Improve shred throughput with adaptive chunksize (#98)

Measure Throughput
     │
     ▼
throughput > best?
     │
 ┌───┴───┐
YES     NO
 │       │
 │       ▼
 │   currentChunk == bestChunk?
 │       │
 │   ┌───┴───┐
 │  YES     NO
 │   │       │
 │   ▼       ▼
 │ throughput < best?  Nichts tun
 │   │
 │ ┌─┴─┐
 │YES NO
 │ │   │
 │ ▼   ▼
 │UPDATE!
 │BEST!
 │
 ▼
UPDATE
BOTH!

Reviewed-on: #98
Co-authored-by: localhorst <localhorst@mosad.xyz>
Co-committed-by: localhorst <localhorst@mosad.xyz>

include/shred.h

@@ -16,9 +16,25 @@
 #include <fcntl.h>
 #include <unistd.h>
 #include <string.h>
+#include <chrono>
 
-#define CHUNK_SIZE 1024 * 1024 * 32 // amount of bytes that are overwritten at once --> 32MB
-#define TFNG_DATA_SIZE CHUNK_SIZE   // amount of bytes used by tfng
+// Adaptive chunk size optimization with multi-armed bandit - always enabled
+// Chunk size configuration
+#define CHUNK_SIZE_START 1024 * 1024 * 32    // Starting chunk size: 32MB
+#define CHUNK_SIZE_MIN 1024 * 1024 * 16      // Minimum chunk size: 16MB (increased from 4MB to prevent premature convergence)
+#define CHUNK_SIZE_MAX 1024 * 1024 * 128     // Maximum chunk size: 128MB
+#define CHUNK_SIZE_STEP_UP 1024 * 1024 * 4   // Increase step: 4MB (symmetric with step down)
+#define CHUNK_SIZE_STEP_DOWN 1024 * 1024 * 4 // Decrease step: 4MB (symmetric exploration)
+#define CHUNK_MEASURE_INTERVAL 64            // Measure performance every 64 chunks
+#define WARMUP_MEASUREMENTS 16               // Skip first 16 measurements (cache writes)
+
+// Multi-armed bandit exploration parameters
+#define EXPLORATION_EPSILON 0.10   // 10% exploration rate (epsilon-greedy)
+#define REEXPLORATION_INTERVAL 500 // Force re-exploration every 500 chunks
+
+// Buffer sizes - always use maximum for adaptive mode
+#define CHUNK_SIZE CHUNK_SIZE_MAX
+#define TFNG_DATA_SIZE CHUNK_SIZE_MAX
 
 // #define DEMO_DRIVE_SIZE 1024*1024*256L // 256MB
 // #define DEMO_DRIVE_SIZE 1024*1024*1024L // 1GB
@@ -38,13 +54,41 @@ public:
 private:
     fileDescriptor randomSrcFileDiscr;
     fileDescriptor driveFileDiscr;
-    unsigned char caTfngData[TFNG_DATA_SIZE];
-    unsigned char caReadBuffer[CHUNK_SIZE];
+
+    unsigned char *caTfngData;
+    unsigned char *caReadBuffer;
+
     unsigned long ulDriveByteSize;
     unsigned long ulDriveByteOverallCount = 0; // all bytes shredded in all iterations + checking -> used for progress calculation
     double d32Percent = 0.0;
     double d32TmpPercent = 0.0;
 
+    // Adaptive chunk size optimization members
+    size_t currentChunkSize;
+    size_t bestChunkSize;
+    unsigned int chunkCounter;
+    unsigned int totalChunkCounter; // Total chunks written (for periodic re-exploration)
+    unsigned int warmupCounter;     // Count warm-up measurements
+    std::chrono::high_resolution_clock::time_point measurementStartTime;
+    double bestThroughputMBps;
+    double lastThroughputMBps;
+    unsigned long bytesWrittenInMeasurement;
+    bool throughputIncreasing;
+
+    // Multi-armed bandit exploration state
+    bool explorationMode;        // Currently in exploration mode?
+    size_t explorationChunkSize; // Chunk size being tested during exploration
+
+    // Adaptive methods
+    void startMeasurement();
+    void evaluateThroughput(Drive *drive);
+    void adjustChunkSize(Drive *drive);
+    size_t getCurrentChunkSize() const;
+
+    // Multi-armed bandit methods
+    bool shouldExplore();                  // Decide: explore or exploit?
+    void performExploration(Drive *drive); // Execute exploration phase
+
     inline double calcProgress();
     int iRewindDrive(fileDescriptor file);
     long getDriveSizeInBytes(fileDescriptor file);