base32Char[b2];

            // Set the file extension separator
            chars[8] = '.';

            // Consume the 5 Least significant bits of the remaining 3 bytes
            chars[9] = s_base32Char[(bytes[5] & 0x1F)];
            chars[10] = s_base32Char[(bytes[6] & 0x1F)];
            chars[11] = s_base32Char[(bytes[7] & 0x1F)];
        }

        /// <summary>
        /// Create a relative path from one path to another. Paths will be resolved before calculating the difference.
        /// Default path comparison for the active platform will be used (OrdinalIgnoreCase for Windows or Mac, Ordinal for Unix).
        /// </summary>
        /// <param name="relativeTo">The source path the output should be relative to. This path is always considered to be a directory.</param>
        /// <param name="path">The destination path.</param>
        /// <returns>The relative path or <paramref name="path"/> if the paths don't share the same root.</returns>
        /// <exception cref="ArgumentNullException">Thrown if <paramref name="relativeTo"/> or <paramref name="path"/> is <c>null</c> or an empty string.</exception>
        public static string GetRelativePath(string relativeTo, string path)
        {
            return GetRelativePath(relativeTo, path, StringComparison);
        }

        private static string GetRelativePath(string relativeTo, string path, StringComparison comparisonType)
        {
            if (string.IsNullOrEmpty(relativeTo)) throw new ArgumentNullException(nameof(relativeTo));
            if (string.IsNullOrWhiteSpace(path)) throw new ArgumentNullException(nameof(path));
            Debug.Assert(comparisonType == StringComparison.Ordinal || comparisonType == StringComparison.OrdinalIgnoreCase);

            relativeTo = GetFullPath(relativeTo);
            path = GetFullPath(path);

            // Need to check if the roots are different- if they are we need to return the "to" path.
            if (!PathInternal.AreRootsEqual(relativeTo, path, comparisonType))
                return path;

            int commonLength = PathInternal.GetCommonPathLength(relativeTo, path, ignoreCase: comparisonType == StringComparison.OrdinalIgnoreCase);

            // If there is nothing in common they can't share the same root, return the "to" path as is.
            if (commonLength == 0)
                return path;

            // Trailing separators aren't significant for comparison
            int relativeToLength = relativeTo.Length;
            if (PathInternal.EndsInDirectorySeparator(relativeTo))
                relativeToLength--;

            bool pathEndsInSeparator = PathInternal.EndsInDirectorySeparator(path);
            int pathLength = path.Length;
            if (pathEndsInSeparator)
                pathLength--;

            // If we have effectively the same path, return "."
            if (relativeToLength == pathLength && commonLength >= relativeToLength) return ".";

            // We have the same root, we need to calculate the difference now using the
            // common Length and Segment count past the length.
            //
            // Some examples:
            //
            //  C:\Foo C:\Bar L3, S1 -> ..\Bar
            //  C:\Foo C:\Foo\Bar L6, S0 -> Bar
            //  C:\Foo\Bar C:\Bar\Bar L3, S2 -> ..\..\Bar\Bar
            //  C:\Foo\Foo C:\Foo\Bar L7, S1 -> ..\Bar

            StringBuilder sb = StringBuilderCache.Acquire(Math.Max(relativeTo.Length, path.Length));

            // Add parent segments for segments past the common on the "from" path
            if (commonLength < relativeToLength)
            {
                sb.Append(PathInternal.ParentDirectoryPrefix);

                for (int i = commonLength; i < relativeToLength; i++)
                {
                    if (PathInternal.IsDirectorySeparator(relativeTo[i]))
                    {
                        sb.Append(PathInternal.ParentDirectoryPrefix);
                    }
                }
            }
            else if (PathInternal.IsDirectorySeparator(path[commonLength]))
            {
                // No parent segments and we need to eat the initial separator
                //  (C:\Foo C:\Foo\Bar case)
                commonLength++;
            }

            // Now add the rest of the "to" path, adding back the trailing separator
            int count = pathLength - commonLength;
            if (pathEndsInSeparator)
                count++;

            sb.Append(path, commonLength, count);
            return StringBuilderCache.GetStringAndRelease(sb);
        }

        // StringComparison and IsCaseSensitive are also available in PathInternal.CaseSensitivity but we are
        // too low in System.Runtime.Extensions to use it (no FileStream, etc.)

        /// <summary>Returns a comparison that can be used to compare file and directory names for equality.</summary>
        internal static StringComparison StringComparison
        {
            get
            {
                return IsCaseSensitive ?
                    StringComparison.Ordinal :
                    StringComparison.OrdinalIgnoreCase;
            }
        }
    }
}