feat: [hx] r to replace with a character

If no selection, the character under the cursor is replaced, if
selection, each character is replaced by the typed character.

src/tui/mode/helix.zig

@@ -441,6 +441,17 @@ const cmds_ = struct {
         try paste_helix(ctx, insert_before);
     }
     pub const paste_clipboard_before_meta: Meta = .{ .description = "Paste from clipboard before selection" };
+
+    pub fn replace_with_character_helix(_: *void, ctx: Ctx) Result {
+        const mv = tui.mainview() orelse return;
+        const ed = mv.get_active_editor() orelse return;
+        var root = ed.buf_root() catch return;
+        root = try ed.with_cursels_mut_once_arg(root, replace_cursel_with_character, ed.allocator, ctx);
+        try ed.update_buf(root);
+        ed.clamp();
+        ed.need_render();
+    }
+    pub const replace_with_character_helix_meta: Meta = .{ .description = "Replace with character" };
 };
 
 fn to_char_helix(ctx: command.Context, move: Editor.cursel_operator_mut_once_arg) command.Result {
@@ -598,6 +609,40 @@ fn move_cursor_word_left_helix(root: Buffer.Root, cursor: *Cursor, metrics: Buff
     }
 }
 
+fn replace_cursel_with_character(ed: *Editor, root: Buffer.Root, cursel: *CurSel, allocator: Allocator, ctx: command.Context) error{Stop}!Buffer.Root {
+    var egc: []const u8 = undefined;
+    if (!(ctx.args.match(.{tp.extract(&egc)}) catch return error.Stop))
+        return error.Stop;
+    const no_selection = try select_char_if_no_selection(cursel, root, ed.metrics);
+    var begin: Cursor = undefined;
+    if (cursel.selection) |*sel| {
+        sel.normalize();
+        begin = sel.*.begin;
+    }
+
+    const sel_length = Editor.cursel_length(root, cursel.*, ed.metrics);
+    const total_length = sel_length * egc.len;
+    var sfa = std.heap.stackFallback(4096, ed.allocator);
+    const sfa_allocator = sfa.get();
+    const replacement = sfa_allocator.alloc(u8, total_length) catch return error.Stop;
+    errdefer allocator.free(replacement);
+    for (0..sel_length) |i| {
+        for (0..egc.len) |j| {
+            replacement[i * egc.len + j] = egc[j];
+        }
+    }
+
+    const root_ = insert_replace_selection(ed, root, cursel, replacement, allocator) catch return error.Stop;
+
+    if (no_selection) {
+        try cursel.cursor.move_left(root, ed.metrics);
+        cursel.disable_selection(root, ed.metrics);
+    } else {
+        cursel.selection = Selection{ .begin = begin, .end = cursel.cursor };
+    }
+    return root_;
+}
+
 fn move_noop(_: Buffer.Root, _: *Cursor, _: Buffer.Metrics) error{Stop}!void {}
 
 fn move_cursor_word_right_end_helix(root: Buffer.Root, cursor: *Cursor, metrics: Buffer.Metrics) error{Stop}!void {
@@ -888,6 +933,21 @@ fn move_cursor_carriage_return(root: Buffer.Root, cursel: CurSel, cursor: *Curso
     try Editor.move_cursor_right(root, cursor, metrics);
 }
 
+fn select_char_if_no_selection(cursel: *CurSel, root: Buffer.Root, metrics: Buffer.Metrics) !bool {
+    if (cursel.selection) |*sel_| {
+        const sel: *Selection = sel_;
+        if (sel.*.empty()) {
+            sel.*.begin = .{ .row = cursel.cursor.row, .col = cursel.cursor.col + 1, .target = cursel.cursor.target + 1 };
+            return true;
+        }
+        return false;
+    } else {
+        const sel = try cursel.enable_selection(root, metrics);
+        sel.begin = .{ .row = cursel.cursor.row, .col = cursel.cursor.col + 1, .target = cursel.cursor.target + 1 };
+        return true;
+    }
+}
+
 fn is_cursel_from_extend_line_below(cursel: CurSel) bool {
     if (cursel.selection) |sel_| {
         var sel = sel_;