fix: history load notifies and never overwrites unreadable file; dictionary keeps ids across external edits, undo safety

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

Sources/SstData/DictionaryFile.swift

@@ -70,6 +70,21 @@ public struct DictionaryFile: Equatable, Sendable {
         return .entry(DictionaryEntry(kind: .term(trimmed)))
     }
 
+    /// Reuses the ids of `old` for entries with equal content (first unused match, in order), so ids survive a reload.
+    public func preservingIDs(from old: DictionaryFile) -> DictionaryFile {
+        var available = old.entries
+        var result = self
+        result.lines = lines.map { line in
+            guard case .entry(let entry) = line else { return line }
+            if let index = available.firstIndex(where: { $0.kind == entry.kind }) {
+                let reused = available.remove(at: index)
+                return .entry(DictionaryEntry(id: reused.id, kind: entry.kind))
+            }
+            return line
+        }
+        return result
+    }
+
     public func serialized() -> String {
         let out = lines.map { line -> String in
             switch line {

Sources/SstData/DictionaryStore.swift

@@ -14,6 +14,8 @@ public final class DictionaryStore {
     public let fileURL: URL
     public private(set) var file = DictionaryFile()
     public var onChange: (() -> Void)?
+    /// Fired (after `onChange`) when the file was changed by something other than this store.
+    public var onExternalChange: (() -> Void)?
 
     private var lastText = ""
     private var watcher: DispatchSourceFileSystemObject?
@@ -53,8 +55,9 @@ public final class DictionaryStore {
     public func reloadIfChanged() {
         guard let text = try? String(contentsOf: fileURL, encoding: .utf8), text != lastText else { return }
         lastText = text
-        file = DictionaryFile(parsing: text)
+        file = DictionaryFile(parsing: text).preservingIDs(from: file)
         onChange?()
+        onExternalChange?()
     }
 
     /// Watches the folder (editors replace files instead of editing them in place).

Sources/SstData/History.swift

@@ -35,6 +35,8 @@ public final class HistoryStore {
         case empty
         case loaded
         case recoveredCorrupt(backup: URL)
+        /// The file exists but cannot be read; it is left alone and nothing is written over it.
+        case unreadable(String)
     }
 
     private struct Document: Codable {
@@ -47,6 +49,7 @@ public final class HistoryStore {
     public var onChange: (() -> Void)?
 
     private let fileURL: URL
+    private var readOnly = false
     private var saveTask: Task<Void, Never>?
 
     public init(fileURL: URL) {
@@ -68,18 +71,29 @@ public final class HistoryStore {
 
     @discardableResult
     public func load() -> LoadOutcome {
-        guard let data = try? Data(contentsOf: fileURL) else {
-            entries = []
-            return .empty
+        let data: Data
+        do {
+            data = try Data(contentsOf: fileURL)
+        } catch {
+            if !FileManager.default.fileExists(atPath: fileURL.path) {
+                entries = []
+                onChange?()
+                return .empty
+            }
+            readOnly = true
+            lastError = error.localizedDescription
+            return .unreadable(error.localizedDescription)
         }
         do {
             entries = try Self.makeDecoder().decode(Document.self, from: data).entries
+            onChange?()
             return .loaded
         } catch {
             let stamp = ISO8601DateFormatter().string(from: Date()).replacingOccurrences(of: ":", with: "-")
             let backup = fileURL.deletingLastPathComponent().appending(path: "history.corrupt-\(stamp).json")
             try? FileManager.default.moveItem(at: fileURL, to: backup)
             entries = []
+            onChange?()
             return .recoveredCorrupt(backup: backup)
         }
     }
@@ -103,6 +117,7 @@ public final class HistoryStore {
     public func flush() {
         saveTask?.cancel()
         saveTask = nil
+        guard !readOnly else { return }
         do {
             try FileManager.default.createDirectory(
                 at: fileURL.deletingLastPathComponent(), withIntermediateDirectories: true)

Sources/SstData/ViewSupport.swift

@@ -161,16 +161,23 @@ public final class DictionaryViewModel: ObservableObject {
     }
 
     public func undo() {
-        guard let snapshot = undoStack.popLast() else { return }
-        canUndo = !undoStack.isEmpty
+        guard let snapshot = undoStack.last else { return }
         do {
             try store.apply { $0 = snapshot }
+            undoStack.removeLast()
+            canUndo = !undoStack.isEmpty
         } catch {
             errorMessage = error.localizedDescription
         }
         reload()
     }
 
+    /// The file was edited elsewhere: older snapshots no longer describe it, so undo is dropped.
+    public func externalChangeDetected() {
+        undoStack.removeAll()
+        canUndo = false
+    }
+
     private func mutate(_ change: (inout DictionaryFile) -> Void) {
         let snapshot = store.file
         do {