NAME
    Data::NDS - routines to work with a perl nested data structure

SYNOPSIS
      use Data::NDS;
      $obj = new Data::NDS;

DESCRIPTION
    This is a module for working with a perl nested data structure (NDS). A
    data structure may consist of any number of nested perl data types
    including:

       lists
       hashes
       scalars
       other (everything else)

    This module can easily perform the following operations:

    Access parts of the NDS
        It is very easy to get a value stored somewhere in an NDS, or to set
        a value somewhere in an NDS.

    Verify structural integrity
        Often, a data structure may have constraints on it (certain parts of
        it may be lists, hashes, or scalars). This module can enforce those
        constraints when setting parts of the NDS.

    Merge multiple NDSs into a single NDS
        Two different NDSs may be merged into a single NDS using a series of
        rules (described below).

    A reasonably complete set of examples for how to do these and other
    tasks is included below.

ACCESSING AN NDS
    Typically, when accessing a nested data structure, you might use
    something like:

      $nds{foo}[5]{bar}

    Although it is very direct, this necessitates putting a great deal of
    information about the structure directly in the program. It also relies
    on the fact that the structure is correctly defined, and all parts are
    present. If there's any possiblitity that this is not the case, you have
    to recurse through the structure to determine this or face some
    unexpected side effects. For example, referencing $nds{foo}[5]{bar} will
    create a lot of structure if it didn't already exist.

    This module will replace access to the value (or substructure) stored
    somewhere in an NDS with a call to a method which will automatically
    check that the structure is correct. It can be used to access, set, or
    delete parts of an NDS. The above example could be replaced with:

      $obj->val($nds,"/foo/5/bar");

    Here, the string "/foo/5/bar"is called a path. It is a series of indices
    separated by a delimiter (which defaults to "/", but which can be set to
    other values using the delim method described below). The indices of
    describe how to traverse through an NDS.

NDS STRUCTURE
    An NDS can have a great deal of structural information associated with
    it. The data stored at every path in a data structure is of a certain
    type, and the some types of structural information imply or prohibit
    other structural characteristics.

    When specifying structural information for a path, some elements in the
    path may be given as a wildcard or as a specific value.

    For example, in a data structure which consists of a hash containing a
    foo key, and that key contains a list of elements, you might specifify
    structural information for specific paths:

      /foo/0
      /foo/1

    or for all elements using a wildcard:

      /foo/*

    In the second case, structural information is set for ALL elements in
    the list.

    It is not allowed to have structural information simultaneously for both
    types of paths. For example, there will never be structural information
    for both:

      /foo/1
      /foo/*

    There MAY be structural information for:

      /foo/1
      /bar/*

    since there is no requirement that /foo and /bar are uniform.

    Most paths in an NDS can have the following pieces of information:

    type
        This refers to the what type of data is storead at the path. Known
        types are scalar, list, hash, and other (which encompasses all other
        types of perl data types).

    uniform or non-uniform lists or hashes
        Hashes and lists can be either uniform or non-uniform. A uniform
        list has elements which are all the same structure. It is not
        required that all elements have every piece of the structure, but
        two elements cannot have a different structure at any level.

        When specifying structural information for uniform list or hashes,
        the wildcard will always be used. When specifying structural
        information for non-uniform lists or hashes, paths will always
        consist of specific elements.

    ordered or unordered lists
        Lists can be ordered or unordered. An ordered list is one in which
        the position in the list has meaning. For example, the 1st and 2nd
        elements in the list are not interchangeable.

        Unordered lists are those in which the order and placement of the
        elements is not important. Because they are interchangeable, all
        unordered lists are uniform.

MERGING NDSes
    One of the more complex tasks of this module is the ability to take two
    NDSes and merge them together recursively. At every level of the merge,
    the data is combined based on the merge method for that path and that
    type of data.

    There are several different methods that can be used for merging NDSes.

    Merging hashes
        Merging hashes is the easiest. Allowed methods are merge, keep,
        keep_warn, replace, replace_warn, or error.

        Merging the two hashes:

          %nds1 = ( a  => NDS1,
                    b  => NDS2 )

          %nds2 = ( b  => NDS3,
                    c  => NDS4 )

        will give a resulting hash:

          %nds  = ( a => NDS1,
                    b => ???
                    c => NDS4 )

        The "a" and "c" keys are the easiest. Since they are only defined in
        one of the two initial hashes, their value is the value they were
        defined with, and it is not necessary to recurse deeper into those
        values.

        The "b" key value depends on the merge method. If the method is
        keep, the first value is used, so

          b => NDS2

        If the method is replace, an existing value will be replaced with a
        second value, so:

          b => NDS3

        In both of these cases, it is not necessary to recurse into the
        structure.

        If the method is merge, the resulting value is obtained by merging
        NDS2 and NDS3.

        If the method is error, an error will occur if the key is defined in
        both hashes, and the program will exit.

        The methods keep_warn and replace_warn are equivalent to keep and
        replace respectively except that a warning will be issued when a key
        is defined in both hashes.

        When merging two hashes, if a value for a key in the first hash is
        empty, or an empty string (""), it is replaced by the value in the
        second hash.

    Merging lists
        When merging lists, allowed methods are: merge, keep, keep_warn,
        replace, replace_warn, append, and error.

        Merging the two lists:

          @list1 = ( NDS1a NDS1b ... )

          @list2 = ( NDS2a NDS2b ... )

        will give the following results depending on the merge method.

        With the keep method, the resulting list will be:

          @list = @list1

        With the replace method, the resulting list will be:

          @list = @list2

        With the append method, the resulting list will be:

          @list = (@list1 @list2)

        With the merge method, the resulting list will be

          @list = ( NDS1 NDS2 ... )

        where NDS1 is a merger of NDS1a and NDS1b, NDS2 is a merger of NDS2a
        and NDS2b, etc.

        If the method is error, an error will occur if both lists have
        elements, and the program will exit.

        The methods keep_warn and replace_warn are equivalent to keep and
        replace respectively except that a warning will be issued when both
        lists have elements.

        The append method is only available with unordered lists. The merge
        method is only available with ordered lists.

        When merging two lists, if a value for an element in the first list
        is empty, or an empty string (""), it is replaced by the value in
        the second list.

    Merging scalars (or other)
        When data of type scalar or other are merged, allowed methods of
        merging are keep, keep_warn, replace, replace_warn, and error.

        Scalars or other types are merged when the parent structures are
        merged recursively, and they include scalars at some level.

        For example, given the two hashes:

          %nds1 = ( a  => 1 )

          %nds2 = ( a  => 2 )

        which are merged using the "merge" method. The "a" key exists in
        both, so the values (1 and 2) are merged. Since they are scalars,
        they will be merged using one of the scalar merge methods listed
        above.

        With the keep and replace methods, the first or second value are
        returned respectively. With the error method, an error is triggered
        if both are defined and the program will exit. With the keep_warn
        and replace_warn methods, a warning will be triggered.

STRUCTURAL INFORMATION
    When handling a data structure, structural information can be kept which
    will allow you to test a data structure to see if it's valid and how it
    should be merged with another NDS. Structural information is optional,
    and reasonable defaults exist.

    Structural information may be given as global defaults (i.e. it applies
    to all paths), or on a path-specific basis (in which case it applies
    only to that one specific

    Structural information may be given for a specific path, in which case
    it applies only to that exact path. It does not apply to structures
    lower OR higher an NDS. Structural information may also be given with no
    path, in which case it provides a default for all paths unless
    overridden with information specific to a path.

    The following structural information may be set:

    ordered BOOLEAN [PATH]
        By default, all lists are treated as unordered, but that can be
        overridden, either on the global level, or path specific level, with
        this descriptor.

        If this is set to 1, the list at the given PATH is ordered, or if
        PATH is omitted, all lists will default to ordered unless explicitly
        set otherwise.

        If this is set to 0, the list(s) will be unordered.

    uniform_hash BOOLEAN
        By default, hash keys are not uniform. By setting this descriptor to
        1, they will default to uniform.

    uniform_ol BOOLEAN
        By default, all ordered lists are uniform. By setting this
        descriptor to 0, they will be treated as non-uniform.

        Note that there is no uniform_ul descriptor because ALL unordered
        lists are treated as uniform (if structural information is being
        used) since there is no consistent way for structural information to
        apply to an unordered list which does not have uniform elements.

    uniform BOOLEAN PATH
        This can apply either to an ordered list or a hash. It is invalid
        for other data types. It sets the element at the given path to be
        explicitly uniform or not uniform.

        With respect to ordered lists, there are two caveats.

        Caveat 1:

        Hashes underneath the list element are uniform if the same key has
        the same structure. It is not required that different keys have the
        same structure.

        For example, if the path "/a" refers to a uniform ordered list, and
        "/a/0" is a hash with a key "key1" in it, and "/a/1" is a hash with
        the key "key1" in it, both "/a/0/key1" and "/a/1/key1" have the same
        structure. "/a/0/key2" can have a different structure however
        (unless the hash is also defined as "uniform").

        Caveat 2:

        Ordered lists underneath the list element are uniform if the
        elements at the same position have the same structure.

        For example, if the path "/a" refers to a uniform ordered list, and
        "/a/0" refers to an ordered list (so "/a/1" must also refer to an
        ordered list, then both "/a/0/0" and "/a/1/0" must have the same
        structure, but "/a/0/1" may be different (unless the second ordered
        list is "uniform" also).

    When specifying the path to set these items, any element in either a
    uniform list or uniform hash should be defined with an asterix (*).

    For example, if "/a" refers to a uniform list of data structures,
    setting values for these elements should be done with "/a/*" instead of
    "/a/0" or some other number. Likewise, uniform hashes should use an
    asterix instead of a hash key.

MERGE INFORMATION
    Merge information is used to specify how different parts of a data
    structure are merged with other structures. Merge information may be
    given for a specific path or as a global default.

    In addition, merge information may be specified for different sets of
    circumstances. For example, one type of "merger" would be to use one
    data structure to provide defaults for another structure, but only when
    that structure didn't already include a value. An alternate type of
    circumstances would be to have the second data structure override values
    in the first structure. Each set of circumstances may be given a ruleset
    name, and merge information can be set (either as global defaults or for
    a specific path) for that set set of circumstances. The named
    circumstances is called a ruleset and is described in more detail below.

    The following merge information may be set:

    merge_hash METHOD [RULESET]
        This provides the default merge method for hash mergers. If RULESET
        is given, it is the default when that set of rules is specified in
        the merger.

        If not specified, the "merge" method is the default.

    merge_ol METHOD [RULESET]
        This provides the default merge method for ordered list mergers. If
        RULESET is given, it is the default when that set of rules is
        specified in the merger.

        If not specified, the "merge" method is the default.

    merge_ul METHOD [RULESET]
        This provides the default merge method for unordered list mergers.
        If RULESET is given, it is the default when that set of rules is
        specified in the merger.

        If not specified, the "append" method is the default.

    merge_scalar METHOD [RULESET]
        This provides the default merge method for ordered list mergers. If
        RULESET is given, it is the default when that set of rules is
        specified in the merger.

        If not specified, the "merge" method is the default.

    merge METHOD PATH [RULESET]
        This provides the merge method for a specific path.

RULE SETS
    It is sometimes desirable to have multiple ways defined to merge two
    NDSes for different sets of circumstances.

    For example, sometimes you want to do a full merge of the NDSes, and
    another time you want one of the NDSes to provide default values for
    anything not defined in the other NDS, but you don't want to override
    any value that is currently there.

    A set of all of the different rules (including both global defaults, and
    path specific methods) which should be applied under a given set of
    circumstances is called a ruleset.

    By default, a single unnamed ruleset is used, and all merging is done
    using the rules defined there. Additional named rulesets may also be
    added. One important difference is that default rules are automatically
    supplied for the unnamed ruleset, but NOT for a named ruleset. If a
    merge method cannot be determined in a named ruleset, it will default to
    that of the unnamed ruleset.

    Any number of named rulesets may be created. There are four reserved
    rule sets named "default", "override", "keep" and "replace" that may not
    be used.

    The "default" ruleset has the following settings:

       merge_hash   = merge
       merge_ul     = keep
       merge_ol     = merge
       merge_scalar = keep

    If you merge two data structures using the "default" ruleset, the second
    structure will provide defaults for the first. In other words, if the
    first includes a scalar at some path, it will keep it, but otherwise, it
    will take the value from the second structure.

    The only exception is that unordered lists are not recursed into. If a
    value is an unordered list, it will use an existing list in it's
    entirety.

    The "override" ruleset has the following settings:

       merge_hash   = merge
       merge_ul     = replace
       merge_ol     = merge
       merge_scalar = replace

    If you merge two data structures using the "override" ruleset, the
    second structure will override the first.

    The "keep" and "replace" rulesets are used to set a value at a given
    path to a new value, possibly completely replacing any existing
    structure. The "keep" ruleset will set the structure to a new value only
    if it doesn't already exist. The "replace" ruleset will remove any
    existing structure and replace it with the new value.

    The "keep" ruleset has all settings set to "keep". The "replace" ruleset
    has them all set to "replace".

METHODS
    When referring to the arguments passed to a method, $path always refers
    to the path in an NDS. $path can be passed in as a delimited string, or
    as a list reference where the list contains the elements of the path. So
    the following are equivalent:

      "/a/b/c"

      [ "a", "b", "c" ]

    When the argument $nds is passed in, it refers to an NDS. The NDS can
    either be a reference to a structure, or the name of an NDS stored in
    the object using the "nds" method.

    new
          $obj = new Data::NDS;

    version
          $version = $obj->version();

        Returns the version of the module.

    warnings
          $obj->warnings(BOOLEAN);

        If a true value is passed in, the module will issue warnings when
        they are encountered.

    structure
          $obj->structure(BOOLEAN);

        If a true value is pushed in, the module will keep track of the
        structure of the NDS and do checks on it where possible. If a false
        value is pushed in, it will not keep track of structure.

        The default is to keep track of structure.

    delim
          $obj->delim();
          $obj->delim($delim);

        When expressing the path as a string, the default delimiter is a
        slash (/). This can be changed using this function. Any string can
        be used as the delimiter. If called with no argument, it returns the
        delimiter.

    ruleset
          $err = $obj->ruleset($name);

        This creates a ruleset of the given name. $name must be
        alphanumeric, and must be created only a single time. The following
        names are reserved and may not be used:

          keep
          replace

        Error codes are:

          0     no error
          1     name not alphanumeric
          2     name previously created
          3     using a reserved ruleset name

    ruleset_valid
          $flag = $obj->ruleset_valid($name);

        This returns 1 if $name is a valid ruleset, 0 otherwise.

    path
          @path = $obj->path($path);
          @path = $obj->path(\@path);

          $path = $obj->path(\@path);
          $path = $obj->path($path);

        A path can be expressed in two different ways: a string with
        elements separted by the path delimiter, or as a list of elements.

        This method will convert between the two. In array context,it will
        return a list of path elements. In scalar context,it will return the
        path as a string with elements separated by the path delimiter.

        It is safe to pass in the list reference in list context, or the
        string version in scalar context. In both cases, the path will be
        returned unmodified.

        In string form, the path can be empty, or can consist only of the
        delimiter, and all of these will return an empty list (i.e. they
        point at the top level).

        In string form, a path may include the delimiter as the first
        character in the path, but it is optional, and the leading delimiter
        does NOT imply anything about where the path starts. In other words:

           /foo/1/bar
           foo/1/bar

        are identical.

    nds There are several different ways in which this method can be called.

          $err = $obj->nds($name,$nds);
          $err = $obj->nds($name,$nds,$new);

        These forms stores an NDS in the object under a given NAME ($name).
        If structural information is kept, it will check the structure of
        the NDS for problems. It will update structural information based on
        the NDS if $new is passed in and is true.

        The error value is the value of the check_structure method or -1 if
        a named NDS doesn't exist.

        If $nds is passed in as a reference, the actual structure (NOT a
        copy of it) is stored. If $nds is passed in as the name of a stored
        NDS, a copy of the structure will be stored under the new name.

          $nds = $obj->nds($name);

        This retrieves the named NDS from the object. If it does not exist,
        it will return nothing. It returns the actual stored NDS, NOT a copy
        of it, so modifying the returned NDS in any way will modify the
        stored copy.

          $nds = $obj->nds($name,"_copy");

        This is similar to the previous function but it will return a copy
        of the named NDS.

          $obj->nds($name,"_delete");

        This will delete the named NDS from the object. If the named NDS
        does not exist, it will return 0, otherwise it will return 1.

    empty
          $isempty = $obj->empty($nds);

        An NDS is empty if it only contains undef values.

        A scalar is empty if it is undef. The empty string "" is NOT treated
        as empty.

        A list is empty if it contains 0 elements, or if every element in it
        is empty.

        A hash is empty if it contains 0 keys, or if the value of every key
        is empty.

        The return value of the method is:

          0   if the NDS is not empty
          1   if the NDS is empty

    valid
          ($valid,$val,$where) = $obj->valid($nds,$path [,$copy]);

        This checks the $nds that is passed in (which can be either a
        structure or a named element) to see if $path is valid.

        If $path exists in $nds, it returns two values. The first is 1
        meaning that the path exists. The second is the value at that path.
        The value at the path is the actual value, NOT a copy of it (unless
        $copy is passed in, in which case it IS a copy).

        If $path does not exist in $nds, it returns three values. The first
        is 0 meaning that the path is not valid. The second one is an error
        code with more information about the error:

         -1   an NDS was passed in by name, but it is
              not valid (in this case, only two elements
              are returned)

          0   the path doesn't exist in $nds
          1   a hash key doesn't exist
          2   a list element doesn't exist

         10   the $nds has a scalar at a point which
              should refer to either a hash or array
         11   the $nds has a reference to an unsupported
              data type where it should refer to either
              a hash or array
         12   a non-integer value was used to access a
              list

        The third value is the path at which the error occurred.

        This method does NOT do any structural checking.

    value
          $val = $obj->value($nds,$path);

        This returns the value of the NDS at the given path. If any error
        ocurs, undef is returned.

        Note: this is simply a wrapper around the valid method, so to figure
        out what error occured, call valid directly.

    keys, values
          @ele = $obj->keys($nds,$path);
          @ele = $obj->values($nds,$path);

        This takes an NDS and returns a list of items at the given path.

        If the object at the path is a scalar, the keys method returns
        nothing. The values method returns the scalar.

        If the object at the path is a list, the keys method returns some of
        the integers 0..N where N is the index of the last element in the
        list. The indices for empty elements are omitted. The values method
        returns the non-empty members of the list.

        If the object at the path is a hash, the kyes method returns the
        non-empty keys of the hash. The values method returns the members of
        the list. The values method returns the non-empty values of the
        hash.

        Undef is returned in the case of an error.

    erase
          $err = $obj->erase($nds,$path);

        This will delete the given path from the NDS. It will delete
        elements from lists, clear elements from ordered lists, or delete
        entries from hashes.

        The return value of the method is:

          0   if there is no error
          1   if an undefined NDS is passed in by name
          2   if the path is invalid in this NDS

    set_structure
          $err = $obj->set_structure($item,$val [,$path]);

        This sets the given item of structural information. If the path is
        given, it sets items for that path, otherwise it sets default
        structural items.

        It returns 0 if there was no error, or else an error code. Several
        warnings may be issued if $obj->warnings has been called. The
        following error codes are used:

            1  Trying to set "type" to an invalid value
            2  Trying to reset "type"
            3  Trying to set "type" to a non-array/hash type when scalar/other
               is not valid

           10  Trying to set an unknown default structural item
           11  Trying to set an unknown structural item for a path

          100  Trying to set an "ordered" flag to something other than 0/1
          101  Trying to use an "ordered" flag on something other than an array
          102  Trying to reset "ordered" (or trying to set a non-uniform list to
               unordered)

          110  Trying to set an "uniform" flag to something other than 0/1
          111  Trying to use an "uniform" flag on something other than an array/hash
          112  Trying to reset "uniform" (or trying to set an unordered list to
               non-uniform)

          130  Trying to set structural information for a child with
               a scalar/other parent

          140  Trying to set structural information for a specific element
               in a "uniform" array
          141  Trying to set structural information for all list elements in
               a non-uniform array

          150  Trying to access a list with a non-integer index

          160  Trying to set structural information for a specific element
               in a uniform hash/array
          161  Trying to set structural information for all elements of a
               non-uniform hash/array

          170  Trying to set the default ordered value to something other than 0/1

          180  Trying to set the default uniform_hash value to something other
               than 0/1
          181  Trying to set the default uniform_ol value to something other
               than 0/1

    get_structure
          $val = $obj->get_structure($path [,$info]);

        This gets a piece of structural information for a path. $info can be
        any of the following:

          type      (this is the default) (returns "unknown" if not set)
          ordered
          uniform
          merge
          keys

        The appropriate value is returned. If information for a specific
        path is not available, default values will be returned. It returns
        nothing if the path has no structural information available.

        The keys information is a list of all known keys that can appear in
        a hash. This may only be used to query keys in a non-uniform hash.

    check_structure
          ($err,$val) = $obj->check_structure($nds [,$new]);

        This will take an NDS and traverse through it, checking the
        structure of every part of it. If $new is passed in, it is allowed
        to contribute new structural information. Otherwise, it must be
        completely defined by previously declared structural information.

        Error codes are:

          1   New structure found (but not allowed)
          2   Structure of invalid type found

        The value returned in the case of an error is the path where the
        error occurred.

    check_value
          ($err,$val) = $obj->check_value($path,$val [,$new]);

        This will check to see if $val has the correct structure to be
        stored at $path in an NDS. It will traverse through the structure of
        $val, similar to how the check_structure method traverses through an
        entire NDS.

        The values of $new, $err, and $val are the same as in the
        check_structure method.

    set_merge
          $err = $obj->set_merge($item,$method [,$ruleset]);
          $err = $obj->set_merge($item,$path,$method [,$ruleset]);

        This will define how to merge values. In the first form, it will set
        the default. $item can be merge_hash, merge_ol, merge_ul, or
        merge_scalar. In the second form, it will set the merge method for
        the given path. Currently, $item must be "merge".

          10   Trying to set an unknown value

          100  Trying to set merge_hash to an invalid value
          101  Trying to set merge_ol to an invalid value
          102  Trying to set merge_ul to an invalid value
          103  Trying to set merge_scalar to an invalid value

          120  Trying to reset "merge" value for a path
          121  Trying to set "merge" for a path with no known type

          130  Invalid merge method for ordered list merging
          131  Invalid merge method for unordered list merging
          132  Invalid merge method for hash merging
          133  Invalid merge method for scalar/other merging

    get_merge
          $method = $obj->get_merge($path [,$ruleset]);

        This gets the merge method for a path.

        The appropriate value is returned. If the method for a specific path
        is not available, default values will be returned. Nothing will be
        returned in the event of a problem.

    merge
          $err = $obj->merge($nds1,$nds2 [,$ruleset] [,$new]);

        This will take two NDSes (each of which can be passed in by name or
        by reference) and will recursively merge the second one into the
        first based on the rules of merging.

        The second NDS will be copied, so no part of the merged NDS will
        contain actual parts of the seconds NDS.

        The name of a ruleset can be passed in. If it is, that set of merge
        rules will be used to do the merging.

        If $new is passed in, it must be 0 or 1. If it is 1, $nds2 may
        provide new structural information. If $new is 0, $nds2 must be
        totally described by existing structural descriptions.

          0   no error
          1   $nds1 refers to a named NDS that does not exist
          2   $nds2 refers to a named NDS that does not exist
          3   $nds1 has an invalid structure
          4   $nds2 has an invalid structure
          5   $nds1/$nds2 not list/hash ref

    merge_path
          $err = $obj->merge_path($nds,$val,$path [,$ruleset] [,$new]);

        This will take an NDS (which can be passed in by name or reference)
        and merge $val into it at the given path. Using the special rulesets
        "replace", the value will replace whatever is there.

        $path must be valid, and $val must be structurally correct if
        structural information is kept.

        It will update structural information based on the NDS if $new is
        passed in and is true.

        The actual value passed in (not a copy) will be merged in.

        The error code is:

          0   no error
          1   a named NDS does not exist
          2   $nds has an invalid structure
          3   $val has an invalid structure

    identical, contains
          $flag = $obj->identical($nds1,$nds2 [,$new] [,$path]);
          $flag = $obj->contains ($nds1,$nds2 [,$new] [,$path]);

        The identical method checks to see if two NDSes are identical. If
        $path is given, only the part that starts at $path is checked.

        When comparing ordered lists, every element must be identical and in
        the same ordered. Unordered lists need to contain the same elements,
        but not necessarily in the same order. This works even if the
        unordered list contains structures instead of scalars.

        The contains method checks to see that $nds2 is a subset (i.e.
        contained in) $nds1. In other words, every scalar in $nds2 is
        identical to one in $nds1.

        undef is returned if there is any error.

        NOTE: because unordered lists must be compared in every possible
        combination, and recursively if the structure contains unordered
        lists which contain other unordered lists deeper in the structure,
        comparing NDSes with unordered lists can be extremely slow.

    which
           %hash = $obj->which($nds,@args)

        This returns a hash of { PATH => VAL } where PATH is is a path in
        $nds and VAL is the value at that path.

        The paths returned all fit the criteria specified in the arguments.

        If no arguments are passed in, a hash of all paths to non-empty
        scalars is returned (note that this means that scalars set to the
        empty string '' ARE returned).

        If @args is passed in, it is a list of criteria. If a scalar matches
        any one of them, it passes. Currently, @args may consist of a list
        of values (scalars) or regular expressions (set using the qr//
        operator). If the value at a path is equal to any of the values
        passed in in @args, or matches any of the regular expressions, then
        it passes.

    print
           $string = $obj->print($nds,%opts);

        This formats an NDS as a string based on a set of options. Known
        options are:

           indent => NUMBER     Specifies the amount of indentation to add
                                at each level. Indent must be 1 or more.
                                Default:  3
           width  => NUMBER     Specifies the width of a printing area. A
                                value of 0 means to not impose any width
                                limit. The minimum allowed width (other than
                                0) is 20.
                                Default:  79
           maxlevel => NUMBER   Specifies the maximum level to print. A value of
                                0 is all levels, but may only be used then a
                                width of 0 is used. Levels beyond this will
                                be pruned.
                                Default:  the number of levels that can be
                                          displayed given indent and width

        The NDS passed in must be a structure, not a named NDS.

EXAMPLES
    All examples assume the following lines:

       use Data::NDS;
       $obj = new Data::NDS;

    path method
        The path function can be used to switch back and forth between a
        path in string format and a path in list format.

           @path = $obj->path("/a/b");
              => ( a b )

           @path = $obj->path("a/b");
              => ( a b )

           @path = $obj->path(["a","b"]);
              => ( a b )

           $path = $obj->path("/a/b");
              => /a/b

           $path = $obj->path("a/b");
              => /a/b

           $path = $obj->path(["a","b"]);
              => /a/b

           @path = $obj->path("/");
              => ( )

           $path = $obj->path([]);
              => "/"

    nds method
        The nds method can be used to store or access a named NDS.

           $nds = { "a" => [ "a1", "a2" ],
                    "b" => [ "b1", "b2" ] };

           $obj->nds("ele1",$nds,1);

           $nds2 = $obj->nds("ele1");
              => { "a" => [ "a1", "a2" ],
                   "b" => [ "b1", "b2" ] }

    valid, value methods
        The valid method is used to check to see if a path is valid in the
        given NDS. It returns a 0/1 if the path is valid. If it is valid, it
        also returns the value at the path. If it is not valid, it returns
        an error code and the path where the error occurred.

           $nds = { "a" => undef,
                    "b" => "foo",
                    "c" => [ "c1", "c2" ],
                    "d" => { "d1k" => "d1v", "d2k" => "d2v" },
                  };

           $obj->valid($nds,"/a");
              => ( 1 undef )

           $obj->valid($nds,"/d/d3k");
              => ( 0 1 /d/d3k )

           $obj->valid($nds,"/f/1/2");
              => ( 0 1 /f )

           $obj->valid($nds,"/c/1");
              => ( 1 c2 )

           $obj->valid($nds,"/c/x");
              => ( 0 12 /c/x )

        The value method is simply a wrapper around valid.

           $val = $obj->value($nds,"/c/1");
              => c2

           $val = $obj->value($nds,"/c/x");
              => undef

    keys, values methods
        Using the samd NDS as defined in the "valid" examples.

           $obj->keys($nds,"/b");
              => ( )

           $obj->keys($nds,"/c");
              => ( 0 1 )

           $obj->keys($nds,"/d");
              => ( d1k d2k )

           $obj->values($nds,"/b");
              => ( foo )

           $obj->values($nds,"/c");
              => ( c1 c2 )

           $obj->values($nds,"/d");
              => ( d1v d2v )

    set_structure, get_structure methods
        These set or report the structure at a path.

        set_structure sets a piece of structural information for a path and
        returns an error code (0 if successful).

        To make sure that the path "/a" refers to a uniform hash, make the
        following two calls:

           $err = $obj->set_structure("type","hash","/a");
           $err = $obj->set_structure("uniform",1,"/a");

        To make sure that "/b" is an ordered list, and all elements in it
        are hashes, use the following calls:

           $err = $obj->set_structure("type","array","/b");
           $err = $obj->set_structure("ordered",1,"/b");
           $err = $obj->set_structure("type","hash","/b/*");

        get_structure will return the structural information for a path:

           $info = $obj->get_structure("/b","type");
              => array

    erase method
           $obj->set_structure("ordered","1","/o");
           $obj->set_structure("ordered","0","/u");

           $nds = { "h" => { "x" => 11, "y" => 22 },
                    "o" => [ qw(alpha beta gamma delta) ],
                    "u" => [ qw(alpha beta gamma delta) ],
                  };

        Erasing a hash key removes the key and value.

           $obj->erase($nds,"/h/x");
             => $nds = { h => { y => 22 },
                         o => [ alpha beta gamma delta ],
                         u => [ alpha beta gamma delta ],
                       }

        Erasing an element in an ordered list replaces it with an undef
        place holder.

           $obj->erase($nds,"/o/1");
             => $nds = { h => { y => 22 },
                         o => [ alpha UNDEF gamma delta ],
                         u => [ alpha beta gamma delta ],
                       }

        Erasing an element from an unordered list removes it completely.

           $obj->erase($nds,"/u/1");
             => $nds = { h => { y => 22 },
                         o => [ alpha UNDEF gamma delta ],
                         u => [ alpha gamma delta ],
                       }

    check_structure method
        You can use the set_structure routine to enforce structure. For
        example, if you want an NDS to be a hash, and in that hash are two
        keys "hu" who's value is a uniform hash, and "ul" who's value is an
        unordered list, use the following:

           $obj->set_structure("type","hash","/hu");
           $obj->set_structure("uniform",1,"/hu");

           $obj->set_structure("type","array","/ul");
           $obj->set_structure("ordered",0,"/ul");

        To check a structure to see if it fits this structure, use the
        check_structure method:

           $a = { "hu" => { "h1" => "h1v" } };
           $obj->check_structure($a,1);
              => ( 0 )

           $b = { "hu" => [ 1, 2 ] };
           $obj->check_structure($b,1);
              => ( 2 /hu )

        You can also add structural information by passing in an NDS that
        goes beyond whatever structure you have defined with set_structure.
        Additional structure will be determined from that structure IF you
        pass in a non-null value as the second argument. If no second
        argument is passed in (or a null value is passed in), the NDS being
        checked must have only the structure that has already been defined.

        For example:

           $b = { "ul" => [ { "aa" => 11 } ] };
           $obj->check_structure($b,0);
              => ( 1 /ul/* )

           $b = { "ul" => [ { "aa" => 11 } ] };
           $obj->check_structure($b,1);
              => ( 0 )

        In the first instance, the check_structure function returns an error
        code since the structure passed in contains structure that was not
        defined in the set_structure calls above.

        In the second instance, the added structure is examined and
        additional structural information is deternubed.

        Since "ul" is defined as an unordered (and therefore uniform) list,
        all of it's members must be identical. They are set to hashes based
        on the above check_structure call, so the following will fail since
        it tries to set them to scalars:

           $c = { "ul" => [ "foo" ] };
           $obj->check_structure($c,1);
              => ( 2 /ul/* )

    set_merge, get_merge methods
        To set the default merge method for a hash to be "keep" (see above
        for description of the various merge methods):

           $obj->set_merge("merge_hash","keep");
              => 0

        To set the merge method for a single element in an NDS, use
        something like the following:

           $err = $obj->set_structure("type","hash","/h");
           $obj->set_merge("merge","/h","keep");
              => 0

        The get_merge method can be used to query the type of merge that is
        done for a path:

           $obj->get_merge("/h");
              => keep

    identical, contains methods
           $a = { "a"  => "foo",
                  "b"  => "bar",
                  "c"  => "baz" };
           $b = { "a"  => "foo",
                  "b"  => "bar",
                  "c"  => "baz" };

           $obj->identical($a,$b,1);
              => 1

           $obj->contains($a,$b,1);
              => 1

           $c = { "a"  => "foo",
                  "c"  => "baz" };

           $obj->identical($a,$c,1);
              => 0

           $obj->contains($a,$c,1);
              => 1

        When looking at unordered lists, elements do not need to be in the
        same order:

           $a = [ qw(a b c) ];
           $b = [ qw(b a c) ];

           $obj->identical($a,$b,1);
              => 1

        Unordered lists can contain unordered lists and they still work:

           $a = [ [ qw(a b c) ], [ qw(d e f) ], [ qw(g h i) ] ];
           $b = [ [ qw(d e f) ], [ qw(a b c) ], [ qw(i g h) ] ];

           $obj->identical($a,$b,1);
              => 1

        This works regardless of the number of unordered lists and the
        intermediate structure (for example: unordered list of hashes
        pointing to unordered lists). Every time an unordered list is
        encountered, every possible combination will be tried. This can be
        very slow so care should be excercised in comparing structures
        containing unordered lists.

    merge method
        Merging hashes using keep, replace, and merge:

           $obj->set_merge("merge_hash","keep");
           $a = { "a"  => 1,
                  "b" => 2 };
           $b = { "a"  => 3,
                  "c" => 4 };
           $obj->merge($a,$b,1);
              => $a = { a => 1,
                        b => 2 }

           $obj->set_merge("merge_hash","replace");
           $a = { "a"  => 1,
                  "b" => 2 };
           $b = { "a"  => 3,
                  "c" => 4 };
           $obj->merge($a,$b,1);
              => $a = { a => 3,
                        c => 4 }

           $obj->set_merge("merge_hash","merge");
           $a = { "a"  => 1,
                  "b" => 2 };
           $b = { "a"  => 3,
                  "c" => 4 };
           $obj->merge($a,$b,1);
              => $a = { a => 1,
                        b => 2,
                        c => 4 }

        Merging unordered lists using keep, replace, and append:

           $obj->set_structure("ordered",0);
           $obj->set_merge("merge_ul","keep");
           $a = [ qw(a b c) ];
           $b = [ qw(d e f) ];
           $obj->merge($a,$b,1);
              => $a = [ a b c ]

           $obj->set_structure("ordered",0);
           $obj->set_merge("merge_ul","replace");
           $a = [ qw(a b c) ];
           $b = [ qw(d e f) ];
           $obj->merge($a,$b,1);
              => $a = [ a b c ]

           $obj->set_structure("ordered",0);
           $obj->set_merge("merge_ul","append");
           $a = [ qw(a b c) ];
           $b = [ qw(d e f) ];
           $obj->merge($a,$b,1);
              => $a = [ a b c d e f ]

        Merging ordered lists using keep, replace, and merge:

           $obj->set_structure("ordered",1);
           $obj->set_merge("merge_ol","keep");
           $a = [ "a", "", "b" ];
           $b = [ "c", "d", "" ];
           $obj->merge($a,$b,1);
              => $a = [ a '' b ]

           $obj->set_structure("ordered",1);
           $obj->set_merge("merge_ol","replace");
           $a = [ "a", "", "b" ];
           $b = [ "c", "d", "" ];
           $obj->merge($a,$b,1);
              => $a = [ c d '' ]

           $obj->set_structure("ordered",1);
           $obj->set_merge("merge_ol","merge");
           $a = [ "a", "", "b" ];
           $b = [ "c", "d", "" ];
           $obj->merge($a,$b,1);
              => $a = [ a d b ]

        A more complex example. Given structures consisting of ordered lists
        of hashes, merge them recursively.

           $a = [ { "a"  => 1,
                    "b"  => 2 },
                  { "c"  => 3 },
                  {},
                  { "d"  => 4,
                    "e"  => 5 } ];

           $b = [ { "a"  => 11,
                    "w"  => 22 },
                  {},
                  { "x"  => 33 },
                  { "d"  => 44 } ];

           $obj->set_structure("type",    "array", "/");
           $obj->set_structure("ordered", 1,       "/");

           $obj->set_structure("type",    "hash",  "/*");

           $obj->set_merge("merge",  "/",  "merge");
           $obj->set_merge("merge",  "/*", "merge");

           $obj->merge($a,$b,1);
              => $a = [ { a => 1, b => 2, w => 22 },
                        { c => 3 },
                        { x => 33 },
                        { d => 4, e => 5 } ]

    merge_path method
        merge_path is very similar to merge except that it merges a value
        into a full NDS starting at a specific path. For example:

           $a = { "a"  => [ 1,2,3 ],
                  "b"  => [ 4,5,6 ] };
           $obj->merge_path($a,[7,8,9],"/c",1);
              => $a = { a => [ 1 2 3 ],
                        b => [ 4 5 6 ],
                        c => [ 7 8 9 ] };

    which method
           $nds = { "b" => "foo",
                    "c" => [ "c1", "c2" ],
                    "d" => { "d1k" => "d1v", "d2k" => "d2v" },
                  };

        You can search for paths for a list of all scalars:

           %p = $obj->which($nds);
              => %p = ( /b     => foo
                        /c/0   => c1
                        /c/1   => c2
                        /d/d1k => d1v
                        /d/d2k => d2v )

        For a subset of scalars:

           %p = $obj->which($nds,"c2","d1v");
              => %p = ( /c/1   => c2
                        /d/d1k => d1v )

        For a set that matches regular expressions:

           %p = $obj->which($nds,qr/^c/);
              => %p = ( /c/0   => c1
                        /c/1   => c2 )

    using rulesets
        Rulesets are powerful tools for determining how you merge data
        structures.

        There are four very common uses of rulesets. They are so commonly
        used that pre-existing rulesets have been defined for them, but any
        number of other rulesets may also be defined.

        The "replace" ruleset may be used to set the structure stored at a
        path overriding any value currently there (but it will NOT replace
        structural information, so it can't be used to redefine what
        constitutes a valid structure).

           $a = { "a"  => [ 1,2,3 ],
                  "b"  => [ 4,5,6 ] };
           $obj->merge_path($a,[7,8,9],"/b","replace",1);
              => $a = { a => [ 1 2 3 ],
                        b => [ 7 8 9 ] }

        The "keep" ruleset will set the structure only if it isn't already
        set.

           $a = { "a"  => [ 1,2,3 ],
                  "b"  => [ 4,5,6 ] };
           $obj->merge_path($a,[7,8,9],"/b","keep",1);
              => $a = { a => [ 1 2 3 ],
                        b => [ 4 5 6 ] }

        The "default" ruleset will set defaults for a structure.

           $a = { "a" => 1,
                  "b" => 2 };
           $d = { "a" => 11,
                  "b" => 22,
                  "c" => 33 };
           $obj->merge($a,$d,"default",1);
              => $a = { a => 1,
                        b => 2,
                        c => 33 }

        The "override" ruleset will recursively override all values.

           $a = { "a" => 1,
                  "b" => 2 };
           $d = { "a" => 11,
                  "c" => 33 };
           $obj->merge($a,$d,"override",1);
              => $a = { a => 11,
                        b => 2,
                        c => 33 }

BACKWARDS INCOMPATIBILITIES
    1.01
        The keys and values methods now only return non-empty elements.

    1.04
        When working with an NDS, sometimes operations were performed on the
        actual structure, sometimes on copies of the structure. It is now
        documented which is which (and some behaviors were changed to be
        more consistent).

KNOWN PROBLEMS
    None at this point.

LICENSE
    This script is free software; you can redistribute it and/or modify it
    under the same terms as Perl itself.

AUTHOR
    Sullivan Beck (sbeck@cpan.org)

