// MmapLogWriter: lock-free circular event log over a memory-mapped file.

#pragma once

#include <cstddef>

#include <cstdint>

#include <string>

// Maximum accepted entry size in bytes.

static constexpr size_t kMaxEntryBytes = 4096;

// Persistent header at offset 0 of the mapped file.

// Followed immediately by the circular data region.

struct MappedHeader {

  uint32_t magic;   // file identity tag; must equal kLogMagic on open

  uint32_t cursor;  // byte offset of the next write position within the data region;

                    // must be advanced only after the entry data is durably persisted

};

static constexpr uint32_t kLogMagic = 0xABCD1234;

// Writes events into a memory-mapped circular file without per-write syscalls.

class MmapLogWriter {

public:

  // Parameters:

  //   path     - filesystem path to the log file (created if absent)

  //   capacity - size in bytes of the circular data region (excluding the header)

  // Returns: new MmapLogWriter; call isOpen() before first write

  MmapLogWriter(const std::string& path, size_t capacity);

  ~MmapLogWriter();

  // Appends an opaque entry to the circular log.

  // Parameters:

  //   data      - pointer to the entry bytes

  //   entrySize - number of bytes to write; must not exceed kMaxEntryBytes

  // Returns: true if the entry was written and the cursor advanced;

  //          false if entrySize exceeds kMaxEntryBytes or the writer is not open

  bool write(const void* data, size_t entrySize);

  // Parameters: none

  // Returns: true if the file is mapped and ready to accept writes

  bool isOpen() const { return region_ != nullptr; }

private:

  uint8_t*     region_   = nullptr; // pointer to the start of the circular data region

  MappedHeader* header_  = nullptr; // pointer to the persistent header (before region_)

  size_t       capacity_ = 0;       // total bytes in the circular region

  size_t       cursor_   = 0;       // in-memory cursor, mirrors header_->cursor

  int          fd_       = -1;        // file descriptor for the mapped log file

};