/*-
 * Copyright (c) 2008-2013 WiredTiger, Inc.
 *	All rights reserved.
 *
 * See the file LICENSE for redistribution information.
 */

/*
 * Transaction ID type: transaction IDs are 32-bit integers that wrap after
 * 4 billion transactions are executed.
 */
typedef uint32_t wt_txnid_t;

#define	WT_TXN_NONE	0		/* No txn running in a session. */
#define	WT_TXN_ABORTED	UINT32_MAX	/* Update rolled back, ignore. */

/*
 * Transaction ID comparison dealing with edge cases and wrapping.
 *
 * WT_TXN_ABORTED is the largest possible ID (never visible to a running
 * transaction), WT_TXN_NONE is smaller than any possible ID (visible to all
 * running transactions).
 *
 * Otherwise, we deal with 32-bit wrapping by looking at the difference between
 * the two IDs.  In what follows, "small" means less than 2^31 and "large"
 * means greater than 2^31.
 *
 * If t2 > t1 and neither has wrapped, then (t2 - t1) is small.  It is
 * certainly smaller than t2, and we assume that we never compare IDs that
 * differ by more than 2^31.  If t2 has wrapped (so it is small) and t1 is
 * large, then (t2 - t1) = (t2 + (-t1)) will be small.
 *
 * In effect, we have a 31-bit window of active transaction IDs: if an update
 * remains in the system after 2 billion transactions it can no longer be
 * compared with current transaction ID.
 */
#define	TXNID_LE(t1, t2)						\
	(((t1) == WT_TXN_ABORTED || (t2) == WT_TXN_NONE) ? 0 :		\
	 ((t1) == WT_TXN_NONE || (t2) == WT_TXN_ABORTED) ? 1 :		\
	 (t2) - (t1) < (UINT32_MAX / 2))

#define	TXNID_LT(t1, t2)						\
	((t1) != (t2) && TXNID_LE(t1, t2))

struct __wt_txn_state {
	volatile wt_txnid_t id;
	volatile wt_txnid_t snap_min;
};

struct __wt_txn_global {
	volatile wt_txnid_t current;	/* Current transaction ID. */
	volatile uint32_t gen;		/* Completed transaction generation */
	WT_TXN_STATE *states;		/* Per-session transaction states */
};

enum __wt_txn_isolation {
	TXN_ISO_READ_UNCOMMITTED,
	TXN_ISO_READ_COMMITTED,
	TXN_ISO_SNAPSHOT
};

struct __wt_txn {
	wt_txnid_t id;

	WT_TXN_ISOLATION isolation;

	/*
	 * Snapshot data:
	 *	ids < snap_min are visible,
	 *	ids > snap_max are invisible,
	 *	everything else is visible unless it is in the snapshot.
	 */
	wt_txnid_t snap_min, snap_max;
	wt_txnid_t *snapshot;
	uint32_t snapshot_count;

	/*
	 * When this transaction started, the oldest transaction ID that was
	 * not yet visible to some transaction in the system.
	 */
	wt_txnid_t oldest_snap_min;

	/* Saved global state, to avoid repeating scans. */
	wt_txnid_t last_id;
	uint32_t last_gen;

	/*
	 * Arrays of txn IDs in WT_UPDATE or WT_REF structures created or
	 * modified by this transaction.
	 */
	wt_txnid_t    **mod;
	size_t		mod_alloc;
	u_int		mod_count;

	WT_REF	      **modref;
	size_t		modref_alloc;
	u_int		modref_count;

#define	TXN_AUTOCOMMIT	0x01
#define	TXN_ERROR	0x02
#define	TXN_OLDEST	0x04
#define	TXN_RUNNING	0x08
	uint32_t flags;
};