Support AVG (average) function.

closes #157

CHANGELOG.md

@@ -1,5 +1,8 @@
 # Change Log
 
+## [HEAD] Unreleased
+- Add `COUNT` and `AVG` functions
+
 ## [v3.1.0] 2017-01-10
 - allow decimals with no leading zero
 - nested hash and array support in bulk expression solver
@@ -151,6 +154,7 @@
 ## [v0.1.0] 2012-01-20
 - initial release
 
+[HEAD]:  https://github.com/rubysolo/dentaku/compare/v3.1.0...HEAD
 [v3.1.0]:  https://github.com/rubysolo/dentaku/compare/v3.0.0...v3.1.0
 [v3.0.0]:  https://github.com/rubysolo/dentaku/compare/v2.0.11...v3.0.0
 [v2.0.11]:  https://github.com/rubysolo/dentaku/compare/v2.0.10...v2.0.11

README.md

@@ -146,7 +146,7 @@ Comparison: `<`, `>`, `<=`, `>=`, `<>`, `!=`, `=`,
 
 Logic: `IF`, `AND`, `OR`, `NOT`, `SWITCH`
 
-Numeric: `MIN`, `MAX`, `SUM`, `ROUND`, `ROUNDDOWN`, `ROUNDUP`
+Numeric: `MIN`, `MAX`, `SUM`, `AVG`, `COUNT`, `ROUND`, `ROUNDDOWN`, `ROUNDUP`
 
 Selections: `CASE` (syntax see [spec](https://github.com/rubysolo/dentaku/blob/master/spec/calculator_spec.rb#L292))
 

lib/dentaku/ast/functions/avg.rb

@@ -0,0 +1,13 @@
+require_relative '../function'
+
+Dentaku::AST::Function.register(:avg, :numeric, ->(*args) {
+  if args.empty?
+    raise Dentaku::ArgumentError.for(
+        :too_few_arguments,
+        function_name: 'AVG()', at_least: 1, given: 0
+    ), 'AVG() requires at least one argument'
+  end
+
+  flatten_args = args.flatten
+  flatten_args.map { |arg| Dentaku::AST::Function.numeric(arg) }.reduce(0, :+) / flatten_args.length
+})

spec/ast/avg_spec.rb

@@ -0,0 +1,33 @@
+require 'spec_helper'
+require 'dentaku/ast/functions/avg'
+require 'dentaku'
+
+describe 'Dentaku::AST::Function::Avg' do
+  it 'returns the average of an array of Numeric values' do
+    result = Dentaku('AVG(1, x, 1.8)', x: 2.3)
+    expect(result).to eq 1.7
+  end
+
+  it 'returns the average of a single entry array of a Numeric value' do
+    result = Dentaku('AVG(x)', x: 2.3)
+    expect(result).to eq 2.3
+  end
+
+  it 'returns the average even if a String is passed' do
+    result = Dentaku('AVG(1, x, 1.8)', x: '2.3')
+    expect(result).to eq 1.7
+  end
+
+  it 'returns the average even if an array is passed' do
+    result = Dentaku('AVG(1, x, 2.3)', x: [4, 5])
+    expect(result).to eq 3.075
+  end
+
+  context 'checking errors' do
+    let(:calculator) { Dentaku::Calculator.new }
+
+    it 'raises an error if no arguments are passed' do
+      expect { calculator.evaluate!('AVG()') }.to raise_error(ArgumentError)
+    end
+  end
+end