From 7be38448bbf116d415f7b8b40697136dae1daf61 Mon Sep 17 00:00:00 2001
From: Stephan Boyer <stephan@stephanboyer.com>
Date: Tue, 19 Dec 2023 03:47:45 -0800
Subject: [PATCH] Demonstrate that the two styles of CRDTs are equivalent

---
 proofs/crdt/equivalence.v | 91 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 91 insertions(+)
 create mode 100644 proofs/crdt/equivalence.v

diff --git a/proofs/crdt/equivalence.v b/proofs/crdt/equivalence.v
new file mode 100644
index 00000000..79974a75
--- /dev/null
+++ b/proofs/crdt/equivalence.v
@@ -0,0 +1,91 @@
+(**************************************************************************)
+(**************************************************************************)
+(****                                                                  ****)
+(****   Equivalence of state-based and operation-based conflict-free   ****)
+(****   replicated data types                                          ****)
+(****                                                                  ****)
+(**************************************************************************)
+(**************************************************************************)
+
+Require Import Coq.Lists.List.
+Require Import Coq.Sorting.Permutation.
+Require Import main.tactics.
+
+Require main.crdt.operation_crdt.
+
+Import Coq.Lists.List.ListNotations.
+
+(* We can emulate a state-based CRDT with an operation-based CRDT. *)
+
+Definition operation_crdt_data_from_state_crdt
+  [A Q]
+  (crdt : state_crdt.Crdt A Q)
+: operation_crdt.CrdtData A Q
+:=
+  {|
+    operation_crdt.State := crdt.(state_crdt.State);
+    operation_crdt.Operation := crdt.(state_crdt.State);
+    operation_crdt.initial := crdt.(state_crdt.initial);
+    operation_crdt.update x s := crdt.(state_crdt.update) x s;
+    operation_crdt.interpret o s := crdt.(state_crdt.merge) o s;
+    operation_crdt.query s := crdt.(state_crdt.query) s;
+    operation_crdt.Precondition _ _ := True;
+  |}.
+
+Program Definition operation_crdt_from_state_crdt
+  [A Q]
+  (crdt : state_crdt.Crdt A Q)
+: operation_crdt.Crdt (operation_crdt_data_from_state_crdt crdt)
+:= {| operation_crdt.commutativity := _ |}.
+Next Obligation.
+  clean.
+  pose proof (crdt.(state_crdt.semilattice _ _)).
+  rewrite state_crdt.associativity
+    with (initial := crdt.(state_crdt.initial)); search.
+  rewrite state_crdt.commutativity
+    with (initial := crdt.(state_crdt.initial)) (x := o1); search.
+Qed.
+
+(*
+  We can emulate an operation-based CRDT with a state-based CRDT if the
+  delivery precondition and equality on operations are decidable and there is a
+  way to order any given set of operations. The order can be completely
+  arbitrary.
+*)
+
+#[local] Obligation Tactic := auto. (* `search` diverges here, sadly. *)
+
+Program Definition state_crdt_from_operation_crdt
+  [A Q]
+  [crdt_data : operation_crdt.CrdtData A Q]
+  (crdt : operation_crdt.Crdt crdt_data)
+  (precondition :
+    forall s o,
+    {crdt_data.(operation_crdt.Precondition) s o} +
+    {~ crdt_data.(operation_crdt.Precondition) s o})
+  (equal :
+    forall (o1 o2 : crdt_data.(operation_crdt.Operation)),
+    {o1 = o2} + {o1 <> o2})
+  (permute :
+    list crdt_data.(operation_crdt.Operation) ->
+    list crdt_data.(operation_crdt.Operation))
+  (H1 : forall os, Permutation os (permute os))
+  (H : forall os1 os2, Permutation os1 os2 -> permute os1 = permute os2)
+: state_crdt.Crdt A Q :=
+  {|
+    state_crdt.State :=
+      list crdt_data.(operation_crdt.Operation) *
+      list crdt_data.(operation_crdt.Operation);
+    state_crdt.initial := ([], []);
+    state_crdt.merge '(_, _) '(_, _) := _;
+    state_crdt.update _ '(_, _) := _;
+    state_crdt.query '(_, delivered) :=
+      crdt_data.(operation_crdt.query)
+        (operation_crdt.run crdt_data delivered);
+  |}.
+Next Obligation.
+  clean.
+  split; search.
+Admitted.
+Next Obligation.
+Admitted.