This library provides:

• Decimal64: Custom decimal number value type of 64-bit size. It supports up to 16 decimal digits of accuracy.
• DecimalFP64: IEEE 754 compliant decimal number value type of 64-bit size. It supports up to 16 decimal digits of accuracy.
• Conversion to and from Swift number types to decimal numbers.

The decimal number types have no dependencies (not even Foundation) and work on any OS supporting Swift 5.1 or later.

Types

• Decimal64.

  Custom decimal number value type focused on speed. It conforms to SignedNumeric, Strideable, but not FloatingPoint. It has no support for +/- Infinity and ignores NaNs.

  let pi: Decimal64 = .pi
  let multiplier: Decimal64 = 2.5
  let bias = Decimal64(7, power: 3)  // 7 * 10^3
  let divisor: Decimal64 = "0.953"   // Supports string-number conversion
  let resultA = pi * multiplier + bias / divisor
  let resultB = resultA >> 2         // Divide by 100 (i.e. 10^-2)

  The decimal number uses an internal 64-bit composed of two values: significand and exponent. Decimal numbers use the formula significand * 10 ^ exponent. The internal values use two's complement.

  

• DecimalFP64.

  Decimal number conforming to the FloatingPoint trying to follow IEEE 754 as closely as possible and supports +/- Infinity, signed zeros, and NaNs.

  The decimal number uses an internal 64-bit composed of three values: exponent, sign, and significand. Decimal numbers use the formula sign * significand * 10 ^ exponent.

  

  DecimalFP64 is a work-in-progress and it still lacks some functionality. Don't use it in production code.

• Foundation's Decimal.

  Reference decimal number type of 160-bit size. It is implemented in Objective-C which doesn't conform to FloatingPoint. It is not very good for performant sensitive processing (overhead on retain/release and general processing).

• Swift's Double.

  Value binary floating-point type with 64-bit backed by fast hardware support. As it is well known, binary floating points are unable to express certain decimal numbers.

Performance

This Swift package contains a small benchmarking command-line application. The benchmark measures the performance of a few basic operations (+, *, /, and string conversion) for the previously defined floating-point types: Decimal64, DecimalFP64, Decimal, and Double.

The results vary depending on the host machine and OS, but in the author's machine (compiling for release):

Type	Duration (secs)
Decimal64	1.026
DecimalFP64	1.087
Double	1.381
Decimal	5.137
Double (generic)	1.823
DecimalFP64 (generic)	1.786

Be sure to compile and run for release. It doesn't make sense to measure performance with debug configuration for several reasons:

• The Swift Standard Library and Foundation linked are already compiled for release.
• The provided decimal numbers heavily rely on inlineable functionality and Swift optimizations.