Add a newer Zstd module for Swift use, with a more idiomatic API, plus nullability annotations

.gitignore

@@ -33,6 +33,10 @@ build-*
 cmakebuild/
 cmake-build/
 
+# Swift build artefacts
+.build/
+.swiftpm/xcode/
+
 # Test artefacts
 tmp*
 *.zst

Package.swift

@@ -1,34 +1,71 @@
-// swift-tools-version:5.0
+// swift-tools-version:5.5
 // The swift-tools-version declares the minimum version of Swift required to build this package.
 
 import PackageDescription
 
 let package = Package(
     name: "zstd",
     platforms: [
-        .macOS(.v10_10), .iOS(.v9), .tvOS(.v9)
+        .macOS(.v10_15), .iOS(.v13), .tvOS(.v13), .watchOS(.v6)
     ],
     products: [
-        // Products define the executables and libraries a package produces, and make them visible to other packages.
-        .library(
-            name: "libzstd",
-            targets: [ "libzstd" ])
-    ],
-    dependencies: [
-        // Dependencies declare other packages that this package depends on.
-        // .package(url: /* package url */, from: "1.0.0"),
+        // Two library products.  Each surfaces only one Swift module to its consumers:
+        //
+        //   • `Zstd`    — modern API (renamed functions, prefix-stripped enum cases).
+        //   • `libzstd` — legacy API (raw `ZSTD_*` names) for source-level backwards
+        //                 compatibility with code that predates the rename.
+        //
+        // A consumer that depends on the `Zstd` product can NOT `import libzstd`
+        // (and vice versa), because each product is backed by a separate facade
+        // target whose modulemap declares only one of the two modules.
+        .library(name: "Zstd",    targets: [ "Zstd" ]),
+        .library(name: "libzstd", targets: [ "libzstd" ]),
+        .executable(name: "zstd-example", targets: [ "zstd-example" ]),
     ],
     targets: [
-        // Targets are the basic building blocks of a package. A target can define a module or a test suite.
-        // Targets can depend on other targets in this package, and on products in packages this package depends on.
+        // ── The actual C library ────────────────────────────────────────────
+        // Compiles the real zstd sources once and exposes the raw headers via
+        // a "private" `_ZstdCore` module that nobody is expected to import.
+        // Both facade targets below depend on this one for the C symbols.
         .target(
-            name: "libzstd",
+            name: "_ZstdCore",
             path: "lib",
             sources: [ "common", "compress", "decompress", "dictBuilder" ],
             publicHeadersPath: ".",
             cSettings: [
-                .headerSearchPath(".")
-            ])
+                .headerSearchPath("."),
+            ]),
+
+        // ── Modern Swift facade ─────────────────────────────────────────────
+        // Owns the `Zstd_module.h` umbrella (which defines
+        // `ZSTD_FOR_SWIFT_MODERN_API` before including the real headers) and
+        // its accompanying `Zstd.apinotes`.  The umbrella + apinotes pair is the
+        // only thing that turns "raw zstd headers" into the modern Swift API.
+        .target(
+            name: "Zstd",
+            dependencies: [ "_ZstdCore" ],
+            path: "Sources/Zstd",
+            publicHeadersPath: "include",
+            cSettings: [
+                .headerSearchPath("../../lib"),  // so the umbrella can find zstd.h
+            ]),
+
+        // ── Legacy Swift facade ─────────────────────────────────────────────
+        // Same headers as `Zstd`, but with an umbrella that doesn't define the
+        // modern-API macro and no apinotes file, so the raw C names come through.
+        .target(
+            name: "libzstd",
+            dependencies: [ "_ZstdCore" ],
+            path: "Sources/libzstd",
+            publicHeadersPath: "include",
+            cSettings: [
+                .headerSearchPath("../../lib"),
+            ]),
+
+        .executableTarget(
+            name: "zstd-example",
+            dependencies: [ "Zstd" ],
+            path: "Sources/zstd-example"),
     ],
     swiftLanguageVersions: [.v5],
     cLanguageStandard: .gnu11,

Sources/Zstd/_empty.c

@@ -0,0 +1 @@
+// SwiftPM requires at least one source file per C target.