#diff #difference #compare #comparison #changes


Differ is a library for finding the differences between two sequences

6 releases

✓ Uses Rust 2018 edition

0.4.1 Feb 10, 2019
0.4.0 Feb 10, 2019
0.3.3 Feb 9, 2019

#136 in Development tools

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Differ is a library for finding the differences between two sequences.

The sequences can be vectors of lines, strings (e.g., words), characters, bytes, or of any custom “item” type so long as it implements the Hash and Eq traits (and uses the same data for both).

The simplest use case is first to create a new Differ using Differ::new(). Then iterate over the Spans returned by Differ::spans(). These spans show how to transform sequence a into sequence b using insertions, deletions, and replacements.

differ is based on the Python difflib.SequenceMatcher.


To use retest, add this line your Cargo.toml file's [dependencies] section:

differ = "0.4"

If you want to be able to serialize or deserialize Matches, Spans, or Tags, then use:

differ = { version = "0.4", features = ["use_serde"] }

Then, in your crate root, for Rust 2015 add extern crate retest, and for Rust 2018 add use retest.


These examples are in the file examples/simple.rs. For more examples see src/tests.rs.

Comparing Text

Here lines are compared, but it could just as easily be strings (e.g., words by splitting on whitespace), or characters or bytes.

use differ::{Differ, Tag};

let a = "foo\nbar\nbaz\nquux".lines().collect::<Vec<_>>();
let b = "foo\nbaz\nbar\nquux".lines().collect::<Vec<_>>();
let differ = Differ::new(&a, &b);
for span in differ.spans() {
match span.tag {
Tag::Equal => print_lines('=', &a[span.a_start..span.a_end]),
Tag::Insert => print_lines('+', &b[span.b_start..span.b_end]),
Tag::Delete => print_lines('-', &a[span.a_start..span.a_end]),
Tag::Replace => print_lines('%', &b[span.b_start..span.b_end]),

fn print_lines(c: char, lines: &[&str]) {
for line in lines {
println!("{} {}", c, line);


= foo
+ baz
= bar
- baz
= quux

Comparing Custom Items

use differ::{Differ, Tag};
use std::hash::{Hash, Hasher};

#[derive(Debug, Clone)]
struct Item<'a> { // Can have any data
x: i32,
y: i32,
text: &'a str, // For this example we've decided to compare the text

impl<'a> Item<'a> {
fn new(x: i32, y: i32, text: &'a str) -> Item<'a> {
Item { x, y, text }

impl<'a> Hash for Item<'a> {
fn hash<H: Hasher>(&self, state: &mut H) {
self.text.hash(state); // MUST use same data as PartialEq

impl<'a> PartialEq for Item<'a> {
fn eq(&self, other: &Item) -> bool {
self.text == other.text // MUST use same data as Hash
impl<'a> Eq for Item<'a> {}

let a = vec![
Item::new(1, 3, "quebec"),
Item::new(2, 4, "alpha"),
Item::new(3, 8, "bravo"),
Item::new(5, 9, "x-ray"),
let b = vec![
Item::new(3, 1, "alpha"),
Item::new(8, 3, "bravo"),
Item::new(9, 5, "yankee"),
Item::new(8, 3, "charlie"),
let differ = Differ::new(&a, &b);
for span in differ.spans() {
match span.tag {
Tag::Equal => (), // ignore
Tag::Insert => println!("{:?}", span),
Tag::Delete => println!("{:?}", span),
Tag::Replace => println!("{:?}", span),


Span { tag: Delete, a_start: 0, a_end: 1, b_start: 0, b_end: 0 }
Span { tag: Replace, a_start: 3, a_end: 4, b_start: 2, b_end: 4 }


To upgrade from 0.3.x to 0.4.x, change method calls to spans_for_matches() to function calls to differ::spans_for_matches().


Differ is free open source software (FOSS) licensed under the GNU General Public License version 3 (GPLv3).