Handle rounding edge case

A lower-rate base currency like IDR previously produced less precise quotes.

Fixes #14

CHANGELOG.md

@@ -4,6 +4,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 ### Changed
+- Handle rounding edge case where a lower-rate base currency like IDR produces less precise quotes
 - Sample weekly when querying over a year
 - Bump PostgreSQL to 12
 

lib/roundable.rb

@@ -7,6 +7,8 @@ module Roundable
   # greater than around 20 are usually quoted to three decimal places and
   # exchange rates greater than 80 are quoted to two decimal places.
   # Currencies over 5000 are usually quoted with no decimal places.
+  #
+  # https://en.wikipedia.org/wiki/Exchange_rate#Quotations
   def round(value)
     if value > 5000
       value.round
@@ -16,8 +18,13 @@ module Roundable
       Float(format('%<value>.3f', value: value))
     elsif value > 1
       Float(format('%<value>.4f', value: value))
-    else
+    # I had originally opted to round smaller numbers simply to five decimal
+    # places but introduced this refinement to handle an edge case where a
+    # lower-rate base currency like IDR produces less precise quotes.
+    elsif value > 0.0001
       Float(format('%<value>.5f', value: value))
+    else
+      Float(format('%<value>.6f', value: value))
     end
   end
 end

spec/roundable_spec.rb

@@ -7,25 +7,38 @@ describe Roundable do
   include Roundable
 
   it 'rounds values over 5,000 to zero decimal places' do
-    round(5000.123456).must_equal 5000.0
+    _(round(5000.123456)).must_equal 5000
   end
 
   it 'rounds values over 80 and below 5,000 to two decimal places' do
-    round(80.123456).must_equal 80.12
-    round(4999.123456).must_equal 4999.12
+    _(round(80.123456)).must_equal 80.12
+    _(round(4999.123456)).must_equal 4999.12
   end
 
   it 'rounds values over 20 and below 80 to three decimal places' do
-    round(79.123456).must_equal 79.123
-    round(20.123456).must_equal 20.123
+    _(round(79.123456)).must_equal 79.123
+    _(round(20.123456)).must_equal 20.123
   end
 
   it 'rounds values over 1 and below 20 to four decimal places' do
-    round(19.123456).must_equal 19.1235
-    round(1.123456).must_equal 1.1235
+    _(round(19.123456)).must_equal 19.1235
+    _(round(1.123456)).must_equal 1.1235
   end
 
   it 'rounds values below 1 to five decimal places' do
-    round(0.123456).must_equal 0.12346
+    _(round(0.123456)).must_equal 0.12346
+  end
+
+  it 'rounds values below 0.0001 to six decimal places' do
+    _(round(0.0000655)).must_equal 0.000066
+  end
+
+  it 'conforms to ECB conventions' do
+    skip "We don't conform ¯\_(ツ)_/¯"
+    require 'day'
+    rates = Day.all.sample.rates.to_a
+    rates.shuffle.each do |_currency, rate|
+      _(round(rate)).must_equal rate
+    end
   end
 end