Optimize JSONL scanner chunk parsing to reduce CPU usage

Sources/CodexBarCore/Vendored/CostUsage/CostUsageJsonl.swift

@@ -23,15 +23,24 @@ enum CostUsageJsonl {
             try handle.seek(toOffset: UInt64(startOffset))
         }
 
-        var buffer = Data()
-        buffer.reserveCapacity(64 * 1024)
-
         var current = Data()
         current.reserveCapacity(4 * 1024)
         var lineBytes = 0
         var truncated = false
         var bytesRead: Int64 = 0
 
+        func appendSegment(_ segment: Data.SubSequence) {
+            guard !segment.isEmpty else { return }
+            lineBytes += segment.count
+            guard !truncated else { return }
+            if lineBytes > maxLineBytes || lineBytes > prefixBytes {
+                truncated = true
+                current.removeAll(keepingCapacity: true)
+                return
+            }
+            current.append(contentsOf: segment)
+        }
+
         func flushLine() {
             guard lineBytes > 0 else { return }
             let line = Line(bytes: current, wasTruncated: truncated)
@@ -49,23 +58,14 @@ enum CostUsageJsonl {
             }
 
             bytesRead += Int64(chunk.count)
-            buffer.append(chunk)
-
-            while true {
-                guard let nl = buffer.firstIndex(of: 0x0A) else { break }
-                let linePart = buffer[..<nl]
-                buffer.removeSubrange(...nl)
-
-                lineBytes += linePart.count
-                if !truncated {
-                    if lineBytes > maxLineBytes || lineBytes > prefixBytes {
-                        truncated = true
-                        current.removeAll(keepingCapacity: true)
-                    } else {
-                        current.append(contentsOf: linePart)
-                    }
-                }
+            var segmentStart = chunk.startIndex
+            while let nl = chunk[segmentStart...].firstIndex(of: 0x0A) {
+                appendSegment(chunk[segmentStart..<nl])
                 flushLine()
+                segmentStart = chunk.index(after: nl)
+            }
+            if segmentStart < chunk.endIndex {
+                appendSegment(chunk[segmentStart..<chunk.endIndex])
             }
         }
 

Tests/CodexBarTests/CostUsageScannerTests.swift

@@ -838,6 +838,60 @@ struct CostUsageScannerTests {
         #expect(report.data[0].outputTokens == 15)
         #expect(report.data[0].totalTokens == 45)
     }
+
+    @Test
+    func jsonlScannerHandlesLinesAcrossReadChunks() throws {
+        let env = try CostUsageTestEnvironment()
+        defer { env.cleanup() }
+
+        let fileURL = env.root.appendingPathComponent("large-lines.jsonl", isDirectory: false)
+        let largeLine = String(repeating: "x", count: 300_000)
+        let contents = "\(largeLine)\nsmall\n"
+        try contents.write(to: fileURL, atomically: true, encoding: .utf8)
+
+        var scanned: [(count: Int, truncated: Bool)] = []
+        let endOffset = try CostUsageJsonl.scan(
+            fileURL: fileURL,
+            maxLineBytes: 400_000,
+            prefixBytes: 400_000)
+        { line in
+            scanned.append((line.bytes.count, line.wasTruncated))
+        }
+
+        #expect(endOffset == Int64(Data(contents.utf8).count))
+        #expect(scanned.count == 2)
+        #expect(scanned[0].count == 300_000)
+        #expect(scanned[0].truncated == false)
+        #expect(scanned[1].count == 5)
+        #expect(scanned[1].truncated == false)
+    }
+
+    @Test
+    func jsonlScannerMarksPrefixLimitedLinesAsTruncated() throws {
+        let env = try CostUsageTestEnvironment()
+        defer { env.cleanup() }
+
+        let fileURL = env.root.appendingPathComponent("truncated-lines.jsonl", isDirectory: false)
+        let shortLine = "ok"
+        let longLine = String(repeating: "a", count: 2000)
+        let contents = "\(shortLine)\n\(longLine)\n"
+        try contents.write(to: fileURL, atomically: true, encoding: .utf8)
+
+        var scanned: [CostUsageJsonl.Line] = []
+        _ = try CostUsageJsonl.scan(
+            fileURL: fileURL,
+            maxLineBytes: 10000,
+            prefixBytes: 64)
+        { line in
+            scanned.append(line)
+        }
+
+        #expect(scanned.count == 2)
+        #expect(String(data: scanned[0].bytes, encoding: .utf8) == "ok")
+        #expect(scanned[0].wasTruncated == false)
+        #expect(scanned[1].bytes.isEmpty)
+        #expect(scanned[1].wasTruncated == true)
+    }
 }
 
 private struct CostUsageTestEnvironment {