nixpkgs/lib/fileset/internal.nix

{ lib ? import ../. }:
let

  inherit (builtins)
    isAttrs
    isPath
    isString
    pathExists
    readDir
    seq
    split
    trace
    typeOf
    ;

  inherit (lib.attrsets)
    attrNames
    attrValues
    mapAttrs
    setAttrByPath
    zipAttrsWith
    ;

  inherit (lib.filesystem)
    pathType
    ;

  inherit (lib.lists)
    all
    commonPrefix
    drop
    elemAt
    filter
    findFirst
    findFirstIndex
    foldl'
    head
    length
    sublist
    tail
    ;

  inherit (lib.path)
    append
    splitRoot
    ;

  inherit (lib.path.subpath)
    components
    join
    ;

  inherit (lib.strings)
    isStringLike
    concatStringsSep
    substring
    stringLength
    ;

in
# Rare case of justified usage of rec:
# - This file is internal, so the return value doesn't matter, no need to make things overridable
# - The functions depend on each other
# - We want to expose all of these functions for easy testing
rec {

  # If you change the internal representation, make sure to:
  # - Increment this version
  # - Add an additional migration function below
  # - Update the description of the internal representation in ./README.md
  _currentVersion = 3;

  # Migrations between versions. The 0th element converts from v0 to v1, and so on
  migrations = [
    # Convert v0 into v1: Add the _internalBase{Root,Components} attributes
    (
      filesetV0:
      let
        parts = splitRoot filesetV0._internalBase;
      in
      filesetV0 // {
        _internalVersion = 1;
        _internalBaseRoot = parts.root;
        _internalBaseComponents = components parts.subpath;
      }
    )

    # Convert v1 into v2: filesetTree's can now also omit attributes to signal paths not being included
    (
      filesetV1:
      # This change is backwards compatible (but not forwards compatible, so we still need a new version)
      filesetV1 // {
        _internalVersion = 2;
      }
    )

    # Convert v2 into v3: filesetTree's now have a representation for an empty file set without a base path
    (
      filesetV2:
      filesetV2 // {
        # All v1 file sets are not the new empty file set
        _internalIsEmptyWithoutBase = false;
        _internalVersion = 3;
      }
    )
  ];

  _noEvalMessage = ''
    lib.fileset: Directly evaluating a file set is not supported.
      To turn it into a usable source, use `lib.fileset.toSource`.
      To pretty-print the contents, use `lib.fileset.trace` or `lib.fileset.traceVal`.'';

  # The empty file set without a base path
  _emptyWithoutBase = {
    _type = "fileset";

    _internalVersion = _currentVersion;

    # The one and only!
    _internalIsEmptyWithoutBase = true;

    # Due to alphabetical ordering, this is evaluated last,
    # which makes the nix repl output nicer than if it would be ordered first.
    # It also allows evaluating it strictly up to this error, which could be useful
    _noEval = throw _noEvalMessage;
  };

  # Create a fileset, see ./README.md#fileset
  # Type: path -> filesetTree -> fileset
  _create = base: tree:
    let
      # Decompose the base into its components
      # See ../path/README.md for why we're not just using `toString`
      parts = splitRoot base;
    in
    {
      _type = "fileset";

      _internalVersion = _currentVersion;

      _internalIsEmptyWithoutBase = false;
      _internalBase = base;
      _internalBaseRoot = parts.root;
      _internalBaseComponents = components parts.subpath;
      _internalTree = tree;

      # Due to alphabetical ordering, this is evaluated last,
      # which makes the nix repl output nicer than if it would be ordered first.
      # It also allows evaluating it strictly up to this error, which could be useful
      _noEval = throw _noEvalMessage;
    };

  # Coerce a value to a fileset, erroring when the value cannot be coerced.
  # The string gives the context for error messages.
  # Type: String -> (fileset | Path) -> fileset
  _coerce = context: value:
    if value._type or "" == "fileset" then
      if value._internalVersion > _currentVersion then
        throw ''
          ${context} is a file set created from a future version of the file set library with a different internal representation:
              - Internal version of the file set: ${toString value._internalVersion}
              - Internal version of the library: ${toString _currentVersion}
              Make sure to update your Nixpkgs to have a newer version of `lib.fileset`.''
      else if value._internalVersion < _currentVersion then
        let
          # Get all the migration functions necessary to convert from the old to the current version
          migrationsToApply = sublist value._internalVersion (_currentVersion - value._internalVersion) migrations;
        in
        foldl' (value: migration: migration value) value migrationsToApply
      else
        value
    else if ! isPath value then
      if isStringLike value then
        throw ''
          ${context} ("${toString value}") is a string-like value, but it should be a file set or a path instead.
              Paths represented as strings are not supported by `lib.fileset`, use `lib.sources` or derivations instead.''
      else
        throw ''
          ${context} is of type ${typeOf value}, but it should be a file set or a path instead.''
    else if ! pathExists value then
      throw ''
        ${context} (${toString value}) does not exist.''
    else
      _singleton value;

  # Coerce many values to filesets, erroring when any value cannot be coerced,
  # or if the filesystem root of the values doesn't match.
  # Type: String -> [ { context :: String, value :: fileset | Path } ] -> [ fileset ]
  _coerceMany = functionContext: list:
    let
      filesets = map ({ context, value }:
        _coerce "${functionContext}: ${context}" value
      ) list;

      # Find the first value with a base, there may be none!
      firstWithBase = findFirst (fileset: ! fileset._internalIsEmptyWithoutBase) null filesets;
      # This value is only accessed if first != null
      firstBaseRoot = firstWithBase._internalBaseRoot;

      # Finds the first element with a filesystem root different than the first element, if any
      differentIndex = findFirstIndex (fileset:
        # The empty value without a base doesn't have a base path
        ! fileset._internalIsEmptyWithoutBase
        && firstBaseRoot != fileset._internalBaseRoot
      ) null filesets;
    in
    # Only evaluates `differentIndex` if there are any elements with a base
    if firstWithBase != null && differentIndex != null then
      throw ''
        ${functionContext}: Filesystem roots are not the same:
            ${(head list).context}: root "${toString firstBaseRoot}"
            ${(elemAt list differentIndex).context}: root "${toString (elemAt filesets differentIndex)._internalBaseRoot}"
            Different roots are not supported.''
    else
      filesets;

  # Create a file set from a path.
  # Type: Path -> fileset
  _singleton = path:
    let
      type = pathType path;
    in
    if type == "directory" then
      _create path type
    else
      # This turns a file path ./default.nix into a fileset with
      # - _internalBase: ./.
      # - _internalTree: {
      #     "default.nix" = <type>;
      #   }
      # See ./README.md#single-files
      _create (dirOf path)
        {
          ${baseNameOf path} = type;
        };

  # Expand a directory representation to an equivalent one in attribute set form.
  # All directory entries are included in the result.
  # Type: Path -> filesetTree -> { <name> = filesetTree; }
  _directoryEntries = path: value:
    if value == "directory" then
      readDir path
    else
      # Set all entries not present to null
      mapAttrs (name: value: null) (readDir path)
      // value;

  /*
    A normalisation of a filesetTree suitable filtering with `builtins.path`:
    - Replace all directories that have no files with `null`.
      This removes directories that would be empty
    - Replace all directories with all files with `"directory"`.
      This speeds up the source filter function

    Note that this function is strict, it evaluates the entire tree

    Type: Path -> filesetTree -> filesetTree
  */
  _normaliseTreeFilter = path: tree:
    if tree == "directory" || isAttrs tree then
      let
        entries = _directoryEntries path tree;
        normalisedSubtrees = mapAttrs (name: _normaliseTreeFilter (path + "/${name}")) entries;
        subtreeValues = attrValues normalisedSubtrees;
      in
      # This triggers either when all files in a directory are filtered out
      # Or when the directory doesn't contain any files at all
      if all isNull subtreeValues then
        null
      # Triggers when we have the same as a `readDir path`, so we can turn it back into an equivalent "directory".
      else if all isString subtreeValues then
        "directory"
      else
        normalisedSubtrees
    else
      tree;

  /*
    A minimal normalisation of a filesetTree, intended for pretty-printing:
    - If all children of a path are recursively included or empty directories, the path itself is also recursively included
    - If all children of a path are fully excluded or empty directories, the path itself is an empty directory
    - Other empty directories are represented with the special "emptyDir" string
      While these could be replaced with `null`, that would take another mapAttrs

    Note that this function is partially lazy.

    Type: Path -> filesetTree -> filesetTree (with "emptyDir"'s)
  */
  _normaliseTreeMinimal = path: tree:
    if tree == "directory" || isAttrs tree then
      let
        entries = _directoryEntries path tree;
        normalisedSubtrees = mapAttrs (name: _normaliseTreeMinimal (path + "/${name}")) entries;
        subtreeValues = attrValues normalisedSubtrees;
      in
      # If there are no entries, or all entries are empty directories, return "emptyDir".
      # After this branch we know that there's at least one file
      if all (value: value == "emptyDir") subtreeValues then
        "emptyDir"

      # If all subtrees are fully included or empty directories
      # (both of which are coincidentally represented as strings), return "directory".
      # This takes advantage of the fact that empty directories can be represented as included directories.
      # Note that the tree == "directory" check allows avoiding recursion
      else if tree == "directory" || all (value: isString value) subtreeValues then
        "directory"

      # If all subtrees are fully excluded or empty directories, return null.
      # This takes advantage of the fact that empty directories can be represented as excluded directories
      else if all (value: isNull value || value == "emptyDir") subtreeValues then
        null

      # Mix of included and excluded entries
      else
        normalisedSubtrees
    else
      tree;

  # Trace a filesetTree in a pretty way when the resulting value is evaluated.
  # This can handle both normal filesetTree's, and ones returned from _normaliseTreeMinimal
  # Type: Path -> filesetTree (with "emptyDir"'s) -> Null
  _printMinimalTree = base: tree:
    let
      treeSuffix = tree:
        if isAttrs tree then
          ""
        else if tree == "directory" then
          " (all files in directory)"
        else
          # This does "leak" the file type strings of the internal representation,
          # but this is the main reason these file type strings even are in the representation!
          # TODO: Consider removing that information from the internal representation for performance.
          # The file types can still be printed by querying them only during tracing
          " (${tree})";

      # Only for attribute set trees
      traceTreeAttrs = prevLine: indent: tree:
        foldl' (prevLine: name:
          let
            subtree = tree.${name};

            # Evaluating this prints the line for this subtree
            thisLine =
              trace "${indent}- ${name}${treeSuffix subtree}" prevLine;
          in
          if subtree == null || subtree == "emptyDir" then
            # Don't print anything at all if this subtree is empty
            prevLine
          else if isAttrs subtree then
            # A directory with explicit entries
            # Do print this node, but also recurse
            traceTreeAttrs thisLine "${indent}  " subtree
          else
            # Either a file, or a recursively included directory
            # Do print this node but no further recursion needed
            thisLine
        ) prevLine (attrNames tree);

      # Evaluating this will print the first line
      firstLine =
        if tree == null || tree == "emptyDir" then
          trace "(empty)" null
        else
          trace "${toString base}${treeSuffix tree}" null;
    in
    if isAttrs tree then
      traceTreeAttrs firstLine "" tree
    else
      firstLine;

  # Pretty-print a file set in a pretty way when the resulting value is evaluated
  # Type: fileset -> Null
  _printFileset = fileset:
    if fileset._internalIsEmptyWithoutBase then
      trace "(empty)" null
    else
      _printMinimalTree fileset._internalBase
        (_normaliseTreeMinimal fileset._internalBase fileset._internalTree);

  # Turn a fileset into a source filter function suitable for `builtins.path`
  # Only directories recursively containing at least one files are recursed into
  # Type: Path -> fileset -> (String -> String -> Bool)
  _toSourceFilter = fileset:
    let
      # Simplify the tree, necessary to make sure all empty directories are null
      # which has the effect that they aren't included in the result
      tree = _normaliseTreeFilter fileset._internalBase fileset._internalTree;

      # The base path as a string with a single trailing slash
      baseString =
        if fileset._internalBaseComponents == [] then
          # Need to handle the filesystem root specially
          "/"
        else
          "/" + concatStringsSep "/" fileset._internalBaseComponents + "/";

      baseLength = stringLength baseString;

      # Check whether a list of path components under the base path exists in the tree.
      # This function is called often, so it should be fast.
      # Type: [ String ] -> Bool
      inTree = components:
        let
          recurse = index: localTree:
            if isAttrs localTree then
              # We have an attribute set, meaning this is a directory with at least one file
              if index >= length components then
                # The path may have no more components though, meaning the filter is running on the directory itself,
                # so we always include it, again because there's at least one file in it.
                true
              else
                # If we do have more components, the filter runs on some entry inside this directory, so we need to recurse
                # We do +2 because builtins.split is an interleaved list of the inbetweens and the matches
                recurse (index + 2) localTree.${elemAt components index}
            else
              # If it's not an attribute set it can only be either null (in which case it's not included)
              # or a string ("directory" or "regular", etc.) in which case it's included
              localTree != null;
        in recurse 0 tree;

      # Filter suited when there's no files
      empty = _: _: false;

      # Filter suited when there's some files
      # This can't be used for when there's no files, because the base directory is always included
      nonEmpty =
        path: _:
        let
          # Add a slash to the path string, turning "/foo" to "/foo/",
          # making sure to not have any false prefix matches below.
          # Note that this would produce "//" for "/",
          # but builtins.path doesn't call the filter function on the `path` argument itself,
          # meaning this function can never receive "/" as an argument
          pathSlash = path + "/";
        in
        # Same as `hasPrefix pathSlash baseString`, but more efficient.
        # With base /foo/bar we need to include /foo:
        # hasPrefix "/foo/" "/foo/bar/"
        if substring 0 (stringLength pathSlash) baseString == pathSlash then
          true
        # Same as `! hasPrefix baseString pathSlash`, but more efficient.
        # With base /foo/bar we need to exclude /baz
        # ! hasPrefix "/baz/" "/foo/bar/"
        else if substring 0 baseLength pathSlash != baseString then
          false
        else
          # Same as `removePrefix baseString path`, but more efficient.
          # From the above code we know that hasPrefix baseString pathSlash holds, so this is safe.
          # We don't use pathSlash here because we only needed the trailing slash for the prefix matching.
          # With base /foo and path /foo/bar/baz this gives
          # inTree (split "/" (removePrefix "/foo/" "/foo/bar/baz"))
          # == inTree (split "/" "bar/baz")
          # == inTree [ "bar" "baz" ]
          inTree (split "/" (substring baseLength (-1) path));
    in
    # Special case because the code below assumes that the _internalBase is always included in the result
    # which shouldn't be done when we have no files at all in the base
    # This also forces the tree before returning the filter, leads to earlier error messages
    if fileset._internalIsEmptyWithoutBase || tree == null then
      empty
    else
      nonEmpty;

  # Transforms the filesetTree of a file set to a shorter base path, e.g.
  # _shortenTreeBase [ "foo" ] (_create /foo/bar null)
  # => { bar = null; }
  _shortenTreeBase = targetBaseComponents: fileset:
    let
      recurse = index:
        # If we haven't reached the required depth yet
        if index < length fileset._internalBaseComponents then
          # Create an attribute set and recurse as the value, this can be lazily evaluated this way
          { ${elemAt fileset._internalBaseComponents index} = recurse (index + 1); }
        else
          # Otherwise we reached the appropriate depth, here's the original tree
          fileset._internalTree;
    in
    recurse (length targetBaseComponents);

  # Transforms the filesetTree of a file set to a longer base path, e.g.
  # _lengthenTreeBase [ "foo" "bar" ] (_create /foo { bar.baz = "regular"; })
  # => { baz = "regular"; }
  _lengthenTreeBase = targetBaseComponents: fileset:
    let
      recurse = index: tree:
        # If the filesetTree is an attribute set and we haven't reached the required depth yet
        if isAttrs tree && index < length targetBaseComponents then
          # Recurse with the tree under the right component (which might not exist)
          recurse (index + 1) (tree.${elemAt targetBaseComponents index} or null)
        else
          # For all values here we can just return the tree itself:
          # tree == null -> the result is also null, everything is excluded
          # tree == "directory" -> the result is also "directory",
          #   because the base path is always a directory and everything is included
          # isAttrs tree -> the result is `tree`
          #   because we don't need to recurse any more since `index == length longestBaseComponents`
          tree;
    in
    recurse (length fileset._internalBaseComponents) fileset._internalTree;

  # Computes the union of a list of filesets.
  # The filesets must already be coerced and validated to be in the same filesystem root
  # Type: [ Fileset ] -> Fileset
  _unionMany = filesets:
    let
      # All filesets that have a base, aka not the ones that are the empty value without a base
      filesetsWithBase = filter (fileset: ! fileset._internalIsEmptyWithoutBase) filesets;

      # The first fileset that has a base.
      # This value is only accessed if there are at all.
      firstWithBase = head filesetsWithBase;

      # To be able to union filesetTree's together, they need to have the same base path.
      # Base paths can be unioned by taking their common prefix,
      # e.g. such that `union /foo/bar /foo/baz` has the base path `/foo`

      # A list of path components common to all base paths.
      # Note that commonPrefix can only be fully evaluated,
      # so this cannot cause a stack overflow due to a build-up of unevaluated thunks.
      commonBaseComponents = foldl'
        (components: el: commonPrefix components el._internalBaseComponents)
        firstWithBase._internalBaseComponents
        # We could also not do the `tail` here to avoid a list allocation,
        # but then we'd have to pay for a potentially expensive
        # but unnecessary `commonPrefix` call
        (tail filesetsWithBase);

      # The common base path assembled from a filesystem root and the common components
      commonBase = append firstWithBase._internalBaseRoot (join commonBaseComponents);

      # A list of filesetTree's that all have the same base path
      # This is achieved by nesting the trees into the components they have over the common base path
      # E.g. `union /foo/bar /foo/baz` has the base path /foo
      # So the tree under `/foo/bar` gets nested under `{ bar = ...; ... }`,
      # while the tree under `/foo/baz` gets nested under `{ baz = ...; ... }`
      # Therefore allowing combined operations over them.
      trees = map (_shortenTreeBase commonBaseComponents) filesetsWithBase;

      # Folds all trees together into a single one using _unionTree
      # We do not use a fold here because it would cause a thunk build-up
      # which could cause a stack overflow for a large number of trees
      resultTree = _unionTrees trees;
    in
    # If there's no values with a base, we have no files
    if filesetsWithBase == [ ] then
      _emptyWithoutBase
    else
      _create commonBase resultTree;

  # The union of multiple filesetTree's with the same base path.
  # Later elements are only evaluated if necessary.
  # Type: [ filesetTree ] -> filesetTree
  _unionTrees = trees:
    let
      stringIndex = findFirstIndex isString null trees;
      withoutNull = filter (tree: tree != null) trees;
    in
    if stringIndex != null then
      # If there's a string, it's always a fully included tree (dir or file),
      # no need to look at other elements
      elemAt trees stringIndex
    else if withoutNull == [ ] then
      # If all trees are null, then the resulting tree is also null
      null
    else
      # The non-null elements have to be attribute sets representing partial trees
      # We need to recurse into those
      zipAttrsWith (name: _unionTrees) withoutNull;

  # Computes the intersection of a list of filesets.
  # The filesets must already be coerced and validated to be in the same filesystem root
  # Type: Fileset -> Fileset -> Fileset
  _intersection = fileset1: fileset2:
    let
      # The common base components prefix, e.g.
      # (/foo/bar, /foo/bar/baz) -> /foo/bar
      # (/foo/bar, /foo/baz) -> /foo
      commonBaseComponentsLength =
        # TODO: Have a `lib.lists.commonPrefixLength` function such that we don't need the list allocation from commonPrefix here
        length (
          commonPrefix
            fileset1._internalBaseComponents
            fileset2._internalBaseComponents
        );

      # To be able to intersect filesetTree's together, they need to have the same base path.
      # Base paths can be intersected by taking the longest one (if any)

      # The fileset with the longest base, if any, e.g.
      # (/foo/bar, /foo/bar/baz) -> /foo/bar/baz
      # (/foo/bar, /foo/baz) -> null
      longestBaseFileset =
        if commonBaseComponentsLength == length fileset1._internalBaseComponents then
          # The common prefix is the same as the first path, so the second path is equal or longer
          fileset2
        else if commonBaseComponentsLength == length fileset2._internalBaseComponents then
          # The common prefix is the same as the second path, so the first path is longer
          fileset1
        else
          # The common prefix is neither the first nor the second path
          # This means there's no overlap between the two sets
          null;

      # Whether the result should be the empty value without a base
      resultIsEmptyWithoutBase =
        # If either fileset is the empty fileset without a base, the intersection is too
        fileset1._internalIsEmptyWithoutBase
        || fileset2._internalIsEmptyWithoutBase
        # If there is no overlap between the base paths
        || longestBaseFileset == null;

      # Lengthen each fileset's tree to the longest base prefix
      tree1 = _lengthenTreeBase longestBaseFileset._internalBaseComponents fileset1;
      tree2 = _lengthenTreeBase longestBaseFileset._internalBaseComponents fileset2;

      # With two filesetTree's with the same base, we can compute their intersection
      resultTree = _intersectTree tree1 tree2;
    in
    if resultIsEmptyWithoutBase then
      _emptyWithoutBase
    else
      _create longestBaseFileset._internalBase resultTree;

  # The intersection of two filesetTree's with the same base path
  # The second element is only evaluated as much as necessary.
  # Type: filesetTree -> filesetTree -> filesetTree
  _intersectTree = lhs: rhs:
    if isAttrs lhs && isAttrs rhs then
      # Both sides are attribute sets, we can recurse for the attributes existing on both sides
      mapAttrs
        (name: _intersectTree lhs.${name})
        (builtins.intersectAttrs lhs rhs)
    else if lhs == null || isString rhs then
      # If the lhs is null, the result should also be null
      # And if the rhs is the identity element
      # (a string, aka it includes everything), then it's also the lhs
      lhs
    else
      # In all other cases it's the rhs
      rhs;

  # Compute the set difference between two file sets.
  # The filesets must already be coerced and validated to be in the same filesystem root.
  # Type: Fileset -> Fileset -> Fileset
  _difference = positive: negative:
    let
      # The common base components prefix, e.g.
      # (/foo/bar, /foo/bar/baz) -> /foo/bar
      # (/foo/bar, /foo/baz) -> /foo
      commonBaseComponentsLength =
        # TODO: Have a `lib.lists.commonPrefixLength` function such that we don't need the list allocation from commonPrefix here
        length (
          commonPrefix
            positive._internalBaseComponents
            negative._internalBaseComponents
        );

      # We need filesetTree's with the same base to be able to compute the difference between them
      # This here is the filesetTree from the negative file set, but for a base path that matches the positive file set.
      # Examples:
      # For `difference /foo /foo/bar`, `negativeTreeWithPositiveBase = { bar = "directory"; }`
      #   because under the base path of `/foo`, only `bar` from the negative file set is included
      # For `difference /foo/bar /foo`, `negativeTreeWithPositiveBase = "directory"`
      #   because under the base path of `/foo/bar`, everything from the negative file set is included
      # For `difference /foo /bar`, `negativeTreeWithPositiveBase = null`
      #   because under the base path of `/foo`, nothing from the negative file set is included
      negativeTreeWithPositiveBase =
        if commonBaseComponentsLength == length positive._internalBaseComponents then
          # The common prefix is the same as the positive base path, so the second path is equal or longer.
          # We need to _shorten_ the negative filesetTree to the same base path as the positive one
          # E.g. for `difference /foo /foo/bar` the common prefix is /foo, equal to the positive file set's base
          # So we need to shorten the base of the tree for the negative argument from /foo/bar to just /foo
          _shortenTreeBase positive._internalBaseComponents negative
        else if commonBaseComponentsLength == length negative._internalBaseComponents then
          # The common prefix is the same as the negative base path, so the first path is longer.
          # We need to lengthen the negative filesetTree to the same base path as the positive one.
          # E.g. for `difference /foo/bar /foo` the common prefix is /foo, equal to the negative file set's base
          # So we need to lengthen the base of the tree for the negative argument from /foo to /foo/bar
          _lengthenTreeBase positive._internalBaseComponents negative
        else
          # The common prefix is neither the first nor the second path.
          # This means there's no overlap between the two file sets,
          # and nothing from the negative argument should get removed from the positive one
          # E.g for `difference /foo /bar`, we remove nothing to get the same as `/foo`
          null;

      resultingTree =
        _differenceTree
        positive._internalBase
        positive._internalTree
        negativeTreeWithPositiveBase;
    in
    # If the first file set is empty, we can never have any files in the result
    if positive._internalIsEmptyWithoutBase then
      _emptyWithoutBase
    # If the second file set is empty, nothing gets removed, so the result is just the first file set
    else if negative._internalIsEmptyWithoutBase then
      positive
    else
      # We use the positive file set base for the result,
      # because only files from the positive side may be included,
      # which is what base path is for
      _create positive._internalBase resultingTree;

  # Computes the set difference of two filesetTree's
  # Type: Path -> filesetTree -> filesetTree
  _differenceTree = path: lhs: rhs:
    # If the lhs doesn't have any files, or the right hand side includes all files
    if lhs == null || isString rhs then
      # The result will always be empty
      null
    # If the right hand side has no files
    else if rhs == null then
      # The result is always the left hand side, because nothing gets removed
      lhs
    else
      # Otherwise we always have two attribute sets to recurse into
      mapAttrs (name: lhsValue:
        _differenceTree (path + "/${name}") lhsValue (rhs.${name} or null)
      ) (_directoryEntries path lhs);

  _fileFilter = predicate: fileset:
    let
      recurse = path: tree:
        mapAttrs (name: subtree:
          if isAttrs subtree || subtree == "directory" then
            recurse (path + "/${name}") subtree
          else if
            predicate {
              inherit name;
              type = subtree;
              # To ensure forwards compatibility with more arguments being added in the future,
              # adding an attribute which can't be deconstructed :)
              "lib.fileset.fileFilter: The predicate function passed as the first argument must be able to handle extra attributes for future compatibility. If you're using `{ name, file }:`, use `{ name, file, ... }:` instead." = null;
            }
          then
            subtree
          else
            null
        ) (_directoryEntries path tree);
    in
    if fileset._internalIsEmptyWithoutBase then
      _emptyWithoutBase
    else
      _create fileset._internalBase
        (recurse fileset._internalBase fileset._internalTree);
}