Allow string section offset to overlap buckets

Sources/HeaderMapCore/BinaryHeaderMap.swift

@@ -63,6 +63,12 @@ struct BinaryHeaderMap {
           MemoryLayout<UInt32>.size +
           MemoryLayout<UInt32>.size;
     }
+    
+    static func headerPlusBucketsSize(
+      bucketCount: DataHeader.BucketCountType) -> Int {
+      let bucketsSectionSize = Int(bucketCount) * BinaryHeaderMap.Bucket.packedSize
+      return packedSize + bucketsSectionSize
+    }
   }
   
   enum Magic: UInt32 {
@@ -174,7 +180,9 @@ extension BinaryHeaderMap {
   
   func getString(at offset: StringSectionOffset) -> String? {
     let begin = Int(header.stringSectionOffset + offset.offset)
-    guard begin < data.count else { return nil }
+    let preambleSize = DataHeader.headerPlusBucketsSize(
+      bucketCount: header.bucketCount)
+    guard preambleSize <= begin, begin < data.count  else { return nil }
     
     let nullByteIndex = data.withUnsafeBytes {
       (bytes: UnsafePointer<UInt8>) -> Int? in
@@ -285,16 +293,11 @@ func parseHeaderMap(data: Data) throws -> DataHeaderParseResult {
       throw HeaderMapParseError.outOfBoundsStringSectionOffset
     }
     
-    let bucketsSectionSize = Int(bucketCount) * BinaryHeaderMap.Bucket.packedSize
-    let headerAndBucketsSectionSize = H.packedSize + bucketsSectionSize
+    let headerAndBucketsSectionSize = H.headerPlusBucketsSize(bucketCount: bucketCount)
     guard headerAndBucketsSectionSize <= data.count else {
       throw HeaderMapParseError.bucketsSectionOverflow
     }
     
-    guard headerAndBucketsSectionSize <= Int(stringSectionOffset) else {
-      throw HeaderMapParseError.invalidStringSectionOffset
-    }
-    
     return DataHeaderParseResult(
       dataHeader: BinaryHeaderMap.DataHeader(
         magic: magic,

Sources/HeaderMapCore/HeaderMapError.swift

@@ -34,7 +34,6 @@ public enum HeaderMapParseError: LocalizedError {
   case outOfBoundsStringSectionOffset
   case bucketCountNotPowerOf2(found: UInt32)
   case bucketsSectionOverflow
-  case invalidStringSectionOffset
 }
 
 public enum HeaderMapCreateError: LocalizedError {
@@ -70,8 +69,6 @@ extension HeaderMapParseError {
       return "Bucket count is not a power of 2, found \(buckets) buckets"
     case .bucketsSectionOverflow:
       return "Bucket section overflows"
-    case .invalidStringSectionOffset:
-      return "The string section offset is invalid"
     }
   }
 }

Tests/HeaderMapCoreTests/HeaderMapTests.swift

@@ -137,5 +137,12 @@ class HeaderMapTests: XCTestCase {
     let hmapEntries = Set<HeaderMap.Entry>(hmap.makeEntryList())
     XCTAssertEqual(Set(), hmapEntries)
   }
+  
+  func testStringOffsetInBucketSection() throws {
+    let hmapData = try loadFile(named: "Empty", extension: "hmap").unwrap()
+    let headerMap = try HeaderMap(data: hmapData)
+    let entries = Set<HeaderMap.Entry>(headerMap.makeEntryList())
+    XCTAssertEqual(entries, Set())
+  }
 }