refactor(language_server): use minimal text edit for ServerFormatter

crates/oxc_language_server/Cargo.toml

@@ -23,6 +23,7 @@ doctest = false
 
 [dependencies]
 oxc_allocator = { workspace = true }
+oxc_data_structures = { workspace = true, features = ["rope"] }
 oxc_diagnostics = { workspace = true }
 oxc_formatter = { workspace = true }
 oxc_linter = { workspace = true, features = ["language_server"] }

crates/oxc_language_server/src/formatter/server_formatter.rs

@@ -1,13 +1,12 @@
 use oxc_allocator::Allocator;
+use oxc_data_structures::rope::{Rope, get_line_column};
 use oxc_formatter::{FormatOptions, Formatter, get_supported_source_type};
 use oxc_parser::{ParseOptions, Parser};
 use tower_lsp_server::{
     UriExt,
     lsp_types::{Position, Range, TextEdit, Uri},
 };
 
-use crate::LSP_MAX_INT;
-
 pub struct ServerFormatter;
 
 impl ServerFormatter {
@@ -49,9 +48,144 @@ impl ServerFormatter {
             return Some(vec![]);
         }
 
+        let (start, end, replacement) = compute_minimal_text_edit(&source_text, &code);
+        let rope = Rope::from(source_text.as_str());
+        let (start_line, start_character) = get_line_column(&rope, start, &source_text);
+        let (end_line, end_character) = get_line_column(&rope, end, &source_text);
+
         Some(vec![TextEdit::new(
-            Range::new(Position::new(0, 0), Position::new(LSP_MAX_INT, 0)),
-            code,
+            Range::new(
+                Position::new(start_line, start_character),
+                Position::new(end_line, end_character),
+            ),
+            replacement.to_string(),
         )])
     }
 }
+
+/// Returns the minimal text edit (start, end, replacement) to transform `source_text` into `formatted_text`
+#[expect(clippy::cast_possible_truncation)]
+fn compute_minimal_text_edit<'a>(
+    source_text: &str,
+    formatted_text: &'a str,
+) -> (u32, u32, &'a str) {
+    debug_assert!(source_text != formatted_text);
+
+    // Find common prefix (byte offset)
+    let mut prefix_byte = 0;
+    for (a, b) in source_text.chars().zip(formatted_text.chars()) {
+        if a == b {
+            prefix_byte += a.len_utf8();
+        } else {
+            break;
+        }
+    }
+
+    // Find common suffix (byte offset from end)
+    let mut suffix_byte = 0;
+    let src_bytes = source_text.as_bytes();
+    let fmt_bytes = formatted_text.as_bytes();
+    let src_len = src_bytes.len();
+    let fmt_len = fmt_bytes.len();
+
+    while suffix_byte < src_len - prefix_byte
+        && suffix_byte < fmt_len - prefix_byte
+        && src_bytes[src_len - 1 - suffix_byte] == fmt_bytes[fmt_len - 1 - suffix_byte]
+    {
+        suffix_byte += 1;
+    }
+
+    let start = prefix_byte as u32;
+    let end = (src_len - suffix_byte) as u32;
+    let replacement_start = prefix_byte;
+    let replacement_end = fmt_len - suffix_byte;
+    let replacement = &formatted_text[replacement_start..replacement_end];
+
+    (start, end, replacement)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::compute_minimal_text_edit;
+
+    #[test]
+    #[should_panic(expected = "assertion failed")]
+    fn test_no_change() {
+        let src = "abc";
+        let formatted = "abc";
+        compute_minimal_text_edit(src, formatted);
+    }
+
+    #[test]
+    fn test_single_char_change() {
+        let src = "abc";
+        let formatted = "axc";
+        let (start, end, replacement) = compute_minimal_text_edit(src, formatted);
+        // Only 'b' replaced by 'x'
+        assert_eq!((start, end, replacement), (1, 2, "x"));
+    }
+
+    #[test]
+    fn test_insert_char() {
+        let src = "abc";
+        let formatted = "abxc";
+        let (start, end, replacement) = compute_minimal_text_edit(src, formatted);
+        // Insert 'x' after 'b'
+        assert_eq!((start, end, replacement), (2, 2, "x"));
+    }
+
+    #[test]
+    fn test_delete_char() {
+        let src = "abc";
+        let formatted = "ac";
+        let (start, end, replacement) = compute_minimal_text_edit(src, formatted);
+        // Delete 'b'
+        assert_eq!((start, end, replacement), (1, 2, ""));
+    }
+
+    #[test]
+    fn test_replace_multiple_chars() {
+        let src = "abcdef";
+        let formatted = "abXYef";
+        let (start, end, replacement) = compute_minimal_text_edit(src, formatted);
+        // Replace "cd" with "XY"
+        assert_eq!((start, end, replacement), (2, 4, "XY"));
+    }
+
+    #[test]
+    fn test_replace_multiple_chars_between_similars_complex() {
+        let src = "aYabYb";
+        let formatted = "aXabXb";
+        let (start, end, replacement) = compute_minimal_text_edit(src, formatted);
+        assert_eq!((start, end, replacement), (1, 5, "XabX"));
+    }
+
+    #[test]
+    fn test_unicode() {
+        let src = "a😀b";
+        let formatted = "a😃b";
+        let (start, end, replacement) = compute_minimal_text_edit(src, formatted);
+        // Replace 😀 with 😃
+        assert_eq!((start, end, replacement), (1, 5, "😃"));
+    }
+
+    #[test]
+    fn test_append() {
+        let src = "a".repeat(100);
+        let mut formatted = src.clone();
+        formatted.push('b'); // Add a character at the end
+
+        let (start, end, replacement) = compute_minimal_text_edit(&src, &formatted);
+        assert_eq!((start, end, replacement), (100, 100, "b"));
+    }
+
+    #[test]
+    fn test_prepend() {
+        let src = "a".repeat(100);
+        let mut formatted = String::from("b");
+        formatted.push_str(&src); // Add a character at the start
+
+        let (start, end, replacement) = compute_minimal_text_edit(&src, &formatted);
+        assert_eq!((start, end, replacement), (0, 0, "b"));
+    }
+}