moduleSpecifiers.ts

import {
    __String,
    allKeysStartWithDot,
    AmbientModuleDeclaration,
    append,
    arrayFrom,
    CharacterCodes,
    combinePaths,
    compareBooleans,
    compareNumberOfDirectorySeparators,
    comparePaths,
    Comparison,
    CompilerOptions,
    containsIgnoredPath,
    containsPath,
    createGetCanonicalFileName,
    Debug,
    directorySeparator,
    emptyArray,
    endsWith,
    ensurePathIsNonModuleName,
    ensureTrailingDirectorySeparator,
    every,
    ExportAssignment,
    Extension,
    extensionFromPath,
    extensionsNotSupportingExtensionlessResolution,
    fileExtensionIsOneOf,
    FileIncludeKind,
    firstDefined,
    flatMap,
    flatten,
    forEach,
    forEachAncestorDirectory,
    getBaseFileName,
    GetCanonicalFileName,
    getConditions,
    getDirectoryPath,
    getEmitModuleResolutionKind,
    getModeForResolutionAtIndex,
    getModuleNameStringLiteralAt,
    getModuleSpecifierEndingPreference,
    getNodeModulePathParts,
    getNormalizedAbsolutePath,
    getOwnKeys,
    getPackageJsonTypesVersionsPaths,
    getPackageNameFromTypesPackageName,
    getPathsBasePath,
    getRelativePathFromDirectory,
    getRelativePathToDirectoryOrUrl,
    getResolvePackageJsonExports,
    getSourceFileOfModule,
    getSupportedExtensions,
    getTextOfIdentifierOrLiteral,
    hasJSFileExtension,
    hasTSFileExtension,
    hostGetCanonicalFileName,
    Identifier,
    isAmbientModule,
    isApplicableVersionedTypesKey,
    isDeclarationFileName,
    isExternalModuleAugmentation,
    isExternalModuleNameRelative,
    isModuleBlock,
    isModuleDeclaration,
    isNonGlobalAmbientModule,
    isRootedDiskPath,
    isSourceFile,
    isString,
    JsxEmit,
    map,
    mapDefined,
    MapLike,
    matchPatternOrExact,
    min,
    ModuleDeclaration,
    ModuleKind,
    ModulePath,
    ModuleResolutionKind,
    ModuleSpecifierCache,
    ModuleSpecifierEnding,
    ModuleSpecifierOptions,
    ModuleSpecifierResolutionHost,
    NodeFlags,
    NodeModulePathParts,
    normalizePath,
    Path,
    pathContainsNodeModules,
    pathIsBareSpecifier,
    pathIsRelative,
    PropertyAccessExpression,
    removeExtension,
    removeFileExtension,
    removeSuffix,
    removeTrailingDirectorySeparator,
    ResolutionMode,
    resolvePath,
    ScriptKind,
    shouldAllowImportingTsExtension,
    some,
    SourceFile,
    startsWith,
    startsWithDirectory,
    stringContains,
    StringLiteral,
    Symbol,
    SymbolFlags,
    toPath,
    tryGetExtensionFromPath,
    tryParsePatterns,
    TypeChecker,
    UserPreferences,
} from "./_namespaces/ts";

// Used by importFixes, getEditsForFileRename, and declaration emit to synthesize import module specifiers.

const enum RelativePreference { Relative, NonRelative, Shortest, ExternalNonRelative }

// Processed preferences
interface Preferences {
    readonly relativePreference: RelativePreference;
    /**
     * @param syntaxImpliedNodeFormat Used when the import syntax implies ESM or CJS irrespective of the mode of the file.
     */
    getAllowedEndingsInPreferredOrder(syntaxImpliedNodeFormat?: SourceFile["impliedNodeFormat"]): ModuleSpecifierEnding[];
}

function getPreferences(
    { importModuleSpecifierPreference, importModuleSpecifierEnding }: UserPreferences,
    compilerOptions: CompilerOptions,
    importingSourceFile: SourceFile,
    oldImportSpecifier?: string,
): Preferences {
    const preferredEnding = getPreferredEnding();
    return {
        relativePreference:
            oldImportSpecifier !== undefined ? (isExternalModuleNameRelative(oldImportSpecifier) ?
                RelativePreference.Relative :
                RelativePreference.NonRelative) :
            importModuleSpecifierPreference === "relative" ? RelativePreference.Relative :
            importModuleSpecifierPreference === "non-relative" ? RelativePreference.NonRelative :
            importModuleSpecifierPreference === "project-relative" ? RelativePreference.ExternalNonRelative :
            RelativePreference.Shortest,
        getAllowedEndingsInPreferredOrder: syntaxImpliedNodeFormat => {
            if ((syntaxImpliedNodeFormat ?? importingSourceFile.impliedNodeFormat) === ModuleKind.ESNext) {
                if (shouldAllowImportingTsExtension(compilerOptions, importingSourceFile.fileName)) {
                    return [ModuleSpecifierEnding.TsExtension, ModuleSpecifierEnding.JsExtension];
                }
                return [ModuleSpecifierEnding.JsExtension];
            }
            if (getEmitModuleResolutionKind(compilerOptions) === ModuleResolutionKind.Classic) {
                return preferredEnding === ModuleSpecifierEnding.JsExtension
                    ? [ModuleSpecifierEnding.JsExtension, ModuleSpecifierEnding.Index]
                    : [ModuleSpecifierEnding.Index, ModuleSpecifierEnding.JsExtension];
            }
            const allowImportingTsExtension = shouldAllowImportingTsExtension(compilerOptions, importingSourceFile.fileName);
            switch (preferredEnding) {
                case ModuleSpecifierEnding.JsExtension: return allowImportingTsExtension
                    ? [ModuleSpecifierEnding.JsExtension, ModuleSpecifierEnding.TsExtension, ModuleSpecifierEnding.Minimal, ModuleSpecifierEnding.Index]
                    : [ModuleSpecifierEnding.JsExtension, ModuleSpecifierEnding.Minimal, ModuleSpecifierEnding.Index];
                case ModuleSpecifierEnding.TsExtension: return [ModuleSpecifierEnding.TsExtension, ModuleSpecifierEnding.Minimal, ModuleSpecifierEnding.JsExtension, ModuleSpecifierEnding.Index];
                case ModuleSpecifierEnding.Index: return allowImportingTsExtension
                    ? [ModuleSpecifierEnding.Index, ModuleSpecifierEnding.Minimal, ModuleSpecifierEnding.TsExtension, ModuleSpecifierEnding.JsExtension]
                    : [ModuleSpecifierEnding.Index, ModuleSpecifierEnding.Minimal, ModuleSpecifierEnding.JsExtension];
                case ModuleSpecifierEnding.Minimal: return allowImportingTsExtension
                    ? [ModuleSpecifierEnding.Minimal, ModuleSpecifierEnding.Index, ModuleSpecifierEnding.TsExtension, ModuleSpecifierEnding.JsExtension]
                    : [ModuleSpecifierEnding.Minimal, ModuleSpecifierEnding.Index, ModuleSpecifierEnding.JsExtension];
                default: Debug.assertNever(preferredEnding);
            }
        },
    };

    function getPreferredEnding(): ModuleSpecifierEnding {
        if (oldImportSpecifier !== undefined) {
            if (hasJSFileExtension(oldImportSpecifier)) return ModuleSpecifierEnding.JsExtension;
            if (endsWith(oldImportSpecifier, "/index")) return ModuleSpecifierEnding.Index;
        }
        return getModuleSpecifierEndingPreference(
            importModuleSpecifierEnding,
            importingSourceFile.impliedNodeFormat,
            compilerOptions,
            importingSourceFile);
    }
}

// `importingSourceFile` and `importingSourceFileName`? Why not just use `importingSourceFile.path`?
// Because when this is called by the file renamer, `importingSourceFile` is the file being renamed,
// while `importingSourceFileName` its *new* name. We need a source file just to get its
// `impliedNodeFormat` and to detect certain preferences from existing import module specifiers.
/** @internal */
export function updateModuleSpecifier(
    compilerOptions: CompilerOptions,
    importingSourceFile: SourceFile,
    importingSourceFileName: Path,
    toFileName: string,
    host: ModuleSpecifierResolutionHost,
    oldImportSpecifier: string,
    options: ModuleSpecifierOptions = {},
): string | undefined {
    const res = getModuleSpecifierWorker(compilerOptions, importingSourceFile, importingSourceFileName, toFileName, host, getPreferences({}, compilerOptions, importingSourceFile, oldImportSpecifier), {}, options);
    if (res === oldImportSpecifier) return undefined;
    return res;
}

// `importingSourceFile` and `importingSourceFileName`? Why not just use `importingSourceFile.path`?
// Because when this is called by the declaration emitter, `importingSourceFile` is the implementation
// file, but `importingSourceFileName` and `toFileName` refer to declaration files (the former to the
// one currently being produced; the latter to the one being imported). We need an implementation file
// just to get its `impliedNodeFormat` and to detect certain preferences from existing import module
// specifiers.
/** @internal */
export function getModuleSpecifier(
    compilerOptions: CompilerOptions,
    importingSourceFile: SourceFile,
    importingSourceFileName: Path,
    toFileName: string,
    host: ModuleSpecifierResolutionHost,
    options: ModuleSpecifierOptions = {},
): string {
    return getModuleSpecifierWorker(compilerOptions, importingSourceFile, importingSourceFileName, toFileName, host, getPreferences({}, compilerOptions, importingSourceFile), {}, options);
}

/** @internal */
export function getNodeModulesPackageName(
    compilerOptions: CompilerOptions,
    importingSourceFile: SourceFile,
    nodeModulesFileName: string,
    host: ModuleSpecifierResolutionHost,
    preferences: UserPreferences,
    options: ModuleSpecifierOptions = {},
): string | undefined {
    const info = getInfo(importingSourceFile.path, host);
    const modulePaths = getAllModulePaths(importingSourceFile.path, nodeModulesFileName, host, preferences, options);
    return firstDefined(modulePaths,
        modulePath => tryGetModuleNameAsNodeModule(modulePath, info, importingSourceFile, host, compilerOptions, preferences, /*packageNameOnly*/ true, options.overrideImportMode));
}

function getModuleSpecifierWorker(
    compilerOptions: CompilerOptions,
    importingSourceFile: SourceFile,
    importingSourceFileName: Path,
    toFileName: string,
    host: ModuleSpecifierResolutionHost,
    preferences: Preferences,
    userPreferences: UserPreferences,
    options: ModuleSpecifierOptions = {}
): string {
    const info = getInfo(importingSourceFileName, host);
    const modulePaths = getAllModulePaths(importingSourceFileName, toFileName, host, userPreferences, options);
    return firstDefined(modulePaths, modulePath => tryGetModuleNameAsNodeModule(modulePath, info, importingSourceFile, host, compilerOptions, userPreferences, /*packageNameOnly*/ undefined, options.overrideImportMode)) ||
        getLocalModuleSpecifier(toFileName, info, compilerOptions, host, options.overrideImportMode || importingSourceFile.impliedNodeFormat, preferences);
}

/** @internal */
export function tryGetModuleSpecifiersFromCache(
    moduleSymbol: Symbol,
    importingSourceFile: SourceFile,
    host: ModuleSpecifierResolutionHost,
    userPreferences: UserPreferences,
    options: ModuleSpecifierOptions = {},
): readonly string[] | undefined {
    return tryGetModuleSpecifiersFromCacheWorker(
        moduleSymbol,
        importingSourceFile,
        host,
        userPreferences,
        options)[0];
}

function tryGetModuleSpecifiersFromCacheWorker(
    moduleSymbol: Symbol,
    importingSourceFile: SourceFile,
    host: ModuleSpecifierResolutionHost,
    userPreferences: UserPreferences,
    options: ModuleSpecifierOptions = {},
): readonly [specifiers?: readonly string[], moduleFile?: SourceFile, modulePaths?: readonly ModulePath[], cache?: ModuleSpecifierCache] {
    const moduleSourceFile = getSourceFileOfModule(moduleSymbol);
    if (!moduleSourceFile) {
        return emptyArray as [];
    }

    const cache = host.getModuleSpecifierCache?.();
    const cached = cache?.get(importingSourceFile.path, moduleSourceFile.path, userPreferences, options);
    return [cached?.moduleSpecifiers, moduleSourceFile, cached?.modulePaths, cache];
}

/**
 * Returns an import for each symlink and for the realpath.
 *
 * @internal
 */
export function getModuleSpecifiers(
    moduleSymbol: Symbol,
    checker: TypeChecker,
    compilerOptions: CompilerOptions,
    importingSourceFile: SourceFile,
    host: ModuleSpecifierResolutionHost,
    userPreferences: UserPreferences,
    options: ModuleSpecifierOptions = {},
): readonly string[] {
    return getModuleSpecifiersWithCacheInfo(
        moduleSymbol,
        checker,
        compilerOptions,
        importingSourceFile,
        host,
        userPreferences,
        options
    ).moduleSpecifiers;
}

/** @internal */
export function getModuleSpecifiersWithCacheInfo(
    moduleSymbol: Symbol,
    checker: TypeChecker,
    compilerOptions: CompilerOptions,
    importingSourceFile: SourceFile,
    host: ModuleSpecifierResolutionHost,
    userPreferences: UserPreferences,
    options: ModuleSpecifierOptions = {},
): { moduleSpecifiers: readonly string[], computedWithoutCache: boolean } {
    let computedWithoutCache = false;
    const ambient = tryGetModuleNameFromAmbientModule(moduleSymbol, checker);
    if (ambient) return { moduleSpecifiers: [ambient], computedWithoutCache };

    // eslint-disable-next-line prefer-const
    let [specifiers, moduleSourceFile, modulePaths, cache] = tryGetModuleSpecifiersFromCacheWorker(
        moduleSymbol,
        importingSourceFile,
        host,
        userPreferences,
        options
    );
    if (specifiers) return { moduleSpecifiers: specifiers, computedWithoutCache };
    if (!moduleSourceFile) return { moduleSpecifiers: emptyArray, computedWithoutCache };

    computedWithoutCache = true;
    modulePaths ||= getAllModulePathsWorker(importingSourceFile.path, moduleSourceFile.originalFileName, host);
    const result = computeModuleSpecifiers(modulePaths, compilerOptions, importingSourceFile, host, userPreferences, options);
    cache?.set(importingSourceFile.path, moduleSourceFile.path, userPreferences, options, modulePaths, result);
    return { moduleSpecifiers: result, computedWithoutCache };
}

function computeModuleSpecifiers(
    modulePaths: readonly ModulePath[],
    compilerOptions: CompilerOptions,
    importingSourceFile: SourceFile,
    host: ModuleSpecifierResolutionHost,
    userPreferences: UserPreferences,
    options: ModuleSpecifierOptions = {},
): readonly string[] {
    const info = getInfo(importingSourceFile.path, host);
    const preferences = getPreferences(userPreferences, compilerOptions, importingSourceFile);
    const existingSpecifier = forEach(modulePaths, modulePath => forEach(
        host.getFileIncludeReasons().get(toPath(modulePath.path, host.getCurrentDirectory(), info.getCanonicalFileName)),
        reason => {
            if (reason.kind !== FileIncludeKind.Import || reason.file !== importingSourceFile.path) return undefined;
            // If the candidate import mode doesn't match the mode we're generating for, don't consider it
            // TODO: maybe useful to keep around as an alternative option for certain contexts where the mode is overridable
            if (importingSourceFile.impliedNodeFormat && importingSourceFile.impliedNodeFormat !== getModeForResolutionAtIndex(importingSourceFile, reason.index)) return undefined;
            const specifier = getModuleNameStringLiteralAt(importingSourceFile, reason.index).text;
            // If the preference is for non relative and the module specifier is relative, ignore it
            return preferences.relativePreference !== RelativePreference.NonRelative || !pathIsRelative(specifier) ?
                specifier :
                undefined;
        }
    ));
    if (existingSpecifier) {
        const moduleSpecifiers = [existingSpecifier];
        return moduleSpecifiers;
    }

    const importedFileIsInNodeModules = some(modulePaths, p => p.isInNodeModules);

    // Module specifier priority:
    //   1. "Bare package specifiers" (e.g. "@foo/bar") resulting from a path through node_modules to a package.json's "types" entry
    //   2. Specifiers generated using "paths" from tsconfig
    //   3. Non-relative specfiers resulting from a path through node_modules (e.g. "@foo/bar/path/to/file")
    //   4. Relative paths
    let nodeModulesSpecifiers: string[] | undefined;
    let pathsSpecifiers: string[] | undefined;
    let redirectPathsSpecifiers: string[] | undefined;
    let relativeSpecifiers: string[] | undefined;
    for (const modulePath of modulePaths) {
        const specifier = modulePath.isInNodeModules
            ? tryGetModuleNameAsNodeModule(modulePath, info, importingSourceFile, host, compilerOptions, userPreferences, /*packageNameOnly*/ undefined, options.overrideImportMode)
            : undefined;
        nodeModulesSpecifiers = append(nodeModulesSpecifiers, specifier);
        if (specifier && modulePath.isRedirect) {
            // If we got a specifier for a redirect, it was a bare package specifier (e.g. "@foo/bar",
            // not "@foo/bar/path/to/file"). No other specifier will be this good, so stop looking.
            return nodeModulesSpecifiers!;
        }

        if (!specifier) {
            const local = getLocalModuleSpecifier(
                modulePath.path,
                info,
                compilerOptions,
                host,
                options.overrideImportMode || importingSourceFile.impliedNodeFormat,
                preferences,
                /*pathsOnly*/ modulePath.isRedirect,
            );
            if (!local) {
                continue;
            }
            if (modulePath.isRedirect) {
                redirectPathsSpecifiers = append(redirectPathsSpecifiers, local);
            }
            else if (pathIsBareSpecifier(local)) {
                pathsSpecifiers = append(pathsSpecifiers, local);
            }
            else if (!importedFileIsInNodeModules || modulePath.isInNodeModules) {
                // Why this extra conditional, not just an `else`? If some path to the file contained
                // 'node_modules', but we can't create a non-relative specifier (e.g. "@foo/bar/path/to/file"),
                // that means we had to go through a *sibling's* node_modules, not one we can access directly.
                // If some path to the file was in node_modules but another was not, this likely indicates that
                // we have a monorepo structure with symlinks. In this case, the non-node_modules path is
                // probably the realpath, e.g. "../bar/path/to/file", but a relative path to another package
                // in a monorepo is probably not portable. So, the module specifier we actually go with will be
                // the relative path through node_modules, so that the declaration emitter can produce a
                // portability error. (See declarationEmitReexportedSymlinkReference3)
                relativeSpecifiers = append(relativeSpecifiers, local);
            }
        }
    }

    return pathsSpecifiers?.length ? pathsSpecifiers :
        redirectPathsSpecifiers?.length ? redirectPathsSpecifiers :
        nodeModulesSpecifiers?.length ? nodeModulesSpecifiers :
        Debug.checkDefined(relativeSpecifiers);
}

interface Info {
    readonly getCanonicalFileName: GetCanonicalFileName;
    readonly importingSourceFileName: Path
    readonly sourceDirectory: Path;
}
// importingSourceFileName is separate because getEditsForFileRename may need to specify an updated path
function getInfo(importingSourceFileName: Path, host: ModuleSpecifierResolutionHost): Info {
    const getCanonicalFileName = createGetCanonicalFileName(host.useCaseSensitiveFileNames ? host.useCaseSensitiveFileNames() : true);
    const sourceDirectory = getDirectoryPath(importingSourceFileName);
    return { getCanonicalFileName, importingSourceFileName, sourceDirectory };
}

function getLocalModuleSpecifier(moduleFileName: string, info: Info, compilerOptions: CompilerOptions, host: ModuleSpecifierResolutionHost, importMode: ResolutionMode, preferences: Preferences): string;
function getLocalModuleSpecifier(moduleFileName: string, info: Info, compilerOptions: CompilerOptions, host: ModuleSpecifierResolutionHost, importMode: ResolutionMode, preferences: Preferences, pathsOnly?: boolean): string | undefined;
function getLocalModuleSpecifier(moduleFileName: string, info: Info, compilerOptions: CompilerOptions, host: ModuleSpecifierResolutionHost, importMode: ResolutionMode, { getAllowedEndingsInPreferredOrder: getAllowedEndingsInPrefererredOrder, relativePreference }: Preferences, pathsOnly?: boolean): string | undefined {
    const { baseUrl, paths, rootDirs } = compilerOptions;
    if (pathsOnly && !paths) {
        return undefined;
    }

    const { sourceDirectory, getCanonicalFileName } = info;
    const allowedEndings = getAllowedEndingsInPrefererredOrder(importMode);
    const relativePath = rootDirs && tryGetModuleNameFromRootDirs(rootDirs, moduleFileName, sourceDirectory, getCanonicalFileName, allowedEndings, compilerOptions) ||
        processEnding(ensurePathIsNonModuleName(getRelativePathFromDirectory(sourceDirectory, moduleFileName, getCanonicalFileName)), allowedEndings, compilerOptions);
    if (!baseUrl && !paths || relativePreference === RelativePreference.Relative) {
        return pathsOnly ? undefined : relativePath;
    }

    const baseDirectory = getNormalizedAbsolutePath(getPathsBasePath(compilerOptions, host) || baseUrl!, host.getCurrentDirectory());
    const relativeToBaseUrl = getRelativePathIfInSameVolume(moduleFileName, baseDirectory, getCanonicalFileName);
    if (!relativeToBaseUrl) {
        return pathsOnly ? undefined : relativePath;
    }

    const fromPaths = paths && tryGetModuleNameFromPaths(relativeToBaseUrl, paths, allowedEndings, host, compilerOptions);
    if (pathsOnly) {
        return fromPaths;
    }

    const maybeNonRelative = fromPaths === undefined && baseUrl !== undefined ? processEnding(relativeToBaseUrl, allowedEndings, compilerOptions) : fromPaths;
    if (!maybeNonRelative) {
        return relativePath;
    }

    if (relativePreference === RelativePreference.NonRelative && !pathIsRelative(maybeNonRelative)) {
        return maybeNonRelative;
    }

    if (relativePreference === RelativePreference.ExternalNonRelative && !pathIsRelative(maybeNonRelative)) {
        const projectDirectory = compilerOptions.configFilePath ?
            toPath(getDirectoryPath(compilerOptions.configFilePath), host.getCurrentDirectory(), info.getCanonicalFileName) :
            info.getCanonicalFileName(host.getCurrentDirectory());
        const modulePath = toPath(moduleFileName, projectDirectory, getCanonicalFileName);
        const sourceIsInternal = startsWith(sourceDirectory, projectDirectory);
        const targetIsInternal = startsWith(modulePath, projectDirectory);
        if (sourceIsInternal && !targetIsInternal || !sourceIsInternal && targetIsInternal) {
            // 1. The import path crosses the boundary of the tsconfig.json-containing directory.
            //
            //      src/
            //        tsconfig.json
            //        index.ts -------
            //      lib/              | (path crosses tsconfig.json)
            //        imported.ts <---
            //
            return maybeNonRelative;
        }

        const nearestTargetPackageJson = getNearestAncestorDirectoryWithPackageJson(host, getDirectoryPath(modulePath));
        const nearestSourcePackageJson = getNearestAncestorDirectoryWithPackageJson(host, sourceDirectory);
        if (nearestSourcePackageJson !== nearestTargetPackageJson) {
            // 2. The importing and imported files are part of different packages.
            //
            //      packages/a/
            //        package.json
            //        index.ts --------
            //      packages/b/        | (path crosses package.json)
            //        package.json     |
            //        component.ts <---
            //
            return maybeNonRelative;
        }

        return relativePath;
    }

    // Prefer a relative import over a baseUrl import if it has fewer components.
    return isPathRelativeToParent(maybeNonRelative) || countPathComponents(relativePath) < countPathComponents(maybeNonRelative) ? relativePath : maybeNonRelative;
}

/** @internal */
export function countPathComponents(path: string): number {
    let count = 0;
    for (let i = startsWith(path, "./") ? 2 : 0; i < path.length; i++) {
        if (path.charCodeAt(i) === CharacterCodes.slash) count++;
    }
    return count;
}

function comparePathsByRedirectAndNumberOfDirectorySeparators(a: ModulePath, b: ModulePath) {
    return compareBooleans(b.isRedirect, a.isRedirect) || compareNumberOfDirectorySeparators(a.path, b.path);
}

function getNearestAncestorDirectoryWithPackageJson(host: ModuleSpecifierResolutionHost, fileName: string) {
    if (host.getNearestAncestorDirectoryWithPackageJson) {
        return host.getNearestAncestorDirectoryWithPackageJson(fileName);
    }
    return !!forEachAncestorDirectory(fileName, directory => {
        return host.fileExists(combinePaths(directory, "package.json")) ? true : undefined;
    });
}

/** @internal */
export function forEachFileNameOfModule<T>(
    importingFileName: string,
    importedFileName: string,
    host: ModuleSpecifierResolutionHost,
    preferSymlinks: boolean,
    cb: (fileName: string, isRedirect: boolean) => T | undefined
): T | undefined {
    const getCanonicalFileName = hostGetCanonicalFileName(host);
    const cwd = host.getCurrentDirectory();
    const referenceRedirect = host.isSourceOfProjectReferenceRedirect(importedFileName) ? host.getProjectReferenceRedirect(importedFileName) : undefined;
    const importedPath = toPath(importedFileName, cwd, getCanonicalFileName);
    const redirects = host.redirectTargetsMap.get(importedPath) || emptyArray;
    const importedFileNames = [...(referenceRedirect ? [referenceRedirect] : emptyArray), importedFileName, ...redirects];
    const targets = importedFileNames.map(f => getNormalizedAbsolutePath(f, cwd));
    let shouldFilterIgnoredPaths = !every(targets, containsIgnoredPath);

    if (!preferSymlinks) {
        // Symlinks inside ignored paths are already filtered out of the symlink cache,
        // so we only need to remove them from the realpath filenames.
        const result = forEach(targets, p => !(shouldFilterIgnoredPaths && containsIgnoredPath(p)) && cb(p, referenceRedirect === p));
        if (result) return result;
    }

    const symlinkedDirectories = host.getSymlinkCache?.().getSymlinkedDirectoriesByRealpath();
    const fullImportedFileName = getNormalizedAbsolutePath(importedFileName, cwd);
    const result = symlinkedDirectories && forEachAncestorDirectory(getDirectoryPath(fullImportedFileName), realPathDirectory => {
        const symlinkDirectories = symlinkedDirectories.get(ensureTrailingDirectorySeparator(toPath(realPathDirectory, cwd, getCanonicalFileName)));
        if (!symlinkDirectories) return undefined; // Continue to ancestor directory

        // Don't want to a package to globally import from itself (importNameCodeFix_symlink_own_package.ts)
        if (startsWithDirectory(importingFileName, realPathDirectory, getCanonicalFileName)) {
            return false; // Stop search, each ancestor directory will also hit this condition
        }

        return forEach(targets, target => {
            if (!startsWithDirectory(target, realPathDirectory, getCanonicalFileName)) {
                return;
            }

            const relative = getRelativePathFromDirectory(realPathDirectory, target, getCanonicalFileName);
            for (const symlinkDirectory of symlinkDirectories) {
                const option = resolvePath(symlinkDirectory, relative);
                const result = cb(option, target === referenceRedirect);
                shouldFilterIgnoredPaths = true; // We found a non-ignored path in symlinks, so we can reject ignored-path realpaths
                if (result) return result;
            }
        });
    });
    return result || (preferSymlinks
        ? forEach(targets, p => shouldFilterIgnoredPaths && containsIgnoredPath(p) ? undefined : cb(p, p === referenceRedirect))
        : undefined);
}

/**
 * Looks for existing imports that use symlinks to this module.
 * Symlinks will be returned first so they are preferred over the real path.
 */
function getAllModulePaths(
    importingFilePath: Path,
    importedFileName: string,
    host: ModuleSpecifierResolutionHost,
    preferences: UserPreferences,
    options: ModuleSpecifierOptions = {},
) {
    const importedFilePath = toPath(importedFileName, host.getCurrentDirectory(), hostGetCanonicalFileName(host));
    const cache = host.getModuleSpecifierCache?.();
    if (cache) {
        const cached = cache.get(importingFilePath, importedFilePath, preferences, options);
        if (cached?.modulePaths) return cached.modulePaths;
    }
    const modulePaths = getAllModulePathsWorker(importingFilePath, importedFileName, host);
    if (cache) {
        cache.setModulePaths(importingFilePath, importedFilePath, preferences, options, modulePaths);
    }
    return modulePaths;
}

function getAllModulePathsWorker(importingFileName: Path, importedFileName: string, host: ModuleSpecifierResolutionHost): readonly ModulePath[] {
    const getCanonicalFileName = hostGetCanonicalFileName(host);
    const allFileNames = new Map<string, { path: string, isRedirect: boolean, isInNodeModules: boolean }>();
    let importedFileFromNodeModules = false;
    forEachFileNameOfModule(
        importingFileName,
        importedFileName,
        host,
        /*preferSymlinks*/ true,
        (path, isRedirect) => {
            const isInNodeModules = pathContainsNodeModules(path);
            allFileNames.set(path, { path: getCanonicalFileName(path), isRedirect, isInNodeModules });
            importedFileFromNodeModules = importedFileFromNodeModules || isInNodeModules;
            // don't return value, so we collect everything
        }
    );

    // Sort by paths closest to importing file Name directory
    const sortedPaths: ModulePath[] = [];
    for (
        let directory = getDirectoryPath(importingFileName);
        allFileNames.size !== 0;
    ) {
        const directoryStart = ensureTrailingDirectorySeparator(directory);
        let pathsInDirectory: ModulePath[] | undefined;
        allFileNames.forEach(({ path, isRedirect, isInNodeModules }, fileName) => {
            if (startsWith(path, directoryStart)) {
                (pathsInDirectory ||= []).push({ path: fileName, isRedirect, isInNodeModules });
                allFileNames.delete(fileName);
            }
        });
        if (pathsInDirectory) {
            if (pathsInDirectory.length > 1) {
                pathsInDirectory.sort(comparePathsByRedirectAndNumberOfDirectorySeparators);
            }
            sortedPaths.push(...pathsInDirectory);
        }
        const newDirectory = getDirectoryPath(directory);
        if (newDirectory === directory) break;
        directory = newDirectory;
    }
    if (allFileNames.size) {
        const remainingPaths = arrayFrom(allFileNames.values());
        if (remainingPaths.length > 1) remainingPaths.sort(comparePathsByRedirectAndNumberOfDirectorySeparators);
        sortedPaths.push(...remainingPaths);
    }

    return sortedPaths;
}

function tryGetModuleNameFromAmbientModule(moduleSymbol: Symbol, checker: TypeChecker): string | undefined {
    const decl = moduleSymbol.declarations?.find(
        d => isNonGlobalAmbientModule(d) && (!isExternalModuleAugmentation(d) || !isExternalModuleNameRelative(getTextOfIdentifierOrLiteral(d.name)))
    ) as (ModuleDeclaration & { name: StringLiteral }) | undefined;
    if (decl) {
        return decl.name.text;
    }

    // the module could be a namespace, which is export through "export=" from an ambient module.
    /**
     * declare module "m" {
     *     namespace ns {
     *         class c {}
     *     }
     *     export = ns;
     * }
     */
    // `import {c} from "m";` is valid, in which case, `moduleSymbol` is "ns", but the module name should be "m"
    const ambientModuleDeclareCandidates = mapDefined(moduleSymbol.declarations,
        d => {
            if (!isModuleDeclaration(d)) return;
            const topNamespace = getTopNamespace(d);
            if (!(topNamespace?.parent?.parent
                && isModuleBlock(topNamespace.parent) && isAmbientModule(topNamespace.parent.parent) && isSourceFile(topNamespace.parent.parent.parent))) return;
            const exportAssignment = ((topNamespace.parent.parent.symbol.exports?.get("export=" as __String)?.valueDeclaration as ExportAssignment)?.expression as PropertyAccessExpression | Identifier);
            if (!exportAssignment) return;
            const exportSymbol = checker.getSymbolAtLocation(exportAssignment);
            if (!exportSymbol) return;
            const originalExportSymbol = exportSymbol?.flags & SymbolFlags.Alias ? checker.getAliasedSymbol(exportSymbol) : exportSymbol;
            if (originalExportSymbol === d.symbol) return topNamespace.parent.parent;

            function getTopNamespace(namespaceDeclaration: ModuleDeclaration) {
                while (namespaceDeclaration.flags & NodeFlags.NestedNamespace) {
                    namespaceDeclaration = namespaceDeclaration.parent as ModuleDeclaration;
                }
                return namespaceDeclaration;
            }
        }
    );
    const ambientModuleDeclare = ambientModuleDeclareCandidates[0] as (AmbientModuleDeclaration & { name: StringLiteral }) | undefined;
    if (ambientModuleDeclare) {
        return ambientModuleDeclare.name.text;
    }
}

function tryGetModuleNameFromPaths(relativeToBaseUrl: string, paths: MapLike<readonly string[]>, allowedEndings: ModuleSpecifierEnding[], host: ModuleSpecifierResolutionHost, compilerOptions: CompilerOptions): string | undefined {
    for (const key in paths) {
        for (const patternText of paths[key]) {
            const pattern = normalizePath(patternText);
            const indexOfStar = pattern.indexOf("*");
            // In module resolution, if `pattern` itself has an extension, a file with that extension is looked up directly,
            // meaning a '.ts' or '.d.ts' extension is allowed to resolve. This is distinct from the case where a '*' substitution
            // causes a module specifier to have an extension, i.e. the extension comes from the module specifier in a JS/TS file
            // and matches the '*'. For example:
            //
            // Module Specifier      | Path Mapping (key: [pattern]) | Interpolation       | Resolution Action
            // ---------------------->------------------------------->--------------------->---------------------------------------------------------------
            // import "@app/foo"    -> "@app/*": ["./src/app/*.ts"] -> "./src/app/foo.ts" -> tryFile("./src/app/foo.ts") || [continue resolution algorithm]
            // import "@app/foo.ts" -> "@app/*": ["./src/app/*"]    -> "./src/app/foo.ts" -> [continue resolution algorithm]
            //
            // (https://github.com/microsoft/TypeScript/blob/ad4ded80e1d58f0bf36ac16bea71bc10d9f09895/src/compiler/moduleNameResolver.ts#L2509-L2516)
            //
            // The interpolation produced by both scenarios is identical, but only in the former, where the extension is encoded in
            // the path mapping rather than in the module specifier, will we prioritize a file lookup on the interpolation result.
            // (In fact, currently, the latter scenario will necessarily fail since no resolution mode recognizes '.ts' as a valid
            // extension for a module specifier.)
            //
            // Here, this means we need to be careful about whether we generate a match from the target filename (typically with a
            // .ts extension) or the possible relative module specifiers representing that file:
            //
            // Filename            | Relative Module Specifier Candidates         | Path Mapping                 | Filename Result    | Module Specifier Results
            // --------------------<----------------------------------------------<------------------------------<-------------------||----------------------------
            // dist/haha.d.ts      <- dist/haha, dist/haha.js                     <- "@app/*": ["./dist/*.d.ts"] <- @app/haha        || (none)
            // dist/haha.d.ts      <- dist/haha, dist/haha.js                     <- "@app/*": ["./dist/*"]      <- (none)           || @app/haha, @app/haha.js
            // dist/foo/index.d.ts <- dist/foo, dist/foo/index, dist/foo/index.js <- "@app/*": ["./dist/*.d.ts"] <- @app/foo/index   || (none)
            // dist/foo/index.d.ts <- dist/foo, dist/foo/index, dist/foo/index.js <- "@app/*": ["./dist/*"]      <- (none)           || @app/foo, @app/foo/index, @app/foo/index.js
            // dist/wow.js.js      <- dist/wow.js, dist/wow.js.js                 <- "@app/*": ["./dist/*.js"]   <- @app/wow.js      || @app/wow, @app/wow.js
            //
            // The "Filename Result" can be generated only if `pattern` has an extension. Care must be taken that the list of
            // relative module specifiers to run the interpolation (a) is actually valid for the module resolution mode, (b) takes
            // into account the existence of other files (e.g. 'dist/wow.js' cannot refer to 'dist/wow.js.js' if 'dist/wow.js'
            // exists) and (c) that they are ordered by preference. The last row shows that the filename result and module
            // specifier results are not mutually exclusive. Note that the filename result is a higher priority in module
            // resolution, but as long criteria (b) above is met, I don't think its result needs to be the highest priority result
            // in module specifier generation. I have included it last, as it's difficult to tell exactly where it should be
            // sorted among the others for a particular value of `importModuleSpecifierEnding`.
            const candidates: { ending: ModuleSpecifierEnding | undefined, value: string }[] = allowedEndings.map(ending => ({
                ending,
                value: processEnding(relativeToBaseUrl, [ending], compilerOptions)
            }));
            if (tryGetExtensionFromPath(pattern)) {
                candidates.push({ ending: undefined, value: relativeToBaseUrl });
            }

            if (indexOfStar !== -1) {
                const prefix = pattern.substring(0, indexOfStar);
                const suffix = pattern.substring(indexOfStar + 1);
                for (const { ending, value } of candidates) {
                    if (value.length >= prefix.length + suffix.length &&
                        startsWith(value, prefix) &&
                        endsWith(value, suffix) &&
                        validateEnding({ ending, value })
                    ) {
                        const matchedStar = value.substring(prefix.length, value.length - suffix.length);
                        return pathIsRelative(matchedStar) ? undefined : key.replace("*", matchedStar);
                    }
                }
            }
            else if (
                some(candidates, c => c.ending !== ModuleSpecifierEnding.Minimal && pattern === c.value) ||
                some(candidates, c => c.ending === ModuleSpecifierEnding.Minimal && pattern === c.value && validateEnding(c))
            ) {
                return key;
            }
        }
    }

    function validateEnding({ ending, value }: { ending: ModuleSpecifierEnding | undefined, value: string }) {
        // Optimization: `removeExtensionAndIndexPostFix` can query the file system (a good bit) if `ending` is `Minimal`, the basename
        // is 'index', and a `host` is provided. To avoid that until it's unavoidable, we ran the function with no `host` above. Only
        // here, after we've checked that the minimal ending is indeed a match (via the length and prefix/suffix checks / `some` calls),
        // do we check that the host-validated result is consistent with the answer we got before. If it's not, it falls back to the
        // `ModuleSpecifierEnding.Index` result, which should already be in the list of candidates if `Minimal` was. (Note: the assumption here is
        // that every module resolution mode that supports dropping extensions also supports dropping `/index`. Like literally
        // everything else in this file, this logic needs to be updated if that's not true in some future module resolution mode.)
        return ending !== ModuleSpecifierEnding.Minimal || value === processEnding(relativeToBaseUrl, [ending], compilerOptions, host);
    }
}

const enum MatchingMode {
    Exact,
    Directory,
    Pattern
}

function tryGetModuleNameFromExports(options: CompilerOptions, targetFilePath: string, packageDirectory: string, packageName: string, exports: unknown, conditions: string[], mode = MatchingMode.Exact): { moduleFileToTry: string } | undefined {
    if (typeof exports === "string") {
        const pathOrPattern = getNormalizedAbsolutePath(combinePaths(packageDirectory, exports), /*currentDirectory*/ undefined);
        const extensionSwappedTarget = hasTSFileExtension(targetFilePath) ? removeFileExtension(targetFilePath) + tryGetJSExtensionForFile(targetFilePath, options) : undefined;
        switch (mode) {
            case MatchingMode.Exact:
                if (comparePaths(targetFilePath, pathOrPattern) === Comparison.EqualTo || (extensionSwappedTarget && comparePaths(extensionSwappedTarget, pathOrPattern) === Comparison.EqualTo)) {
                    return { moduleFileToTry: packageName };
                }
                break;
            case MatchingMode.Directory:
                if (containsPath(pathOrPattern, targetFilePath)) {
                    const fragment = getRelativePathFromDirectory(pathOrPattern, targetFilePath, /*ignoreCase*/ false);
                    return { moduleFileToTry: getNormalizedAbsolutePath(combinePaths(combinePaths(packageName, exports), fragment), /*currentDirectory*/ undefined) };
                }
                break;
            case MatchingMode.Pattern:
                const starPos = pathOrPattern.indexOf("*");
                const leadingSlice = pathOrPattern.slice(0, starPos);
                const trailingSlice = pathOrPattern.slice(starPos + 1);
                if (startsWith(targetFilePath, leadingSlice) && endsWith(targetFilePath, trailingSlice)) {
                    const starReplacement = targetFilePath.slice(leadingSlice.length, targetFilePath.length - trailingSlice.length);
                    return { moduleFileToTry: packageName.replace("*", starReplacement) };
                }
                if (extensionSwappedTarget && startsWith(extensionSwappedTarget, leadingSlice) && endsWith(extensionSwappedTarget, trailingSlice)) {
                    const starReplacement = extensionSwappedTarget.slice(leadingSlice.length, extensionSwappedTarget.length - trailingSlice.length);
                    return { moduleFileToTry: packageName.replace("*", starReplacement) };
                }
                break;
        }
    }
    else if (Array.isArray(exports)) {
        return forEach(exports, e => tryGetModuleNameFromExports(options, targetFilePath, packageDirectory, packageName, e, conditions));
    }
    else if (typeof exports === "object" && exports !== null) { // eslint-disable-line no-null/no-null
        if (allKeysStartWithDot(exports as MapLike<unknown>)) {
            // sub-mappings
            // 3 cases:
            // * directory mappings (legacyish, key ends with / (technically allows index/extension resolution under cjs mode))
            // * pattern mappings (contains a *)
            // * exact mappings (no *, does not end with /)
            return forEach(getOwnKeys(exports as MapLike<unknown>), k => {
                const subPackageName = getNormalizedAbsolutePath(combinePaths(packageName, k), /*currentDirectory*/ undefined);
                const mode = endsWith(k, "/") ? MatchingMode.Directory
                    : stringContains(k, "*") ? MatchingMode.Pattern
                    : MatchingMode.Exact;
                return tryGetModuleNameFromExports(options, targetFilePath, packageDirectory, subPackageName, (exports as MapLike<unknown>)[k], conditions, mode);
            });
        }
        else {
            // conditional mapping
            for (const key of getOwnKeys(exports as MapLike<unknown>)) {
                if (key === "default" || conditions.indexOf(key) >= 0 || isApplicableVersionedTypesKey(conditions, key)) {
                    const subTarget = (exports as MapLike<unknown>)[key];
                    const result = tryGetModuleNameFromExports(options, targetFilePath, packageDirectory, packageName, subTarget, conditions);
                    if (result) {
                        return result;
                    }
                }
            }
        }
    }
    return undefined;
}

function tryGetModuleNameFromRootDirs(rootDirs: readonly string[], moduleFileName: string, sourceDirectory: string, getCanonicalFileName: (file: string) => string, allowedEndings: readonly ModuleSpecifierEnding[], compilerOptions: CompilerOptions): string | undefined {
    const normalizedTargetPaths = getPathsRelativeToRootDirs(moduleFileName, rootDirs, getCanonicalFileName);
    if (normalizedTargetPaths === undefined) {
        return undefined;
    }

    const normalizedSourcePaths = getPathsRelativeToRootDirs(sourceDirectory, rootDirs, getCanonicalFileName);
    const relativePaths = flatMap(normalizedSourcePaths, sourcePath => {
        return map(normalizedTargetPaths, targetPath => ensurePathIsNonModuleName(getRelativePathFromDirectory(sourcePath, targetPath, getCanonicalFileName)));
    });
    const shortest = min(relativePaths, compareNumberOfDirectorySeparators);
    if (!shortest) {
        return undefined;
    }
    return processEnding(shortest, allowedEndings, compilerOptions);
}

function tryGetModuleNameAsNodeModule({ path, isRedirect }: ModulePath, { getCanonicalFileName, sourceDirectory }: Info, importingSourceFile: SourceFile, host: ModuleSpecifierResolutionHost, options: CompilerOptions, userPreferences: UserPreferences, packageNameOnly?: boolean, overrideMode?: ResolutionMode): string | undefined {
    if (!host.fileExists || !host.readFile) {
        return undefined;
    }
    const parts: NodeModulePathParts = getNodeModulePathParts(path)!;
    if (!parts) {
        return undefined;
    }

    // Simplify the full file path to something that can be resolved by Node.

    const preferences = getPreferences(userPreferences, options, importingSourceFile);
    const allowedEndings = preferences.getAllowedEndingsInPreferredOrder();
    let moduleSpecifier = path;
    let isPackageRootPath = false;
    if (!packageNameOnly) {
        let packageRootIndex = parts.packageRootIndex;
        let moduleFileName: string | undefined;
        while (true) {
            // If the module could be imported by a directory name, use that directory's name
            const { moduleFileToTry, packageRootPath, blockedByExports, verbatimFromExports } = tryDirectoryWithPackageJson(packageRootIndex);
            if (getEmitModuleResolutionKind(options) !== ModuleResolutionKind.Classic) {
                if (blockedByExports) {
                    return undefined; // File is under this package.json, but is not publicly exported - there's no way to name it via `node_modules` resolution
                }
                if (verbatimFromExports) {
                    return moduleFileToTry;
                }
            }
            if (packageRootPath) {
                moduleSpecifier = packageRootPath;
                isPackageRootPath = true;
                break;
            }
            if (!moduleFileName) moduleFileName = moduleFileToTry;

            // try with next level of directory
            packageRootIndex = path.indexOf(directorySeparator, packageRootIndex + 1);
            if (packageRootIndex === -1) {
                moduleSpecifier = processEnding(moduleFileName, allowedEndings, options, host);
                break;
            }
        }
    }

    if (isRedirect && !isPackageRootPath) {
        return undefined;
    }

    const globalTypingsCacheLocation = host.getGlobalTypingsCacheLocation && host.getGlobalTypingsCacheLocation();
    // Get a path that's relative to node_modules or the importing file's path
    // if node_modules folder is in this folder or any of its parent folders, no need to keep it.
    const pathToTopLevelNodeModules = getCanonicalFileName(moduleSpecifier.substring(0, parts.topLevelNodeModulesIndex));
    if (!(startsWith(sourceDirectory, pathToTopLevelNodeModules) || globalTypingsCacheLocation && startsWith(getCanonicalFileName(globalTypingsCacheLocation), pathToTopLevelNodeModules))) {
        return undefined;
    }

    // If the module was found in @types, get the actual Node package name
    const nodeModulesDirectoryName = moduleSpecifier.substring(parts.topLevelPackageNameIndex + 1);
    const packageName = getPackageNameFromTypesPackageName(nodeModulesDirectoryName);
    // For classic resolution, only allow importing from node_modules/@types, not other node_modules
    return getEmitModuleResolutionKind(options) === ModuleResolutionKind.Classic && packageName === nodeModulesDirectoryName ? undefined : packageName;

    function tryDirectoryWithPackageJson(packageRootIndex: number): { moduleFileToTry: string, packageRootPath?: string, blockedByExports?: true, verbatimFromExports?: true } {
        const packageRootPath = path.substring(0, packageRootIndex);
        const packageJsonPath = combinePaths(packageRootPath, "package.json");
        let moduleFileToTry = path;
        let maybeBlockedByTypesVersions = false;
        const cachedPackageJson = host.getPackageJsonInfoCache?.()?.getPackageJsonInfo(packageJsonPath);
        if (typeof cachedPackageJson === "object" || cachedPackageJson === undefined && host.fileExists(packageJsonPath)) {
            const packageJsonContent = cachedPackageJson?.contents.packageJsonContent || JSON.parse(host.readFile!(packageJsonPath)!);
            const importMode = overrideMode || importingSourceFile.impliedNodeFormat;
            if (getResolvePackageJsonExports(options)) {
                // The package name that we found in node_modules could be different from the package
                // name in the package.json content via url/filepath dependency specifiers. We need to
                // use the actual directory name, so don't look at `packageJsonContent.name` here.
                const nodeModulesDirectoryName = packageRootPath.substring(parts.topLevelPackageNameIndex + 1);
                const packageName = getPackageNameFromTypesPackageName(nodeModulesDirectoryName);
                const conditions = getConditions(options, importMode === ModuleKind.ESNext);
                const fromExports = packageJsonContent.exports
                    ? tryGetModuleNameFromExports(options, path, packageRootPath, packageName, packageJsonContent.exports, conditions)
                    : undefined;
                if (fromExports) {
                    const withJsExtension = !hasTSFileExtension(fromExports.moduleFileToTry)
                        ? fromExports
                        : { moduleFileToTry: removeFileExtension(fromExports.moduleFileToTry) + tryGetJSExtensionForFile(fromExports.moduleFileToTry, options) };
                    return { ...withJsExtension, verbatimFromExports: true };
                }
                if (packageJsonContent.exports) {
                    return { moduleFileToTry: path, blockedByExports: true };
                }
            }
            const versionPaths = packageJsonContent.typesVersions
                ? getPackageJsonTypesVersionsPaths(packageJsonContent.typesVersions)
                : undefined;
            if (versionPaths) {
                const subModuleName = path.slice(packageRootPath.length + 1);
                const fromPaths = tryGetModuleNameFromPaths(
                    subModuleName,
                    versionPaths.paths,
                    allowedEndings,
                    host,
                    options
                );
                if (fromPaths === undefined) {
                    maybeBlockedByTypesVersions = true;
                }
                else {
                    moduleFileToTry = combinePaths(packageRootPath, fromPaths);
                }
            }
            // If the file is the main module, it can be imported by the package name
            const mainFileRelative = packageJsonContent.typings || packageJsonContent.types || packageJsonContent.main || "index.js";
            if (isString(mainFileRelative) && !(maybeBlockedByTypesVersions && matchPatternOrExact(tryParsePatterns(versionPaths!.paths), mainFileRelative))) {
                // The 'main' file is also subject to mapping through typesVersions, and we couldn't come up with a path
                // explicitly through typesVersions, so if it matches a key in typesVersions now, it's not reachable.
                // (The only way this can happen is if some file in a package that's not resolvable from outside the
                // package got pulled into the program anyway, e.g. transitively through a file that *is* reachable. It
                // happens very easily in fourslash tests though, since every test file listed gets included. See
                // importNameCodeFix_typesVersions.ts for an example.)
                const mainExportFile = toPath(mainFileRelative, packageRootPath, getCanonicalFileName);
                const canonicalModuleFileToTry = getCanonicalFileName(moduleFileToTry);
                if (removeFileExtension(mainExportFile) === removeFileExtension(canonicalModuleFileToTry)) {
                    // ^ An arbitrary removal of file extension for this comparison is almost certainly wrong
                    return { packageRootPath, moduleFileToTry };
                }
                else if (
                    packageJsonContent.type !== "module" &&
                    !fileExtensionIsOneOf(canonicalModuleFileToTry, extensionsNotSupportingExtensionlessResolution) &&
                    startsWith(canonicalModuleFileToTry, mainExportFile) &&
                    getDirectoryPath(canonicalModuleFileToTry) === removeTrailingDirectorySeparator(mainExportFile) &&
                    removeFileExtension(getBaseFileName(canonicalModuleFileToTry)) === "index"
                ) {
                    // if mainExportFile is a directory, which contains moduleFileToTry, we just try index file
                    // example mainExportFile: `pkg/lib` and moduleFileToTry: `pkg/lib/index`, we can use packageRootPath
                    // but this behavior is deprecated for packages with "type": "module", so we only do this for packages without "type": "module"
                    // and make sure that the extension on index.{???} is something that supports omitting the extension
                    return { packageRootPath, moduleFileToTry };
                }
            }
        }
        else {
            // No package.json exists; an index.js will still resolve as the package name
            const fileName = getCanonicalFileName(moduleFileToTry.substring(parts.packageRootIndex + 1));
            if (fileName === "index.d.ts" || fileName === "index.js" || fileName === "index.ts" || fileName === "index.tsx") {
                return { moduleFileToTry, packageRootPath };
            }
        }
        return { moduleFileToTry };
    }
}

function tryGetAnyFileFromPath(host: ModuleSpecifierResolutionHost, path: string) {
    if (!host.fileExists) return;
    // We check all js, `node` and `json` extensions in addition to TS, since node module resolution would also choose those over the directory
    const extensions = flatten(getSupportedExtensions({ allowJs: true }, [{ extension: "node", isMixedContent: false }, { extension: "json", isMixedContent: false, scriptKind: ScriptKind.JSON }]));
    for (const e of extensions) {
        const fullPath = path + e;
        if (host.fileExists(fullPath)) {
            return fullPath;
        }
    }
}

function getPathsRelativeToRootDirs(path: string, rootDirs: readonly string[], getCanonicalFileName: GetCanonicalFileName): string[] | undefined {
    return mapDefined(rootDirs, rootDir => {
        const relativePath = getRelativePathIfInSameVolume(path, rootDir, getCanonicalFileName);
        return relativePath !== undefined && isPathRelativeToParent(relativePath) ? undefined : relativePath;
    });
}

function processEnding(fileName: string, allowedEndings: readonly ModuleSpecifierEnding[], options: CompilerOptions, host?: ModuleSpecifierResolutionHost): string {
    if (fileExtensionIsOneOf(fileName, [Extension.Json, Extension.Mjs, Extension.Cjs])) {
        return fileName;
    }

    const noExtension = removeFileExtension(fileName);
    if (fileName === noExtension) {
        return fileName;
    }

    const jsPriority = allowedEndings.indexOf(ModuleSpecifierEnding.JsExtension);
    const tsPriority = allowedEndings.indexOf(ModuleSpecifierEnding.TsExtension);
    if (fileExtensionIsOneOf(fileName, [Extension.Mts, Extension.Cts]) && tsPriority !== -1 && tsPriority < jsPriority) {
        return fileName;
    }
    else if (fileExtensionIsOneOf(fileName, [Extension.Dmts, Extension.Mts, Extension.Dcts, Extension.Cts])) {
        return noExtension + getJSExtensionForFile(fileName, options);
    }
    else if (!fileExtensionIsOneOf(fileName, [Extension.Dts]) && fileExtensionIsOneOf(fileName, [Extension.Ts]) && stringContains(fileName, ".d.")) {
        // `foo.d.json.ts` and the like - remap back to `foo.json`
        return tryGetRealFileNameForNonJsDeclarationFileName(fileName)!;
    }

    switch (allowedEndings[0]) {
        case ModuleSpecifierEnding.Minimal:
            const withoutIndex = removeSuffix(noExtension, "/index");
            if (host && withoutIndex !== noExtension && tryGetAnyFileFromPath(host, withoutIndex)) {
                // Can't remove index if there's a file by the same name as the directory.
                // Probably more callers should pass `host` so we can determine this?
                return noExtension;
            }
            return withoutIndex;
        case ModuleSpecifierEnding.Index:
            return noExtension;
        case ModuleSpecifierEnding.JsExtension:
            return noExtension + getJSExtensionForFile(fileName, options);
        case ModuleSpecifierEnding.TsExtension:
            // For now, we don't know if this import is going to be type-only, which means we don't
            // know if a .d.ts extension is valid, so use no extension or a .js extension
            if (isDeclarationFileName(fileName)) {
                const extensionlessPriority = allowedEndings.findIndex(e => e === ModuleSpecifierEnding.Minimal || e === ModuleSpecifierEnding.Index);
                return extensionlessPriority !== -1 && extensionlessPriority < jsPriority
                    ? noExtension
                    : noExtension + getJSExtensionForFile(fileName, options);
            }
            return fileName;
        default:
            return Debug.assertNever(allowedEndings[0]);
    }
}

/** @internal */
export function tryGetRealFileNameForNonJsDeclarationFileName(fileName: string) {
    const baseName = getBaseFileName(fileName);
    if (!endsWith(fileName, Extension.Ts) || !stringContains(baseName, ".d.") || fileExtensionIsOneOf(baseName, [Extension.Dts])) return undefined;
    const noExtension = removeExtension(fileName, Extension.Ts);
    const ext = noExtension.substring(noExtension.lastIndexOf("."));
    return noExtension.substring(0, noExtension.indexOf(".d.")) + ext;
}

function getJSExtensionForFile(fileName: string, options: CompilerOptions): Extension {
    return tryGetJSExtensionForFile(fileName, options) ?? Debug.fail(`Extension ${extensionFromPath(fileName)} is unsupported:: FileName:: ${fileName}`);
}

/** @internal */
export function tryGetJSExtensionForFile(fileName: string, options: CompilerOptions): Extension | undefined {
    const ext = tryGetExtensionFromPath(fileName);
    switch (ext) {
        case Extension.Ts:
        case Extension.Dts:
            return Extension.Js;
        case Extension.Tsx:
            return options.jsx === JsxEmit.Preserve ? Extension.Jsx : Extension.Js;
        case Extension.Js:
        case Extension.Jsx:
        case Extension.Json:
            return ext;
        case Extension.Dmts:
        case Extension.Mts:
        case Extension.Mjs:
            return Extension.Mjs;
        case Extension.Dcts:
        case Extension.Cts:
        case Extension.Cjs:
            return Extension.Cjs;
        default:
            return undefined;
    }
}

function getRelativePathIfInSameVolume(path: string, directoryPath: string, getCanonicalFileName: GetCanonicalFileName): string | undefined {
    const relativePath = getRelativePathToDirectoryOrUrl(directoryPath, path, directoryPath, getCanonicalFileName, /*isAbsolutePathAnUrl*/ false);
    return isRootedDiskPath(relativePath) ? undefined : relativePath;
}

function isPathRelativeToParent(path: string): boolean {
    return startsWith(path, "..");
}