---
epic: 1
story: 1.4
title: "Arbitrary Precision Arithmetic"
status: draft
---

## Epic 1 — Core Calculation Engine (Rust Crate)
**Goal:** Build `calcpad-engine` as a standalone Rust crate that powers all platforms. This is the foundation.

### Story 1.4: Arbitrary Precision Arithmetic

As a CalcPad user,
I want calculations precise to at least 30 significant digits with no floating-point drift,
So that financial calculations and large factorials produce exact, trustworthy results.

**Acceptance Criteria:**

**Given** the expression `0.1 + 0.2`
**When** the engine evaluates it
**Then** the result is exactly `0.3`
**And** there is no floating-point representation error

**Given** the expression `100!`
**When** the engine evaluates it
**Then** it returns the exact integer value (158 digits)
**And** no precision is lost

**Given** the expression `1 / 3`
**When** the engine evaluates it
**Then** it returns a result precise to at least 30 significant digits
**And** the display is appropriately rounded for the user

**Given** the expression `0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1`
**When** the engine evaluates it
**Then** the result is exactly `1.0`

**Given** a financial expression such as `$1000000.01 * 365`
**When** the engine evaluates it
**Then** the result preserves cents-level precision with no drift
**And** the result is `$365000003.65`

**Given** the engine is compiled to WASM
**When** arbitrary precision operations execute
**Then** the `dashu` crate functions correctly in the WASM target
**And** no platform-specific numeric behavior differs from native

**Given** an expression involving very large numbers such as `2^1000`
**When** the engine evaluates it
**Then** the exact integer result is returned
**And** the engine does not overflow or panic