AncDec

Anchored Decimal

A fast, precise fixed-point decimal type for no_std environments with independent 19-digit integer and 19-digit fractional parts.

Why AncDec?

AncDec's Solution

• Independent storage: 19-digit integer + 19-digit fraction (not shared)
• Exact arithmetic: No floating-point rounding errors
• Overflow-safe: Wide arithmetic (u256) for mul/div prevents overflow
• Fast: 1.2x - 1.8x faster than rust_decimal
• no_std: Zero heap allocation, embedded-friendly
• Zero dependencies: No external crates required (serde optional)
• Safe: All public APIs return Result, internal panics are unreachable by design

Core Structure

#[repr(C)]
#[derive(Clone, Copy, Debug)]
pub struct AncDec {
    pub int: u64,   // Integer part (up to 19 digits)
    pub frac: u64,  // Fractional part (up to 19 digits)
    pub scale: u8,  // Number of decimal places (0-19)
    pub neg: bool,  // Sign flag
}

The #[repr(C)] layout enables easy FFI bindings for other languages.

Installation

[dependencies]

ancdec = "0.1"

Zero dependencies by default. Only core is used (no std, no alloc).

With serde support:

[dependencies]

ancdec = { version = "0.1", features = ["serde"] }

Usage

Creating AncDec values

use ancdec::AncDec;

// From Rust integer primitives via From trait (infallible)
let a: AncDec = 123i64.into();
let b: AncDec = (-456i32).into();

// From string via parse (returns Result)
let c: AncDec = "456.789".parse()?;
let d = AncDec::parse("123.456")?;

// TryFrom for &str and floats
let e = AncDec::try_from("789.012")?;
let f = AncDec::try_from(3.14f64)?;

Arithmetic

let a: AncDec = "123.456".parse()?;
let b: AncDec = "78.9".parse()?;

let sum = a + b;        // 202.356
let diff = a - b;       // 44.556
let product = a * b;    // 9740.6784
let quotient = a / b;   // 1.5647148288973384030
let remainder = a % b;  // 44.556

Comparison

let a: AncDec = "100.5".parse()?;
let b: AncDec = "100.50".parse()?;

assert!(a == b);  // true (trailing zeros normalized)
assert!(a >= b);

Rounding

use ancdec::RoundMode;

let a: AncDec = "123.456789".parse()?;

a.round(2, RoundMode::HalfUp);   // 123.46
a.round(0, RoundMode::Floor);    // 123
a.floor();                        // 123
a.ceil();                         // 124
a.trunc();                        // 123
a.fract();                        // 0.456789

Conversion

let a: AncDec = "123.456".parse()?;

let f: f64 = a.to_f64();      // 123.456
let i: i64 = a.to_i64();      // 123
let i128: i128 = a.to_i128(); // 123

Supported Types & Traits

From Trait (Infallible)

Integer conversions never fail:

Type	Example
i8, i16, i32, i64, i128, isize	(-123i64).into()
u8, u16, u32, u64, u128, usize	123u64.into()

TryFrom / Parse (Fallible)

String and float conversions return Result<AncDec, ParseError>:

// FromStr trait
let a: AncDec = "123.456".parse()?;

// TryFrom<&str>
let b = AncDec::try_from("123.456")?;

// TryFrom<f64> - rejects NaN and Infinity
let c = AncDec::try_from(3.14f64)?;
let err = AncDec::try_from(f64::NAN);  // Err(InvalidFloat)

// parse method (works with any Display type)
let d = AncDec::parse("123.456")?;
let e = AncDec::parse(my_custom_type)?;

ParseError

Variant	Description
Empty	Input string is empty
NoDigits	No valid digits found
TrailingChars	Invalid characters after number
InvalidFloat	NaN or Infinity

Operator Traits

Trait	Operators
Add, Sub, Mul, Div, Rem	+, -, *, /, %
AddAssign, SubAssign, MulAssign, DivAssign, RemAssign	+=, -=, *=, /=, %=
Neg	-a

All operators work with AncDec, &AncDec, and integer primitives:

let a: AncDec = 10i64.into();
let b = a + 5;          // AncDec + i32
let c = 3 * a;          // i32 * AncDec
let d = &a + &a;        // &AncDec + &AncDec

Comparison Traits

Trait	Usage
PartialEq, Eq	==, !=
PartialOrd, Ord	<, >, <=, >=, .cmp()

Other Traits

Trait	Usage
Default	AncDec::default() returns ZERO
Hash	Usable in HashMap, HashSet
FromStr	"123.45".parse::<AncDec>()
TryFrom<&str>	AncDec::try_from("123.45")
TryFrom<f32>	AncDec::try_from(3.14f32)
TryFrom<f64>	AncDec::try_from(3.14f64)
Display	format!("{}", a), format!("{:.2}", a)
Sum	iter.sum::<AncDec>()
Product	iter.product::<AncDec>()

Constants

AncDec::ZERO   // 0
AncDec::ONE    // 1
AncDec::TWO    // 2
AncDec::TEN    // 10
AncDec::MAX    // Maximum representable value

Utility Methods

let a: AncDec = "-123.456".parse()?;

a.abs();          // 123.456
a.signum();       // -1
a.is_positive();  // false
a.is_negative();  // true
a.is_zero();      // false
a.min(b);         // minimum of a and b
a.max(b);         // maximum of a and b
a.clamp(lo, hi);  // clamp to range
a.pow(3);         // a³ (binary exponentiation)

Safety Design

AncDec is designed with a layered safety model:

Public API - Always Safe

All public APIs return Result for fallible operations:

Function	Return Type	Failure Case
"123".parse::<AncDec>()	Result<AncDec, ParseError>	Invalid input
AncDec::try_from("123")	Result<AncDec, ParseError>	Invalid input
AncDec::try_from(3.14f64)	Result<AncDec, ParseError>	NaN, Infinity
AncDec::parse(value)	Result<AncDec, ParseError>	Invalid input

Integer conversions via From trait are infallible by design.

Internal Panic - Unreachable by Design

Internal functions like pow10(exp) contain panic for out-of-range values:

// Internal only (pub(crate))
const fn pow10(exp: u8) -> u64 {
    match exp {
        0..=19 => { /* valid values */ },
        _ => panic!("scale overflow"),  // Unreachable
    }
}

Why this is safe:

1. pow10 is pub(crate) - External code cannot call it directly
2. Scale is bounded at parse time - Fractional digits are truncated to 19
3. Arithmetic preserves bounds - All operations maintain scale ≤ 19
4. The panic exists only to catch internal bugs - If reached, it indicates a logic error in the library itself

This design provides:

• Zero runtime overhead for range checks in hot paths
• Compile-time guarantees through type system
• Defense in depth against internal bugs

Division by Zero

Division by zero panics (consistent with Rust's integer division):

let a: AncDec = 10i64.into();
let b = AncDec::ZERO;
let c = a / b;  // Panics

Use is_zero() to check before division if needed.

Performance Optimizations

Compile-time Power of 10

Power of 10 values are const fn with match expressions:

• Compile-time evaluation when exponent is constant
• Zero heap allocation
• LLVM optimizes to jump table or direct substitution

Aggressive Inlining

All hot-path functions are #[inline(always)]:

• Arithmetic operations
• Comparisons
• Conversions

Benchmarks

Compared against rust_decimal (lower is better):

Operation	AncDec	rust_decimal	Speedup
add	6.3 ns	11.1 ns	1.76x
sub	6.3 ns	11.3 ns	1.79x
mul	9.5 ns	11.4 ns	1.20x
div	13.7 ns	19.8 ns	1.45x
cmp	4.5 ns	5.3 ns	1.18x
parse	11.3 ns	11.4 ns	1.01x

Benchmarked on Intel Core i7-10750H @ 2.60GHz, Rust 1.87.0, release mode Benchmarked on Intel Core i7-10750H @ 2.60GHz, Rust 1.60.0, release mode

Precision Limits

Component	Limit	Note
Integer part	19 digits	u64::MAX ≈ 1.8 × 10¹⁹
Fractional part	19 digits	Independent of integer
Total precision	38 digits	Not shared like rust_decimal

Precision Behavior

• Parsing: Fractional digits beyond 19 are truncated, integer part saturates at u64::MAX
• Multiplication: Results clamped to 19 decimal places
• Division: Results have exactly 19 decimal places

// Fractional truncation during parse
let a: AncDec = "1.1234567890123456789999".parse()?;
// Stored as 1.1234567890123456789 (19 digits)

// Integer saturation during parse
let b: AncDec = "99999999999999999999".parse()?;
// Stored as u64::MAX (18446744073709551615)

// Multiplication precision
let c: AncDec = "0.1234567890123456789".parse()?;
let d = c * c;  // Result has 19 decimal places

Features

Feature	Dependencies	Description
(default)	None	Core functionality, only uses core
serde	serde	Serialization/deserialization support

Serde

When enabled, AncDec serializes as decimal string:

use serde::{Serialize, Deserialize};

#[derive(Serialize, Deserialize)]
struct Price {
    amount: AncDec,
}

// JSON: {"amount": "123.456"}

Use Cases

• Finance: Exact monetary calculations
• Accounting: Tax, invoicing, ledger systems
• Cryptocurrency: Wei/Satoshi precision arithmetic
• Embedded: Payment terminals, IoT devices (no_std, zero dependencies)
• Games: In-game currency systems
• Scientific: When exact decimal representation matters

Comparison with Alternatives

Feature	AncDec	rust_decimal	f64
Precision	19+19 digits (independent)	28 digits (shared)	~15 digits (shared)
Exact decimal	✅	✅	❌
Overflow handling	✅ (u256 wide arithmetic)	✅	❌ (silent)
no_std	✅	⚠️ (feature flag)	✅
Zero dependencies	✅	❌	✅
Speed (vs rust_decimal)	1.2-1.8x faster	baseline	~10x faster
FFI-friendly	✅ (repr(C))	❌	✅
Size	24 bytes	16 bytes	8 bytes

License

MIT License

Contributing

Contributions welcome! Please ensure:

• All tests pass (cargo test)
• Serde tests pass (cargo test --features serde)
• Benchmarks don't regress (cargo bench)
• Code follows existing style