Tool Overview

This tool compares two versions of source code and shows you exactly how they differ. You can paste code directly or upload files for the original and modified versions. The viewer then runs a line-based diff and renders the result either side by side or inline. Added, removed, moved, and unchanged lines are clearly marked, with line numbers and change statistics to help you understand the scope of the change.

The problem it solves is seeing what changed between code versions in a way that is easy to understand. Reading raw diffs from command line tools can be hard, and setting up heavy desktop software is not always convenient. This browser-based viewer focuses on clear, visual presentation of code changes, while also supporting copy and download of the diff output and an optional AI analysis that summarizes the change in simple language.

The code diff viewer is designed for developers, reviewers, team leads, and students. A beginner can paste code and see highlights for additions and deletions. Technical users can switch between inline and side-by-side views, ignore whitespace changes, upload code files, and use the smart analysis to classify changes as refactors, bug fixes, or new features.

Background & Concept Explanation

When code changes, you often want to know exactly what lines were added, removed, or moved. A diff is a structured way to express this. At its core, a diff algorithm takes two sequences of lines and computes a minimal set of edits (additions and deletions) that turn one into the other. These edits can then be rendered in different layouts for humans to read.

This tool uses a well-known line-based diff algorithm. It splits each version of the code into lines, looks for common sequences, and marks each line as added, removed, or unchanged. On top of this, it performs a simple move detection step by matching identical lines that appear in different positions, tagging them as moved where appropriate. This helps you see not only what content was changed, but also when code blocks were relocated.

People struggle with code diffs when the output is dense or lacks visual cues. Plain text with plus and minus signs is powerful, but it can be tiring to scan for longer changes. This tool adds colors, symbols, and layout options to make the diff easier to read. Side-by-side mode shows old lines on the left and new lines on the right. Inline mode shows a single unified sequence with change markers. Line numbers help you cross reference back to your own editor or version control system.

Key Features

• Dual code editors. The main interface provides two separate code areas: one for the original code and one for the modified code. You can paste text, type directly, or load content from files. Each editor shows a live character count and warns you when you near the maximum length limit used for performance.
• File upload support with size limits. Each editor has an associated file input. You can click the upload icon to choose a text-based code file. The tool enforces a maximum file size of 5 MB per file and reports a clear error if the file is too large. File contents are read as text and placed into the corresponding editor, trimmed to the internal length limit if necessary.
• Length safeguards. To protect the browser, the tool limits each code field to 50,000 characters. It blocks pastes that would exceed this limit and truncates overlong file content while showing a message. Visual counters under each editor show how many characters are used and how many are left.
• Side-by-side and inline diff modes. A mode switch lets you choose between side-by-side and inline views. In side-by-side mode, old code appears in one column and new code in another. In inline mode, all lines appear in a single list, with markers for additions, deletions, moves, and unchanged lines. You can switch modes at any time; the underlying diff result is reused.
• Ignore whitespace toggle. When the ignore whitespace option is enabled, the diff engine runs in a mode where spaces and formatting changes are treated as less significant. This is useful when comparing code that has been re-indented or reformatted but not otherwise changed. Turning this option on or off recalculates the diff.
• Change statistics bar. Above the diff result display, a small statistics strip shows how many additions, deletions, and moved lines were detected. This gives you a quick snapshot of change size and type before you dive into the detailed view.
• Diff download. A download button in the diff header generates a plain text representation of the current diff. Each line is prefixed with a symbol: + for added lines, - for removed lines, and a space for unchanged lines. The text is saved as "code_diff.txt" through the browser download flow.
• Clear and new comparison controls. A "New" button resets only the diff result so that you can re-run comparisons after editing the code fields. A separate "Clear" button clears both code editors, the diff, and any AI analysis so you can start again from an empty state.
• Per-field copy actions. Each editor has a copy icon that copies its current contents to the clipboard. A brief visual state change shows that the copy succeeded. This makes it easy to move code between tools or share it without manually selecting text.
• Optional AI-based semantic analysis. The "Smart Analysis" button sends both code versions to a backend analysis service. The service returns a structured result with a short summary, a change type (such as Refactor, Bug Fix, Feature, Documentation, or Style), and an impact level (High, Medium, or Low). The viewer displays this in a colored summary box at the top of the page.
• Inline error banner. Any issues, such as invalid AI responses, file read errors, or size limit violations, are shown in a dismissible error banner at the top. This makes it clear when the tool could not process input as expected and why.

Common Use Cases

Code reviews outside a version control UI. You might receive code snippets by email or chat, or you may want to review changes without setting up a full repository. You paste the original and modified code into the two editors, select side-by-side view, and review changes visually. The statistics and move detection help you see whether the change is mostly additions, deletions, or reorganized code.

Explaining a change to a teammate. When preparing a handoff or explaining a complex change, you can use the Smart Analysis feature. After pasting both versions, you run the analysis and read the summary. It classifies the change and gives an impact level, which you can quote in messages or documentation.

Checking the effect of an auto-formatter. If your formatter has restructured code, you can toggle "Ignore Whitespace" to see whether it changed anything beyond layout. If the diff becomes small or empty with whitespace ignored, you know that the tool did not introduce functional changes. A related operation involves comparing XML documents as part of a similar workflow.

Studying refactors. For learning or teaching, you can place pre-refactor code on the left and refactored code on the right. Move detection highlights lines moved to new positions, and the Smart Analysis summary might identify the change type as a refactor with a certain impact level. This helps make refactoring steps visible and easier to discuss.

How to Use This Tool (Step-by-Step)

1. Open the code diff viewer in your browser. You will see two code editors labeled "Original Code" and "Modified Code", as well as controls for diff mode, whitespace handling, and smart analysis.
2. Paste or type your original code into the first editor. You can also click the upload icon to load a code file from your system. If the file is larger than the allowed size, an error message appears and the file is not loaded.
3. Paste, type, or upload the modified code into the second editor. The viewer tracks character counts for both fields and alerts you if either is close to the maximum supported size.
4. Optionally adjust the "Ignore Whitespace" toggle if you want to filter out formatting-only differences. Turn it on to collapse whitespace changes, off to see every difference including spacing.
5. Choose your preferred display mode from the mode switch. Select "Side-by-Side" to see old and new code in separate columns. Select "Inline" to view a unified sequence of diff lines.
6. Scroll down to the statistics bar to see counts of additions, deletions, and moved lines. Use these numbers to get a first idea of how large and complex the change is.
7. Scroll through the diff result. In both modes, each line shows its line number (from the original or modified code), a symbol indicating its type (plus, minus, move arrow, or blank), and the code content. Background colors indicate added, removed, moved, or unchanged lines.
8. If you want to save the diff as text, click the download icon in the diff header. A file named "code_diff.txt" is downloaded containing the diff with +, -, and space prefixes. You can open it in any text editor or attach it to emails and tickets.
9. To reset the current comparison while keeping code in the editors, click the "New" button in the diff header. To clear both code editors and any analysis completely, click the "Clear" button in the toolbar.
10. If you want semantic insight, click the "Smart Analysis" button. Wait for the analysis result to appear at the top. It will display a summary sentence and labels for change type and impact. Use this information to document or discuss the change. You can run new analyses after editing the code.

Calculations & Logic

The core diff function starts by enforcing a per-input length limit. If either the original or new code exceeds the maximum number of characters, it throws an error that the UI surfaces. When inputs are within limits, it calls a line-based diff routine that returns an array of change objects. Each change object contains a text block and flags indicating whether the lines are added, removed, or unchanged.

To build the detailed diff lines, the function iterates over the changes, splitting each block into individual lines. It assigns line numbers from the original and new code using counters. Added lines get new line numbers only, removed lines get old line numbers only, and unchanged lines get both. Each line is stored as a diff line with type and content fields.

After the first pass, a second step detects simple moves. It builds a map from line content to indices of removed lines. Then it scans for added lines with identical content. When it finds such a match, it marks both the removed line and the added line as moved and assigns a shared move identifier. This flag is used to style moved lines differently, and the addition and deletion counts are adjusted to avoid double counting.

The statistics in the result object summarize the number of additions, deletions, and moves. Additions and deletions count only lines whose type remains "added" or "removed" after move detection. The move count is derived from the number of pairs of lines marked as moved. These statistics power the bar that appears above the diff view.

The AI analysis function sends both code strings to a backend service under a dedicated key. The backend is expected to respond with a structured object containing a free-form summary string, a changeType label, and an impact label. The frontend validates that these fields are present and are strings, then casts them to a fixed set of known values (Refactor, Bug Fix, Feature, Documentation, Style, and High, Medium, Low) before storing them. If validation fails or an error occurs, the function throws a descriptive error which is caught and displayed in the error banner.

Reference Tables or Scales

The tool is built for text-based code only. Binary files and non-text formats are not supported and may produce unreadable results if loaded.

Tips, Limitations & Best Practices

Use the ignore whitespace option when comparing code that has been reformatted by tools, but be cautious for languages where whitespace is significant. If in doubt, compare with the option off first, then turn it on to see which changes are purely cosmetic. For adjacent tasks, comparing files addresses a complementary step.

Keep files within the documented size limits. For very large files or entire projects, consider using your version control system’s built-in diffs and apply this viewer to specific files or sections where you need extra clarity.

Rely on the change statistics and move detection to quickly understand the nature of a change. Many additions and few deletions might indicate a new feature; many deletions might suggest cleanup. Moves can signal refactors or reorganizations.

Treat AI analysis as a helper, not a judge. The smart summary can highlight intent and impact, but final review decisions should still be based on your own reading of the diff and project guidelines.

If you see repeated errors, such as invalid AI responses or file read failures, check that your browser supports the required features and that your inputs are plain text. When issues persist, you can still use the core diff without the AI layer, as the comparison logic is self-contained and deterministic.