fix(chat): use diff lib to get diffs (aws#6940)

## Problem

Use https://www.npmjs.com/package/diff for diffs


## Solution

- Update diffs for FsWrite
- Remove hardcoded changed lines
- Remove hardcoded diff language
- Use file name instead of file path in card header

<img width="754" alt="image"
src="https://github.com/user-attachments/assets/aa876778-3fca-48be-99c4-697983f3aabe"
/>

---

- Treat all work as PUBLIC. Private `feature/x` branches will not be
squash-merged at release time.
- Your code changes must meet the guidelines in
[CONTRIBUTING.md](https://github.com/aws/aws-toolkit-vscode/blob/master/CONTRIBUTING.md#guidelines).
- License: I confirm that my contribution is made under the terms of the
Apache 2.0 license.

Author: ctlai95

packages/core/src/codewhispererChat/controllers/chat/messenger/messenger.ts

@@ -44,9 +44,10 @@ import { ChatItemButton, ChatItemContent, ChatItemFormItem, MynahIconsType, Myna
 import { ChatHistoryManager } from '../../../storages/chatHistory'
 import { ToolType, ToolUtils } from '../../../tools/toolUtils'
 import { ChatStream } from '../../../tools/chatStream'
-import { getWorkspaceForFile } from '../../../../shared/utilities/workspaceUtils'
 import path from 'path'
 import { CommandValidation } from '../../../tools/executeBash'
+import { Change } from 'diff'
+import { FsWriteParams } from '../../../tools/fsWrite'
 
 export type StaticTextResponseType = 'quick-action-help' | 'onboarding-help' | 'transform' | 'help'
 
@@ -219,8 +220,10 @@ export class Messenger {
 
                         const tool = ToolUtils.tryFromToolUse(toolUse)
                         if ('type' in tool) {
+                            let changeList: Change[] | undefined = undefined
                             if (tool.type === ToolType.FsWrite) {
                                 session.setShowDiffOnFileWrite(true)
+                                changeList = await tool.tool.getDiffChanges()
                             }
                             if (
                                 tool.type === ToolType.FsWrite ||
@@ -239,7 +242,8 @@ export class Messenger {
                                 triggerID,
                                 toolUse,
                                 validation,
-                                session.messageIdToUpdate
+                                session.messageIdToUpdate,
+                                changeList
                             )
                             await ToolUtils.queueDescription(tool, chatStream)
 
@@ -480,7 +484,8 @@ export class Messenger {
         triggerID: string,
         toolUse: ToolUse | undefined,
         validation: CommandValidation,
-        messageIdToUpdate: string | undefined
+        messageIdToUpdate: string | undefined,
+        changeList?: Change[]
     ) {
         const buttons: ChatItemButton[] = []
         let fileList: ChatItemContent['fileList'] = undefined
@@ -495,24 +500,29 @@ export class Messenger {
                 message = validation.warning + message
             }
         } else if (toolUse?.name === ToolType.FsWrite) {
-            const absoluteFilePath = (toolUse?.input as any).path
-            const projectPath = getWorkspaceForFile(absoluteFilePath)
-            const relativePath = projectPath ? path.relative(projectPath, absoluteFilePath) : absoluteFilePath
+            const input = toolUse.input as unknown as FsWriteParams
+            const fileName = path.basename(input.path)
+            const changes = changeList?.reduce(
+                (acc, { count = 0, added, removed }) => {
+                    if (added) {
+                        acc.added += count
+                    } else if (removed) {
+                        acc.deleted += count
+                    }
+                    return acc
+                },
+                { added: 0, deleted: 0 }
+            )
             // FileList
             fileList = {
                 fileTreeTitle: '',
                 hideFileCount: true,
-                filePaths: [relativePath],
+                filePaths: [fileName],
                 details: {
-                    [relativePath]: {
+                    [fileName]: {
                         // eslint-disable-next-line unicorn/no-null
                         icon: null,
-                        label: 'Created',
-                        changes: {
-                            added: 36,
-                            deleted: 0,
-                            total: 36,
-                        },
+                        changes: changes,
                     },
                 },
             }

packages/core/src/codewhispererChat/tools/chatStream.ts

@@ -8,6 +8,7 @@ import { getLogger } from '../../shared/logger/logger'
 import { Messenger } from '../controllers/chat/messenger/messenger'
 import { ToolUse } from '@amzn/codewhisperer-streaming'
 import { CommandValidation } from './executeBash'
+import { Change } from 'diff'
 
 /**
  * A writable stream that feeds each chunk/line to the chat UI.
@@ -23,6 +24,7 @@ export class ChatStream extends Writable {
         private readonly toolUse: ToolUse | undefined,
         private readonly validation: CommandValidation,
         private readonly messageIdToUpdate: string | undefined,
+        private readonly changeList?: Change[],
         private readonly logger = getLogger('chatStream')
     ) {
         super()
@@ -44,7 +46,8 @@ export class ChatStream extends Writable {
             this.triggerID,
             this.toolUse,
             this.validation,
-            this.messageIdToUpdate
+            this.messageIdToUpdate,
+            this.changeList
         )
         callback()
     }

packages/core/src/codewhispererChat/tools/fsWrite.ts

@@ -8,6 +8,9 @@ import { getLogger } from '../../shared/logger/logger'
 import vscode from 'vscode'
 import { fs } from '../../shared/fs/fs'
 import { Writable } from 'stream'
+import { Change, diffLines } from 'diff'
+import { getDiffMarkdown } from '../../shared/utilities/diffUtils'
+import { getLanguageForFilePath } from '../../shared/utilities/textDocumentUtilities'
 
 interface BaseParams {
     path: string
@@ -69,114 +72,40 @@ export class FsWrite {
         }
     }
 
-    // TODO:  Refactor the fsWrite.ts file "queueDescription" method to use existing diffLines and diff library to get the diff preview. or reuse existing diff view logic in cwchat. This will be part of next PR.
-    private showStrReplacePreview(oldStr: string, newStr: string): string {
-        // Split both strings into arrays of lines
-        const oldStrLines = oldStr.split('\n')
-        const newStrLines = newStr.split('\n')
-        let result = ''
-
-        // If strings are identical, return empty string
-        if (oldStr === newStr) {
-            return result
-        }
-
-        let oldLineIndex = 0
-        let newLineIndex = 0
-        // Loop through both arrays until we've processed all lines
-        while (oldLineIndex < oldStrLines.length || newLineIndex < newStrLines.length) {
-            if (
-                oldLineIndex < oldStrLines.length &&
-                newLineIndex < newStrLines.length &&
-                oldStrLines[oldLineIndex] === newStrLines[newLineIndex]
-            ) {
-                // Line is unchanged - prefix with space
-                result += ` ${oldStrLines[oldLineIndex]}\n`
-                oldLineIndex++
-                newLineIndex++
-            } else {
-                // Line is different
-                if (oldLineIndex < oldStrLines.length) {
-                    // Remove line - prefix with minus
-                    result += `- ${oldStrLines[oldLineIndex]}\n`
-                    oldLineIndex++
-                }
-                if (newLineIndex < newStrLines.length) {
-                    // Add line - prefix with plus
-                    result += `+ ${newStrLines[newLineIndex]}\n`
-                    newLineIndex++
-                }
-            }
-        }
-
-        return result
-    }
-
-    private async showInsertPreview(path: string, insertLine: number, newStr: string): Promise<string> {
-        const fileContent = await fs.readFileText(path)
-        const lines = fileContent.split('\n')
-        const startLine = Math.max(0, insertLine - 2)
-        const endLine = Math.min(lines.length, insertLine + 3)
-
-        const contextLines: string[] = []
-
-        // Add lines before insertion point
-        for (let index = startLine; index < insertLine; index++) {
-            contextLines.push(` ${lines[index]}`)
-        }
-
-        // Add the new line with a '+' prefix
-        contextLines.push(`+ ${newStr}`)
-
-        // Add lines after insertion point
-        for (let index = insertLine; index < endLine; index++) {
-            contextLines.push(` ${lines[index]}`)
-        }
+    public async queueDescription(updates: Writable): Promise<void> {
+        const sanitizedPath = sanitizePath(this.params.path)
+        const changes = await this.getDiffChanges()
+        const language = await getLanguageForFilePath(sanitizedPath)
 
-        return contextLines.join('\n')
+        const diff = getDiffMarkdown(changes, language)
+        updates.write(diff)
+        updates.end()
     }
 
-    private async showAppendPreview(sanitizedPath: string, newStr: string) {
-        const fileContent = await fs.readFileText(sanitizedPath)
-        const needsNewline = fileContent.length !== 0 && !fileContent.endsWith('\n')
-
-        let contentToAppend = newStr
-        if (needsNewline) {
-            contentToAppend = '\n' + contentToAppend
+    public async getDiffChanges(): Promise<Change[]> {
+        const sanitizedPath = sanitizePath(this.params.path)
+        let newContent
+        let oldContent
+        try {
+            oldContent = await fs.readFileText(sanitizedPath)
+        } catch (err) {
+            oldContent = ''
         }
-
-        // Get the last 3 lines from existing content for better UX
-        const lines = fileContent.split('\n')
-        const linesForContext = lines.slice(-3)
-
-        return `${linesForContext.join('\n')}\n+ ${contentToAppend.trim()}`
-    }
-
-    public async queueDescription(updates: Writable): Promise<void> {
         switch (this.params.command) {
             case 'create':
-                updates.write(`\`\`\`diff-typescript
-${'+' + this.params.fileText?.replace(/\n/g, '\n+')}
-                    `)
+                newContent = this.getCreateCommandText(this.params)
                 break
             case 'strReplace':
-                updates.write(`\`\`\`diff-typescript
-${this.showStrReplacePreview(this.params.oldStr, this.params.newStr)}
-\`\`\`
-`)
+                newContent = await this.getStrReplaceContent(this.params, sanitizedPath)
                 break
             case 'insert':
-                updates.write(`\`\`\`diff-typescript
-${await this.showInsertPreview(this.params.path, this.params.insertLine, this.params.newStr)}
-\`\`\``)
+                newContent = await this.getInsertContent(this.params, sanitizedPath)
                 break
             case 'append':
-                updates.write(`\`\`\`diff-typescript
-${await this.showAppendPreview(this.params.path, this.params.newStr)}
-\`\`\``)
+                newContent = await this.getAppendContent(this.params, sanitizedPath)
                 break
         }
-        updates.end()
+        return diffLines(oldContent, newContent)
     }
 
     public async validate(): Promise<void> {
@@ -224,6 +153,11 @@ ${await this.showAppendPreview(this.params.path, this.params.newStr)}
     }
 
     private async handleStrReplace(params: StrReplaceParams, sanitizedPath: string): Promise<void> {
+        const newContent = await this.getStrReplaceContent(params, sanitizedPath)
+        await fs.writeFile(sanitizedPath, newContent)
+    }
+
+    private async getStrReplaceContent(params: StrReplaceParams, sanitizedPath: string): Promise<string> {
         const fileContent = await fs.readFileText(sanitizedPath)
 
         const matches = [...fileContent.matchAll(new RegExp(this.escapeRegExp(params.oldStr), 'g'))]
@@ -235,11 +169,15 @@ ${await this.showAppendPreview(this.params.path, this.params.newStr)}
             throw new Error(`${matches.length} occurrences of oldStr were found when only 1 is expected`)
         }
 
-        const newContent = fileContent.replace(params.oldStr, params.newStr)
-        await fs.writeFile(sanitizedPath, newContent)
+        return fileContent.replace(params.oldStr, params.newStr)
     }
 
     private async handleInsert(params: InsertParams, sanitizedPath: string): Promise<void> {
+        const newContent = await this.getInsertContent(params, sanitizedPath)
+        await fs.writeFile(sanitizedPath, newContent)
+    }
+
+    private async getInsertContent(params: InsertParams, sanitizedPath: string): Promise<string> {
         const fileContent = await fs.readFileText(sanitizedPath)
         const lines = fileContent.split('\n')
 
@@ -252,11 +190,15 @@ ${await this.showAppendPreview(this.params.path, this.params.newStr)}
         } else {
             newContent = [...lines.slice(0, insertLine), params.newStr, ...lines.slice(insertLine)].join('\n')
         }
+        return newContent
+    }
 
+    private async handleAppend(params: AppendParams, sanitizedPath: string): Promise<void> {
+        const newContent = await this.getAppendContent(params, sanitizedPath)
         await fs.writeFile(sanitizedPath, newContent)
     }
 
-    private async handleAppend(params: AppendParams, sanitizedPath: string): Promise<void> {
+    private async getAppendContent(params: AppendParams, sanitizedPath: string): Promise<string> {
         const fileContent = await fs.readFileText(sanitizedPath)
         const needsNewline = fileContent.length !== 0 && !fileContent.endsWith('\n')
 
@@ -265,8 +207,7 @@ ${await this.showAppendPreview(this.params.path, this.params.newStr)}
             contentToAppend = '\n' + contentToAppend
         }
 
-        const newContent = fileContent + contentToAppend
-        await fs.writeFile(sanitizedPath, newContent)
+        return fileContent + contentToAppend
     }
 
     private getCreateCommandText(params: CreateParams): string {

packages/core/src/shared/utilities/diffUtils.ts

@@ -149,3 +149,31 @@ export function getDiffCharsAndLines(
         addedLines,
     }
 }
+
+/**
+ * Converts diff changes into a markdown-formatted string with syntax highlighting.
+ * This function takes an array of diff changes and formats them as a markdown code block
+ * with diff syntax, optionally including language-specific syntax highlighting.
+ *
+ * @param {Change[]} changes - An array of diff Change objects containing added, removed, or unchanged content
+ * @param {string} [language] - Optional language identifier for syntax highlighting in the markdown output
+ * @returns {string} A markdown-formatted string representing the diff with proper syntax highlighting
+ * @example
+ * // Generate diff markdown for JavaScript changes
+ * const diffChanges = diffLines(originalCode, newCode);
+ * const markdown = getDiffMarkdown(diffChanges, 'javascript');
+ * // Result will be a markdown code block with diff syntax and JavaScript highlighting
+ */
+export function getDiffMarkdown(changes: Change[], language?: string): string {
+    return ['```diff' + (language ? `-${language}` : ''), ...changes.flatMap(formatDiffPart), '```'].join('\n')
+}
+
+function formatDiffPart(part: Change) {
+    const prefix = part.added ? '+' : part.removed ? '-' : ' '
+    const lines = part.value.split('\n')
+
+    if (lines.length > 0 && lines[lines.length - 1] === '') {
+        lines.pop()
+    }
+    return lines.map((line) => `${prefix}${line}`)
+}

packages/core/src/shared/utilities/textDocumentUtilities.ts

@@ -224,6 +224,27 @@ export async function addEofNewline(editor: vscode.TextEditor) {
     }
 }
 
+/**
+ * Determines the language identifier for a file at the specified path.
+ * This function attempts to open the file as a VS Code document to retrieve its language ID.
+ *
+ * @param {string} filePath - The absolute or relative path to the file whose language should be determined.
+ * @returns {Promise<string | undefined>} A promise that resolves to the language identifier of the file,
+ *                                        or undefined if the file cannot be opened or the language cannot be determined.
+ * @example
+ * // Get language for a JavaScript file
+ * const language = await getLanguageForFilePath('/path/to/file.js');
+ * // language will be 'javascript'
+ */
+export async function getLanguageForFilePath(filePath: string): Promise<string | undefined> {
+    try {
+        const document = await vscode.workspace.openTextDocument(filePath)
+        return document.languageId
+    } catch (err) {
+        return
+    }
+}
+
 class ReadonlyTextDocumentProvider implements vscode.TextDocumentContentProvider {
     private content = ''