commit 05d26285e3fac39fa65b75851201103488f1c293
Author: Dustin J. Mitchell <dustin@mozilla.com>
Date:   Wed Dec 25 10:51:44 2019 -0500

    implement TextOperation

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 000000000..53eaa2196
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+/target
+**/*.rs.bk
diff --git a/Cargo.lock b/Cargo.lock
new file mode 100644
index 000000000..d7e8046f3
--- /dev/null
+++ b/Cargo.lock
@@ -0,0 +1,6 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+[[package]]
+name = "ot"
+version = "0.1.0"
+
diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 000000000..93ba0d087
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,9 @@
+[package]
+name = "ot"
+version = "0.1.0"
+authors = ["Dustin J. Mitchell <dustin@mozilla.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
diff --git a/src/main.rs b/src/main.rs
new file mode 100644
index 000000000..d4a28f0a7
--- /dev/null
+++ b/src/main.rs
@@ -0,0 +1,7 @@
+mod operation;
+
+// see https://raw.githubusercontent.com/Operational-Transformation/ot.js/master/dist/ot.js
+
+fn main() {
+    println!("Hello, world!");
+}
diff --git a/src/operation.rs b/src/operation.rs
new file mode 100644
index 000000000..e643b228f
--- /dev/null
+++ b/src/operation.rs
@@ -0,0 +1,576 @@
+#[derive(PartialEq, Debug, Clone)]
+enum Op {
+    Retain(usize),
+    Insert(String),
+    Delete(usize),
+}
+
+#[derive(PartialEq, Debug, Clone)]
+struct TextOperation {
+    // When an operation is applied to an input string, you can think of this as if an imaginary
+    // cursor runs over the entire string and skips over some parts, deletes some parts and inserts
+    // characters at some positions. These actions (skip/delete/insert) are stored as an array in
+    // the "ops" property.
+    ops: Vec<Op>,
+
+    // An operation's base_length is the length of every string the operation can be applied to.
+    base_length: usize,
+
+    // The target_length is the length of every string that results from applying the operation on
+    // a valid input string.
+    target_length: usize,
+}
+
+impl TextOperation {
+    // Create a new, empty TextOperation
+    pub fn new() -> Self {
+        Self {
+            ops: vec![],
+            base_length: 0,
+            target_length: 0,
+        }
+    }
+
+    // Add a retain op to this TextOperation (used in the builder pattern)
+    pub fn retain(mut self: Self, n: usize) -> Self {
+        if n == 0 {
+            return self;
+        }
+
+        self.base_length += n;
+        self.target_length += n;
+
+        if let Some(&mut Op::Retain(ref mut l)) = self.ops.last_mut() {
+            *l += n;
+        } else {
+            self.ops.push(Op::Retain(n));
+        }
+
+        self
+    }
+
+    // Add an insert op to this TextOperation (used in the builder pattern)
+    pub fn insert(mut self: Self, s: impl Into<String>) -> Self {
+        let s = s.into();
+        let l = s.len();
+
+        if  l == 0 {
+            return self;
+        }
+
+        if let Some(&Op::Delete(_)) = self.ops.last() {
+            // maintain the invariant that inserts never follow a delete by popping the
+            // delete. adding the insert, and then re-adding the delete
+            let del = self.ops.pop().unwrap();
+            self = self.insert(s);
+            self.ops.push(del);
+            return self;
+        }
+
+        self.target_length += l;
+
+        if let Some(&mut Op::Insert(ref mut l)) = self.ops.last_mut() {
+            l.push_str(&s)
+        } else {
+            self.ops.push(Op::Insert(s));
+        }
+
+        self
+    }
+
+    // Add a delete op to this TextOperation (used in the builder pattern)
+    pub fn delete(mut self: Self, n: usize) -> Self {
+        if n == 0 {
+            return self;
+        }
+
+        self.base_length += n;
+
+        if let Some(&mut Op::Delete(ref mut l)) = self.ops.last_mut() {
+            *l += n;
+        } else {
+            self.ops.push(Op::Delete(n));
+        }
+
+        self
+    }
+
+    // Apply an operation to a string. Returns an error if there's a mismatch between the input string
+    // and the operation.
+    pub fn apply(self: &Self, input: &str) -> Result<String, String> {
+        let strlen = input.len();
+
+        if strlen != self.base_length {
+            return Err(String::from("The operation's base length must be equal to the string's length"))
+        }
+
+        let mut res = String::new();
+        let mut i = 0;
+        for op in &self.ops {
+            match op {
+                Op::Retain(n) => {
+                    assert!(i + n <= strlen);
+                    res.push_str(&input[i..i + n]);
+                    i += n;
+                },
+                Op::Insert(s) => {
+                    res.push_str(&s);
+                },
+                Op::Delete(n) => {
+                    i += n;
+                },
+            }
+        }
+
+        Ok(res)
+    }
+
+    // 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 the heart of OT.  That is, given a state map
+    //
+    //        *
+    //       / \
+    //  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(mut operation1: TextOperation, mut operation2: TextOperation) -> (TextOperation, TextOperation) {
+        // both must start at the same state, and thus have the same base length
+        assert_eq!(operation1.base_length, operation2.base_length);
+
+        let (mut operation1p, mut operation2p) = (TextOperation::new(), TextOperation::new());
+        let (mut ops1, mut ops2) = (operation1.ops.iter_mut(), operation2.ops.iter_mut());
+        let (mut op1, mut op2) = (ops1.next(), ops2.next());
+
+        loop {
+            match (&mut op1, &mut op2) {
+                // end condition: both ops1 and ops2 have been processed
+                (None, None) => break,
+
+                // handle inserts, preferring op1 if both are inserts
+                (Some(&mut Op::Insert(ref s)), _) => {
+                    operation1p = operation1p.insert(s.clone());
+                    operation2p = operation2p.retain(s.len());
+                    op1 = ops1.next();
+                    continue;
+                },
+
+                (_, Some(&mut Op::Insert(ref s))) => {
+                    operation1p = operation1p.retain(s.len());
+                    operation2p = operation2p.insert(s.clone());
+                    op2 = ops2.next();
+                    continue;
+                },
+
+                (None, _) | (_, None) => {
+                    unreachable!();
+                }
+
+                (Some(&mut Op::Retain(ref mut n1)), Some(&mut Op::Retain(ref mut n2))) => {
+                    // retain the minimum of the two, and rewrite the larger input op
+                    // to only retain what's left
+                    let min;
+                    if n1 > n2 {
+                        min = *n2;
+                        *n1 = *n1 - *n2;
+                        op2 = ops2.next();
+                    } else if n1 == n2 {
+                        min = *n2;
+                        op1 = ops1.next();
+                        op2 = ops2.next();
+                    } else {
+                        min = *n1;
+                        *n2 = *n2 - *n1;
+                        op1 = ops1.next();
+                    }
+                    operation1p = operation1p.retain(min);
+                    operation2p = operation2p.retain(min);
+                },
+                (Some(Op::Delete(n1)), Some(Op::Delete(n2))) => {
+                    // Both operations delete the same string at the same position. We don't need
+                    // to produce any operations, we just skip over the delete ops and handle the
+                    // case that one operation deletes more than the other.
+                    if n1 > n2 {
+                        *n1 = *n2 - *n1;
+                        op2 = ops2.next();
+                    } else if n1 == n2 {
+                        op1 = ops1.next();
+                        op2 = ops2.next();
+                    } else {
+                        *n2 = *n2 - *n1;
+                        op1 = ops1.next();
+                    }
+                },
+                (Some(Op::Delete(n1)), Some(Op::Retain(n2))) => {
+                    let min;
+                    if n1 > n2 {
+                        min = *n2;
+                        *n1 = *n1 - *n2;
+                        op2 = ops2.next();
+                    } else if n1 == n2 {
+                        min = *n1;
+                        op1 = ops1.next();
+                        op2 = ops2.next();
+                    } else {
+                        min = *n1;
+                        *n2 = *n2 - *n1;
+                        op1 = ops1.next();
+                    }
+                    operation1p = operation1p.delete(min);
+                },
+                (Some(Op::Retain(n1)), Some(Op::Delete(n2))) => {
+                    let min;
+                    if n1 > n2 {
+                        min = *n2;
+                        *n1 = *n1 - *n2;
+                        op2 = ops2.next();
+                    } else if n1 == n2 {
+                        min = *n1;
+                        op1 = ops1.next();
+                        op2 = ops2.next();
+                    } else {
+                        min = *n1;
+                        *n2 = *n2 - *n1;
+                        op1 = ops1.next();
+                    }
+                    operation2p = operation2p.delete(min);
+                },
+            }
+        }
+
+        (operation1p, operation2p)
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::{TextOperation, Op};
+
+    #[test]
+    fn test_build_retain() {
+        let op = TextOperation::new()
+            .retain(10);
+        assert_eq!(op.base_length, 10);
+        assert_eq!(op.target_length, 10);
+        assert_eq!(op.ops, vec![Op::Retain(10)]);
+    }
+
+    #[test]
+    fn test_build_retain_null() {
+        let op = TextOperation::new()
+            .retain(0);
+        assert_eq!(op.base_length, 0);
+        assert_eq!(op.target_length, 0);
+        assert_eq!(op.ops, vec![]);
+    }
+
+    #[test]
+    fn test_build_retain_merge() {
+        let op = TextOperation::new()
+            .retain(10)
+            .retain(20);
+        assert_eq!(op.base_length, 30);
+        assert_eq!(op.target_length, 30);
+        assert_eq!(op.ops, vec![Op::Retain(30)]);
+    }
+
+    #[test]
+    fn test_build_retain_after_insert() {
+        let op = TextOperation::new()
+            .insert(String::from("hello"))
+            .retain(20);
+        assert_eq!(op.base_length, 20);
+        assert_eq!(op.target_length, 25);
+        assert_eq!(op.ops, vec![Op::Insert(String::from("hello")), Op::Retain(20)]);
+    }
+
+    #[test]
+    fn test_build_retain_after_delete() {
+        let op = TextOperation::new()
+            .delete(10)
+            .retain(20);
+        assert_eq!(op.base_length, 30);
+        assert_eq!(op.target_length, 20);
+        assert_eq!(op.ops, vec![Op::Delete(10), Op::Retain(20)]);
+    }
+
+    #[test]
+    fn test_build_insert() {
+        let op = TextOperation::new()
+            .insert(String::from("hello"));
+        assert_eq!(op.base_length, 0);
+        assert_eq!(op.target_length, 5);
+        assert_eq!(op.ops, vec![Op::Insert(String::from("hello"))]);
+    }
+
+    #[test]
+    fn test_build_insert_null() {
+        let op = TextOperation::new()
+            .insert(String::from(""));
+        assert_eq!(op.base_length, 0);
+        assert_eq!(op.target_length,0);
+        assert_eq!(op.ops, vec![]);
+    }
+
+    #[test]
+    fn test_build_insert_merge() {
+        let op = TextOperation::new()
+            .insert(String::from("hello"))
+            .insert(String::from(" world"));
+        assert_eq!(op.base_length, 0);
+        assert_eq!(op.target_length, 11);
+        assert_eq!(op.ops, vec![Op::Insert(String::from("hello world"))]);
+    }
+
+    #[test]
+    fn test_build_insert_after_retain() {
+        let op = TextOperation::new()
+            .retain(10)
+            .insert(String::from("hello"));
+        assert_eq!(op.base_length, 10);
+        assert_eq!(op.target_length, 15);
+        assert_eq!(op.ops, vec![Op::Retain(10), Op::Insert(String::from("hello"))]);
+    }
+
+    #[test]
+    fn test_build_insert_after_delete() {
+        let op = TextOperation::new()
+            .delete(10)
+            .insert(String::from("hello"));
+        assert_eq!(op.base_length, 10);
+        assert_eq!(op.target_length, 5);
+        assert_eq!(op.ops, vec![
+            Op::Insert(String::from("hello")),
+            Op::Delete(10),
+        ]);
+    }
+
+    #[test]
+    fn test_build_insert_after_delete_merge() {
+        let op = TextOperation::new()
+            .insert(String::from("hello"))
+            .delete(10)
+            .insert(String::from(" world"));
+        assert_eq!(op.base_length, 10);
+        assert_eq!(op.target_length, 11);
+        assert_eq!(op.ops, vec![
+            Op::Insert(String::from("hello world")),
+            Op::Delete(10),
+        ]);
+    }
+
+    #[test]
+    fn test_build_delete() {
+        let op = TextOperation::new()
+            .delete(17);
+        assert_eq!(op.base_length, 17);
+        assert_eq!(op.target_length, 0);
+        assert_eq!(op.ops, vec![Op::Delete(17)]);
+    }
+
+    #[test]
+    fn test_build_delete_null() {
+        let op = TextOperation::new()
+            .delete(0);
+        assert_eq!(op.base_length, 0);
+        assert_eq!(op.target_length, 0);
+        assert_eq!(op.ops, vec![]);
+    }
+
+    #[test]
+    fn test_build_delete_merge() {
+        let op = TextOperation::new()
+            .delete(5)
+            .delete(8);
+        assert_eq!(op.base_length, 13);
+        assert_eq!(op.target_length, 0);
+        assert_eq!(op.ops, vec![Op::Delete(13)]);
+    }
+
+    #[test]
+    fn test_build_delete_after_insert() {
+        let op = TextOperation::new()
+            .insert(String::from("hello"))
+            .delete(8);
+        assert_eq!(op.base_length, 8);
+        assert_eq!(op.target_length, 5);
+        assert_eq!(op.ops, vec![Op::Insert(String::from("hello")), Op::Delete(8)]);
+    }
+
+    #[test]
+    fn test_build_delete_after_retain() {
+        let op = TextOperation::new()
+            .retain(10)
+            .delete(8);
+        assert_eq!(op.base_length, 18);
+        assert_eq!(op.target_length, 10);
+        assert_eq!(op.ops, vec![Op::Retain(10), Op::Delete(8)]);
+    }
+
+    #[test]
+    fn test_apply_retain() {
+        let op = TextOperation::new()
+            .retain(5);
+        assert_eq!(
+            op.apply("hello"),
+            Ok(String::from("hello")));
+    }
+
+    #[test]
+    fn test_apply_insert() {
+        let op = TextOperation::new()
+            .retain(5)
+            .insert(String::from(" cruel"))
+            .retain(6);
+        assert_eq!(
+            op.apply("hello world"),
+            Ok(String::from("hello cruel world")));
+    }
+
+    #[test]
+    fn test_apply_delete() {
+        let op = TextOperation::new()
+            .retain(5)
+            .delete(6)
+            .retain(6);
+        assert_eq!(
+            op.apply("hello cruel world"),
+            Ok(String::from("hello world")));
+    }
+
+    #[test]
+    fn test_apply_wrong_length() {
+        let op = TextOperation::new()
+            .retain(5)
+            .insert(String::from(" cruel"))
+            .retain(6);
+        assert_eq!(
+            op.apply("hello cruel world"),
+            Err(String::from("The operation's base length must be equal to the string's length")));
+    }
+
+    mod transform {
+        use super::{TextOperation};
+
+        const STARTING_STATE: &str ="abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
+
+        fn test_transform(o1: TextOperation, o2: TextOperation, exp1p: TextOperation, exp2p: TextOperation) {
+            // first check that the resulting operation is as expected..
+            let (o1p, o2p) = TextOperation::transform(o1.clone(), o2.clone());
+            assert_eq!((&o1p, &o2p), (&exp1p, &exp2p));
+
+            // then check that the definition of `transform` is satisfied, by applying to an
+            // arbitrary string
+            let input = &STARTING_STATE[0..o1.base_length];
+            // B' composed with A
+            let first = o2p.apply(&o1.apply(&input).unwrap()).unwrap();
+            // A' composed with B
+            let second = o1p.apply(&o2.apply(&input).unwrap()).unwrap();
+            print!("{} -> {}\n", input, first);
+            assert_eq!(first, second);
+        }
+
+        #[test]
+        fn test_transform_empty() {
+            test_transform(
+                TextOperation::new(),
+                TextOperation::new(),
+                TextOperation::new(),
+                TextOperation::new());
+        }
+
+        #[test]
+        fn test_transform_one_noop() {
+            test_transform(
+                TextOperation::new().retain(2).insert("123").retain(10),
+                TextOperation::new().retain(12),
+                TextOperation::new().retain(2).insert("123").retain(10),
+                TextOperation::new().retain(15));
+        }
+
+        #[test]
+        fn test_transform_two_inserts() {
+            test_transform(
+                TextOperation::new().insert("123"),
+                TextOperation::new().insert("567"),
+                TextOperation::new().insert("123").retain(3),
+                TextOperation::new().retain(3).insert("567"));
+        }
+
+        #[test]
+        fn test_transform_two_retains() {
+            test_transform(
+                TextOperation::new().retain(10),
+                TextOperation::new().retain(10),
+                TextOperation::new().retain(10),
+                TextOperation::new().retain(10));
+        }
+
+        #[test]
+        fn test_transform_insert_two_different_spots() {
+            test_transform(
+                TextOperation::new().retain(5).insert("123").retain(10),
+                TextOperation::new().retain(10).insert("567").retain(5),
+                TextOperation::new().retain(5).insert("123").retain(13),
+                TextOperation::new().retain(13).insert("567").retain(5));
+        }
+
+        #[test]
+        fn test_transform_insert_two_different_spots_reversed() {
+            test_transform(
+                TextOperation::new().retain(10).insert("567").retain(5),
+                TextOperation::new().retain(5).insert("123").retain(10),
+                TextOperation::new().retain(13).insert("567").retain(5),
+                TextOperation::new().retain(5).insert("123").retain(13));
+        }
+
+        #[test]
+        fn test_transform_two_deletes() {
+            test_transform(
+                TextOperation::new().delete(10),
+                TextOperation::new().delete(10),
+                TextOperation::new(),
+                TextOperation::new());
+        }
+
+        #[test]
+        fn test_transform_delete_retain() {
+            test_transform(
+                TextOperation::new().retain(10),
+                TextOperation::new().delete(3).retain(7),
+                TextOperation::new().retain(7),
+                TextOperation::new().delete(3).retain(7));
+        }
+
+        #[test]
+        fn test_transform_retain_delete() {
+            test_transform(
+                TextOperation::new().delete(3).retain(7),
+                TextOperation::new().retain(10),
+                TextOperation::new().delete(3).retain(7),
+                TextOperation::new().retain(7));
+        }
+
+        #[test]
+        fn test_transform_delete_insert() {
+            test_transform(
+                TextOperation::new().delete(10),
+                TextOperation::new().retain(3).insert("123").retain(7),
+                TextOperation::new().delete(3).retain(3).delete(7),
+                TextOperation::new().insert("123"));
+        }
+    }
+}