anomaly

const (
	// WW is a write-dependency: Ti installs a version of x, and Tj installs the
	// version of x that immediately follows it in the version order.
	WW Dep = iota
	// WR is a read-dependency: Ti installs a version of x, and Tj reads it.
	WR
	// RW is an anti-dependency: Ti reads a version of x, and Tj installs the
	// version of x that immediately follows the one Ti read.
	RW
)

const (
	// ReadUncommitted is Adya's PL-1: forbids G0 only.
	ReadUncommitted Level = iota
	// ReadCommitted is Adya's PL-2: forbids G0 and G1 (= G1a ∪ G1b ∪ G1c).
	ReadCommitted
	// SnapshotIsolation is Adya's PL-SI, and it is the level GoGraph targets.
	//
	// It forbids G0, G1 and G-nonadjacent — and PERMITS G2-item cycles whose
	// anti-dependency edges are adjacent, which is exactly write skew.
	//
	// THE BOUNDARY IS G-NONADJACENT, NOT G-SINGLE, and getting that right is the
	// substance of this checker. Adya's PL-SI forbids G-SIb, "a cycle with
	// exactly one anti-dependency edge" — Elle's G-single. Cerone, Bernardi &
	// Gotsman's characterisation of GENERALIZED snapshot isolation is stronger:
	// it forbids any cycle whose anti-dependency edges are pairwise
	// non-adjacent, of which a single anti-dependency is the degenerate case.
	// Elle adopts the stronger form and says why, in
	// src/elle/consistency_model.clj (read 2026-08-08): "Chatting with Alexey
	// Gotsman about this confirms my suspicion: generalized SI forbids *any*
	// history where all rw edges are nonadjacent, not just G-single."
	//
	// The two classic anomalies fall on opposite sides of it, and that is the
	// check that matters:
	//
	//   - LOST UPDATE (P4): T1 -rw-> T2 -ww-> T1. One anti-dependency, so the
	//     cycle is G-single ⊂ G-nonadjacent ⇒ FORBIDDEN. Snapshot isolation
	//     prevents it by first-committer-wins on the shared key.
	//
	//   - WRITE SKEW (A5B): T1 -rw-> T2 -rw-> T1. Two anti-dependencies, and
	//     they are ADJACENT — each transaction is both entered and left by one —
	//     so the cycle is G2-item but NOT G-nonadjacent ⇒ PERMITTED. The two
	//     transactions write different keys, so nothing conflicts.
	//
	// A checker that flagged legal write skew would be worse than none, because
	// every clean run would carry a false violation and the real ones would stop
	// being read.
	SnapshotIsolation
	// Serializable is Adya's PL-3: forbids G0, G1 and G2 — any cycle at all.
	Serializable
)

const (
	// Read observes a version of a key.
	Read OpKind = iota
	// Write installs a version of a key.
	Write
)

const (
	// G1a is an ABORTED READ: a transaction read a version written by a
	// transaction that aborted.
	G1a Phenomenon = iota
	// G1b is an INTERMEDIATE READ: a transaction read a version of a key that
	// its writer superseded before committing, so no committed state ever held
	// it.
	G1b
	// G0 is a WRITE CYCLE: a cycle in the DSG made entirely of write-
	// dependencies. It means two transactions' writes to different keys were
	// applied in opposite orders.
	G0
	// G1c is CIRCULAR INFORMATION FLOW: a cycle made of write- and read-
	// dependencies. Each transaction in the cycle depends on information the
	// next one produced.
	G1c
	// GSingle (Elle's G-single; Adya's G-SIb, "missed effects") is a cycle with
	// EXACTLY ONE anti-dependency edge. Lost update has this shape.
	GSingle
	// GNonadjacent is a cycle whose anti-dependency edges are never adjacent to
	// one another. It generalises GSingle and is the boundary snapshot
	// isolation actually draws; see [SnapshotIsolation].
	GNonadjacent
	// G2Item is an ANTI-DEPENDENCY CYCLE over individual items: any cycle
	// carrying at least one anti-dependency edge. Write skew has this shape, and
	// snapshot isolation PERMITS it.
	G2Item
	// Unwritten is not one of Adya's phenomena. It reports a read of a version
	// no transaction in the history wrote, which means the history is
	// INCOMPLETE. It is surfaced rather than skipped because an incomplete
	// history silently produces a clean verdict, and a clean verdict from
	// missing data is the worst output this package could give.
	Unwritten
)

const InitVersion Version = 0