add ABS function (#284)

* feat: abs function

* test: cover abs

* feat: require new function

* chore: update readme

* test: cover within calculator spec

README.md

@@ -145,7 +145,7 @@ Comparison: `<`, `>`, `<=`, `>=`, `<>`, `!=`, `=`,
 
 Logic: `IF`, `AND`, `OR`, `XOR`, `NOT`, `SWITCH`
 
-Numeric: `MIN`, `MAX`, `SUM`, `AVG`, `COUNT`, `ROUND`, `ROUNDDOWN`, `ROUNDUP`
+Numeric: `MIN`, `MAX`, `SUM`, `AVG`, `COUNT`, `ROUND`, `ROUNDDOWN`, `ROUNDUP`, `ABS`
 
 Selections: `CASE` (syntax see [spec](https://github.com/rubysolo/dentaku/blob/master/spec/calculator_spec.rb#L593))
 

lib/dentaku/ast.rb

@@ -15,6 +15,7 @@
 require_relative './ast/grouping'
 require_relative './ast/case'
 require_relative './ast/function_registry'
+require_relative './ast/functions/abs'
 require_relative './ast/functions/all'
 require_relative './ast/functions/and'
 require_relative './ast/functions/any'

lib/dentaku/ast/functions/abs.rb

@@ -0,0 +1,5 @@
+require_relative '../function'
+
+Dentaku::AST::Function.register(:abs, :numeric, lambda { |numeric|
+  Dentaku::AST::Function.numeric(numeric).abs
+})

spec/ast/abs_spec.rb

@@ -0,0 +1,26 @@
+require 'spec_helper'
+require 'dentaku/ast/functions/abs'
+require 'dentaku'
+
+describe 'Dentaku::AST::Function::Abs' do
+  it 'returns the absolute value of number' do
+    result = Dentaku('ABS(-4.2)')
+    expect(result).to eq(4.2)
+  end
+
+  it 'returns the correct value for positive number' do
+    result = Dentaku('ABS(1.3)')
+    expect(result).to eq(1.3)
+  end
+
+  it 'returns the correct value for zero' do
+    result = Dentaku('ABS(0)')
+    expect(result).to eq(0)
+  end
+
+  context 'checking errors' do
+    it 'raises an error if argument is not numeric' do
+      expect { Dentaku!("ABS(2020-1-1)") }.to raise_error(Dentaku::ArgumentError)
+    end
+  end
+end

spec/calculator_spec.rb

@@ -471,6 +471,13 @@
       expect(calculator.evaluate('ROUND(apples * 0.93)', apples: 10)).to eq(9)
     end
 
+    it 'include ABS' do
+      expect(calculator.evaluate('abs(-2.2)')).to eq(2.2)
+      expect(calculator.evaluate('abs(5)')).to eq(5)
+
+      expect(calculator.evaluate('ABS(x * -1)', x: 10)).to eq(10)
+    end
+
     it 'include NOT' do
       expect(calculator.evaluate('NOT(some_boolean)', some_boolean: true)).to be_falsey
       expect(calculator.evaluate('NOT(some_boolean)', some_boolean: false)).to be_truthy