[Sketch]Arbitrary-precision BigInt

Sources/BigInt/BigInt.swift

@@ -395,7 +395,17 @@ extension BigInt: BinaryInteger {
 
   public var bitWidth: Int { words.count * UInt.bitWidth }
 
-  public var trailingZeroBitCount: Int { words.first?.trailingZeroBitCount ?? 0 }
+  public var trailingZeroBitCount: Int {
+    var totalZeros = 0
+    for word in words {
+      let zeros = word.trailingZeroBitCount
+      totalZeros += zeros
+      if zeros < UInt.bitWidth {
+        break
+      }
+    }
+    return totalZeros
+  }
 
   @inlinable
   public static func / (lhs: BigInt, rhs: BigInt) -> BigInt {

Tests/BigIntTests/BigIntTests.swift

@@ -201,6 +201,14 @@ final class BigIntTests: XCTestCase {
     XCTAssertEqual(BigInt(Int64.max).signum(), +1)
     XCTAssertEqual(BigInt(+0x1p1023).signum(), +1)
   }
+
+  func testTrailingZeroCount() {
+    let foo = BigInt(1) << 300
+    XCTAssertEqual(foo.trailingZeroBitCount, 300)
+
+    let bar = (BigInt(1) << 300) + 0b101000
+    XCTAssertEqual(bar.trailingZeroBitCount, 3)
+  }
 
   // MARK: - Comparing and hashing