improve Decimal conversion performance using _mantissa (#124)

* improve Decimal conversion performance using `_mantissa`

* fix linux builds by keeping the decimal string init in a compiler directive

* more efficient zero out assurance after buffer creation

* Decimal (from bigint) initializer

Author: mredig

Sources/Data Conversion.swift

@@ -53,7 +53,6 @@ extension BigUInt {
         let byteCount = (self.bitWidth + 7) / 8
 
         let buffer = UnsafeMutableBufferPointer<UInt8>.allocate(capacity: byteCount)
-        buffer.initialize(repeating: 0)
 
         guard byteCount > 0 else { return UnsafeRawBufferPointer(start: buffer.baseAddress, count: 0) }
 
@@ -69,6 +68,8 @@ extension BigUInt {
                 i -= 1
             }
         }
+        let zeroOut = UnsafeMutableBufferPointer<UInt8>(start: buffer.baseAddress, count: i)
+        zeroOut.initialize(repeating: 0)
         return UnsafeRawBufferPointer(start: buffer.baseAddress, count: byteCount)
     }
 

Sources/Floating Point Conversion.swift

@@ -49,7 +49,20 @@ extension BigUInt {
         guard integer.sign == .plus else { return nil }
         assert(integer.floatingPointClass == .positiveNormal)
 
+        #if os(Linux)
+        // `Decimal._mantissa` has an internal access level on linux, and it might get
+        // deprecated in the future, so keeping the string implementation around for now.
         let significand = BigUInt("\(integer.significand)")!
+        #else
+        let significand = {
+            var start = BigUInt(0)
+            for (place, value) in integer.significand.mantissaParts.enumerated() {
+                guard value > 0 else { continue }
+                start += (1 << (place * 16)) * BigUInt(value)
+            }
+            return start
+        }()
+        #endif
         let exponent = BigUInt(10).power(integer.exponent)
 
         self = significand * exponent
@@ -124,3 +137,45 @@ extension BigInt.Sign {
         }
     }
 }
+
+#if canImport(Foundation)
+public extension Decimal {
+    init(_ value: BigUInt) {
+        guard
+            value < BigUInt(exactly: Decimal.greatestFiniteMagnitude)!
+        else {
+            self = .greatestFiniteMagnitude
+            return
+        }
+        guard !value.isZero else { self = 0; return }
+
+        self.init(string: "\(value)")!
+    }
+
+    init(_ value: BigInt) {
+        if value >= 0 {
+            self.init(BigUInt(value))
+        } else {
+            self.init(value.magnitude)
+            self *= -1
+        }
+    }
+}
+#endif
+
+#if canImport(Foundation) && !os(Linux)
+private extension Decimal {
+    var mantissaParts: [UInt16] {
+        [
+            _mantissa.0,
+            _mantissa.1,
+            _mantissa.2,
+            _mantissa.3,
+            _mantissa.4,
+            _mantissa.5,
+            _mantissa.6,
+            _mantissa.7,
+        ]
+    }
+}
+#endif

Tests/BigIntTests/BigIntTests.swift

@@ -185,6 +185,18 @@ class BigIntTests: XCTestCase {
         test(BigInt(1) << 1024, Float.infinity)
         test(BigInt(words: convertWords([0, 0xFFFFFF0000000000, 0])),
              Float.greatestFiniteMagnitude)
+
+        XCTAssertEqual(Decimal(BigInt(0)), 0)
+        XCTAssertEqual(Decimal(BigInt(20)), 20)
+        XCTAssertEqual(Decimal(BigInt(123456789)), 123456789)
+        XCTAssertEqual(Decimal(BigInt(exactly: Decimal.greatestFiniteMagnitude)!), .greatestFiniteMagnitude)
+        XCTAssertEqual(Decimal(BigInt(exactly: Decimal.greatestFiniteMagnitude)! * 2), .greatestFiniteMagnitude)
+        XCTAssertEqual(Decimal(-BigInt(0)), 0)
+        XCTAssertEqual(Decimal(-BigInt(20)), -20)
+        XCTAssertEqual(Decimal(-BigInt(123456789)), -123456789)
+        XCTAssertEqual(Decimal(-BigInt(exactly: Decimal.greatestFiniteMagnitude)!), -.greatestFiniteMagnitude)
+        XCTAssertEqual(Decimal(-BigInt(exactly: Decimal.greatestFiniteMagnitude)! * 2), -.greatestFiniteMagnitude)
+
     }
 
     func testTwosComplement() {

Tests/BigIntTests/BigUIntTests.swift

@@ -264,6 +264,12 @@ class BigUIntTests: XCTestCase {
         test(BigUInt(0x8000027FFFFFFFFF as UInt64), 0x800002p40 as Float)
         test(BigUInt(0x8000028000000000 as UInt64), 0x800002p40 as Float)
         test(BigUInt(0x800002FFFFFFFFFF as UInt64), 0x800002p40 as Float)
+
+        XCTAssertEqual(Decimal(BigUInt(0)), 0)
+        XCTAssertEqual(Decimal(BigUInt(20)), 20)
+        XCTAssertEqual(Decimal(BigUInt(123456789)), 123456789)
+        XCTAssertEqual(Decimal(BigUInt(exactly: Decimal.greatestFiniteMagnitude)!), .greatestFiniteMagnitude)
+        XCTAssertEqual(Decimal(BigUInt(exactly: Decimal.greatestFiniteMagnitude)! * 2), .greatestFiniteMagnitude)
     }
 
     func testInit_Misc() {
@@ -688,6 +694,7 @@ class BigUIntTests: XCTestCase {
         test(BigUInt(), [])
         test(BigUInt(1), [0x01])
         test(BigUInt(2), [0x02])
+        test(BigUInt(0x010203), [0x1, 0x2, 0x3])
         test(BigUInt(0x0102030405060708), [0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08])
         test(BigUInt(0x01) << 64 + BigUInt(0x0203040506070809), [0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09])
     }