[SE-0368] StaticBigInt: rename subscript parameter

Author: benrimmington

stdlib/public/core/StaticBigInt.swift

@@ -84,8 +84,8 @@ extension StaticBigInt {
 
   /// Returns a 32-bit or 64-bit word of this value's binary representation.
   ///
-  /// The words are ordered from least significant to most significant, with an
-  /// infinite sign extension. Negative values are in two's complement.
+  /// The words are ordered from least significant to most significant, with
+  /// an infinite sign extension. Negative values are in two's complement.
   ///
   ///     let negative: StaticBigInt = -0x0011223344556677_8899AABBCCDDEEFF
   ///     negative.signum()  //-> -1
@@ -101,15 +101,18 @@ extension StaticBigInt {
   ///     positive[1]        //-> 0x0011223344556677
   ///     positive[2]        //-> 0x0000000000000000
   ///
-  /// - Parameter index: A nonnegative zero-based index.
+  /// - Parameter wordIndex: A nonnegative zero-based offset.
   @available(SwiftStdlib 5.8, *)
   @inlinable
-  public subscript(_ index: Int) -> UInt {
-    _precondition(index >= 0, "index out of range")
-    guard (index * UInt.bitWidth) < bitWidth else {
+  public subscript(_ wordIndex: Int) -> UInt {
+    _precondition(wordIndex >= 0, "Negative word index is out of range")
+    let bitIndex = wordIndex.multipliedReportingOverflow(by: UInt.bitWidth)
+    guard !bitIndex.overflow, bitIndex.partialValue < bitWidth else {
       return _isNegative ? ~0 : 0
     }
-    return UInt(Builtin.wordAtIndex_IntLiteral(_value, index._builtinWordValue))
+    return UInt(
+      Builtin.wordAtIndex_IntLiteral(_value, wordIndex._builtinWordValue)
+    )
   }
 }
 

test/stdlib/StaticBigInt.swift

@@ -83,6 +83,7 @@ extension StaticBigIntTests {
       expectEqual(0x7766554433221101, negative.actual[0])
       expectEqual(0xFFEEDDCCBBAA9988, negative.actual[1])
       expectEqual(0xFFFFFFFFFFFFFFFF, negative.actual[2])
+      expectEqual(0xFFFFFFFFFFFFFFFF, negative.actual[.max])
     }
     do {
       let positive = Wrapper(0x0011223344556677_8899AABBCCDDEEFF)
@@ -91,6 +92,7 @@ extension StaticBigIntTests {
       expectEqual(0x8899AABBCCDDEEFF, positive.actual[0])
       expectEqual(0x0011223344556677, positive.actual[1])
       expectEqual(0x0000000000000000, positive.actual[2])
+      expectEqual(0x0000000000000000, positive.actual[.max])
     }
   }
 }