Big File Viewer

Open huge text and log files — gigabytes — without loading them into memory. Built for the files the built-in editor chokes on: multi-gigabyte logs, exports and data dumps.

Features

• Instant open — the viewer reads only the bytes it renders; nothing is loaded wholesale.
• Virtualized scrolling — 40-million-line files scroll smoothly. Row positions map onto a capped spacer height, so the Chromium scroll clamp (~33.5M px) is never a limit.
• Background line index — a one-pass streaming scan counts lines and samples line offsets; while it runs you already see content and an estimated line count.
• Streaming search (Ctrl+F) — whole-file search with progress, case toggle, next/previous hit navigation (F3 / Shift+F3) and jump-to-hit. UTF-8 files support case-insensitive search; UTF-16 files are searched in the byte-exact domain (case-sensitive only).
• Go to line (Ctrl+G) — 1-based line jump.
• Encoding support — auto-detect (BOM + 64 KB sample) or choose UTF-8, UTF-16LE/BE, GBK, GB18030, Big5, Shift-JIS, EUC-KR, Windows-1252, ISO-8859-1 or ASCII from the toolbar. ASCII-compatible encodings keep the same 0x0A line grid; a UTF-16 endianness or 1-byte↔2-byte switch rebuilds the line index automatically. Legacy CJK encodings decode for viewing and search, and accept ASCII-only line edits (Node ships no encoder for those code pages).
• Syntax highlighting — a lightweight per-line highlighter for JavaScript/TypeScript, JSON, C/C++, Python, shell, Go, Rust, Java, SQL, YAML, INI, markup, CSS and Markdown, plus timestamp/log-level coloring in .log files. Toggle with viewbigfile.syntaxHighlight (default on).
• Editor tab width — Tab characters render at your editor.tabSize (display only: the file is never rewritten with expanded spaces).
• Top-anchored, draggable dialogs — Find (Ctrl+F) and Go to Line (Ctrl+G) open just below the toolbar and can be moved anywhere; the viewer below stays fully interactive while the dialog is open. Transient messages appear in a centered, draggable toast.
• Horizontal scrolling — long lines get a real horizontal scrollbar; line numbers stay pinned to the left while you scroll sideways.
• Word wrap (default on) — toolbar Wrap toggle or viewbigfile.wordWrap; turn it off to switch to the horizontal scrollbar. Files up to 10,000 lines render every line at its natural height (each long line wraps, short lines stay compact — no blank gaps). Larger files keep a fixed row height so virtual scrolling still works; size it with viewbigfile.wrapLines (0 = derive from the longest line in the file).
• Line editing, opt-in — the viewer is read-only until you explicitly enable editing (with a confirmation). Editing replaces whole lines; Save (Ctrl+S) patches same-length lines in place and rewrites the file when any line changed length. If the file changed on disk since it was opened, the save is refused.
• Big-file prompt — opening a text file larger than viewbigfile.autoOpenThresholdMB in the built-in editor offers to reopen it here instead.
• English / Chinese UI — follows the display language.

Keyboard & zoom

Zoom only changes the row font of this viewer — the workbench zoom level is untouched. The chosen size is remembered across tabs and restarts, and Ctrl+0 returns to the theme's editor font size. In a virtualized view Ctrl+A can only select the rendered rows, so the viewer says so instead of silently copying a partial selection.

Extension Settings

• viewbigfile.autoOpenThresholdMB (default 100) — files above this size trigger the "open in Big File Viewer?" prompt when opened with the built-in editor. 0 disables the prompt.
• viewbigfile.maxLineDisplayChars (default 2000) — characters fetched and rendered per line; longer lines are truncated with an ellipsis marker (display only — the file is never modified).
• viewbigfile.encoding (default auto) — text encoding used for line decoding and search. auto uses a BOM and a 64 KB heuristic; explicit values: utf8, utf16le, utf16be, gbk, gb18030, big5, shift_jis, euc-kr, windows-1252, iso-8859-1, ascii.
• viewbigfile.syntaxHighlight (default true) — per-line syntax coloring of the visible rows (see the language list above).
• viewbigfile.wordWrap (default true) — wrap long lines instead of using the horizontal scrollbar. The toolbar Wrap button toggles the same mode for the current panel; turn it off to get the horizontal scrollbar.
• viewbigfile.wrapLines (default 0) — visual lines per wrapped row for files above 10,000 lines: 0 = derive from the longest line in the file, 1–100 = fixed cap. Files up to 10,000 lines use natural heights and ignore this setting.

Performance (measured)

Environment: Windows 11, AMD Ryzen 7 7840HS, NVMe SSD (F:), Node 26.4.0, VSCode 1.144.0. The 1 GiB text is a synthetic log; the 12.4 GB and 53.3 GB files are VMware disk images used as large stress files — for a big-file viewer a "line" is a byte 0x0A/0x0D split, so their line counts describe the stress input, not log content.

Search is a cold full scan for a needle that does not occur (0 hits), so the number is the whole-file throughput; the 1 GiB log search finds the planted needle 120 times.

How it stays fast

• Positioned reads: every fetch is an fs.read at an explicit offset through a bounded LRU of 64 KB-aligned windows — memory does not grow with file size.
• Background line index: one streaming pass records every 1024th line start in a compact array; line lookups seek to the nearest sample and walk forward.
• Virtualized rows: only the visible rows (plus a small overscan) exist in the DOM; scrolling maps positions onto a capped spacer instead of a native total height.
• Streaming search: fixed-size chunks with overlap for boundary matches; hit collection is capped and cancellable.
• All file offsets are handled as plain 64-bit-safe numbers — multi-gigabyte files never truncate at 2 GB.

Editing limitations (by design)

• Editing is line-level: click a line, type, Enter commits (and moves to the next line), Esc cancels. There is no free-form text editing.
• Saving lines whose byte length changed rewrites the entire file via a same-directory temp file and an atomic rename; unchanged-length lines are patched in place.
• No automatic reload on external changes: the save-time size + mtime check refuses to overwrite a file that changed on disk.
• Legacy CJK encodings (GBK/Big5/Shift-JIS/EUC-KR) have no encoder in Node: lines are editable only while the new text stays ASCII; non-ASCII edits are refused rather than written as garbage.
• Display-truncated lines (longer than viewbigfile.maxLineDisplayChars) are not editable — committing the visible slice would delete the rest of the line. Raise the limit or use the built-in editor.
• Syntax highlighting is per line: block comments and multi-line strings are not tracked across lines (a big-file viewer must never scan the file to color a viewport row).

Build & install locally

cd bigfile
npm install
npm run typecheck
npm run compile
npm test                    # unit + smoke tests (no VSCode needed)
npm run test:integration    # real VSCode host via @vscode/test-electron
npx vsce package --allow-missing-repository
code --install-extension viewbigfile-0.2.4.vsix --force

Support

See SUPPORT.md.

License

MIT