add INTERCEPT function

Closes #296

README.md

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

lib/dentaku/ast.rb

@@ -24,6 +24,7 @@
 require_relative './ast/functions/duration'
 require_relative './ast/functions/filter'
 require_relative './ast/functions/if'
+require_relative './ast/functions/intercept'
 require_relative './ast/functions/map'
 require_relative './ast/functions/max'
 require_relative './ast/functions/min'
@@ -38,4 +39,4 @@
 require_relative './ast/functions/string_functions'
 require_relative './ast/functions/sum'
 require_relative './ast/functions/switch'
-require_relative './ast/functions/xor'
+require_relative './ast/functions/xor'

lib/dentaku/ast/functions/intercept.rb

@@ -0,0 +1,35 @@
+require_relative '../function'
+
+Dentaku::AST::Function.register(:intercept, :list, ->(*args) {
+  flatten_args = args.flatten
+  if flatten_args.length != 2 || !flatten_args.all? { |arg| arg.is_a?(Array) } || flatten_args.any? { |arg| arg.empty? }
+    raise Dentaku::ArgumentError.for(
+        :wrong_number_of_arguments,
+        function_name: 'INTERCEPT()', exact: 2, given: flatten_args.length
+    ), 'INTERCEPT() requires exactly two arrays of numbers'
+  end
+
+  x_values, y_values = flatten_args
+  if x_values.length != y_values.length
+    raise Dentaku::ArgumentError.for(
+        :unequal_array_lengths,
+        function_name: 'INTERCEPT()'
+    ), 'INTERCEPT() requires arrays of equal length'
+  end
+
+  x_values = x_values.map { |arg| Dentaku::AST::Function.numeric(arg) }
+  y_values = y_values.map { |arg| Dentaku::AST::Function.numeric(arg) }
+
+  x_sum = x_values.reduce(0, :+)
+  y_sum = y_values.reduce(0, :+)
+  xy_sum = x_values.zip(y_values).map { |x, y| x * y }.reduce(0, :+)
+  x_square_sum = x_values.map { |x| x**2 }.reduce(0, :+)
+
+  n = x_values.length
+
+  slope = (n * xy_sum - x_sum * y_sum) / (n * x_square_sum - x_sum**2)
+  intercept = y_values.reduce(:+) / n - slope * x_values.reduce(:+) / n
+
+  BigDecimal(intercept)
+})
+

spec/ast/intercept_spec.rb

@@ -0,0 +1,30 @@
+require 'spec_helper'
+require 'dentaku/ast/functions/intercept'
+require 'dentaku'
+
+describe 'Dentaku::AST::Function::Intercept' do
+  it 'returns the correct intercept for given x and y arrays' do
+    x_values = [1, 2, 3, 4, 5]
+    y_values = [2, 3, 5, 4, 6]
+    result = Dentaku('INTERCEPT(?, ?)', x_values, y_values)
+    expect(result).to be_within(0.001).of(1.2)
+  end
+
+  context 'checking errors' do
+    it 'raises an error if arguments are not arrays' do
+      expect { Dentaku!("INTERCEPT(1, 2)") }.to raise_error(Dentaku::ArgumentError)
+    end
+
+    it 'raises an error if the arrays are not of equal length' do
+      x_values = [1, 2, 3]
+      y_values = [2, 3, 5, 4]
+      expect { Dentaku!("INTERCEPT(?, ?)", x_values, y_values) }.to raise_error(Dentaku::ArgumentError)
+    end
+
+    it 'raises an error if any of the arrays is empty' do
+      x_values = []
+      y_values = [2, 3, 5, 4]
+      expect { Dentaku!("INTERCEPT(?, ?)", x_values, y_values) }.to raise_error(Dentaku::ArgumentError)
+    end
+  end
+end

spec/calculator_spec.rb

@@ -522,6 +522,13 @@
       expect(calculator.evaluate('NOT(some_boolean) AND -1 > 3', some_boolean: true)).to be_falsey
     end
 
+    it 'calculates intercept correctly' do
+      x_values = [1, 2, 3, 4, 5]
+      y_values = [2, 3, 5, 4, 6]
+      result = calculator.evaluate('INTERCEPT(x_values, y_values)', x_values: x_values, y_values: y_values)
+      expect(result).to be_within(0.001).of(1.2)
+    end
+
     describe "any" do
       it "enumerates values and returns true if any evaluation is truthy" do
         expect(calculator.evaluate!('any(xs, x, x > 3)', xs: [1, 2, 3, 4])).to be_truthy