@hastom/fixed-point/README.md

Light lib for fixed point math made around native bigint

# Fixed Point Library

A high-precision fixed-point math library for JavaScript and TypeScript, built around native `bigint` for maximum performance and precision.

## Features

- Arbitrary precision fixed-point arithmetic
- Built on JavaScript's native `bigint` type
- Comprehensive math operations
- No external dependencies
- TypeScript support

## Installation

```bash
# Using npm
npm install @hastom/fixed-point

# Using yarn
yarn add @hastom/fixed-point

# Using pnpm
pnpm add @hastom/fixed-point
```

## Basic Usage

```typescript
import { fpFromDecimal, fpFromInt } from '@hastom/fixed-point';

// Create fixed-point numbers with precision of 18 decimal places
const a = fpFromDecimal('123.456789', 18);
const b = fpFromDecimal('0.000000000000000001', 18); // Smallest possible value with precision of 18
const c = fpFromInt(100000000000000000000n, 18, 18); // Already scaled value with source and destination precision of 18

// Perform arithmetic operations
const sum = a.add(c);
console.log(sum.toDecimalString()); // '223.456789000000000000'

// Comparison
if (a.gt(b)) {
  console.log('a is greater than b');
}

// Convert to decimal string
console.log(a.toDecimalString()); // '123.456789000000000000'
```

## API Reference

### Factory Functions

#### `fpFromDecimal(src: number | string | bigint, dstPrecision: number): FixedPoint`

Creates a fixed-point number from a decimal value.

Parameters:
- `src`: The decimal value to convert (as a number, string, or bigint)
- `dstPrecision`: The precision (number of decimal places) for the resulting FixedPoint

```typescript
import { fpFromDecimal } from '@hastom/fixed-point';

// Create a fixed-point number with 18 decimal places
const num1 = fpFromDecimal('123.456789', 18);

// Create a fixed-point number with 10 decimal places
const num2 = fpFromDecimal('3.1415926535', 10);

// Can also use a number as input (but be aware of JS floating point limitations)
const num3 = fpFromDecimal(123.456, 18);

// Or use a bigint (which will be scaled by the precision)
const num4 = fpFromDecimal(123n, 18); // Equivalent to 123.000000000000000000
```

#### `fpFromInt(src: number | string | bigint, srcPrecision: number, dstPrecision: number): FixedPoint`

Creates a fixed-point number from an already scaled integer value.

Parameters:
- `src`: The integer value (in an already scaled form)
- `srcPrecision`: The precision of the source integer
- `dstPrecision`: The precision for the resulting FixedPoint

```typescript
import { fpFromInt } from '@hastom/fixed-point';

// Create a fixed-point number from a scaled integer
// 100000000000000000000n represents 100.0 with 18 decimal places
const num1 = fpFromInt(100000000000000000000n, 18, 18);

// Convert from one precision to another
// 12345 represents 1.2345 with 4 decimal places, converting to 8 decimal places
const num2 = fpFromInt(12345, 4, 8); // Will be 1.23450000 with 8 decimal places

// Using a string for a very large number
const num3 = fpFromInt('123456789012345678901234567890', 18, 18);
```

### FixedPoint Methods

#### Arithmetic Operations

##### `add(other: FixedPoint): FixedPoint`

Adds two fixed-point numbers.

```typescript
const a = fpFromDecimal('10.5', 18);
const b = fpFromDecimal('20.3', 18);
const result = a.add(b);
console.log(result.toDecimalString()); // '30.800000000000000000'
```

##### `sub(other: FixedPoint): FixedPoint`

Subtracts a value from the fixed-point number.

```typescript
const a = fpFromDecimal('30.5', 18);
const b = fpFromDecimal('10.3', 18);
const result = a.sub(b);
console.log(result.toDecimalString()); // '20.200000000000000000'
```

##### `mul(other: FixedPoint): FixedPoint`

Multiplies the fixed-point number by another value.

```typescript
const a = fpFromDecimal('2.5', 18);
const b = fpFromDecimal('3', 18);
const result = a.mul(b);
console.log(result.toDecimalString()); // '7.500000000000000000'
```

##### `div(other: FixedPoint): FixedPoint`

Divides the fixed-point number by another value.

```typescript
const a = fpFromDecimal('10', 18);
const b = fpFromDecimal('2.5', 18);
const result = a.div(b);
console.log(result.toDecimalString()); // '4.000000000000000000'
```


##### `neg(): FixedPoint`

Negates the fixed-point number.

```typescript
const a = fpFromDecimal('10.5', 18);
const result = a.neg();
console.log(result.toDecimalString()); // '-10.500000000000000000'
```

##### `abs(): FixedPoint`

Returns the absolute value of the fixed-point number.

```typescript
const a = fpFromDecimal('-10.5', 18);
const result = a.abs();
console.log(result.toDecimalString()); // '10.5'
```


#### Comparison Operations

##### `eq(other: FixedPoint): boolean`

Checks if the fixed-point number is equal to another value.

```typescript
const a = fpFromDecimal('10.5', 18);
const b = fpFromDecimal('10.5', 18);
console.log(a.eq(b)); // true
```

##### `gt(other: FixedPoint): boolean`

Checks if the fixed-point number is greater than another value.

```typescript
const a = fpFromDecimal('10.5', 18);
const b = fpFromDecimal('5.2', 18);
console.log(a.gt(b)); // true
```

##### `gte(other: FixedPoint): boolean`

Checks if the fixed-point number is greater than or equal to another value.

```typescript
const a = fpFromDecimal('10.5', 18);
const b = fpFromDecimal('10.5', 18);
console.log(a.gte(b)); // true
```

##### `lt(other: FixedPoint): boolean`

Checks if the fixed-point number is less than another value.

```typescript
const a = fpFromDecimal('5.2', 18);
const b = fpFromDecimal('10.5', 18);
console.log(a.lt(b)); // true
```

##### `lte(other: FixedPoint): boolean`

Checks if the fixed-point number is less than or equal to another value.

```typescript
const a = fpFromDecimal('10.5', 18);
const b = fpFromDecimal('10.5', 18);
console.log(a.lte(b)); // true
```

#### Utility Methods

##### `toString(): string`

Returns the base fixed-point representation (the internal scaled value as a string).

```typescript
const a = fpFromDecimal('10.5', 18);
console.log(a.toString()); // '10500000000000000000' (for precision of 18)
```

##### `toDecimalString(): string`

Converts the fixed-point number to a decimal string.

```typescript
const a = fpFromDecimal('10.5', 18);
console.log(a.toDecimalString()); // '10.500000000000000000'
```

##### `toDecimal(): number`

Converts the fixed-point number to a JavaScript number.

```typescript
const a = fpFromDecimal('10.54321', 18);
console.log(a.toDecimal()); // 10.54321
```

##### `isZero(): boolean`

Checks if the fixed-point number is zero.

```typescript
const a = fpFromDecimal('0', 18);
console.log(a.isZero()); // true
```

##### `isPositive(): boolean`

Checks if the fixed-point number is positive.

```typescript
const a = fpFromDecimal('10.5', 18);
console.log(a.isPositive()); // true
```

##### `isNegative(): boolean`

Checks if the fixed-point number is negative.

```typescript
const a = fpFromDecimal('-10.5', 18);
console.log(a.isNegative()); // true
```

##### `toPrecision(precision: number): FixedPoint`

Converts the fixed-point number to a different precision.

```typescript
const a = fpFromDecimal('10.5', 18);
const result = a.toPrecision(10);
console.log(result.toDecimalString()); // '10.5000000000' (but with 10 digits of precision)
```