# Utility functions.

let

  inherit (builtins)
    head tail isList stringLength substring lessThan sub;

in

rec {

  # "Fold" a binary function `op' between successive elements of
  # `list' with `nul' as the starting value, i.e., `fold op nul [x_1
  # x_2 ... x_n] == op x_1 (op x_2 ... (op x_n nul))'.  (This is
  # Haskell's foldr).
  fold = op: nul: list:
    if list == []
    then nul
    else op (head list) (fold op nul (tail list));

    
  # Concatenate a list of lists.
  concatLists =
    fold (x: y: x ++ y) [];


  # Concatenate a list of strings.
  concatStrings =
    fold (x: y: x + y) "";


  # Place an element between each element of a list, e.g.,
  # `intersperse "," ["a" "b" "c"]' returns ["a" "," "b" "," "c"].
  intersperse = separator: list:
    if list == [] || tail list == []
    then list
    else [(head list) separator]
         ++ (intersperse separator (tail list));


  concatStringsSep = separator: list:
    concatStrings (intersperse separator list);


  # Flatten the argument into a single list; that is, nested lists are
  # spliced into the top-level lists.  E.g., `flatten [1 [2 [3] 4] 5]
  # == [1 2 3 4 5]' and `flatten 1 == [1]'.
  flatten = x:
    if isList x
    then fold (x: y: (flatten x) ++ y) [] x
    else [x];


  # Return an attribute from nested attribute sets.  For instance ["x"
  # "y"] applied to some set e returns e.x.y, if it exists.  The
  # default value is returned otherwise.
  getAttr = attrPath: default: e:
    let {
      attr = head attrPath;
      body =
        if attrPath == [] then e
        else if builtins ? hasAttr && builtins.hasAttr attr e
        then getAttr (tail attrPath) default (builtins.getAttr attr e)
        else default;
    };


  # Filter a list using a predicate; that is, return a list containing
  # every element from `list' for which `pred' returns true.
  filter = pred: list:
    fold (x: y: if pred x then [x] ++ y else y) [] list;


  # Return true if `list' has an element `x':
  elem = x: list: fold (a: bs: x == a || bs) false list;


  # Find the sole element in the list matching the specified
  # predicate, or returns the default value.
  findSingle = pred: default: list:
    let found = filter pred list;
    in if found == [] then default
       else if tail found != [] then
         abort "Multiple elements match predicate in findSingle."
       else head found;


  # Return true if each element of a list is equal, false otherwise.
  eqLists = xs: ys:
    if xs == [] && ys == [] then true
    else if xs == [] || ys == [] then false
    else head xs == head ys && eqLists (tail xs) (tail ys);


  # Determine whether a filename ends in the given suffix.
  hasSuffix = ext: fileName:
    let lenFileName = stringLength fileName;
        lenExt = stringLength ext;
    in !(lessThan lenFileName lenExt) &&
       substring (sub lenFileName lenExt) lenFileName fileName == ext;

       
  # Bring in a path as a source, filtering out all Subversion and CVS
  # directories, as well as backup files (*~).
  cleanSource =
    let filter = name: type: let baseName = baseNameOf (toString name); in ! (
      # Filter out Subversion and CVS directories.
      (type == "directory" && (name == ".svn" || name == "CVS")) ||
      # Filter out backup files.
      (hasSuffix "~" name)
    );
    in src: builtins.filterSource filter src;


  # Return a singleton list or an empty list, depending on a boolean
  # value.  Useful when building lists with optional elements
  # (e.g. `++ optional (system == "i686-linux") flashplayer').
  optional = cond: elem: if cond then [elem] else [];


  # Return a list of integers from `first' up to and including `last'.
  range = first: last:
    if builtins.lessThan last first
    then []
    else [first] ++ range (builtins.add first 1) last;

  # Return true only if there is an attribute and it is true.
  checkFlag = attrSet: name:
	if (name == "true") then true else
	if (name == "false") then false else
	if (isInList (getAttr ["flags"] [] attrSet) name) then true else
	getAttr [name] false attrSet ;

  logicalOR = x: y: x || y;
  logicalAND = x: y: x && y;

  # Input : attrSet, [ [name default] ... ], name
  # Output : its value or default.
  getValue = attrSet: argList: name:
  ( getAttr [name] (if checkFlag attrSet name then true else
	if argList == [] then null else
	let x = builtins.head argList; in
		if (head x) == name then 
			(head (tail x))
		else (getValue attrSet 
			(tail argList) name)) attrSet );

  # Input : attrSet, [[name default] ...], [ [flagname reqs..] ... ]
  # Output : are reqs satisfied? It's asserted.
  checkReqs = attrSet : argList : condList :
  (
    fold logicalAND true 
      (map (x: let name = (head x) ; in
	
	((checkFlag attrSet name) -> 
	(fold logicalAND true
	(map (y: let val=(getValue attrSet argList y); in
		(val!=null) && (val!=false)) 
	(tail x))))) condList)) ;
	
   
  isInList = list: x:
	if (list == []) then false else
	if (x == (head list)) then true else
	isInList (tail list) x;
  
  uniqList = {inputList, outputList ? []}:
	if (inputList == []) then outputList else
	let x=head inputList; 
	newOutputList = outputList ++
	 (if (isInList outputList x) then [] else [x]);
	in uniqList {outputList=newOutputList; 
		inputList = (tail inputList);};

  condConcat = name: list: checker:
	if list == [] then name else
	if checker (head list) then 
		condConcat 
			(name + (head (tail list))) 
			(tail (tail list)) 
			checker
	else condConcat
		name (tail (tail list)) checker;

}