BigDecimal.with_decimal_precision

bigdecimal_extensions.rb

class BigDecimal
   # `BigDecimal(123.456, 2)` becomes 120.0. It keeps 2 *significant digits*.
   # If you want two *decimals* of precision, use this method.
   def self.with_decimal_precision(value, precision)
     if value.is_a?(Float)
       length_before_decimal = Math.log10(value.abs).to_i + 1  # log for performance.
       BigDecimal(value, length_before_decimal + precision)
     else
       BigDecimal(value, precision)
     end
   end
end

bigdecimal_extensions_spec.rb

require "rails_helper"

RSpec.describe BigDecimal do
  describe ".with_decimal_precision" do
    it "turns a float into a BigDecimal with the given decimal precision" do
      expect(BigDecimal.with_decimal_precision(1234.5678, 0)).to eq(BigDecimal("1235"))
      expect(BigDecimal.with_decimal_precision(1234.5678, 1)).to eq(BigDecimal("1234.6"))
      expect(BigDecimal.with_decimal_precision(1234.5678, 2)).to eq(BigDecimal("1234.57"))
      expect(BigDecimal.with_decimal_precision(1234.5678, 5)).to eq(BigDecimal("1234.56780"))
    end

    it "handles negative floats" do
      expect(BigDecimal.with_decimal_precision(-1234.5678, 2)).to eq(BigDecimal("-1234.57"))
    end

    it "handles negative precisions" do
      expect(BigDecimal.with_decimal_precision(1234.5678, -1)).to eq(BigDecimal("1230"))
      expect(BigDecimal.with_decimal_precision(1234.5678, -2)).to eq(BigDecimal("1200"))
    end

    it "turns non-floats into BigDecimal, ignoring the precision" do
      expect(BigDecimal.with_decimal_precision(1234, 2)).to eq(BigDecimal("1234"))
      expect(BigDecimal.with_decimal_precision("1234.567", 2)).to eq(BigDecimal("1234.567"))
      expect(BigDecimal.with_decimal_precision(BigDecimal("1234.567"), 2)).to eq(BigDecimal("1234.567"))
    end
  end
end