ExtremeAndy.CombinatoryFilters 1.0.4

CombinatoryFilters

Functional filter abstraction for creating, applying, mapping, and reducing combinatory filter structures

Installation

dotnet add package ExtremeAndy.CombinatoryFilters

Usage

1. Define your filter interface(s) and/or class(es). Here's an example of a simple filter which checks whether an integer is between UpperBound and LowerBound.

public class NumericRangeFilter : LeafFilterNode<NumericRangeFilter>, IRealisableLeafFilterNode<int>
{
    public NumericRangeFilter(int lowerBound, int upperBound)
    {
        LowerBound = lowerBound;
        UpperBound = upperBound;
    }

    public int LowerBound { get; }

    public int UpperBound { get; }

    public bool IsMatch(int item) => LowerBound <= item && item <= UpperBound;
}

2. Optionally implement IEquatable<IFilterNode> on your filter class. If this is not done, then calling .Equals() on an IFilterNode in your filter tree will default to reference equality when comparing your leaf filters.

3. Create an instance of your filter and apply it to some values

var filter = new NumericRangeFilter(5, 10);
var values = new[] { 1, 3, 5, 9, 11 };
var expectedFilteredValues = new[] { 5, 9 };

var filterPredicate = filter.GetPredicate<NumericRangeFilter, int>();
var filteredValues = values.Where(filterPredicate);

Assert.Equal(expectedFilteredValues, filteredValues);

Complex filters

You can assemble arbitrarily complex filters as follows:

var filter5To10 = new NumericRangeFilter(5, 10);
var filter8To15 = new NumericRangeFilter(8, 15);
var filter5To10Or8To15 = new CombinationFilter<NumericRangeFilter>(new[] { filter5To10, filter8To15 }, CombinationOperator.Or);
var filter9To12 = new NumericRangeFilter(9, 12);
var filter = new CombinationFilter<NumericRangeFilter>(new IFilterNode<NumericRangeFilter>[] { filter5To10Or8To15, filter9To12 }, CombinationOperator.And);

Inversion

Any filter can be inverted using .Invert().

Testing a single value

You can test a single value as follows:

var filter = new NumericRangeFilter(5, 10);
var isMatch = filter.IsMatch(7);

Preserving ordering of filters

CombinationFilter stores filters as an IImmutableSet. If you wish to preserve the order of your filters, use OrderedCombinationFilter instead.

Advanced usage

IFilterNode<> supports Map and Match for mapping and reducing filters.

Map usage

In this example, we reduce the range of the leaf node filters by increasing the lower bound by 1 and decreasing the upper bound by 1. The structure of all the And, Or and Invert operations remains unchanged.

var shortenedFilters = filter.Map(f =>
{
    var newLowerBound = f.LowerBound + 1;
    var newUpperBound = f.UpperBound - 1;
    return new NumericRangeFilter(newLowerBound, newUpperBound);
});

Match usage

In this example, we want to compute the length of the longest filter interval, or infinity if any filter is inverted.

var longestIntervalLength = filter.Match<double>(
    (lengths, _) => lengths.Max(),
    length => double.PositiveInfinity,
    f => f.UpperBound - f.LowerBound);