{ lib }:
# Operations on attribute sets.

let
  inherit (builtins) head tail length;
  inherit (lib.trivial) and;
  inherit (lib.strings) concatStringsSep;
  inherit (lib.lists) fold concatMap concatLists;
in

rec {
  inherit (builtins) attrNames listToAttrs hasAttr isAttrs getAttr;


  /* Return an attribute from nested attribute sets.

     Example:
       x = { a = { b = 3; }; }
       attrByPath ["a" "b"] 6 x
       => 3
       attrByPath ["z" "z"] 6 x
       => 6
  */
  attrByPath = attrPath: default: e:
    let attr = head attrPath;
    in
      if attrPath == [] then e
      else if e ? ${attr}
      then attrByPath (tail attrPath) default e.${attr}
      else default;

  /* Return if an attribute from nested attribute set exists.

     Example:
       x = { a = { b = 3; }; }
       hasAttrByPath ["a" "b"] x
       => true
       hasAttrByPath ["z" "z"] x
       => false

  */
  hasAttrByPath = attrPath: e:
    let attr = head attrPath;
    in
      if attrPath == [] then true
      else if e ? ${attr}
      then hasAttrByPath (tail attrPath) e.${attr}
      else false;


  /* Return nested attribute set in which an attribute is set.

     Example:
       setAttrByPath ["a" "b"] 3
       => { a = { b = 3; }; }
  */
  setAttrByPath = attrPath: value:
    if attrPath == [] then value
    else listToAttrs
      [ { name = head attrPath; value = setAttrByPath (tail attrPath) value; } ];


  /* Like `getAttrPath' without a default value. If it doesn't find the
     path it will throw.

     Example:
       x = { a = { b = 3; }; }
       getAttrFromPath ["a" "b"] x
       => 3
       getAttrFromPath ["z" "z"] x
       => error: cannot find attribute `z.z'
  */
  getAttrFromPath = attrPath: set:
    let errorMsg = "cannot find attribute `" + concatStringsSep "." attrPath + "'";
    in attrByPath attrPath (abort errorMsg) set;


  /* Return the specified attributes from a set.

     Example:
       attrVals ["a" "b" "c"] as
       => [as.a as.b as.c]
  */
  attrVals = nameList: set: map (x: set.${x}) nameList;


  /* Return the values of all attributes in the given set, sorted by
     attribute name.

     Example:
       attrValues {c = 3; a = 1; b = 2;}
       => [1 2 3]
  */
  attrValues = builtins.attrValues or (attrs: attrVals (attrNames attrs) attrs);


  /* Given a set of attribute names, return the set of the corresponding
     attributes from the given set.

     Example:
       getAttrs [ "a" "b" ] { a = 1; b = 2; c = 3; }
       => { a = 1; b = 2; }
  */
  getAttrs = names: attrs: genAttrs names (name: attrs.${name});

  /* Collect each attribute named `attr' from a list of attribute
     sets.  Sets that don't contain the named attribute are ignored.

     Example:
       catAttrs "a" [{a = 1;} {b = 0;} {a = 2;}]
       => [1 2]
  */
  catAttrs = builtins.catAttrs or
    (attr: l: concatLists (map (s: if s ? ${attr} then [s.${attr}] else []) l));


  /* Filter an attribute set by removing all attributes for which the
     given predicate return false.

     Example:
       filterAttrs (n: v: n == "foo") { foo = 1; bar = 2; }
       => { foo = 1; }
  */
  filterAttrs = pred: set:
    listToAttrs (concatMap (name: let v = set.${name}; in if pred name v then [(nameValuePair name v)] else []) (attrNames set));


  /* Filter an attribute set recursively by removing all attributes for
     which the given predicate return false.

     Example:
       filterAttrsRecursive (n: v: v != null) { foo = { bar = null; }; }
       => { foo = {}; }
  */
  filterAttrsRecursive = pred: set:
    listToAttrs (
      concatMap (name:
        let v = set.${name}; in
        if pred name v then [
          (nameValuePair name (
            if isAttrs v then filterAttrsRecursive pred v
            else v
          ))
        ] else []
      ) (attrNames set)
    );

  /* Apply fold functions to values grouped by key.

     Example:
       foldAttrs (n: a: [n] ++ a) [] [{ a = 2; } { a = 3; }]
       => { a = [ 2 3 ]; }
  */
  foldAttrs = op: nul: list_of_attrs:
    fold (n: a:
        fold (name: o:
          o // { ${name} = op n.${name} (a.${name} or nul); }
        ) a (attrNames n)
    ) {} list_of_attrs;


  /* Recursively collect sets that verify a given predicate named `pred'
     from the set `attrs'.  The recursion is stopped when the predicate is
     verified.

     Type:
       collect ::
         (AttrSet -> Bool) -> AttrSet -> [x]

     Example:
       collect isList { a = { b = ["b"]; }; c = [1]; }
       => [["b"] [1]]

       collect (x: x ? outPath)
          { a = { outPath = "a/"; }; b = { outPath = "b/"; }; }
       => [{ outPath = "a/"; } { outPath = "b/"; }]
  */
  collect = pred: attrs:
    if pred attrs then
      [ attrs ]
    else if isAttrs attrs then
      concatMap (collect pred) (attrValues attrs)
    else
      [];


  /* Utility function that creates a {name, value} pair as expected by
     builtins.listToAttrs.

     Example:
       nameValuePair "some" 6
       => { name = "some"; value = 6; }
  */
  nameValuePair = name: value: { inherit name value; };


  /* Apply a function to each element in an attribute set.  The
     function takes two arguments --- the attribute name and its value
     --- and returns the new value for the attribute.  The result is a
     new attribute set.

     Example:
       mapAttrs (name: value: name + "-" + value)
          { x = "foo"; y = "bar"; }
       => { x = "x-foo"; y = "y-bar"; }
  */
  mapAttrs = builtins.mapAttrs or
    (f: set:
      listToAttrs (map (attr: { name = attr; value = f attr set.${attr}; }) (attrNames set)));


  /* Like `mapAttrs', but allows the name of each attribute to be
     changed in addition to the value.  The applied function should
     return both the new name and value as a `nameValuePair'.

     Example:
       mapAttrs' (name: value: nameValuePair ("foo_" + name) ("bar-" + value))
          { x = "a"; y = "b"; }
       => { foo_x = "bar-a"; foo_y = "bar-b"; }
  */
  mapAttrs' = f: set:
    listToAttrs (map (attr: f attr set.${attr}) (attrNames set));


  /* Call a function for each attribute in the given set and return
     the result in a list.

     Example:
       mapAttrsToList (name: value: name + value)
          { x = "a"; y = "b"; }
       => [ "xa" "yb" ]
  */
  mapAttrsToList = f: attrs:
    map (name: f name attrs.${name}) (attrNames attrs);


  /* Like `mapAttrs', except that it recursively applies itself to
     attribute sets.  Also, the first argument of the argument
     function is a *list* of the names of the containing attributes.

     Type:
       mapAttrsRecursive ::
         ([String] -> a -> b) -> AttrSet -> AttrSet

     Example:
       mapAttrsRecursive (path: value: concatStringsSep "-" (path ++ [value]))
         { n = { a = "A"; m = { b = "B"; c = "C"; }; }; d = "D"; }
       => { n = { a = "n-a-A"; m = { b = "n-m-b-B"; c = "n-m-c-C"; }; }; d = "d-D"; }
  */
  mapAttrsRecursive = mapAttrsRecursiveCond (as: true);


  /* Like `mapAttrsRecursive', but it takes an additional predicate
     function that tells it whether to recursive into an attribute
     set.  If it returns false, `mapAttrsRecursiveCond' does not
     recurse, but does apply the map function.  It is returns true, it
     does recurse, and does not apply the map function.

     Type:
       mapAttrsRecursiveCond ::
         (AttrSet -> Bool) -> ([String] -> a -> b) -> AttrSet -> AttrSet

     Example:
       # To prevent recursing into derivations (which are attribute
       # sets with the attribute "type" equal to "derivation"):
       mapAttrsRecursiveCond
         (as: !(as ? "type" && as.type == "derivation"))
         (x: ... do something ...)
         attrs
  */
  mapAttrsRecursiveCond = cond: f: set:
    let
      recurse = path: set:
        let
          g =
            name: value:
            if isAttrs value && cond value
              then recurse (path ++ [name]) value
              else f (path ++ [name]) value;
        in mapAttrs g set;
    in recurse [] set;


  /* Generate an attribute set by mapping a function over a list of
     attribute names.

     Example:
       genAttrs [ "foo" "bar" ] (name: "x_" + name)
       => { foo = "x_foo"; bar = "x_bar"; }
  */
  genAttrs = names: f:
    listToAttrs (map (n: nameValuePair n (f n)) names);


  /* Check whether the argument is a derivation. Any set with
     { type = "derivation"; } counts as a derivation.

     Example:
       nixpkgs = import <nixpkgs> {}
       isDerivation nixpkgs.ruby
       => true
       isDerivation "foobar"
       => false
  */
  isDerivation = x: isAttrs x && x ? type && x.type == "derivation";

  /* Converts a store path to a fake derivation. */
  toDerivation = path:
    let
      path' = builtins.storePath path;
      res =
        { type = "derivation";
          name = builtins.unsafeDiscardStringContext (builtins.substring 33 (-1) (baseNameOf path'));
          outPath = path';
          outputs = [ "out" ];
          out = res;
          outputName = "out";
        };
    in res;


  /* If `cond' is true, return the attribute set `as',
     otherwise an empty attribute set.

     Example:
       optionalAttrs (true) { my = "set"; }
       => { my = "set"; }
       optionalAttrs (false) { my = "set"; }
       => { }
  */
  optionalAttrs = cond: as: if cond then as else {};


  /* Merge sets of attributes and use the function f to merge attributes
     values.

     Example:
       zipAttrsWithNames ["a"] (name: vs: vs) [{a = "x";} {a = "y"; b = "z";}]
       => { a = ["x" "y"]; }
  */
  zipAttrsWithNames = names: f: sets:
    listToAttrs (map (name: {
      inherit name;
      value = f name (catAttrs name sets);
    }) names);

  /* Implementation note: Common names  appear multiple times in the list of
     names, hopefully this does not affect the system because the maximal
     laziness avoid computing twice the same expression and listToAttrs does
     not care about duplicated attribute names.

     Example:
       zipAttrsWith (name: values: values) [{a = "x";} {a = "y"; b = "z";}]
       => { a = ["x" "y"]; b = ["z"] }
  */
  zipAttrsWith = f: sets: zipAttrsWithNames (concatMap attrNames sets) f sets;
  /* Like `zipAttrsWith' with `(name: values: value)' as the function.

    Example:
      zipAttrs [{a = "x";} {a = "y"; b = "z";}]
      => { a = ["x" "y"]; b = ["z"] }
  */
  zipAttrs = zipAttrsWith (name: values: values);

  /* Does the same as the update operator '//' except that attributes are
     merged until the given predicate is verified.  The predicate should
     accept 3 arguments which are the path to reach the attribute, a part of
     the first attribute set and a part of the second attribute set.  When
     the predicate is verified, the value of the first attribute set is
     replaced by the value of the second attribute set.

     Example:
       recursiveUpdateUntil (path: l: r: path == ["foo"]) {
         # first attribute set
         foo.bar = 1;
         foo.baz = 2;
         bar = 3;
       } {
         #second attribute set
         foo.bar = 1;
         foo.quz = 2;
         baz = 4;
       }

       returns: {
         foo.bar = 1; # 'foo.*' from the second set
         foo.quz = 2; #
         bar = 3;     # 'bar' from the first set
         baz = 4;     # 'baz' from the second set
       }

     */
  recursiveUpdateUntil = pred: lhs: rhs:
    let f = attrPath:
      zipAttrsWith (n: values:
        let here = attrPath ++ [n]; in
        if tail values == []
        || pred here (head (tail values)) (head values) then
          head values
        else
          f here values
      );
    in f [] [rhs lhs];

  /* A recursive variant of the update operator ‘//’.  The recursion
     stops when one of the attribute values is not an attribute set,
     in which case the right hand side value takes precedence over the
     left hand side value.

     Example:
       recursiveUpdate {
         boot.loader.grub.enable = true;
         boot.loader.grub.device = "/dev/hda";
       } {
         boot.loader.grub.device = "";
       }

       returns: {
         boot.loader.grub.enable = true;
         boot.loader.grub.device = "";
       }

     */
  recursiveUpdate = lhs: rhs:
    recursiveUpdateUntil (path: lhs: rhs:
      !(isAttrs lhs && isAttrs rhs)
    ) lhs rhs;

  /* Returns true if the pattern is contained in the set. False otherwise.

     Example:
       matchAttrs { cpu = {}; } { cpu = { bits = 64; }; }
       => true
   */
  matchAttrs = pattern: attrs: assert isAttrs pattern;
    fold and true (attrValues (zipAttrsWithNames (attrNames pattern) (n: values:
      let pat = head values; val = head (tail values); in
      if length values == 1 then false
      else if isAttrs pat then isAttrs val && matchAttrs pat val
      else pat == val
    ) [pattern attrs]));

  /* Override only the attributes that are already present in the old set
    useful for deep-overriding.

    Example:
      overrideExisting {} { a = 1; }
      => {}
      overrideExisting { b = 2; } { a = 1; }
      => { b = 2; }
      overrideExisting { a = 3; b = 2; } { a = 1; }
      => { a = 1; b = 2; }
  */
  overrideExisting = old: new:
    mapAttrs (name: value: new.${name} or value) old;

  /* Get a package output.
     If no output is found, fallback to `.out` and then to the default.

     Example:
       getOutput "dev" pkgs.openssl
       => "/nix/store/9rz8gxhzf8sw4kf2j2f1grr49w8zx5vj-openssl-1.0.1r-dev"
  */
  getOutput = output: pkg:
    if pkg.outputUnspecified or false
      then pkg.${output} or pkg.out or pkg
      else pkg;

  getBin = getOutput "bin";
  getLib = getOutput "lib";
  getDev = getOutput "dev";

  /* Pick the outputs of packages to place in buildInputs */
  chooseDevOutputs = drvs: builtins.map getDev drvs;

  /* Make various Nix tools consider the contents of the resulting
     attribute set when looking for what to build, find, etc.
   */
  recurseIntoAttrs =
    attrs: attrs // { recurseForDerivations = true; };

  /*** deprecated stuff ***/

  zipWithNames = zipAttrsWithNames;
  zip = builtins.trace
    "lib.zip is deprecated, use lib.zipAttrsWith instead" zipAttrsWith;

}