fix(core): round Decimal.doubleValue correctly

FlipcashCore/Sources/FlipcashCore/Extensions/Decimal+Double.swift

@@ -8,59 +8,18 @@
 
 import Foundation
 
-/// A copy from Swift Foundation
-/// Reference:
-/// https://github.com/apple/swift-corelibs-foundation/blob/main/Sources/Foundation/Decimal.swift
 extension Decimal {
-
-    fileprivate subscript(index: UInt32) -> UInt16 {
-        get {
-            switch index {
-            case 0: return _mantissa.0
-            case 1: return _mantissa.1
-            case 2: return _mantissa.2
-            case 3: return _mantissa.3
-            case 4: return _mantissa.4
-            case 5: return _mantissa.5
-            case 6: return _mantissa.6
-            case 7: return _mantissa.7
-            default: fatalError("Invalid index \(index) for _mantissa")
-            }
-        }
-        set {
-            switch index {
-            case 0: _mantissa.0 = newValue
-            case 1: _mantissa.1 = newValue
-            case 2: _mantissa.2 = newValue
-            case 3: _mantissa.3 = newValue
-            case 4: _mantissa.4 = newValue
-            case 5: _mantissa.5 = newValue
-            case 6: _mantissa.6 = newValue
-            case 7: _mantissa.7 = newValue
-            default: fatalError("Invalid index \(index) for _mantissa")
-            }
-        }
-    }
-
+
+    /// The nearest `Double` to this value, matching Android's `BigDecimal.toDouble()`.
+    ///
+    /// `native_amount` in intent exchange data is a double, so both platforms must land on the
+    /// same one. `NSDecimalNumber.doubleValue` scales the mantissa by ten once per decimal digit
+    /// and can miss by an ulp or more (0.007 becomes 0.006999999999999999). `description` is
+    /// locale-independent and `Double(String)` rounds correctly.
     public var doubleValue: Double {
         if _length == 0 {
             return _isNegative == 1 ? Double.nan : 0
         }
-
-        var d = 0.0
-        for idx in (0..<min(_length, 8)).reversed() {
-            d = d * 65536 + Double(self[idx])
-        }
-
-        if _exponent < 0 {
-            for _ in _exponent..<0 {
-                d /= 10.0
-            }
-        } else {
-            for _ in 0..<_exponent {
-                d *= 10.0
-            }
-        }
-        return _isNegative != 0 ? -d : d
+        return Double(description) ?? NSDecimalNumber(decimal: self).doubleValue
     }
 }

FlipcashCore/Tests/FlipcashCoreTests/DecimalDoubleTests.swift

@@ -0,0 +1,52 @@
+//
+//  DecimalDoubleTests.swift
+//  FlipcashCoreTests
+//
+
+import Foundation
+import Testing
+@testable import FlipcashCore
+
+@Suite("Decimal.doubleValue")
+struct DecimalDoubleTests {
+
+    // `native_amount` goes on the wire as a double, and Android's
+    // `BigDecimal.toDouble()` is correctly rounded. Each value must land on the
+    // same double `Double(String)` parses. 0.007, 0.000001 and the Int64-max
+    // amount were each off by an ulp or more when the conversion divided by ten
+    // once per decimal digit.
+    @Test("Converts to the correctly rounded double", arguments: [
+        "0.001",
+        "0.007",
+        "0.000001",
+        "0.8",
+        "0.2",
+        "4.95",
+        "123456789.123456789",
+        "9223372036854.775807",
+        "-0.007",
+        "100000000000000000000",
+    ])
+    func correctlyRounded(_ string: String) throws {
+        let decimal = try #require(Decimal(string: string))
+        let expected = try #require(Double(string))
+        #expect(decimal.doubleValue == expected)
+    }
+
+    @Test("Native amounts in the multi-payment intent vectors convert exactly", arguments: [
+        "0.800000", "0.200000", "9223372036854.775807", "0.000001",
+        "0.001000", "0.002000", "0.003000", "0.004000", "0.005000",
+        "0.006000", "0.007000", "0.008000", "0.009000", "0.010000",
+    ])
+    func multiPaymentVectorAmounts(_ string: String) throws {
+        let decimal = try #require(Decimal(string: string))
+        let expected = try #require(Double(string))
+        #expect(decimal.doubleValue == expected)
+    }
+
+    @Test("Zero and NaN")
+    func zeroAndNaN() {
+        #expect(Decimal.zero.doubleValue == 0)
+        #expect(Decimal.nan.doubleValue.isNaN)
+    }
+}