From 71dcee80bdf8c90c98f288371905237ea33fb758 Mon Sep 17 00:00:00 2001
From: Kevin Wooten <kevin@wooten.com>
Date: Mon, 16 Jun 2025 14:54:47 -0700
Subject: [PATCH] CBOR: Use Data.startIndex to ensure correct offset for slices

---
 Sources/PotentCBOR/CBORStream.swift | 5 +++--
 Tests/CBORDecoderTests.swift        | 6 ++++++
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/Sources/PotentCBOR/CBORStream.swift b/Sources/PotentCBOR/CBORStream.swift
index d0a72a359..fc07212d6 100644
--- a/Sources/PotentCBOR/CBORStream.swift
+++ b/Sources/PotentCBOR/CBORStream.swift
@@ -62,15 +62,16 @@ public class CBORDataStream: CBORInputStream, CBOROutputStream {
 
   public init(data: Data = Data(), offset: Int = 0) {
     self.data = data
-    self.offset = offset
+    self.offset = offset + data.startIndex
   }
 
   public func reset() {
     data.removeAll()
+    offset = data.startIndex
   }
 
   func checkAvailable(count: Int) throws {
-    if (offset + count) > data.count {
+    if (offset + count) > data.endIndex {
       throw CBORSerialization.Error.unexpectedEndOfStream
     }
   }
diff --git a/Tests/CBORDecoderTests.swift b/Tests/CBORDecoderTests.swift
index 0281f6e40..7b191b0df 100644
--- a/Tests/CBORDecoderTests.swift
+++ b/Tests/CBORDecoderTests.swift
@@ -16,6 +16,12 @@ import XCTest
 
 class CBORDecoderTests: XCTestCase {
 
+  func testDecodeFromSlice() {
+    let data = Data([0x1, 0x2, 0x3, 0x64, 0x49, 0x45, 0x54, 0x46, 0x3, 0x2, 0x1])
+    let slice = data[3..<data.count-3]
+    XCTAssertEqual(try CBORDecoder.default.decode(String.self, from: slice), "IETF")
+  }
+
   func testDecodeEmpty() {
     struct Empty: Equatable, Codable {}
     XCTAssertEqual(try CBORDecoder.default.decode(Empty.self, from: Data([0xA0])), Empty())