Luis Muñoz > Hash-Filler > HashFiller

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NAME ^

HashFiller - Programatically fill elements of a hash based in prerequisites

SYNOPSIS ^

  use Hash::Filler;

  my $hf = new Hash::Filler;

                                # Show how a ->fill() method executes
                                # the rules
  $Hash::Filler::DEBUG = 1;

                                # Add a set of rules

  $hf->add('key1', sub { my $hr = shift; ... }, ['key2', 'key3'], $pref);
  $hf->add('key1', sub { my $hr = shift; ... }, [], $pref);
  $hf->add('key2', sub { my $hr = shift; ... }, ['key1', 'key3'], $pref);
  $hf->add('key3', sub { my $hr = shift; ... }, ['key1', 'key2'], $pref);
  $hf->add(undef, sub { ... }, [], $pref);

                                # Remove rules

  $hf->remove($rule_id);

  $hf->loop(0);                 # Don't try to avoid infinite loops

                                # Test if a key exists using defined()
  $hf->method($Hash::Filler::DEFINED);

  my %hash;

  $hf->fill(\%hash, 'key1');    # Calculate the value of $hash{key1}
  $hash{'key2'} = 'foo';        # Manually fill a hash position
  $hf->fill(\%hash, 'key2');    # Calculate the value of $hash{key2}
  $hr->dump_r_tree();           # Print the key tree

  my @stats = $hf->stats();     # Obtain statistics about rule invocation
  my @prof = $hf->profile();    # Obtain profiling information about the rules

DESCRIPTION ^

Hash::Filler provides an interface so that hash elements can be calculated depending in the existence of other hash elements, using user-supplied code references.

One of the first uses of this module was inside a server. In this server, the responses to commands came from external sources. For each request, the server needed to contact a number of external sources to calculate the proper answer. These calculations sometimes attempted redundant external accesses, thus increased the response time and load.

To help in this situation, the calculations were rewritten to access a hash instead of the external sources directly and this module was used to fill the hash depending on the requirements of the calculations. The external accesses were also improved so that more than one choice or rule existed for each datum, depending on wether prerequisites existed already in the hash or not.

Hopefully this explanation will make it easier to understand what this module is about :)

There are a few relevant methods, described below:

->add($key, $code, $r_prereq, $pref)

Adds a new rule to the Hash::Filler object. The rule will be used to fill the hash bucket identified with key $key. To fill this bucket, the code referenced by $code will be invoked, passing it a reference to the hash being worked on and the key that is being filled. This will only be called if all of the hash buckets whose keys are in the list referenced by $r_prereq exist.

If the user-supplied code returns a false value, failure is assumed.

An optional preference can be supplied. This is used to help the internal rule selection choose the better rule.

Multiple rules for the same $key and the same $r_prereq can be added. The module will attempt to use them both but the execution order will be undefined unless you use $pref. The default $pref is 100.

A special case occurs when $key is undefined. In this case, this rule is said to be a 'wildcard'. This means that the rule applies to any key that needs to be filled. Wildcard rules are applied after any matching rules (ie, after rules that apply specifically to a given $key). Multiple wildcard rules are selected based in the preference and availability of prerequisites.

This function returns a 'rule identifier'. This identifier is the index that designates a given rule. Generally, it is only used in conjunction with profiling.

->remove($id)

Removes the rule whose identifier matches $id. The implementation actually does not remove the rule. Instead it marks the rule as non-usable.

->dump_r_tree

This method prints out a representation of the rule tree kept by the object. The tree lists the rules in the order they would be preferred for a given key.

Output is sent to STDOUT.

->profile

Returns an array containing time profiles for the execution of each rule. The index in the array is the identifier assigned for each rule. Each slot in the array contain the accumulated time for all the invocations of that particular rule.

Slot 0 in the array contains the accumulated time for ALL the invoked rules. This make it easier to find the most important contributors to the accumulated time.

Note that time is only computed if the user-supplied method must be called. Whenever the hash bucket to be filled by a rule already has a value, this method will not be called and no time will be added to this rule.

->stats

This method returns an array which counts the number of times a given rule has been invoked and its user-supplied method has been called. The index into the array is the rule identifier, just as in the case of ->profile. The 0th element of the array contains the total number of times that ->fill has been called. This is useful to deduce how many times the rules needed to be invoked.

->method($val)

Which method to use to decide if a given key is present in the hash. The accepted values are:

$Hash::Filler::EXISTS (default)
    The existence of a hash element or key is calculated using a
    construct like C<exists($hash{$key})>.
$Hash::Filler::DEFINED
    The existence of a hash element or key is calculated using a
    construct like C<defined($hash{$key})>.
$Hash::Filler::TRUE
    The existence of a hash element or key is calculated using a
    construct like C<$hash{$key}>.
Reference to a sub
    This allows the user to specify a function to determine wether a
    hash bucket must be calculated or not. The function is invoked by
    passing it a reference to the hash and the key that must be
    checked. The function must return a TRUE value is the bucket is
    already populated or false if the corresponding rules must be
    applied.

This allow this module to be customized to the particular application in which it is being used. Be advised that changing this might cause a change in which and when the rules are invoked for a particular hash so probably it should only be used before the first call to ->fill.

By defult, the module uses exists() to do this check.

->loop($val)

Controls if the module should try to avoid infinite loops. A true $val means that it must try (the default). A false value means otherwise.

->fill($r_hash, $key)

Attempts to fill the bucket $key of the hash referenced by $r_hash using the supplied rules.

This method will return a true value if there are rules that allow the requested $key to be calculated (or the $key is in the hash) and the user supplied code returned true.

To avoid infinite loops, the code will not invoke a rule twice unless ->loop is called with a true value. The rules will be used starting with the ones with less prerequisites, as these are assumed to be lighter. To use a different ordering, specify $pref. Higher values of $pref are used first.

CAVEATS ^

This code uses recursion to resolve rules. This allows it to figure out the value for a given key with only an incomplete rule specification. Be warned that this might be costly if used with large sets of rules.

AUTHOR ^

Luis E. Munoz < lem@cantv.net>

SEE ALSO ^

perl(1).

WARRANTY ^

Absolutely none.

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