Fix BufferReader.lineNumber to correctly count newlines

Sources/FoundationEssentials/CodableUtilities.swift

@@ -599,7 +599,7 @@ struct BufferReader {
                 if nextIndex < readIndex && fullBuffer[unchecked: nextIndex] == ._newline {
                     p = nextIndex
                 }
-            } else if fullBuffer[offset: 1] == ._newline {
+            } else if fullBuffer[unchecked: p] == ._newline {
                 count += 1
             }
             fullBuffer.formIndex(&p, offsetBy: 1)

Tests/FoundationEssentialsTests/BufferViewTests.swift

@@ -362,3 +362,107 @@ private struct BufferViewTests {
         }
     }
 }
+
+@Suite("BufferReader")
+private struct BufferReaderTests {
+
+    @Test("Line number counting with standalone newlines")
+    func lineNumberCounting() {
+        // Test case designed to expose the bug: second byte is 'A' (not a newline),
+        // but there are newlines at positions 3, 5, and 7.
+        // With the bug, fullBuffer[offset: 1] would check byte 1 ('A') and never count
+        // the newlines at positions 3, 5, and 7.
+        let testString = "A\nB\nC\nD"
+        let bytes = Array(testString.utf8)
+
+        bytes.withUnsafeBufferPointer {
+            let bufferView = BufferView(unsafeBufferPointer: $0)
+            #expect(bufferView != nil)
+            guard let bufferView else { return }
+
+            var reader = BufferReader(bytes: bufferView)
+
+            // Advance reader to the end to test lineNumber calculation
+            while !reader.isAtEnd {
+                _ = reader.read()
+            }
+
+            // Should count 4 lines: initial line + 3 newlines
+            #expect(reader.lineNumber == 4, "Expected 4 lines, got \(reader.lineNumber)")
+        }
+    }
+
+    @Test("Line number counting with CRLF sequences")
+    func lineNumberCountingWithCRLF() {
+        // Test CRLF handling: \r\n should count as one line break
+        let testString = "Line1\r\nLine2\r\nLine3"
+        let bytes = Array(testString.utf8)
+
+        bytes.withUnsafeBufferPointer {
+            let bufferView = BufferView(unsafeBufferPointer: $0)
+            #expect(bufferView != nil)
+            guard let bufferView else { return }
+
+            var reader = BufferReader(bytes: bufferView)
+
+            while !reader.isAtEnd {
+                _ = reader.read()
+            }
+
+            // Should count 3 lines: initial line + 2 CRLF sequences
+            #expect(reader.lineNumber == 3, "Expected 3 lines, got \(reader.lineNumber)")
+        }
+    }
+
+    @Test("Line number counting with mixed newlines")
+    func lineNumberCountingMixed() {
+        // Test mixed \n and \r\n sequences
+        // Second byte is 'a' (not a newline) to ensure bug would be exposed
+        let testString = "a\nb\r\nc\nd"
+        let bytes = Array(testString.utf8)
+
+        bytes.withUnsafeBufferPointer {
+            let bufferView = BufferView(unsafeBufferPointer: $0)
+            #expect(bufferView != nil)
+            guard let bufferView else { return }
+
+            var reader = BufferReader(bytes: bufferView)
+
+            while !reader.isAtEnd {
+                _ = reader.read()
+            }
+
+            // Should count 4 lines: initial line + 3 line breaks (one \n, one \r\n, one \n)
+            #expect(reader.lineNumber == 4, "Expected 4 lines, got \(reader.lineNumber)")
+        }
+    }
+
+    @Test("Line number at different read positions")
+    func lineNumberAtPosition() {
+        let testString = "Line1\nLine2\nLine3"
+        let bytes = Array(testString.utf8)
+
+        bytes.withUnsafeBufferPointer {
+            let bufferView = BufferView(unsafeBufferPointer: $0)
+            #expect(bufferView != nil)
+            guard let bufferView else { return }
+
+            var reader = BufferReader(bytes: bufferView)
+
+            // Read up to first newline
+            while reader.readIndex < bufferView.endIndex {
+                let byte = reader.read()
+                if byte == UInt8(ascii: "\n") {
+                    break
+                }
+            }
+            #expect(reader.lineNumber == 2, "After first newline, expected 2 lines, got \(reader.lineNumber)")
+
+            // Read to end
+            while !reader.isAtEnd {
+                _ = reader.read()
+            }
+            #expect(reader.lineNumber == 3, "At end, expected 3 lines, got \(reader.lineNumber)")
+        }
+    }
+}