diff --git a/src/diff.zig b/src/diff.zig
index c688cd2..c73e4b9 100644
--- a/src/diff.zig
+++ b/src/diff.zig
@@ -4,30 +4,46 @@ const dizzy = @import("dizzy");
 const Buffer = @import("Buffer");
 const tracy = @import("tracy");
 
-const Self = @This();
-const module_name = @typeName(Self);
+const module_name = @typeName(@This());
 
 pub const Kind = enum { insert, delete };
-pub const Edit = struct {
+pub const Diff = struct {
     kind: Kind,
     line: usize,
     offset: usize,
+    start: usize,
+    end: usize,
     bytes: []const u8,
 };
 
-pid: ?tp.pid,
+pub const Edit = struct {
+    kind: Kind,
+    start: usize,
+    end: usize,
+    bytes: []const u8,
+};
 
-pub fn create() !Self {
+pub fn create() !AsyncDiffer {
     return .{ .pid = try Process.create() };
 }
 
-pub fn deinit(self: *Self) void {
-    if (self.pid) |pid| {
-        pid.send(.{"shutdown"}) catch {};
-        pid.deinit();
-        self.pid = null;
+pub const AsyncDiffer = struct {
+    pid: ?tp.pid,
+
+    pub fn deinit(self: *@This()) void {
+        if (self.pid) |pid| {
+            pid.send(.{"shutdown"}) catch {};
+            pid.deinit();
+            self.pid = null;
+        }
     }
-}
+
+    pub const CallBack = fn (from: tp.pid_ref, edits: []Diff) void;
+
+    pub fn diff(self: @This(), cb: *const CallBack, root_dst: Buffer.Root, root_src: Buffer.Root, eol_mode: Buffer.EolMode) tp.result {
+        if (self.pid) |pid| try pid.send(.{ "D", @intFromPtr(cb), @intFromPtr(root_dst), @intFromPtr(root_src), @intFromEnum(eol_mode) });
+    }
+};
 
 const Process = struct {
     receiver: Receiver,
@@ -61,95 +77,188 @@ const Process = struct {
         var eol_mode: Buffer.EolModeTag = @intFromEnum(Buffer.EolMode.lf);
 
         return if (try m.match(.{ "D", tp.extract(&cb), tp.extract(&root_dst), tp.extract(&root_src), tp.extract(&eol_mode) }))
-            do_diff(from, cb, root_dst, root_src, @enumFromInt(eol_mode)) catch |e| tp.exit_error(e, @errorReturnTrace())
+            do_diff_async(from, cb, root_dst, root_src, @enumFromInt(eol_mode)) catch |e| tp.exit_error(e, @errorReturnTrace())
         else if (try m.match(.{"shutdown"}))
             tp.exit_normal();
     }
 
-    fn do_diff(from: tp.pid_ref, cb_addr: usize, root_new_addr: usize, root_old_addr: usize, eol_mode: Buffer.EolMode) !void {
-        var arena = std.heap.ArenaAllocator.init(allocator);
-        defer arena.deinit();
-        const a = arena.allocator();
-        const frame = tracy.initZone(@src(), .{ .name = "diff" });
-        defer frame.deinit();
-        const cb: *CallBack = if (cb_addr == 0) return else @ptrFromInt(cb_addr);
-        const root_dst: Buffer.Root = if (root_new_addr == 0) return else @ptrFromInt(root_new_addr);
-        const root_src: Buffer.Root = if (root_old_addr == 0) return else @ptrFromInt(root_old_addr);
+    fn do_diff_async(from_: tp.pid_ref, cb_addr: usize, root_dst_addr: usize, root_src_addr: usize, eol_mode: Buffer.EolMode) !void {
+        const cb_: *AsyncDiffer.CallBack = if (cb_addr == 0) return else @ptrFromInt(cb_addr);
+        const root_dst: Buffer.Root = if (root_dst_addr == 0) return else @ptrFromInt(root_dst_addr);
+        const root_src: Buffer.Root = if (root_src_addr == 0) return else @ptrFromInt(root_src_addr);
 
-        var dizzy_edits = std.ArrayListUnmanaged(dizzy.Edit){};
-        var dst = std.ArrayList(u8).init(a);
-        var src = std.ArrayList(u8).init(a);
-        var scratch = std.ArrayListUnmanaged(u32){};
-        var edits = std.ArrayList(Edit).init(a);
+        var arena_ = std.heap.ArenaAllocator.init(allocator);
+        defer arena_.deinit();
+        const arena = arena_.allocator();
 
+        var dst = std.ArrayList(u8).init(arena);
+        var src = std.ArrayList(u8).init(arena);
         try root_dst.store(dst.writer(), eol_mode);
         try root_src.store(src.writer(), eol_mode);
 
-        const scratch_len = 4 * (dst.items.len + src.items.len) + 2;
-        try scratch.ensureTotalCapacity(a, scratch_len);
-        scratch.items.len = scratch_len;
-
-        try dizzy.PrimitiveSliceDiffer(u8).diff(a, &dizzy_edits, src.items, dst.items, scratch.items);
-
-        if (dizzy_edits.items.len > 2)
-            try edits.ensureTotalCapacity((dizzy_edits.items.len - 1) / 2);
-
-        var lines_dst: usize = 0;
-        var pos_src: usize = 0;
-        var pos_dst: usize = 0;
-        var last_offset: usize = 0;
-
-        for (dizzy_edits.items) |dizzy_edit| {
-            switch (dizzy_edit.kind) {
-                .equal => {
-                    const dist = dizzy_edit.range.end - dizzy_edit.range.start;
-                    pos_src += dist;
-                    pos_dst += dist;
-                    scan_char(src.items[dizzy_edit.range.start..dizzy_edit.range.end], &lines_dst, '\n', &last_offset);
-                },
-                .insert => {
-                    const dist = dizzy_edit.range.end - dizzy_edit.range.start;
-                    pos_src += 0;
-                    pos_dst += dist;
-                    const line_start_dst: usize = lines_dst;
-                    scan_char(dst.items[dizzy_edit.range.start..dizzy_edit.range.end], &lines_dst, '\n', null);
-                    (try edits.addOne()).* = .{
-                        .kind = .insert,
-                        .line = line_start_dst,
-                        .offset = last_offset,
-                        .bytes = dst.items[dizzy_edit.range.start..dizzy_edit.range.end],
-                    };
-                },
-                .delete => {
-                    const dist = dizzy_edit.range.end - dizzy_edit.range.start;
-                    pos_src += dist;
-                    pos_dst += 0;
-                    (try edits.addOne()).* = .{
-                        .kind = .delete,
-                        .line = lines_dst,
-                        .offset = last_offset,
-                        .bytes = src.items[dizzy_edit.range.start..dizzy_edit.range.end],
-                    };
-                },
-            }
-        }
-        cb(from, edits.items);
-    }
-
-    fn scan_char(chars: []const u8, lines: *usize, char: u8, last_offset: ?*usize) void {
-        var pos = chars;
-        while (pos.len > 0) {
-            if (pos[0] == char) {
-                if (last_offset) |off| off.* = pos.len - 1;
-                lines.* += 1;
-            }
-            pos = pos[1..];
-        }
+        const edits = try diff(arena, dst.items, src.items);
+        cb_(from_, edits);
     }
 };
 
-pub const CallBack = fn (from: tp.pid_ref, edits: []Edit) void;
+pub fn diff(allocator: std.mem.Allocator, dst: []const u8, src: []const u8) ![]Diff {
+    var arena_ = std.heap.ArenaAllocator.init(allocator);
+    defer arena_.deinit();
+    const arena = arena_.allocator();
+    const frame = tracy.initZone(@src(), .{ .name = "diff" });
+    defer frame.deinit();
 
-pub fn diff(self: Self, cb: *const CallBack, root_dst: Buffer.Root, root_src: Buffer.Root, eol_mode: Buffer.EolMode) tp.result {
-    if (self.pid) |pid| try pid.send(.{ "D", @intFromPtr(cb), @intFromPtr(root_dst), @intFromPtr(root_src), @intFromEnum(eol_mode) });
+    var dizzy_edits = std.ArrayListUnmanaged(dizzy.Edit){};
+    var scratch = std.ArrayListUnmanaged(u32){};
+    var diffs = std.ArrayList(Diff).init(allocator);
+
+    const scratch_len = 4 * (dst.len + src.len) + 2;
+    try scratch.ensureTotalCapacity(arena, scratch_len);
+    scratch.items.len = scratch_len;
+
+    try dizzy.PrimitiveSliceDiffer(u8).diff(arena, &dizzy_edits, src, dst, scratch.items);
+
+    if (dizzy_edits.items.len > 2)
+        try diffs.ensureTotalCapacity((dizzy_edits.items.len - 1) / 2);
+
+    var lines_dst: usize = 0;
+    var pos_src: usize = 0;
+    var pos_dst: usize = 0;
+    var last_offset: usize = 0;
+
+    for (dizzy_edits.items) |dizzy_edit| {
+        switch (dizzy_edit.kind) {
+            .equal => {
+                const dist = dizzy_edit.range.end - dizzy_edit.range.start;
+                pos_src += dist;
+                pos_dst += dist;
+                scan_char(src[dizzy_edit.range.start..dizzy_edit.range.end], &lines_dst, '\n', &last_offset);
+            },
+            .insert => {
+                const dist = dizzy_edit.range.end - dizzy_edit.range.start;
+                pos_src += 0;
+                pos_dst += dist;
+                const line_start_dst: usize = lines_dst;
+                scan_char(dst[dizzy_edit.range.start..dizzy_edit.range.end], &lines_dst, '\n', null);
+                (try diffs.addOne()).* = .{
+                    .kind = .insert,
+                    .line = line_start_dst,
+                    .offset = last_offset,
+                    .start = dizzy_edit.range.start,
+                    .end = dizzy_edit.range.end,
+                    .bytes = dst[dizzy_edit.range.start..dizzy_edit.range.end],
+                };
+            },
+            .delete => {
+                const dist = dizzy_edit.range.end - dizzy_edit.range.start;
+                pos_src += dist;
+                pos_dst += 0;
+                (try diffs.addOne()).* = .{
+                    .kind = .delete,
+                    .line = lines_dst,
+                    .offset = last_offset,
+                    .start = dizzy_edit.range.start,
+                    .end = dizzy_edit.range.end,
+                    .bytes = src[dizzy_edit.range.start..dizzy_edit.range.end],
+                };
+            },
+        }
+    }
+    return diffs.toOwnedSlice();
+}
+
+pub fn get_edits(allocator: std.mem.Allocator, dst: []const u8, src: []const u8) ![]Edit {
+    var arena_ = std.heap.ArenaAllocator.init(allocator);
+    defer arena_.deinit();
+    const arena = arena_.allocator();
+    const frame = tracy.initZone(@src(), .{ .name = "diff" });
+    defer frame.deinit();
+
+    var dizzy_edits = std.ArrayListUnmanaged(dizzy.Edit){};
+    var scratch = std.ArrayListUnmanaged(u32){};
+    var edits = std.ArrayList(Edit).init(allocator);
+
+    const scratch_len = 4 * (dst.len + src.len) + 2;
+    try scratch.ensureTotalCapacity(arena, scratch_len);
+    scratch.items.len = scratch_len;
+
+    try dizzy.PrimitiveSliceDiffer(u8).diff(arena, &dizzy_edits, src, dst, scratch.items);
+
+    if (dizzy_edits.items.len > 2)
+        try edits.ensureTotalCapacity((dizzy_edits.items.len - 1) / 2);
+
+    var pos: usize = 0;
+
+    for (dizzy_edits.items) |dizzy_edit| {
+        switch (dizzy_edit.kind) {
+            .equal => {
+                const dist = dizzy_edit.range.end - dizzy_edit.range.start;
+                pos += dist;
+            },
+            .insert => {
+                (try edits.addOne()).* = .{
+                    .kind = .insert,
+                    .start = pos,
+                    .end = pos,
+                    .bytes = dst[dizzy_edit.range.start..dizzy_edit.range.end],
+                };
+                const dist = dizzy_edit.range.end - dizzy_edit.range.start;
+                pos += dist;
+            },
+            .delete => {
+                const dist = dizzy_edit.range.end - dizzy_edit.range.start;
+                pos += 0;
+                (try edits.addOne()).* = .{
+                    .kind = .delete,
+                    .start = pos,
+                    .end = pos + dist,
+                    .bytes = "",
+                };
+            },
+        }
+    }
+    return edits.toOwnedSlice();
+}
+
+fn scan_char(chars: []const u8, lines: *usize, char: u8, last_offset: ?*usize) void {
+    var pos = chars;
+    while (pos.len > 0) {
+        if (pos[0] == char) {
+            if (last_offset) |off| off.* = pos.len - 1;
+            lines.* += 1;
+        }
+        pos = pos[1..];
+    }
+}
+
+pub fn assert_edits_valid(allocator: std.mem.Allocator, dst: []const u8, src: []const u8, edits: []Edit) void {
+    const frame = tracy.initZone(@src(), .{ .name = "diff validate" });
+    defer frame.deinit();
+    var result = std.ArrayListUnmanaged(u8){};
+    var tmp = std.ArrayListUnmanaged(u8){};
+    defer result.deinit(allocator);
+    defer tmp.deinit(allocator);
+    result.appendSlice(allocator, src) catch @panic("assert_edits_valid OOM");
+
+    for (edits) |edit| {
+        tmp.clearRetainingCapacity();
+        tmp.appendSlice(allocator, result.items[0..edit.start]) catch @panic("assert_edits_valid OOM");
+        tmp.appendSlice(allocator, edit.bytes) catch @panic("assert_edits_valid OOM");
+        tmp.appendSlice(allocator, result.items[edit.end..]) catch @panic("assert_edits_valid OOM");
+        result.clearRetainingCapacity();
+        result.appendSlice(allocator, tmp.items) catch @panic("assert_edits_valid OOM");
+    }
+
+    if (!std.mem.eql(u8, dst, result.items)) {
+        write_file(src, "bad_diff_src") catch @panic("invalid edits write failed");
+        write_file(dst, "bad_diff_dst") catch @panic("invalid edits write failed");
+        write_file(result.items, "bad_diff_result") catch @panic("invalid edits write failed");
+        @panic("invalid edits");
+    }
+}
+
+fn write_file(data: []const u8, file_name: []const u8) !void {
+    var file = try std.fs.cwd().createFile(file_name, .{ .truncate = true });
+    defer file.close();
+    return file.writeAll(data);
 }
diff --git a/src/tui/editor_gutter.zig b/src/tui/editor_gutter.zig
index c061530..d120a90 100644
--- a/src/tui/editor_gutter.zig
+++ b/src/tui/editor_gutter.zig
@@ -31,7 +31,7 @@ relative: bool,
 highlight: bool,
 width: usize = 4,
 editor: *ed.Editor,
-diff: diff,
+diff: diff.AsyncDiffer,
 diff_symbols: std.ArrayList(Symbol),
 
 const Self = @This();
@@ -310,11 +310,11 @@ fn diff_update(self: *Self) !void {
     return self.diff.diff(diff_result, new, old, eol_mode);
 }
 
-fn diff_result(from: tp.pid_ref, edits: []diff.Edit) void {
+fn diff_result(from: tp.pid_ref, edits: []diff.Diff) void {
     diff_result_send(from, edits) catch |e| @import("log").err(@typeName(Self), "diff", e);
 }
 
-fn diff_result_send(from: tp.pid_ref, edits: []diff.Edit) !void {
+fn diff_result_send(from: tp.pid_ref, edits: []diff.Diff) !void {
     var buf: [tp.max_message_size]u8 = undefined;
     var stream = std.io.fixedBufferStream(&buf);
     const writer = stream.writer();