Allow reading and writing of empty header maps

Sources/HeaderMapCore/BinaryHeaderMap.swift

@@ -277,17 +277,17 @@ func parseHeaderMap(data: Data) throws -> DataHeaderParseResult {
         found: reseredValue)
     }
     
-    guard bucketCount.isPowerOf2 else {
+    guard bucketCount == 0 || bucketCount.isPowerOf2 else {
       throw HeaderMapParseError.bucketCountNotPowerOf2(found: bucketCount)
     }
     
-    guard Int(stringSectionOffset) < data.count else {
+    guard Int(stringSectionOffset) <= data.count else {
       throw HeaderMapParseError.outOfBoundsStringSectionOffset
     }
     
     let bucketsSectionSize = Int(bucketCount) * BinaryHeaderMap.Bucket.packedSize
     let headerAndBucketsSectionSize = H.packedSize + bucketsSectionSize
-    guard headerAndBucketsSectionSize < data.count else {
+    guard headerAndBucketsSectionSize <= data.count else {
       throw HeaderMapParseError.bucketsSectionOverflow
     }
     

Sources/HeaderMapCore/HeaderMapError.swift

@@ -39,7 +39,6 @@ public enum HeaderMapParseError: LocalizedError {
 
 public enum HeaderMapCreateError: LocalizedError {
   case invalidStringSectionOffset
-  case noEntries
   case stringWithoutOffsetInTable
   case hashTableFull
   case unhashableKey
@@ -82,8 +81,6 @@ extension HeaderMapCreateError {
     switch self {
     case .invalidStringSectionOffset:
       return "The string section offset is invalid"
-    case .noEntries:
-      return "Header map needs to contain at least one entry"
     case .stringWithoutOffsetInTable:
       return "String not present in string section"
     case .hashTableFull:

Sources/HeaderMapCore/HeaderMapMaker.swift

@@ -25,10 +25,6 @@ import Foundation
 func makeHeaderMapBinaryData(
   withEntries unsafeEntries: [HeaderMap.Entry]) throws -> Data {
   
-  guard unsafeEntries.count > 0 else {
-    throw HeaderMapCreateError.noEntries
-  }
-  
   let safeEntries = sanitize(headerEntries: unsafeEntries)
   let allStrings = Set(safeEntries.flatMap { [$0.key, $0.prefix, $0.suffix] })
   let stringSection = try makeStringSection(allStrings: allStrings)
@@ -81,9 +77,12 @@ fileprivate func makeStringSection(
   allStrings: Set<String>) throws -> StringSection {
   
   var buffer = Data()
-  buffer.append(UInt8(BinaryHeaderMap.StringSectionOffset.Reserved))
   var offsets = Dictionary<String, BinaryHeaderMap.StringSectionOffset>()
   
+  if !allStrings.isEmpty {
+    buffer.append(UInt8(BinaryHeaderMap.StringSectionOffset.Reserved))
+  }
+  
   for string in allStrings {
     guard let stringBytes = string.data(using: BinaryHeaderMap.StringEncoding) else {
       throw HeaderMapError.unencodableString
@@ -134,8 +133,6 @@ fileprivate func makeBucketSection(
   forEntries entries: [HeaderMap.Entry],
   stringSection: StringSection) throws -> BucketSection {
   
-  assert(entries.count > 0)
-  
   let bucketCount = numberOfBuckets(forEntryCount: entries.count)
   var bytes = Data(count: bucketCount * BinaryHeaderMap.Bucket.packedSize)
   try bytes.withUnsafeMutableBytes {
@@ -188,8 +185,6 @@ fileprivate func makeBucketSection(
 }
 
 fileprivate func numberOfBuckets(forEntryCount entryCount: Int) -> Int {
-  assert(entryCount > 0)
-  
   let minimumSlots = Int(ceil(Double(entryCount) * (1.0 / 0.7)))
   var bucketCount = 1
   while bucketCount < minimumSlots {

Sources/hmap/Print.swift

@@ -36,7 +36,8 @@ func addPrintCommand(to group: Group) {
       let output = try FilePrintCommand.perform(
         input: FilePrintCommand.Input(headerMapFile: url, argumentPath: file)
       )
-      Swift.print(output.text)
+      let text = output.text.isEmpty ? "Empty header map" : output.text
+      Swift.print(text)
     } catch (let error) {
       Swift.print(error.localizedDescription)
       exit(ToolReturnCode.failure.rawValue)

Tests/HeaderMapCoreTests/HeaderMapTests.swift

@@ -129,5 +129,13 @@ class HeaderMapTests: XCTestCase {
     let decodedEntries = hmap.makeJSONHeaderMap().entries
     XCTAssertEqual(entries, decodedEntries)
   }
+  
+  func testEmptyHeaderMap() throws {
+    let hmapBinaryData = try HeaderMap.makeBinary(withEntries: [])
+    XCTAssertNotNil(hmapBinaryData)
+    let hmap = try HeaderMap(data: hmapBinaryData)
+    let hmapEntries = Set<HeaderMap.Entry>(hmap.makeEntryList())
+    XCTAssertEqual(Set(), hmapEntries)
+  }
 }