//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2023 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//

#if canImport(Darwin)
internal import os
#elseif os(Android)
import Android
#elseif canImport(Glibc)
import Glibc
#elseif os(Windows)
import WinSDK
#endif

internal import _FoundationCShims

extension StringProtocol {
    fileprivate func _standardizingSlashes() -> String {
        #if os(Windows)
        // The string functions below all assume that the path separator is a forward slash
        // Standardize the path to use forward slashes before processing for consistency
        return self.replacing(._backslash, with: ._slash)
        #else
        if let str = _specializingCast(self, to: String.self) {
            return str
        } else {
            return String(self)
        }
        #endif
    }
}

extension String {
    internal func deletingLastPathComponent() -> String {
        _standardizingSlashes()._deletingLastPathComponent()
    }
    
    private func _deletingLastPathComponent() -> String {
        let lastSlash = self.lastIndex { $0 == "/" }
        guard let lastSlash else {
            // No slash
            return ""
        }
        
        if lastSlash == startIndex {
            // Only a first slash, return a bare slash.
            return "/"
        }
        
        if lastSlash == index(before: endIndex) {
            // This is a trailing slash. Ignore it.
            let beforeLastSlash = self[startIndex..<lastSlash].lastIndex { $0 == "/" }
            if let beforeLastSlash {
                return String(self[startIndex..<beforeLastSlash])
            } else {
                // No other slash. Return empty string.
                return ""
            }
        } else {
            return String(self[startIndex..<lastSlash])
        }
    }
        
    internal func appendingPathComponent(_ component: String) -> String {
        _standardizingSlashes()._appendingPathComponent(component)
    }
    
    private func _appendingPathComponent(_ component: String) -> String {
        var result = self
        if !component.isEmpty {
            var needsSlash = true
            if isEmpty {
                needsSlash = false
            } else if count == 1 {
                needsSlash = first! != "/"
            } else if count == 2 {
                // "net"
                needsSlash = !(self[startIndex] == "\\" && self[index(after: startIndex)] == "\\")
            }
            
            if needsSlash {
                result = result + "/"
            }
            
            result = result + component
        }
        
        result = result.reduce(into: "") { partialResult, c in
            guard c == "/" else {
                partialResult += String(c)
                return
            }
            
            guard !partialResult.isEmpty else {
                partialResult += "/"
                return
            }
            
            let lastCharacter = partialResult.last!
            if lastCharacter != "/" {
                // Append the slash
                partialResult += "/"
            }
        }
        
        if result.isEmpty { return "" }
        
        if result.last! != "/" {
            return result
        }
        
        var idx = result.endIndex
        idx = result.index(before: idx)
        while idx != result.startIndex && result[idx] == "/" {
            idx = result.index(before: idx)
        }
        
        return String(result[result.startIndex..<result.index(after: idx)])
    }

    internal var lastPathComponent: String {
        _standardizingSlashes()._lastPathComponent
    }
    
    private var _lastPathComponent: String {
        let lastSlash = self.lastIndex { $0 == "/" }
        guard let lastSlash else {
            // No slash, just return self
            return self
        }
        
        if lastSlash == startIndex {
            if count == 1 {
                // Only a first slash, return a bare slash.
                return "/"
            } else {
                return String(self[index(after: startIndex)..<endIndex])
            }
        }
        
        if lastSlash == index(before: endIndex) {
            // This is a trailing slash. Ignore it and all slashes that directly precede it.
            let lastNonSlash = self[startIndex..<lastSlash].lastIndex { $0 != "/" }
            guard let lastNonSlash else {
                // String is all slashes, return a bare slash.
                return "/"
            }
            let slashBeforeLastComponent = self[startIndex..<lastNonSlash].lastIndex { $0 == "/" }
            if let slashBeforeLastComponent {
                return String(self[index(after: slashBeforeLastComponent)...lastNonSlash])
            } else {
                // No other slash. Return string up to the last non-slash character.
                return String(self[startIndex...lastNonSlash])
            }
        } else {
            return String(self[index(after: lastSlash)..<endIndex])
        }
    }

    internal static let invalidExtensionScalars = Set<Unicode.Scalar>([
        " ",
        "/",
        "\u{061c}", // ARABIC LETTER MARK
        "\u{200e}", // LEFT-TO-RIGHT MARK
        "\u{200f}", // RIGHT-TO-LEFT MARK
        "\u{202a}", // LEFT-TO-RIGHT EMBEDDING
        "\u{202b}", // RIGHT-TO-LEFT EMBEDDING
        "\u{202c}", // POP DIRECTIONAL FORMATTING
        "\u{202d}", // LEFT-TO-RIGHT OVERRIDE
        "\u{202e}", // RIGHT-TO-LEFT OVERRIDE
        "\u{2066}", // LEFT-TO-RIGHT ISOLATE
        "\u{2067}", // RIGHT-TO-LEFT ISOLATE
        "\u{2068}", // FIRST STRONG ISOLATE
        "\u{2069}", // POP DIRECTIONAL ISOLATE
    ])

    internal func deletingPathExtension() -> String {
        guard !pathExtension.isEmpty else {
            return self
        }
        let dot = UInt8(ascii: ".")
        guard let lastDot = self.utf8.lastIndex(of: dot) else {
            return self
        }
        return String(self[..<lastDot])
    }

    private func validatePathExtension(_ pathExtension: String) -> Bool {
        guard pathExtension.utf8.last != UInt8(ascii: ".") else {
            return false
        }
        if let lastDot = pathExtension.utf8.lastIndex(of: UInt8(ascii: ".")) {
            let beforeDot = pathExtension[..<lastDot]._standardizingSlashes().unicodeScalars
            let afterDot = pathExtension[pathExtension.index(after: lastDot)...]._standardizingSlashes().unicodeScalars
            return beforeDot.allSatisfy { $0 != "/" } && afterDot.allSatisfy { !String.invalidExtensionScalars.contains($0) }
        } else {
            return pathExtension._standardizingSlashes().unicodeScalars.allSatisfy { !String.invalidExtensionScalars.contains($0) }
        }
    }

    internal func appendingPathExtension(_ pathExtension: String) -> String {
        guard validatePathExtension(pathExtension) else {
            return self
        }
        return self + ".\(pathExtension)"
    }

    internal var pathExtension: String {
        let dot = UInt8(ascii: ".")
        let lastComponent = lastPathComponent.utf8
        guard lastComponent.last != dot,
              !lastComponent.starts(with: [dot, dot]),
              let lastDot = lastComponent.lastIndex(of: dot),
              lastDot != lastComponent.startIndex else {
            return ""
        }
        let result = String(lastPathComponent[lastComponent.index(after: lastDot)...])
        guard validatePathExtension(result) else {
            return ""
        }
        return result
    }

    internal func merging(relativePath: String) -> String {
        _standardizingSlashes()._merging(relativePath: relativePath)
    }
    
    private func _merging(relativePath: String) -> String {
        guard relativePath.utf8.first != UInt8(ascii: "/") else {
            return relativePath
        }
        guard let basePathEnd = self.utf8.lastIndex(of: UInt8(ascii: "/")) else {
            return relativePath
        }
        return self[...basePathEnd] + relativePath
    }

    internal var removingDotSegments: String {
        _standardizingSlashes()._removingDotSegments
    }
    
    private var _removingDotSegments: String {
        let input = self.utf8
        guard !input.isEmpty else {
            return ""
        }
        var output = [UInt8]()

        enum DotState {
            case initial
            case dot
            case dotDot
            case slash
            case slashDot
            case slashDotDot
            case appendUntilSlash
        }
        let dot = UInt8(ascii: ".")
        let slash = UInt8(ascii: "/")

        var state = DotState.initial
        for v in input {
            switch state {
            case .initial:
                if v == dot {
                    state = .dot
                } else if v == slash {
                    state = .slash
                } else {
                    output.append(v)
                    state = .appendUntilSlash
                }
                break
            case .dot:
                if v == dot {
                    state = .dotDot
                } else if v == slash {
                    state = .initial
                } else {
                    output.append(contentsOf: [dot, v])
                    state = .appendUntilSlash
                }
                break
            case .dotDot:
                if v == slash {
                    state = .initial
                } else {
                    output.append(contentsOf: [dot, dot, v])
                    state = .appendUntilSlash
                }
                break
            case .slash:
                if v == dot {
                    state = .slashDot
                } else if v == slash {
                    output.append(slash)
                } else {
                    output.append(contentsOf: [slash, v])
                    state = .appendUntilSlash
                }
                break
            case .slashDot:
                if v == dot {
                    state = .slashDotDot
                } else if v == slash {
                    state = .slash
                } else {
                    output.append(contentsOf: [slash, dot, v])
                    state = .appendUntilSlash
                }
                break
            case .slashDotDot:
                if v == slash {
                    while let last = output.popLast(), last != slash { }
                    state = .slash
                } else {
                    output.append(contentsOf: [slash, dot, dot, v])
                    state = .appendUntilSlash
                }
                break
            case .appendUntilSlash:
                if v == slash {
                    state = .slash
                } else {
                    output.append(v)
                }
                break
            }
        }

        switch state {
        case .initial:
            break
        case .dot:
            break
        case .dotDot:
            break
        case .slash:
            output.append(slash)
            break
        case .slashDot:
            output.append(slash)
            break
        case .slashDotDot:
            while let last = output.popLast(), last != slash { }
            output.append(slash)
            break
        case .appendUntilSlash:
            break
        }

        output.append(0) // NULL-terminated

        return String(cString: output)
    }

    #if !NO_FILESYSTEM
    internal static func homeDirectoryPath(forUser user: String? = nil) -> String {
#if os(Windows)
        func GetUserProfile() -> String? {
            return "USERPROFILE".withCString(encodedAs: UTF16.self) { pwszVariable in
                let dwLength: DWORD = GetEnvironmentVariableW(pwszVariable, nil, 0)
                // Ensure that `USERPROFILE` is defined.
                if dwLength == 0 { return nil }
                return withUnsafeTemporaryAllocation(of: WCHAR.self, capacity: Int(dwLength)) {
                    guard GetEnvironmentVariableW(pwszVariable, $0.baseAddress, dwLength) == dwLength - 1 else {
                        return nil
                    }
                    return String(decoding: $0, as: UTF16.self)
                }
            }
        }

        if let user {
            return user.withCString(encodedAs: UTF16.self) { pwszUserName in
                var cbSID: DWORD = 0
                var cchReferencedDomainName: DWORD = 0
                var eUse: SID_NAME_USE = SidTypeUnknown
                guard LookupAccountNameW(nil, pwszUserName, nil, &cbSID, nil, &cchReferencedDomainName, &eUse) else {
                    fatalError("unable to lookup SID for user \(user)")
                }

                return withUnsafeTemporaryAllocation(of: CChar.self, capacity: Int(cbSID)) { pSID in
                    return withUnsafeTemporaryAllocation(of: WCHAR.self, capacity: Int(cchReferencedDomainName)) { pwszReferencedDomainName in
                        guard LookupAccountNameW(nil, pwszUserName, pSID.baseAddress, &cbSID, pwszReferencedDomainName.baseAddress, &cchReferencedDomainName, &eUse) else {
                            fatalError("unable to lookup SID for user \(user)")
                        }

                        var pwszSID: LPWSTR? = nil
                        guard ConvertSidToStringSidW(pSID.baseAddress, &pwszSID) else {
                            fatalError("unable to convert SID to string for user \(user)")
                        }

                        return #"SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList\#\(String(decodingCString: pwszSID!, as: UTF16.self))"#.withCString(encodedAs: UTF16.self) { pwszKeyPath in
                            return "ProfileImagePath".withCString(encodedAs: UTF16.self) { pwszKey in
                                var cbData: DWORD = 0
                                guard RegGetValueW(HKEY_LOCAL_MACHINE, pwszKeyPath, pwszKey, RRF_RT_REG_SZ, nil, nil, &cbData) == ERROR_SUCCESS else {
                                    fatalError("unable to query ProfileImagePath for user \(user)")
                                }
                                return withUnsafeTemporaryAllocation(of: WCHAR.self, capacity: Int(cbData)) { pwszData in
                                    guard RegGetValueW(HKEY_LOCAL_MACHINE, pwszKeyPath, pwszKey, RRF_RT_REG_SZ, nil, pwszData.baseAddress, &cbData) == ERROR_SUCCESS else {
                                        fatalError("unable to query ProfileImagePath for user \(user)")
                                    }
                                    return String(decodingCString: pwszData.baseAddress!, as: UTF16.self)
                                }
                            }
                        }

                    }
                }
            }
        }

        var hToken: HANDLE? = nil
        guard OpenProcessToken(GetCurrentProcess(), TOKEN_QUERY, &hToken) else {
            guard let UserProfile = GetUserProfile() else {
                fatalError("unable to evaluate `%UserProfile%`")
            }
            return UserProfile
        }
        defer { CloseHandle(hToken) }

        var dwcchSize: DWORD = 0
        _ = GetUserProfileDirectoryW(hToken, nil, &dwcchSize)

        return withUnsafeTemporaryAllocation(of: WCHAR.self, capacity: Int(dwcchSize)) {
            var dwcchSize: DWORD = DWORD($0.count)
            guard GetUserProfileDirectoryW(hToken, $0.baseAddress, &dwcchSize) else {
                fatalError("unable to query user profile directory")
            }
            return String(decoding: $0, as: UTF16.self)
        }
#else
        #if targetEnvironment(simulator)
        if user == nil, let envValue = getenv("CFFIXED_USER_HOME") ?? getenv("HOME") {
            return String(cString: envValue).standardizingPath
        }
        #endif
        
        // First check CFFIXED_USER_HOME if the environment is not considered tainted
        if let envVar = Platform.getEnvSecure("CFFIXED_USER_HOME") {
            return envVar.standardizingPath
        }
        
        #if !os(WASI) // WASI does not have user concept
        // Next, attempt to find the home directory via getpwnam/getpwuid
        var pass: UnsafeMutablePointer<passwd>?
        if let user {
            pass = getpwnam(user)
        } else {
            // We use the real UID instead of the EUID here when the EUID is the root user (i.e. a process has called seteuid(0))
            // In this instance, we historically do this to ensure a stable home directory location for processes that call seteuid(0)
            pass = getpwuid(Platform.getUGIDs(allowEffectiveRootUID: false).uid)
        }
        
        if let dir = pass?.pointee.pw_dir {
            return String(cString: dir).standardizingPath
        }
        #endif
        
        // Fallback to HOME for the current user if possible
        if user == nil, let home = getenv("HOME") {
            return String(cString: home).standardizingPath
        }
        
        // If all else fails, log and fall back to /var/empty
        return "/var/empty"
#endif
    }
    
    // From swift-corelibs-foundation's NSTemporaryDirectory. Internal for now, pending a better public API.
    internal static var temporaryDirectoryPath: String {
        func normalizedPath(with path: String) -> String {
            var result = path._standardizingSlashes()
            guard result.utf8.last != ._slash else {
                return result
            }
            return result + "/"
        }
#if os(Windows)
        let cchLength: DWORD = GetTempPathW(0, nil)
        var wszPath: [WCHAR] = Array<WCHAR>(repeating: 0, count: Int(cchLength + 1))
        guard GetTempPathW(DWORD(wszPath.count), &wszPath) <= cchLength else {
            preconditionFailure("GetTempPathW mutation race")
        }
        return normalizedPath(with: String(decodingCString: wszPath, as: UTF16.self).standardizingPath)
#else
#if canImport(Darwin)
        // If confstr returns 0 it either failed or the variable had no content
        // If the variable had no content, we should continue on below
        // If it fails, we should also silently ignore the error and continue on below. This API can fail for non-programmer reasons such as the device being out of storage space when libSystem attempts to create the directory
        let length: Int = confstr(_CS_DARWIN_USER_TEMP_DIR, nil, 0)
        if length > 0 {
            let result: String? = withUnsafeTemporaryAllocation(of: UInt8.self, capacity: length) { buffer in
                let finalLength = confstr(_CS_DARWIN_USER_TEMP_DIR, buffer.baseAddress!, buffer.count)
                assert(length == finalLength, "Value of _CS_DARWIN_USER_TEMP_DIR changed?")
                if length > 0 && length < buffer.count {
                    return String(decoding: buffer, as: UTF8.self)
                }
                return nil
            }
            
            if let result {
                return result
            }
        }
#endif
#if !os(WASI)
        if let envValue = Platform.getEnvSecure("TMPDIR") {
            return normalizedPath(with: envValue)
        }
#endif
#if os(Android)
        // Bionic uses /data/local/tmp/ as temporary directory. TMPDIR is rarely
        // defined.
        return "/data/local/tmp/"
#else
        return "/tmp/"
#endif
#endif
    }
    /// Replaces any number of sequential `/`
    /// characters with /
    /// NOTE: Internal so it's testable
    /// - Returns: The replaced String
    internal func _transmutingCompressingSlashes() -> String {
        let input = self.utf8
        guard input.count > 1 else {
            return self
        }

        enum SlashState {
            case initial
            case slash
        }

        return String(unsafeUninitializedCapacity: input.count) { buffer in
            var state = SlashState.initial
            var i = 0
            for v in input {
                switch state {
                case .initial:
                    buffer[i] = v
                    i += 1
                    if v == ._slash {
                        state = .slash
                    }
                case .slash:
                    if v != ._slash {
                        buffer[i] = v
                        i += 1
                        state = .initial
                    }
                }
            }
            return i
        }
    }

    private var _droppingTrailingSlashes: String {
        guard !self.isEmpty else {
            return self
        }
        guard let lastNonSlash = self.lastIndex(where: { $0 != "/"}) else {
            // It's all /'s so just return a single slash
            return "/"
        }
        return String(self[...lastNonSlash])
    }
    
    static var NETWORK_PREFIX: String { #"\\"# }
    
    private var _standardizingPath: String {
        var result = _standardizingSlashes()._transmutingCompressingSlashes()._droppingTrailingSlashes
        let postNetStart = if result.starts(with: String.NETWORK_PREFIX) {
            result.firstIndex(of: "/") ?? result.endIndex
        } else {
            result.startIndex
        }
        let hasDotDot = result[postNetStart...]._hasDotDotComponent()
        if hasDotDot, let resolved = result._resolvingSymlinksInPath() {
            result = resolved
        }

        result = result._removingDotSegments

        // Automounted paths need to be stripped for various flavors of paths
        let exclusionList = ["/Applications", "/Library", "/System", "/Users", "/Volumes", "/bin", "/cores", "/dev", "/opt", "/private", "/sbin", "/usr"]
        for path in ["/private/var/automount", "/var/automount", "/private"] {
            if result.starts(with: "\(path)/") {
                let newPath = String(result.dropFirst(path.count))
                let isExcluded = exclusionList.contains {
                    newPath == $0 || newPath.starts(with: "\($0)/")
                }
                if !isExcluded && FileManager.default.fileExists(atPath: newPath) {
                    result = newPath
                }
                break
            }
        }
        return result
    }

    var standardizingPath: String {
        expandingTildeInPath._standardizingPath
    }
    #endif // !NO_FILESYSTEM
    
    // _NSPathComponents
    var pathComponents: [String] {
        _standardizingSlashes()._pathComponents
    }
    
    private var _pathComponents: [String] {
        var components = self.components(separatedBy: "/").filter { !$0.isEmpty }
        if self.first == "/" {
            components.insert("/", at: 0)
        }
        if self.last == "/" && self.count > 1 {
            components.append("/")
        }
        return components
    }
    
    #if !NO_FILESYSTEM
    var abbreviatingWithTildeInPath: String {
        _standardizingSlashes()._abbreviatingWithTildeInPath
    }
    
    private var _abbreviatingWithTildeInPath: String {
        guard !self.isEmpty && self != "/" else { return self }
        let homeDir = String.homeDirectoryPath()
        guard self.starts(with: homeDir) else { return self }
        let nextIdxInOriginal = self.unicodeScalars.index(self.startIndex, offsetBy: homeDir.unicodeScalars.count)
        guard nextIdxInOriginal == self.endIndex || self[nextIdxInOriginal] == "/" else { return self }
        return "~" + self[nextIdxInOriginal...]
    }
    
    var expandingTildeInPath: String {
        _standardizingSlashes()._expandingTildeInPath
    }
    
    private var _expandingTildeInPath: String {
        guard self.first == "~" else { return self }
        var user: String? = nil
        let firstSlash = self.firstIndex(of: "/") ?? self.endIndex
        let indexAfterTilde = self.index(after: self.startIndex)
        if firstSlash != indexAfterTilde {
            user = String(self[indexAfterTilde ..< firstSlash])
        }
        let userDir = String.homeDirectoryPath(forUser: user)
        return userDir + self[firstSlash...]
    }
    
    private static func _resolvingSymlinksInPathUsingFullPathAttribute(_ fsRep: UnsafePointer<CChar>) -> String? {
        #if canImport(Darwin)
        var attrs = attrlist()
        attrs.bitmapcount = u_short(ATTR_BIT_MAP_COUNT)
        attrs.commonattr = attrgroup_t(ATTR_CMN_FULLPATH)
        var buffer = FullPathAttributes()
        guard getattrlist(fsRep, &attrs, &buffer, MemoryLayout<FullPathAttributes>.size, 0) == 0, buffer.fullPathAttr.attr_length > 0 else {
            return nil
        }
        
        let length = Int(buffer.fullPathAttr.attr_length) // Includes null byte
        return withUnsafePointer(to: buffer.fullPathBuf) { pathPtr in
            pathPtr.withMemoryRebound(to: CChar.self, capacity: length) { ccharPtr in
                String(cString: ccharPtr)
            }
        }
        #else
        return nil
        #endif
    }
    
    func _resolvingSymlinksInPath() -> String? {
        guard !isEmpty else { return nil }

#if os(Windows)
        return try? self.withNTPathRepresentation {
            let hFile: HANDLE = CreateFileW($0, GENERIC_READ, FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE, nil, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, nil)
            if hFile == INVALID_HANDLE_VALUE {
                return nil
            }
            defer { CloseHandle(hFile) }

            let dwLength: DWORD = GetFinalPathNameByHandleW(hFile, nil, 0, VOLUME_NAME_DOS)
            return withUnsafeTemporaryAllocation(of: WCHAR.self, capacity: Int(dwLength)) {
                guard GetFinalPathNameByHandleW(hFile, $0.baseAddress, dwLength, VOLUME_NAME_DOS) == dwLength - 1 else {
                    return nil
                }

                // When using `VOLUME_NAME_DOS`, the returned path uses `\\?\`.
                return String(decodingCString: $0.baseAddress!.advanced(by: 4), as: UTF16.self)
            }
        }
#else
        return self.withFileSystemRepresentation { fsPtr -> String? in
            guard let fsPtr else { return nil }
            // If not using the cache (which may not require hitting the disk at all if it's warm), try getting the full path from getattrlist.
            // If it succeeds, this approach always returns an absolute path starting from the root. Since this function returns relative paths when given a relative path to a relative symlink, dont use this approach unless the path is absolute.
            
            var path = self
            if URL.isAbsolute(standardizing: &path), let resolved = Self._resolvingSymlinksInPathUsingFullPathAttribute(fsPtr) {
                return resolved
            }
            
            return withUnsafeTemporaryAllocation(of: CChar.self, capacity: FileManager.MAX_PATH_SIZE) { buffer -> String? in
                buffer.initialize(repeating: 0)
                defer { buffer.deinitialize() }
                var fullLength = Platform.copyCString(dst: buffer.baseAddress!, src: fsPtr, size: FileManager.MAX_PATH_SIZE) + 1 // Includes null byte
                return withUnsafeTemporaryAllocation(of: CChar.self, capacity: FileManager.MAX_PATH_SIZE) { linkBuffer -> String? in
                    linkBuffer.initialize(repeating: 0)
                    defer { linkBuffer.deinitialize() }
                    var scanLoc = buffer.baseAddress!
                    var links = 0
                    while true {
                        var linkResultLen = 0
                        let lastScanLoc = scanLoc
                        while scanLoc.pointee == 0x2F /* U+002F Solidus (/) */ {
                            scanLoc = scanLoc.advanced(by: 1)
                        }
                        while scanLoc.pointee != 0x2F && scanLoc.pointee != 0 {
                            scanLoc = scanLoc.advanced(by: 1)
                        }
                        let slash = scanLoc.pointee
                        scanLoc.pointee = 0
                        var statBuf = stat()
                        if lstat(buffer.baseAddress!, &statBuf) < 0 {
                            return nil
                        }
                        if statBuf.st_mode & S_IFMT == S_IFLNK {
                            /* Examples:
                             *   fspath == /foo/bar0baz/quux/froboz
                             *   linkx == /tic/tac/toe
                             *   result == /tic/tac/toe/baz/quux/froboz
                             *
                             *   fspath == /foo/bar0baz/quux/froboz
                             *   linkx == tic/tac/toe
                             *   result == /foo/tic/tac/toe/baz/quux/froboz
                             */
                            if links > MAXSYMLINKS {
                                return nil
                            }
                            linkResultLen = readlink(buffer.baseAddress!, linkBuffer.baseAddress!, FileManager.MAX_PATH_SIZE - 1)
                            if linkResultLen < 0 {
                                return nil
                            }
                            linkBuffer[linkResultLen] = 0
                        }
                        
                        scanLoc.pointee = slash
                        if linkResultLen > 0 {
                            links += 1
                            // If the link is not an absolute path, preserve the prefix
                            let preservedPrefixLength = linkBuffer[0] == 0x2F ? 0 : (buffer.baseAddress!.distance(to: lastScanLoc) + 1)
                            
                            let scanLocIdx = buffer.baseAddress!.distance(to: scanLoc)
                            let suffixLength = fullLength - scanLocIdx // Includes null byte
                            if preservedPrefixLength + linkResultLen + suffixLength > FileManager.MAX_PATH_SIZE {
                                return nil
                            }
                            
                            // Shift the suffix + null byte to the correct location
                            let afterSuffixIdx = buffer[(preservedPrefixLength + linkResultLen)...].update(fromContentsOf: buffer[scanLocIdx ..< (fullLength)])
                            
                            // Replace the component with the link
                            _ = buffer[preservedPrefixLength ..< (preservedPrefixLength + linkResultLen)].update(fromContentsOf: linkBuffer[..<linkResultLen])
                            
                            fullLength = afterSuffixIdx
                            scanLoc = linkBuffer[0] == 0x2F ? buffer.baseAddress! : lastScanLoc
                        } else {
                            if scanLoc.pointee == 0 {
                                break
                            }
                        }
                    }
                    return String(cString: buffer.baseAddress!)
                }
            }
        }
#endif
    }
    
    var resolvingSymlinksInPath: String {
        var result = expandingTildeInPath
        if let resolved = result._resolvingSymlinksInPath() {
            result = resolved
        }
        return result._standardizingPath
    }
    #endif // !NO_FILESYSTEM
}

fileprivate enum DotState {
    case initial
    case dot
    case dotDot
    case lookingForSlash
}
extension StringProtocol {
    internal func replacing(_ a: UInt8, with b: UInt8) -> String {
        var utf8Array = Array(self.utf8)
        var didReplace = false
        // ~300x faster than Array.replace([UInt8], with: [UInt8]) for one element
        for i in 0..<utf8Array.count {
            if utf8Array[i] == a {
                utf8Array[i] = b
                didReplace = true
            }
        }
        guard didReplace else {
            return String(self)
        }
        return String(unsafeUninitializedCapacity: utf8Array.count) { buffer in
            buffer.initialize(fromContentsOf: utf8Array)
        }
    }

    // Internal for testing purposes
    internal func _hasDotDotComponent() -> Bool {
        let input = self.utf8
        guard input.count >= 2 else {
            return false
        }

        var state = DotState.initial
        for v in input {
            switch state {
            case .initial:
                if v == ._dot {
                    state = .dot
                } else if v == ._slash {
                    continue
                } else {
                    state = .lookingForSlash
                }
            case .dot:
                if v == ._dot {
                    state = .dotDot
                } else if v == ._slash {
                    state = .initial
                } else {
                    state = .lookingForSlash
                }
            case .dotDot:
                if v == ._slash {
                    return true // Starts with "../"
                } else {
                    state = .lookingForSlash
                }
            case .lookingForSlash:
                if v == ._slash {
                    state = .initial
                } else {
                    continue
                }
            }
        }
        return state == .dotDot
    }
}