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+---
+epic: 7
+story: 7.2
+title: "Rust FFI Bridge"
+status: draft
+---
+
+## Epic 7 — macOS App (Swift/SwiftUI)
+**Goal:** A beautiful, native macOS app that feels like it belongs on the platform.
+
+### Story 7.2: Rust FFI Bridge
+
+As a **macOS developer** integrating CalcPad,
+I want a Swift package that wraps the Rust engine's C FFI into idiomatic Swift types,
+So that the macOS app can call the engine safely and efficiently without manual pointer management.
+
+**Acceptance Criteria:**
+
+**Given** the Rust engine is compiled as a C-compatible dynamic library
+**When** the Swift package is imported
+**Then** a `CalcPadEngine` class is available with methods `evaluateLine(_ line: String) -> LineResult` and `evaluateSheet(_ text: String) -> SheetResult`
+
+**Given** the Swift app calls `evaluateLine("2 + 2")`
+**When** the FFI bridge processes the call
+**Then** the result is returned as a Swift `LineResult` struct (not a raw pointer)
+**And** all memory allocated by Rust is properly freed (no leaks)
+
+**Given** the Swift app calls `evaluateSheet` with a multi-line string
+**When** the FFI bridge processes the call
+**Then** an array of `LineResult` values is returned, one per line, in order
+
+**Given** the Rust engine returns an error for a malformed expression
+**When** the Swift bridge receives the error
+**Then** it is converted to a Swift `Error` type or an error variant in `LineResult` (not a crash or nil)
+
+**Given** the app is profiled with Instruments (Leaks tool)
+**When** `evaluateLine` and `evaluateSheet` are called repeatedly over many cycles
+**Then** no memory leaks are detected from the FFI boundary
+
+**Given** the bridge is used on a background thread
+**When** multiple calls are made concurrently
+**Then** the bridge handles thread safety correctly (either by being thread-safe or by documenting single-threaded use with appropriate assertions)