chore: import upstream snapshot with attribution

Sources/ContainerizationExtras/Prefix.swift

@@ -0,0 +1,88 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2025-2026 Apple Inc. and the Containerization project authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//===----------------------------------------------------------------------===//
+
+/// CIDR prefix length (e.g., `/24` for a 24-bit network mask).
+@frozen
+public struct Prefix: Sendable, CustomStringConvertible, Hashable, Codable {
+    public let length: UInt8
+
+    /// Create a prefix (0-128). Use `ipv4(_:)` or `ipv6(_:)` for version-specific validation.
+    public init?(length: UInt8) {
+        guard length <= 128 else { return nil }
+        self.length = length
+    }
+
+    /// Create an IPv4 prefix (0-32). Returns `nil` if length > 32.
+    public static func ipv4(_ length: UInt8) -> Prefix? {
+        guard length <= 32 else { return nil }
+        return Prefix(unchecked: length)
+    }
+
+    /// Create an IPv6 prefix (0-128). Returns `nil` if length > 128.
+    public static func ipv6(_ length: UInt8) -> Prefix? {
+        guard length <= 128 else { return nil }
+        return Prefix(unchecked: length)
+    }
+
+    /// Internal unchecked initializer for known-valid values.
+    internal init(unchecked length: UInt8) {
+        self.length = length
+    }
+
+    public var description: String {
+        "\(length)"
+    }
+}
+
+extension Prefix {
+    /// Computes a 32-bit mask for the suffix (host) portion of an IPv4 address.
+    ///
+    /// Example: Prefix `/24` → `0x0000_00FF` (255 host addresses)
+    @inlinable
+    public var suffixMask32: UInt32 {
+        if self.length <= 0 {
+            return 0xffff_ffff
+        }
+        return self.length >= 32 ? 0x0000_0000 : (1 << (32 - self.length)) - 1
+    }
+
+    /// Network portion mask (high-order bits) for IPv4.
+    ///
+    /// Example: Prefix `/24` → `0xFFFF_FF00` (255.255.255.0)
+    @inlinable
+    public var prefixMask32: UInt32 {
+        ~self.suffixMask32
+    }
+
+    /// Computes a 128-bit mask for the suffix (host) portion of an IPv6 address.
+    ///
+    /// Example: Prefix `/64` → `0x0000_0000_0000_0000_FFFF_FFFF_FFFF_FFFF`
+    @inlinable
+    public var suffixMask128: UInt128 {
+        if self.length <= 0 {
+            return UInt128.max
+        }
+        return self.length >= 128 ? 0 : (1 << (128 - self.length)) - 1
+    }
+
+    /// Network portion mask (high-order bits) for IPv6.
+    ///
+    /// Example: Prefix `/64` → `0xFFFF_FFFF_FFFF_FFFF_0000_0000_0000_0000`
+    @inlinable
+    public var prefixMask128: UInt128 {
+        ~self.suffixMask128
+    }
+}