FixedPointDecimal

Financial systems need exact decimal arithmetic -- binary floating-point (Double) cannot represent 0.1 exactly, causing accumulation errors. Foundation.Decimal solves exactness but carries inherent overhead: it is a 20-byte variable-precision type whose arithmetic must operate on a multi-word mantissa, handle variable exponents, and manage intermediate results that may exceed the mantissa width.

FixedPointDecimal takes a different approach: fix the precision at compile time (8 fractional digits) and use a plain Int64 as the backing store.

let price: FixedPointDecimal = 123.45
let quantity: FixedPointDecimal = 1000
let notional = price * quantity  // 123450

Full API documentation is available on the Swift Package Index.

Features

• Zero heap allocations for all arithmetic, comparison, conversion, and rounding operations
• @frozen for cross-module inlining and optimal ContiguousArray layout
• Pure Swift core -- no Foundation dependency except for Decimal conversions
• Cross-platform -- Linux + all Apple platforms, x86 + ARM, Swift 6.2
• Safe by default -- trapping arithmetic (matching Swift Int), with wrapping and overflow-reporting variants
• NaN support -- sentinel-based NaN that traps all operations
• Banker's rounding everywhere -- all entry points (string parsing, Double conversion, Decimal conversion, arithmetic) use banker's rounding (round half to even). Explicit rounded(scale:_:) supports six modes

Performance

Operation	FixedPointDecimal	Foundation.Decimal	Speedup
Addition	0.67 ns	240 ns	359x
Subtraction	0.67 ns	283 ns	424x
Multiplication	8 ns	607 ns	79x
Division	8 ns	1,285 ns	168x
Comparison (<)	0.33 ns	300 ns	901x
Equality (==)	0.34 ns	317 ns	943x
Hash	5 ns	261 ns	48x
To Double	0.49 ns	271 ns	551x
String description	44 ns	1,045 ns	24x
JSON encode	320 ns	1,215 ns	3.8x
JSON decode	457 ns	831 ns	1.8x
init(significand:exponent:)	0.41 ns	—	—
init(Double)	1.4 ns	2,319 ns	1,622x
rounded(scale:)	2 ns	705 ns	349x

Zero heap allocations across all operations. 8 bytes in-memory and on the wire (vs 20 for Decimal).

Measured on Apple M4 Max, Swift 6.2, p50 wall clock, using package-benchmark. See the full performance analysis for instruction counts, allocation breakdowns, and memory layout details.

Range and Precision

Property	Value
Fractional digits	8 (fixed)
Minimum value	-92,233,720,368.54775807
Maximum value	92,233,720,368.54775807
Smallest positive	0.00000001
Storage	@frozen struct, Int64 (8 bytes)

Eight fractional digits cover all practical financial instruments: cents (2), mils (3), basis points (4), FX pips (5), and cryptocurrency satoshis (8). The range (~92 billion) is sufficient for individual prices and quantities but may require Int128 backing for aggregated notional values (see 06-future-128bit.md).

Installation

Add to your Package.swift:

dependencies: [
    .package(url: "https://github.com/ordo-one/FixedPoint.git", from: "1.0.0"),
]

Then add the dependency to your target:

.target(
    name: "MyTarget",
    dependencies: [
        .product(name: "FixedPointDecimal", package: "FixedPoint"),
    ]
)

Protocol Conformances

Core (all platforms): Sendable, BitwiseCopyable, AtomicRepresentable, Equatable, Hashable, Comparable, AdditiveArithmetic, Numeric, SignedNumeric, Strideable, ExpressibleByIntegerLiteral, ExpressibleByFloatLiteral, Codable, CustomStringConvertible, CustomDebugStringConvertible, LosslessStringConvertible, CustomReflectable

SwiftUI (macOS/iOS): VectorArithmetic, Plottable

Foundation (all platforms): Decimal.FormatStyle forwarding (.number, .percent, .currency(code:) and all modifiers), plus a dedicated FixedPointDecimalFormatStyle conforming to FormatStyle and ParseableFormatStyle

Usage

Arithmetic

let a: FixedPointDecimal = 10.25
let b: FixedPointDecimal = 3

a + b           // 13.25
a - b           // 7.25
a * b           // 30.75
a / b           // 3.41666667 (banker's rounding)
a % b           // 1.25

// Wrapping (when you need non-trapping overflow — not faster than checked operators)
a &+ b
a &- b

Integer and float literals work directly via ExpressibleByIntegerLiteral and ExpressibleByFloatLiteral:

a * 100        // integer literal
a * 0.5        // float literal

Float literals go through Double, but this is safe for all values within FixedPointDecimal's 8-digit fractional range — values like 0.1, 0.2, 0.3 all convert correctly because round(value * 10^8) produces the exact integer. For values with more than 15 total significant digits, use the string initializer: FixedPointDecimal("12345678901.12345678").

Conversions

// From/to Double (pure Swift, no Foundation)
let fromDouble: FixedPointDecimal = 123.45
let toDouble = Double(fromDouble)              // 123.45

// From/to Foundation.Decimal (for UI presentation)
let fromDecimal = FixedPointDecimal(someDecimal)
let toDecimal = Decimal(fromDecimal)

// Failable exact conversions (nil if not exactly representable)
let exact = Double(exactly: someFixedPoint)    // Optional(123.45)
let intVal = Int(exactly: someFixedPoint)      // nil if has fractional part

// Truncating integer conversions (matching Int(someDouble) semantics)
let truncated = Int(someFixedPoint)            // truncates fractional part

String Parsing

let price: FixedPointDecimal = 99.95              // literal syntax
let parsed = FixedPointDecimal("99.95")!        // failable init (runtime)
String(price)                                   // "99.95"

Rounding

let value: FixedPointDecimal = 123.456789
value.rounded()                                // 123 (integer rounding, banker's)
value.rounded(scale: 2)                        // 123.46 (banker's rounding)
value.rounded(scale: 2, .towardZero)           // 123.45
value.rounded(scale: 0, .up)                   // 124
value.rounded(scale: 0, .toNearestOrAwayFromZero)  // 123 (schoolbook rounding)

SwiftUI Integration

// FormatStyle for TextField binding
TextField("Price", value: $price, format: .fixedPointDecimal)
price.formatted(.fixedPointDecimal.precision(2))    // "123.46"

// Decimal.FormatStyle forwarding — full locale-aware formatting
price.formatted(.number)                            // "123.45" (default locale)
price.formatted(.number.locale(Locale(identifier: "de_DE")))  // "123,45"
price.formatted(.currency(code: "USD"))             // "$123.45"
price.formatted(.currency(code: "SEK"))             // "123,45 kr"
price.formatted(.percent)                           // "12,345%"

// VectorArithmetic — animated transitions work automatically
struct PriceView: View {
    var price: FixedPointDecimal
    var body: some View {
        Text(price, format: .fixedPointDecimal.precision(2))
    }
}

// Plottable — direct use in Swift Charts
Chart(data) { item in
    LineMark(
        x: .value("Time", item.timestamp),
        y: .value("Price", item.price)  // FixedPointDecimal
    )
}

Codable

Encodes as a human-readable JSON string. Decodes flexibly from String, integer, or floating-point JSON values:

// Encoding: always a string for precision safety
let data = try JSONEncoder().encode(price)  // "123.45"

// Decoding: accepts multiple formats for interoperability
// "123.45"  -- string (canonical)
// 123       -- integer (face value, not raw storage)
// 123.45    -- floating-point (from external APIs)
let decoded = try JSONDecoder().decode(FixedPointDecimal.self, from: data)

Atomic Operations

AtomicRepresentable enables lock-free atomic operations via the Synchronization module:

import Synchronization

let bestBid = Atomic<FixedPointDecimal>(FixedPointDecimal(100.50))
bestBid.store(FixedPointDecimal(100.55), ordering: .releasing)
let current = bestBid.load(ordering: .acquiring)

NaN

let nan = FixedPointDecimal.nan
nan.isNaN                    // true
nan == nan                   // true (sentinel semantics)
(nan + someValue).isNaN      // true (propagates)
nan.description              // "nan"

Overflow Handling

Default operators trap on overflow, matching Swift Int:

// Trapping (default -- catches bugs in development)
let result = a + b  // traps if overflow

// Overflow-reporting (for defensive checks)
let (value, overflow) = a.addingReportingOverflow(b)

// Wrapping (non-trapping overflow — not faster than checked operators)
let wrapped = a &+ b

Building and Testing

swift build
swift test        # 541 tests across 14 suites

Benchmarks

Benchmarks use package-benchmark and compare every operation against Foundation.Decimal:

cd Benchmarks
swift package benchmark

Metrics collected: wall clock time, CPU instructions, heap allocations (malloc count).

Fuzz Testing

Fuzz testing uses libFuzzer via Swift's -sanitize=fuzzer flag. This requires the open-source Swift toolchain on Linux (not available in Xcode on macOS).

# Build only
bash Fuzz/run.sh

# Build and run (Ctrl-C to stop)
bash Fuzz/run.sh run

# Run for 60 seconds
bash Fuzz/run.sh run -max_total_time=60

# Debug build (for lldb)
bash Fuzz/run.sh debug run

The fuzzer validates invariants across all operations:

• Arithmetic: commutativity, NaN propagation, no silent NaN sentinel creation
• Comparisons: strict total order (exactly one of <, ==, >)
• Conversions: String, Double, Decimal, Codable round-trips
• Rounding: scale-8 identity, no overflow
• Hashing: equal values produce equal hashes

Crash artifacts are saved as Fuzz/crash-* files for reproduction.

Acknowledgments

• Benchmark infrastructure powered by package-benchmark
• Entirely built with Claude Code with careful guidance and coaching

License

Apache License 2.0. See LICENSE for details.