VIM-2074 Backspace behaviour is incorrect in Replace mode

src/main/java/com/maddyhome/idea/vim/action/editor/VimEditorActions.kt

@@ -22,11 +22,6 @@ import com.maddyhome.idea.vim.handler.VimActionHandler
 import com.maddyhome.idea.vim.helper.enumSetOf
 import java.util.*
 
-@CommandOrMotion(keys = ["<C-H>", "<BS>"], modes = [Mode.INSERT])
-internal class VimEditorBackSpace : IdeActionHandler(IdeActions.ACTION_EDITOR_BACKSPACE) {
-  override val type: Command.Type = Command.Type.DELETE
-}
-
 @CommandOrMotion(keys = ["<Del>"], modes = [Mode.INSERT])
 internal class VimEditorDelete : IdeActionHandler(IdeActions.ACTION_EDITOR_DELETE) {
   override val type: Command.Type = Command.Type.DELETE

src/main/java/com/maddyhome/idea/vim/group/ChangeGroup.kt

@@ -104,6 +104,11 @@ class ChangeGroup : VimChangeGroupBase() {
     }
   }
 
+  override fun processBackspace(editor: VimEditor, context: ExecutionContext) {
+    injector.actionExecutor.executeAction(editor, name = IdeActions.ACTION_EDITOR_BACKSPACE, context = context)
+    injector.scroll.scrollCaretIntoView(editor)
+  }
+
   private fun restoreCursor(editor: VimEditor, caret: VimCaret, startLine: Int) {
     if (caret != editor.primaryCaret()) {
       (editor as IjVimEditor).editor.caretModel.addCaret(

src/main/java/com/maddyhome/idea/vim/helper/UserDataManager.kt

@@ -25,6 +25,7 @@ import com.maddyhome.idea.vim.ex.ExOutputModel
 import com.maddyhome.idea.vim.group.visual.VisualChange
 import com.maddyhome.idea.vim.group.visual.vimLeadSelectionOffset
 import com.maddyhome.idea.vim.common.InsertSequence
+import com.maddyhome.idea.vim.common.VimEditorReplaceMask
 import com.maddyhome.idea.vim.newapi.vim
 import com.maddyhome.idea.vim.state.VimStateMachine
 import com.maddyhome.idea.vim.state.mode.Mode
@@ -126,6 +127,7 @@ internal var Editor.vimExOutput: ExOutputModel? by userData()
 internal var Editor.vimTestInputModel: TestInputModel? by userData()
 
 internal var Editor.vimChangeActionSwitchMode: Mode? by userData()
+internal var Editor.replaceMask: VimEditorReplaceMask? by userData()
 
 internal var Caret.currentInsert: InsertSequence? by userData()
 internal val Caret.insertHistory: MutableList<InsertSequence> by userDataOr { mutableListOf() }

src/main/java/com/maddyhome/idea/vim/newapi/IjLiveRange.kt

@@ -17,6 +17,24 @@ internal class IjLiveRange(val marker: RangeMarker) : LiveRange {
 
   override val endOffset: Int
     get() = marker.endOffset
+
+  override fun equals(other: Any?): Boolean {
+    if (this === other) return true
+    if (javaClass != other?.javaClass) return false
+
+    other as IjLiveRange
+
+    if (startOffset != other.startOffset) return false
+    if (endOffset != other.endOffset) return false
+
+    return true
+  }
+
+  override fun hashCode(): Int {
+    var result = startOffset
+    result = 31 * result + endOffset
+    return result
+  }
 }
 
 val RangeMarker.vim: LiveRange

src/main/java/com/maddyhome/idea/vim/newapi/IjVimEditor.kt

@@ -44,6 +44,8 @@ import com.maddyhome.idea.vim.common.IndentConfig.Companion.create
 import com.maddyhome.idea.vim.common.LiveRange
 import com.maddyhome.idea.vim.common.ModeChangeListener
 import com.maddyhome.idea.vim.common.TextRange
+import com.maddyhome.idea.vim.common.VimEditorReplaceMask
+import com.maddyhome.idea.vim.common.forgetAllReplaceMasks
 import com.maddyhome.idea.vim.group.visual.vimSetSystemBlockSelectionSilently
 import com.maddyhome.idea.vim.helper.EditorHelper
 import com.maddyhome.idea.vim.helper.StrictMode
@@ -53,6 +55,7 @@ import com.maddyhome.idea.vim.helper.fileSize
 import com.maddyhome.idea.vim.helper.getTopLevelEditor
 import com.maddyhome.idea.vim.helper.inExMode
 import com.maddyhome.idea.vim.helper.isTemplateActive
+import com.maddyhome.idea.vim.helper.replaceMask
 import com.maddyhome.idea.vim.helper.vimChangeActionSwitchMode
 import com.maddyhome.idea.vim.helper.vimLastSelectionType
 import com.maddyhome.idea.vim.impl.state.VimStateMachineImpl
@@ -75,6 +78,11 @@ internal class IjVimEditor(editor: Editor) : MutableLinearEditor, VimEditorBase(
   // TBH, I don't like the names. Need to think a bit more about this
   val editor = editor.getTopLevelEditor()
   val originalEditor = editor
+  override var replaceMask: VimEditorReplaceMask?
+    get() = editor.replaceMask
+    set(value) {
+      editor.replaceMask = value
+    }
 
   override fun updateMode(mode: Mode) {
     (injector.vimState as VimStateMachineImpl).mode = mode
@@ -454,6 +462,7 @@ internal class IjVimEditor(editor: Editor) : MutableLinearEditor, VimEditorBase(
     get() = (editor as? EditorEx)?.isInsertMode ?: false
     set(value) {
       (editor as? EditorEx)?.isInsertMode = value
+      forgetAllReplaceMasks()
     }
 
   override val document: VimDocument

src/test/java/org/jetbrains/plugins/ideavim/action/ChangeActionTest.kt

@@ -7,6 +7,7 @@
  */
 package org.jetbrains.plugins.ideavim.action
 
+import com.intellij.idea.TestFor
 import com.maddyhome.idea.vim.api.injector
 import com.maddyhome.idea.vim.state.mode.Mode
 import com.maddyhome.idea.vim.state.mode.ReturnTo
@@ -1068,4 +1069,14 @@ foobaz
       Mode.NORMAL(),
     )
   }
+
+  @Test
+  @TestFor(issues = ["VIM-2074"])
+  fun `backspace with replace mode`() {
+    configureByText("${c}Hello world")
+    typeText("R1111")
+    assertState("1111o world")
+    typeText("<BS><BS><BS>")
+    assertState("1ello world")
+  }
 }

vim-engine/src/main/kotlin/com/maddyhome/idea/vim/action/change/change/ChangeReplaceAction.kt

@@ -15,6 +15,7 @@ import com.maddyhome.idea.vim.api.injector
 import com.maddyhome.idea.vim.command.Argument
 import com.maddyhome.idea.vim.command.Command
 import com.maddyhome.idea.vim.command.OperatorArguments
+import com.maddyhome.idea.vim.common.VimEditorReplaceMask
 import com.maddyhome.idea.vim.handler.ChangeEditorActionHandler
 
 @CommandOrMotion(keys = ["R"], modes = [Mode.NORMAL])
@@ -39,4 +40,5 @@ class ChangeReplaceAction : ChangeEditorActionHandler.SingleExecution() {
  */
 private fun changeReplace(editor: VimEditor, context: ExecutionContext) {
   injector.changeGroup.initInsert(editor, context, com.maddyhome.idea.vim.state.mode.Mode.REPLACE)
+  editor.replaceMask = VimEditorReplaceMask()
 }

vim-engine/src/main/kotlin/com/maddyhome/idea/vim/action/change/insert/InsertBackspaceAction.kt

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2003-2024 The IdeaVim authors
+ *
+ * Use of this source code is governed by an MIT-style
+ * license that can be found in the LICENSE.txt file or at
+ * https://opensource.org/licenses/MIT.
+ */
+
+package com.maddyhome.idea.vim.action.change.insert
+
+import com.intellij.vim.annotations.CommandOrMotion
+import com.intellij.vim.annotations.Mode
+import com.maddyhome.idea.vim.api.ExecutionContext
+import com.maddyhome.idea.vim.api.VimEditor
+import com.maddyhome.idea.vim.api.getLineStartForOffset
+import com.maddyhome.idea.vim.api.injector
+import com.maddyhome.idea.vim.command.Command
+import com.maddyhome.idea.vim.command.OperatorArguments
+import com.maddyhome.idea.vim.handler.VimActionHandler
+
+@CommandOrMotion(keys = ["<C-H>", "<BS>"], modes = [Mode.INSERT])
+internal class InsertBackspaceAction : VimActionHandler.SingleExecution() {
+  override val type: Command.Type = Command.Type.OTHER_WRITABLE
+
+  override fun execute( editor: VimEditor, context: ExecutionContext, cmd: Command, operatorArguments: OperatorArguments, ): Boolean {
+    if (editor.insertMode) {
+      injector.changeGroup.processBackspace(editor, context)
+    } else {
+      for (caret in editor.carets()) {
+        val offset = (caret.offset - 1).takeIf { it > 0 } ?: continue
+        val oldChar = editor.replaceMask?.popChange(editor, offset)
+        if (oldChar != null) {
+          injector.changeGroup.replaceText(editor, caret, offset, offset + 1, oldChar.toString())
+        }
+        caret.moveToOffset(offset)
+      }
+    }
+    return true
+  }
+}

vim-engine/src/main/kotlin/com/maddyhome/idea/vim/api/VimChangeGroup.kt

@@ -41,6 +41,7 @@ interface VimChangeGroup {
 
   fun processEnter(editor: VimEditor, caret: VimCaret, context: ExecutionContext)
   fun processEnter(editor: VimEditor, context: ExecutionContext)
+  fun processBackspace(editor: VimEditor, context: ExecutionContext)
 
   fun processPostChangeModeSwitch(editor: VimEditor, context: ExecutionContext, toSwitch: Mode)
 

vim-engine/src/main/kotlin/com/maddyhome/idea/vim/api/VimChangeGroupBase.kt

@@ -731,12 +731,14 @@ abstract class VimChangeGroupBase : VimChangeGroup {
   ): Boolean {
     logger.debug { "processKey($key)" }
     if (key.keyChar != KeyEvent.CHAR_UNDEFINED) {
+      editor.replaceMask?.recordChangeAtCaret(editor)
       processResultBuilder.addExecutionStep { _, lambdaEditor, lambdaContext -> type(lambdaEditor, lambdaContext, key.keyChar) }
       return true
     }
 
     // Shift-space
     if (key.keyCode == 32 && key.modifiers and KeyEvent.SHIFT_DOWN_MASK != 0) {
+      editor.replaceMask?.recordChangeAtCaret(editor)
       processResultBuilder.addExecutionStep { _, lambdaEditor, lambdaContext -> type(lambdaEditor, lambdaContext, ' ') }
       return true
     }

vim-engine/src/main/kotlin/com/maddyhome/idea/vim/api/VimEditor.kt

@@ -11,12 +11,11 @@ package com.maddyhome.idea.vim.api
 import com.maddyhome.idea.vim.command.OperatorArguments
 import com.maddyhome.idea.vim.common.LiveRange
 import com.maddyhome.idea.vim.common.TextRange
-import com.maddyhome.idea.vim.impl.state.VimStateMachineImpl
+import com.maddyhome.idea.vim.common.VimEditorReplaceMask
 import com.maddyhome.idea.vim.state.mode.Mode
 import com.maddyhome.idea.vim.state.mode.ReturnTo
 import com.maddyhome.idea.vim.state.mode.SelectionType
 import com.maddyhome.idea.vim.state.mode.returnTo
-import org.jetbrains.annotations.ApiStatus.Internal
 
 /**
  * Every line in [VimEditor] ends with a new line TODO <- this is probably not true already
@@ -130,6 +129,7 @@ interface VimEditor {
   val lfMakesNewLine: Boolean
   var vimChangeActionSwitchMode: Mode?
   val indentConfig: VimIndentConfig
+  var replaceMask: VimEditorReplaceMask?
 
   fun fileSize(): Long
 

vim-engine/src/main/kotlin/com/maddyhome/idea/vim/api/VimEditorBase.kt

@@ -8,6 +8,7 @@
 
 package com.maddyhome.idea.vim.api
 
+import com.maddyhome.idea.vim.common.forgetAllReplaceMasks
 import com.maddyhome.idea.vim.state.mode.Mode
 
 abstract class VimEditorBase : VimEditor {
@@ -18,6 +19,9 @@ abstract class VimEditorBase : VimEditor {
       if (vimState.mode == value) return
 
       val oldValue = vimState.mode
+      if (oldValue == Mode.REPLACE) {
+        forgetAllReplaceMasks()
+      }
       updateMode(value)
       injector.listenersNotifier.notifyModeChanged(this, oldValue)
     }

vim-engine/src/main/kotlin/com/maddyhome/idea/vim/common/VimEditorReplaceMask.kt

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2003-2024 The IdeaVim authors
+ *
+ * Use of this source code is governed by an MIT-style
+ * license that can be found in the LICENSE.txt file or at
+ * https://opensource.org/licenses/MIT.
+ */
+
+package com.maddyhome.idea.vim.common
+
+import com.maddyhome.idea.vim.api.VimEditor
+import com.maddyhome.idea.vim.api.injector
+
+class VimEditorReplaceMask {
+  private val changedChars = mutableMapOf<LiveRange, Char>()
+
+  fun recordChangeAtCaret(editor: VimEditor) {
+    for (caret in editor.carets()) {
+      val offset = caret.offset
+      val marker = editor.createLiveMarker(offset, offset)
+      changedChars[marker] = editor.charAt(offset)
+    }
+  }
+
+  fun popChange(editor: VimEditor, offset: Int): Char? {
+    val marker = editor.createLiveMarker(offset, offset)
+    val change = changedChars[marker]
+    changedChars.remove(marker)
+    return change
+  }
+}
+
+fun forgetAllReplaceMasks() {
+  injector.editorGroup.getEditors().forEach { it.replaceMask = null }
+}

vim-engine/src/main/kotlin/com/maddyhome/idea/vim/regexp/VimRegex.kt

@@ -26,6 +26,7 @@ import com.maddyhome.idea.vim.api.VirtualFile
 import com.maddyhome.idea.vim.command.OperatorArguments
 import com.maddyhome.idea.vim.common.LiveRange
 import com.maddyhome.idea.vim.common.TextRange
+import com.maddyhome.idea.vim.common.VimEditorReplaceMask
 import com.maddyhome.idea.vim.ex.ExException
 import com.maddyhome.idea.vim.helper.noneOfEnum
 import com.maddyhome.idea.vim.regexp.engine.VimRegexEngine
@@ -606,6 +607,7 @@ class VimRegex(pattern: String) {
     override var vimChangeActionSwitchMode: Mode? = null
     override val indentConfig: VimIndentConfig
       get() = TODO("Not yet implemented")
+    override var replaceMask: VimEditorReplaceMask? = null
 
     override fun fileSize(): Long {
       return text.length.toLong()