ruby 43 lines · 7 steps

Handling money safely with BigDecimal in Ruby

A module that converts, rounds, taxes, and splits money using exact decimal arithmetic instead of error-prone floats.

Explained by highlit
Intermediate bigdecimal rounding currency module-function precision
1require "bigdecimal"
2 
3module Money
4 CENTS = BigDecimal("0.01")
5 
6 module_function
7 
8 def to_decimal(amount)
9 case amount
10 when BigDecimal then amount
11 when Integer then BigDecimal(amount)
12 when Float then BigDecimal(amount.to_s)
13 when String then BigDecimal(amount)
14 else
15 raise ArgumentError, "cannot convert #{amount.class} to money"
16 end
17 end
18 
19 def round(amount)
20 to_decimal(amount).round(2, BigDecimal::ROUND_HALF_EVEN)
21 end
22 
23 def apply_tax(subtotal, rate)
24 tax = to_decimal(subtotal) * to_decimal(rate)
25 round(tax)
26 end
27 
28 def split(total, ways)
29 raise ArgumentError, "ways must be positive" unless ways.positive?
30 
31 cents = (to_decimal(total) / CENTS).round
32 base, remainder = cents.divmod(ways)
33 
34 Array.new(ways) do |i|
35 share = i < remainder ? base + 1 : base
36 (share * CENTS)
37 end
38 end
39 
40 def format(amount)
41 format("$%.2f", round(amount))
42 end
43end
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Representing money as BigDecimal avoids the rounding errors floats introduce, especially when parsing floats via their string form.
  2. 2Banker's rounding (ROUND_HALF_EVEN) reduces systematic bias when rounding many monetary values.
  3. 3Splitting money in integer cents and distributing the remainder guarantees the shares add back to the exact total.

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