add FILTER function

Pretty much a copy-paste of the existing MAP function.

Author: dleavitt

README.md

@@ -152,6 +152,8 @@ Selections: `CASE` (syntax see [spec](https://github.com/rubysolo/dentaku/blob/m
 
 String: `LEFT`, `RIGHT`, `MID`, `LEN`, `FIND`, `SUBSTITUTE`, `CONCAT`, `CONTAINS`
 
+Collection: `MAP`, `FILTER`, `ALL`, `ANY`
+
 RESOLVING DEPENDENCIES
 ----------------------
 

lib/dentaku/ast/functions/filter.rb

@@ -0,0 +1,36 @@
+require_relative '../function'
+require_relative '../../exceptions'
+
+module Dentaku
+  module AST
+    class Filter < Function
+      def self.min_param_count
+        3
+      end
+
+      def self.max_param_count
+        3
+      end
+
+      def deferred_args
+        [1, 2]
+      end
+
+      def value(context = {})
+        collection      = @args[0].value(context)
+        item_identifier = @args[1].identifier
+        expression      = @args[2]
+
+        Array(collection).filter do |item_value|
+          expression.value(
+            context.merge(
+              FlatHash.from_hash_with_intermediates(item_identifier => item_value)
+            )
+          )
+        end
+      end
+    end
+  end
+end
+
+Dentaku::AST::Function.register_class(:filter, Dentaku::AST::Filter)

spec/ast/filter_spec.rb

@@ -0,0 +1,18 @@
+require 'spec_helper'
+require 'dentaku/ast/functions/filter'
+require 'dentaku'
+
+describe Dentaku::AST::Filter do
+  it 'excludes unmatched values' do
+    result = Dentaku('SUM(FILTER(vals, val, val > 1))', vals: [1, 2, 3])
+    expect(result).to eq(5)
+  end
+
+  it 'works with a single value if needed for some reason' do
+    result = Dentaku('FILTER(vals, val, val > 1)', vals: 1)
+    expect(result).to eq([])
+
+    result = Dentaku('FILTER(vals, val, val > 1)', vals: 2)
+    expect(result).to eq([2])
+  end
+end