add a transform function for operations

src/lib.rs

@@ -2,4 +2,5 @@
 #![allow(dead_code)]
 
 mod errors;
+mod operation;
 mod taskdb;

src/operation.rs

@@ -0,0 +1,129 @@
+use chrono::{DateTime, Utc};
+use serde_json::Value;
+use uuid::Uuid;
+
+#[derive(PartialEq, Clone, Debug)]
+pub enum Operation {
+    Create {
+        uuid: Uuid,
+    },
+    Update {
+        uuid: Uuid,
+        property: String,
+        value: Value,
+        timestamp: DateTime<Utc>,
+    },
+}
+
+use Operation::*;
+
+impl Operation {
+    // Transform takes two operations A and B that happened concurrently and produces two
+    // operations A' and B' such that `apply(apply(S, A), B') = apply(apply(S, B), A')`. This
+    // function is used to serialize operations in a process similar to a Git "rebase".
+    //
+    //        *
+    //       / \
+    //  op1 /   \ op2
+    //     /     \
+    //    *       *
+    //
+    // this function "completes the diamond:
+    //
+    //    *       *
+    //     \     /
+    // op2' \   / op1'
+    //       \ /
+    //        *
+    //
+    // such that applying op2' after op1 has the same effect as applying op1' after op2.  This
+    // allows two different systems which have already applied op1 and op2, respectively, and thus
+    // reached different states, to return to the same state by applying op2' and op1',
+    // respectively.
+    pub fn transform(
+        operation1: Operation,
+        operation2: Operation,
+    ) -> (Option<Operation>, Option<Operation>) {
+        match (&operation1, &operation2) {
+            // Two creations of the same uuid reach the same state, so there's no need for any
+            // further operations to bring the state together.
+            (&Create { uuid: uuid1 }, &Create { uuid: uuid2 }) if uuid1 == uuid2 => (None, None),
+
+            // Two updates to the same property of the same task might conflict.
+            (
+                &Update {
+                    uuid: ref uuid1,
+                    property: ref property1,
+                    value: ref value1,
+                    timestamp: ref timestamp1,
+                },
+                &Update {
+                    uuid: ref uuid2,
+                    property: ref property2,
+                    value: ref value2,
+                    timestamp: ref timestamp2,
+                },
+            ) if uuid1 == uuid2 && property1 == property2 => {
+                // if the value is the same, there's no conflict
+                if value1 == value2 {
+                    (None, None)
+                } else if timestamp1 < timestamp2 {
+                    // prefer the later modification
+                    (None, Some(operation2))
+                } else if timestamp1 > timestamp2 {
+                    // prefer the later modification
+                    (Some(operation1), None)
+                } else {
+                    // arbitrarily resolve in favor of the first operation
+                    (Some(operation1), None)
+                }
+            }
+
+            // anything else is not a conflict of any sort, so return the operations unchanged
+            (_, _) => (Some(operation1), Some(operation2)),
+        }
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::taskdb::DB;
+
+    fn test_transform(
+        o1: Operation,
+        o2: Operation,
+        exp1p: Option<Operation>,
+        exp2p: Option<Operation>,
+    ) {
+        let (o1p, o2p) = Operation::transform(o1.clone(), o2.clone());
+        assert_eq!((&o1p, &o2p), (&exp1p, &exp2p));
+
+        // check that the two operation sequences have the same effect, enforcing the invariant of
+        // the transform function.
+        let mut db1 = DB::new();
+        db1.apply(o1).unwrap();
+        if let Some(o) = o2p {
+            db1.apply(o).unwrap();
+        }
+        let mut db2 = DB::new();
+        db2.apply(o2).unwrap();
+        if let Some(o) = o1p {
+            db2.apply(o).unwrap();
+        }
+        assert_eq!(db1.tasks(), db2.tasks());
+    }
+
+    #[test]
+    fn test_unrelated_create() {
+        let uuid1 = Uuid::new_v4();
+        let uuid2 = Uuid::new_v4();
+
+        test_transform(
+            Operation::Create { uuid: uuid1 },
+            Operation::Create { uuid: uuid2 },
+            Some(Operation::Create { uuid: uuid1 }),
+            Some(Operation::Create { uuid: uuid2 }),
+        );
+    }
+}

src/taskdb.rs

@@ -1,24 +1,12 @@
 use crate::errors::Error;
-use chrono::{DateTime, Utc};
+use crate::operation::Operation;
 use serde_json::Value;
 use std::collections::hash_map::Entry;
 use std::collections::HashMap;
 use uuid::Uuid;
 
-#[derive(PartialEq, Clone, Debug)]
-enum Operation {
-    Create {
-        uuid: Uuid,
-    },
-    Update {
-        uuid: Uuid,
-        property: String,
-        value: Value,
-        timestamp: DateTime<Utc>,
-    },
-}
-
-struct DB {
+#[derive(PartialEq, Debug)]
+pub struct DB {
     // The current state, with all pending operations applied
     tasks: HashMap<Uuid, HashMap<String, Value>>,
 
@@ -33,7 +21,7 @@ struct DB {
 
 impl DB {
     /// Create a new, empty database
-    fn new() -> DB {
+    pub fn new() -> DB {
         DB {
             tasks: HashMap::new(),
             base_version: 0,
@@ -43,7 +31,7 @@ impl DB {
 
     /// Apply an operation to the DB.  Aside from synchronization operations, this
     /// is the only way to modify the DB.
-    fn apply(&mut self, op: Operation) -> Result<(), Error> {
+    pub fn apply(&mut self, op: Operation) -> Result<(), Error> {
         match op {
             Operation::Create { uuid } => {
                 match self.tasks.entry(uuid) {
@@ -78,7 +66,7 @@ impl DB {
     /// Get a read-only reference to the underlying set of tasks.
     ///
     /// This API is temporary, but provides query access to the DB.
-    fn tasks(&self) -> &HashMap<Uuid, HashMap<String, Value>> {
+    pub fn tasks(&self) -> &HashMap<Uuid, HashMap<String, Value>> {
         &self.tasks
     }
 }
@@ -86,6 +74,7 @@ impl DB {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use chrono::Utc;
     use uuid::Uuid;
 
     #[test]