subdataChunks optimization

Sources/tart/Data+Chunks.swift

@@ -0,0 +1,18 @@
+import Foundation
+
+extension Data {
+  /*
+   * Performant version of splitting a Data into chunks of a given size.
+   * It appers that "Data.chunks` is not as performant as chunking the range of the data
+   * into subranges and getting subdata directly.
+   */
+  func subdataChunks(ofCount: Int) -> [Data] {
+    var chunks: [Data] = []
+
+    for subrange in (0..<self.count).chunks(ofCount: ofCount) {
+      chunks.append(self.subdata(in: subrange))
+    }
+
+    return chunks
+  }
+}

Sources/tart/Fetcher.swift

@@ -52,8 +52,8 @@ class Fetcher {
     try FileManager.default.removeItem(at: fileURL)
 
     Task {
-      for chunk in (0 ..< mappedFile.count).chunks(ofCount: 64 * 1024 * 1024) {
-        await dataCh.send(mappedFile.subdata(in: chunk))
+      for chunk in mappedFile.subdataChunks(ofCount: 64 * 1024 * 1024) {
+        await dataCh.send(chunk)
       }
 
       dataCh.finish()

Sources/tart/OCI/Layerizer/DiskV2.swift

@@ -15,7 +15,7 @@ class DiskV2: Disk {
     // Compress the disk file as multiple individually decompressible streams,
     // each equal ``Self.layerLimitBytes`` bytes or less due to LZ4 compression
     try await withThrowingTaskGroup(of: (Int, OCIManifestLayer).self) { group in
-      for (index, data) in mappedDisk.chunks(ofCount: layerLimitBytes).enumerated() {
+      for (index, data) in mappedDisk.subdataChunks(ofCount: layerLimitBytes).enumerated() {
         // Respect the concurrency limit
         if index >= concurrency {
           if let (index, pushedLayer) = try await group.next() {
@@ -209,7 +209,7 @@ class DiskV2: Disk {
 
     var offset = offset
 
-    for chunk in data.chunks(ofCount: holeGranularityBytes) {
+    for chunk in data.subdataChunks(ofCount: holeGranularityBytes) {
       // If the local layer cache is used, only write chunks that differ
       // since the base disk can contain anything at any position
       if let rdisk = rdisk {

Sources/tart/OCI/Registry.swift

@@ -223,7 +223,7 @@ class Registry {
 
     // chunked upload
     var uploadedBytes = 0
-    let chunks = fromData.chunks(ofCount: chunkSizeMb == 0 ? fromData.count : chunkSizeMb * 1_000_000)
+    let chunks = fromData.subdataChunks(ofCount: chunkSizeMb == 0 ? fromData.count : chunkSizeMb * 1_000_000)
     for (index, chunk) in chunks.enumerated() {
       let lastChunk = index == (chunks.count - 1)
       let (data, response) = try await dataRequest(