co-mono/packages/coding-agent/src/core/tools/edit-diff.ts

/**
 * Shared diff computation utilities for the edit tool.
 * Used by both edit.ts (for execution) and tool-execution.ts (for preview rendering).
 */

import * as Diff from "diff";
import { constants } from "fs";
import { access, readFile } from "fs/promises";
import { resolveToCwd } from "./path-utils.js";

export function detectLineEnding(content: string): "\r\n" | "\n" {
	const crlfIdx = content.indexOf("\r\n");
	const lfIdx = content.indexOf("\n");
	if (lfIdx === -1) return "\n";
	if (crlfIdx === -1) return "\n";
	return crlfIdx < lfIdx ? "\r\n" : "\n";
}

export function normalizeToLF(text: string): string {
	return text.replace(/\r\n/g, "\n").replace(/\r/g, "\n");
}

export function restoreLineEndings(text: string, ending: "\r\n" | "\n"): string {
	return ending === "\r\n" ? text.replace(/\n/g, "\r\n") : text;
}

/** Strip UTF-8 BOM if present, return both the BOM (if any) and the text without it */
export function stripBom(content: string): { bom: string; text: string } {
	return content.startsWith("\uFEFF") ? { bom: "\uFEFF", text: content.slice(1) } : { bom: "", text: content };
}

/**
 * Generate a unified diff string with line numbers and context.
 * Returns both the diff string and the first changed line number (in the new file).
 */
export function generateDiffString(
	oldContent: string,
	newContent: string,
	contextLines = 4,
): { diff: string; firstChangedLine: number | undefined } {
	const parts = Diff.diffLines(oldContent, newContent);
	const output: string[] = [];

	const oldLines = oldContent.split("\n");
	const newLines = newContent.split("\n");
	const maxLineNum = Math.max(oldLines.length, newLines.length);
	const lineNumWidth = String(maxLineNum).length;

	let oldLineNum = 1;
	let newLineNum = 1;
	let lastWasChange = false;
	let firstChangedLine: number | undefined;

	for (let i = 0; i < parts.length; i++) {
		const part = parts[i];
		const raw = part.value.split("\n");
		if (raw[raw.length - 1] === "") {
			raw.pop();
		}

		if (part.added || part.removed) {
			// Capture the first changed line (in the new file)
			if (firstChangedLine === undefined) {
				firstChangedLine = newLineNum;
			}

			// Show the change
			for (const line of raw) {
				if (part.added) {
					const lineNum = String(newLineNum).padStart(lineNumWidth, " ");
					output.push(`+${lineNum} ${line}`);
					newLineNum++;
				} else {
					// removed
					const lineNum = String(oldLineNum).padStart(lineNumWidth, " ");
					output.push(`-${lineNum} ${line}`);
					oldLineNum++;
				}
			}
			lastWasChange = true;
		} else {
			// Context lines - only show a few before/after changes
			const nextPartIsChange = i < parts.length - 1 && (parts[i + 1].added || parts[i + 1].removed);

			if (lastWasChange || nextPartIsChange) {
				// Show context
				let linesToShow = raw;
				let skipStart = 0;
				let skipEnd = 0;

				if (!lastWasChange) {
					// Show only last N lines as leading context
					skipStart = Math.max(0, raw.length - contextLines);
					linesToShow = raw.slice(skipStart);
				}

				if (!nextPartIsChange && linesToShow.length > contextLines) {
					// Show only first N lines as trailing context
					skipEnd = linesToShow.length - contextLines;
					linesToShow = linesToShow.slice(0, contextLines);
				}

				// Add ellipsis if we skipped lines at start
				if (skipStart > 0) {
					output.push(` ${"".padStart(lineNumWidth, " ")} ...`);
					// Update line numbers for the skipped leading context
					oldLineNum += skipStart;
					newLineNum += skipStart;
				}

				for (const line of linesToShow) {
					const lineNum = String(oldLineNum).padStart(lineNumWidth, " ");
					output.push(` ${lineNum} ${line}`);
					oldLineNum++;
					newLineNum++;
				}

				// Add ellipsis if we skipped lines at end
				if (skipEnd > 0) {
					output.push(` ${"".padStart(lineNumWidth, " ")} ...`);
					// Update line numbers for the skipped trailing context
					oldLineNum += skipEnd;
					newLineNum += skipEnd;
				}
			} else {
				// Skip these context lines entirely
				oldLineNum += raw.length;
				newLineNum += raw.length;
			}

			lastWasChange = false;
		}
	}

	return { diff: output.join("\n"), firstChangedLine };
}

export interface EditDiffResult {
	diff: string;
	firstChangedLine: number | undefined;
}

export interface EditDiffError {
	error: string;
}

/**
 * Compute the diff for an edit operation without applying it.
 * Used for preview rendering in the TUI before the tool executes.
 */
export async function computeEditDiff(
	path: string,
	oldText: string,
	newText: string,
	cwd: string,
): Promise<EditDiffResult | EditDiffError> {
	const absolutePath = resolveToCwd(path, cwd);

	try {
		// Check if file exists and is readable
		try {
			await access(absolutePath, constants.R_OK);
		} catch {
			return { error: `File not found: ${path}` };
		}

		// Read the file
		const rawContent = await readFile(absolutePath, "utf-8");

		// Strip BOM before matching (LLM won't include invisible BOM in oldText)
		const { text: content } = stripBom(rawContent);

		const normalizedContent = normalizeToLF(content);
		const normalizedOldText = normalizeToLF(oldText);
		const normalizedNewText = normalizeToLF(newText);

		// Check if old text exists
		if (!normalizedContent.includes(normalizedOldText)) {
			return {
				error: `Could not find the exact text in ${path}. The old text must match exactly including all whitespace and newlines.`,
			};
		}

		// Count occurrences
		const occurrences = normalizedContent.split(normalizedOldText).length - 1;
		if (occurrences > 1) {
			return {
				error: `Found ${occurrences} occurrences of the text in ${path}. The text must be unique. Please provide more context to make it unique.`,
			};
		}

		// Compute the new content
		const index = normalizedContent.indexOf(normalizedOldText);
		const normalizedNewContent =
			normalizedContent.substring(0, index) +
			normalizedNewText +
			normalizedContent.substring(index + normalizedOldText.length);

		// Check if it would actually change anything
		if (normalizedContent === normalizedNewContent) {
			return {
				error: `No changes would be made to ${path}. The replacement produces identical content.`,
			};
		}

		// Generate the diff
		return generateDiffString(normalizedContent, normalizedNewContent);
	} catch (err) {
		return { error: err instanceof Error ? err.message : String(err) };
	}
}