Don't crash on binary files

CHANGELOG.md

@@ -4,6 +4,9 @@
 
 Fixed handling of `@`, `<` and `>` in elisp.
 
+Fixed crash on binary files. Difftastic now simply shows "binary" for
+files that don't look like text.
+
 ### Diffing
 
 Fixed an issue where comment replacements were not detected.

README.md

@@ -41,7 +41,7 @@ terminal. It will try to align unchanged nodes (see screenshot above).
 ## Known Problems
 
 Crashes. The code is underdocumented, undertested, and
-unfinished. Binary files are known to make difftastic panic.
+unfinished.
 
 Performance. Difftastic scales relatively poorly on files with a large
 number of changes, and can use a lot of memory. This might be solved

src/main.rs

@@ -23,6 +23,20 @@ fn term_width() -> Option<usize> {
     term_size::dimensions().map(|(w, _)| w)
 }
 
+/// Do these bytes look like a binary format that we can't do anything
+/// useful with?
+fn is_probably_binary(bytes: &[u8]) -> bool {
+    // If more than 20 of the first 1,000 characters are not valid
+    // UTF-8, we assume it's binary.
+    let num_replaced = String::from_utf8_lossy(&bytes)
+        .to_string()
+        .chars()
+        .take(1000)
+        .filter(|c| *c == std::char::REPLACEMENT_CHARACTER)
+        .count();
+    num_replaced > 20
+}
+
 const VERSION: &str = env!("CARGO_PKG_VERSION");
 
 fn main() {
@@ -57,16 +71,31 @@ fn main() {
         _ => panic!("Expected 2 arguments or 7 arguments"),
     };
 
-    let lhs_src = read_or_die(&lhs_path);
-    let rhs_src = read_or_die(&rhs_path);
-
     let syntax_toml = ConfigDir::read_default_toml();
-
     let lang = match Path::new(&display_path).extension() {
         Some(extension) => find_lang(syntax_toml, &OsStr::to_string_lossy(extension)),
         None => None,
     };
 
+    let lhs_bytes = read_or_die(&lhs_path);
+    let rhs_bytes = read_or_die(&rhs_path);
+    let lhs_binary = is_probably_binary(&lhs_bytes);
+    let rhs_binary = is_probably_binary(&rhs_bytes);
+
+    let lang_name = match &lang {
+        _ if lhs_binary || rhs_binary => "binary".to_string(),
+        Some(lang) => lang.name.clone(),
+        None => "plain text".to_string(),
+    };
+    println!("{}", style::header(&display_path, &lang_name));
+
+    if lhs_binary || rhs_binary {
+        return;
+    }
+
+    let lhs_src = String::from_utf8_lossy(&lhs_bytes).to_string();
+    let rhs_src = String::from_utf8_lossy(&rhs_bytes).to_string();
+
     let terminal_width = match matches.value_of("COLUMNS") {
         Some(width) => width.parse::<usize>().unwrap(),
         None => term_width().unwrap_or(80),
@@ -91,12 +120,6 @@ fn main() {
 
     let lhs_matched_lines = matching_lines(&lhs);
 
-    let lang_name = match &lang {
-        Some(lang) => lang.name.clone(),
-        None => "plain text".to_string(),
-    };
-    println!("{}", style::header(&display_path, &lang_name));
-
     let mut groups = visible_groups(&lhs_positions, &rhs_positions);
     if groups.is_empty() {
         if lang.is_some() {

src/parse.rs

@@ -7,9 +7,9 @@ use std::fs;
 use toml::Value;
 use typed_arena::Arena;
 
-pub fn read_or_die(path: &str) -> String {
+pub fn read_or_die(path: &str) -> Vec<u8> {
     match fs::read(path) {
-        Ok(src) => String::from_utf8_lossy(&src).to_string(),
+        Ok(src) => src,
         Err(e) => {
             match e.kind() {
                 std::io::ErrorKind::NotFound => {