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#pragma once
#include "Api.h"
#include "HandlerManager.h"
#include "IStorageBackend.h"
#include "Types.h"
#include <cstddef>
#include <string>
#include <vector>
/**
* @file CursorManager.h
* @brief Declares paging and cursor orchestration for query execution.
*/
namespace nt
{
/**
* @class CursorManager
* @brief Orchestrates paginated data retrieval from a storage backend.
*
* CursorManager depends only on IStorageBackend. The concrete backend
* (SqliteBackend, InMemoryBackend, …) is injected at construction time.
*
* Tuples are never loaded all at once. Open() fetches the first page;
* each subsequent Next() call transparently fetches the next page from
* the backend once the current page is exhausted.
*
* Pagination is driven by the Merkle B-tree: each cursor carries the hex
* hash of the relation's Merkle root at open time, and LoadPage calls
* Merkle::Page(backend, merkle_root, fetch_offset, PAGE_SIZE) to retrieve
* the next page of tuple hashes without loading sibling subtrees.
*/
class NT_API CursorManager
{
public:
static constexpr std::size_t PAGE_SIZE = 1;
/** @brief Constructs a CursorManager that reads from the given backend. */
explicit CursorManager(IStorageBackend& backend);
/**
* @brief Active scan state for a single relation.
*
* Holds one page of deserialized tuples at a time. When page_position
* reaches the end of the page, Next() discards the page and fetches
* the next one from the backend.
*
* The pointer returned by Next() is valid only until the next Next()
* or Close() call — a page replacement invalidates all prior pointers.
*
* merkle_root is seeded from ObjectManager::Relation::merkle_root at
* open time and does not change during iteration; the cursor reads the
* snapshot captured when it was opened.
*/
struct cursor {
HandlerManager::handle* handle = nullptr;
/**
* Hex hash of the relation's Merkle B-tree root node.
* Empty string means the relation contains no tuples.
*/
std::string merkle_root;
/**
* Bound argument values for EPHEMERAL_RELATION cursors.
* Written by the JOIN operator before each probe; ignored for
* stored RELATION cursors.
*/
std::vector<std::string> args;
std::vector<Tuple> page;
std::size_t page_position = 0;
std::size_t fetch_offset = 0; // next Merkle::Page() call starts here
bool exhausted = false;
};
/**
* @brief Opens a cursor on the relation referenced by the handle.
*
* Fetches the first page immediately for non-empty relations. Returns
* a valid exhausted cursor when @p merkle_root is empty (empty relation).
* Returns nullptr when the handle does not refer to a RELATION or
* EPHEMERAL_RELATION.
*
* @param handle Open RELATION or EPHEMERAL_RELATION handle.
* @param merkle_root Hex hash of the relation's Merkle root at open
* time. Ignored for EPHEMERAL_RELATION handles.
*/
cursor* Open(HandlerManager::handle* handle,
const std::string& merkle_root = "");
/**
* @brief Pulls the next tuple from the cursor.
*
* Fetches the next page from the backend transparently when the
* current page is exhausted. Returns nullptr when all pages are done.
*/
Tuple* Next(cursor* cursor);
/**
* @brief Closes the cursor and releases its resources.
* @param cursor Cursor to close.
*/
void Close(cursor* cursor);
private:
IStorageBackend& backend_;
/**
* Fetches a page of hashes via Merkle::Page, resolves each from the
* KV store, and deserializes them into cursor->page. Clears the
* existing page. Advances fetch_offset by the number of hashes
* returned; sets exhausted when fewer than PAGE_SIZE hashes come back.
*/
void LoadPage(cursor* c);
};
}
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