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/**
* `<DiffViewer />` — the React shell (SPEC §3.3, §4.5).
*
* The component is a *rendering of the model and nothing more*: it computes
* a {@link DiffResult} (or takes one), flattens it into an explicit row
* list, and draws that list. All the diff intelligence lives in
* `./headless`; everything here is presentation, expansion state, and
* accessibility.
*
* The row list is deliberately materialised rather than nested-mapped: it is
* what makes `aria-rowcount`/`aria-rowindex` truthful, and it is the array
* Phase 3's virtualization will window over.
*/
import {
useCallback,
useEffect,
useMemo,
useRef,
useState,
type ReactNode,
} from "react";
import styles from "./DiffViewer.module.scss";
import { Gap, type ExpandDirection, type GapLabels } from "./components/Gap";
import {
SplitRowView,
UnifiedRowView,
cx,
type RenderContent,
type RowClassNames,
} from "./components/Row";
import { observeSize, useVirtualRows } from "./components/useVirtualRows";
import { computeDiff } from "./diff/computeDiff";
import { resolveOptions, type DiffLine, type DiffOptions, type DiffResult } from "./types";
import { alignRows, type SplitRow } from "./view/rows";
/**
* Coerce a numeric prop, keeping `Infinity` only where it is meaningful.
*
* Mirrors the headless `toExtent`/`toFinite` split (SPEC resolved decision
* #8) for the two props that never reach `resolveOptions`.
*/
function toPositive(value: number | undefined, fallback: number, allowInfinity = false): number {
if (typeof value !== "number" || Number.isNaN(value)) return fallback;
if (!Number.isFinite(value)) return allowInfinity && value > 0 ? value : fallback;
return value;
}
/** Every user-facing string, so the viewer can be localised (SPEC §3.4). */
export type DiffViewerLabels = Partial<{
/** Visible text on the "reveal from the top of the gap" control. */
expandUp: string;
/** Visible text on the "reveal from the bottom of the gap" control. */
expandDown: string;
/** Visible text on the "reveal everything" control. */
expandAll: string;
/** Accessible name for the up control — the visible text is only a glyph. */
ariaExpandUp: (count: number) => string;
/** Accessible name for the down control. */
ariaExpandDown: (count: number) => string;
/** Accessible name for the expand-all control. */
ariaExpandAll: string;
/** Receives the number of hidden lines. */
hiddenLines: (count: number) => string;
/** Accessible name for the diff table itself. */
tableLabel: string;
/** Accessible name for the horizontally scrollable code region. */
scrollRegionLabel: string;
noChanges: string;
noChangesDetail: string;
tooLarge: string;
tooDifferent: string;
diffAnyway: string;
added: string;
removed: string;
/** Receives the stats; renders the header summary. */
stats: (stats: { added: number; removed: number; unchanged: number }) => ReactNode;
}>;
/** Class-name slots (SPEC §4.5). */
export type DiffViewerClassNames = Partial<
Record<
"root" | "header" | "row" | "gutter" | "lineNumber" | "content" | "gap" | "marker",
string
>
>;
export interface DiffViewerProps extends DiffOptions {
/** The "before" document. Ignored when `diff` is supplied. */
oldText?: string;
/** The "after" document. Ignored when `diff` is supplied. */
newText?: string;
/**
* A pre-computed model — skips `computeDiff` entirely (e.g. computed in a
* worker or on the server). Expansion still works: the hidden lines travel
* inside the model (SPEC resolved decision #11).
*/
diff?: DiffResult;
/** @default "split" */
view?: "split" | "unified";
/** @default true */
showLineNumbers?: boolean;
/**
* Wrap long lines instead of scrolling horizontally.
* @default false
*/
wrap?: boolean;
/**
* Row count above which only the visible window is rendered (SPEC §3.3).
*
* Virtualization needs a bounded viewport to window against, so above the
* threshold the component also caps its own height
* (`--usefy-diff-max-height`, default `70vh`) and scrolls internally.
* Below it the diff renders in full and grows with its content.
*
* `Infinity` disables windowing; so does `wrap` (resolved decision #6).
* @default 200
*/
virtualizeThreshold?: number;
/**
* Height of one row in pixels — the unit windowing counts in.
*
* Must match the rendered row height, which is why it is written to
* `--usefy-diff-row-height` rather than merely assumed.
* @default 22
*/
rowHeight?: number;
/** Reveal every collapsed region on first render. @default false */
defaultExpandAll?: boolean;
/** The syntax-highlighting seam (SPEC §4.5). */
renderContent?: RenderContent;
labels?: DiffViewerLabels;
/** `"system"` follows `prefers-color-scheme`. @default "system" */
theme?: "light" | "dark" | "system";
classNames?: DiffViewerClassNames;
className?: string;
/** Fired after a gap is expanded, with how many lines were revealed. */
onExpand?: (hunkIndex: number, lines: number) => void;
}
const DEFAULT_LABELS = {
expandUp: "↑ {count}",
expandDown: "↓ {count}",
expandAll: "Expand all",
ariaExpandUp: (count: number) => `Expand ${count} lines above`,
ariaExpandDown: (count: number) => `Expand ${count} lines below`,
ariaExpandAll: "Expand all hidden lines",
hiddenLines: (count: number) => `⋯ ${count} unchanged ${count === 1 ? "line" : "lines"}`,
tableLabel: "Diff",
scrollRegionLabel: "Diff content, scrollable horizontally",
noChanges: "No changes",
noChangesDetail: "The two versions are identical.",
tooLarge: "This diff is too large to display.",
tooDifferent: "These two versions have too little in common to diff quickly.",
diffAnyway: "Diff anyway",
added: "Added",
removed: "Removed",
} as const;
/**
* Resolve the effective dark-mode boolean for the chosen theme.
*
* `"system"` is resolved in **CSS** first (a `prefers-color-scheme` media
* query in the stylesheet), so the very first paint — including a server
* render, and including a page with JS disabled — is already correct. This
* hook exists only so a *live* OS theme change repaints; it therefore starts
* from `false` for `"system"` and never causes the initial flash.
*/
function useResolvedDark(theme: "light" | "dark" | "system"): boolean {
const [dark, setDark] = useState(theme === "dark");
useEffect(() => {
if (theme === "light") {
setDark(false);
return;
}
if (theme === "dark") {
setDark(true);
return;
}
if (typeof window === "undefined" || !window.matchMedia) {
setDark(false);
return;
}
const mq = window.matchMedia("(prefers-color-scheme: dark)");
const update = () => setDark(mq.matches);
update();
mq.addEventListener?.("change", update);
return () => mq.removeEventListener?.("change", update);
}, [theme]);
return dark;
}
/**
* How much of one collapsed region has been revealed.
*
* `top` counts lines revealed from the start of the run, `bottom` from the
* end, so the remaining gap marker stays *between* them — the shape every
* code-review tool uses.
*/
interface Revealed {
top: number;
bottom: number;
}
const NOTHING_REVEALED: Revealed = { top: 0, bottom: 0 };
const NO_REVEALS: Record<string, Revealed> = {};
/** Key for one expandable region: which hunk, and which side of it. */
function regionKey(hunkIndex: number, side: "before" | "after"): string {
return `${hunkIndex}:${side}`;
}
/** One rendered table row. The list of these is the render plan. */
type RenderRow =
| { kind: "split"; key: string; row: SplitRow }
| { kind: "unified"; key: string; line: DiffLine }
| {
kind: "gap";
key: string;
hunkIndex: number;
side: "before" | "after";
hidden: number;
state: Revealed;
};
/**
* Track whether the scroll container actually overflows.
*
* A scrollable region has to be keyboard-reachable or a keyboard-only user
* simply cannot see the right-hand side of a long line (Chromium only
* scrolls a container that can take focus). But an always-tabbable div adds
* a dead tab stop to every short diff, so the affordance is applied only
* when there is something to scroll.
*/
function useOverflowing(ref: React.RefObject<HTMLElement | null>, deps: unknown[]): boolean {
const [overflowing, setOverflowing] = useState(false);
useEffect(() => {
const node = ref.current;
if (!node) return;
const measure = () => setOverflowing(node.scrollWidth > node.clientWidth + 1);
measure();
return observeSize(node, measure);
// eslint-disable-next-line react-hooks/exhaustive-deps
}, deps);
return overflowing;
}
/**
* Render a text diff as a side-by-side or unified table.
*
* ## What it renders
*
* A real `<table>`: rows are `<tr>`, the first line-number cell of each row
* is a `<th scope="row">` and everything else is a `<td>`, so assistive
* technology can navigate the diff as the tabular data it is (SPEC §8).
* Every changed row carries a visually-hidden "Added"/"Removed" label **and**
* a `+`/`−` gutter glyph, so the change type is never signalled by colour
* alone. `aria-rowcount`/`aria-rowindex` describe the rows actually drawn.
*
* Content always renders as **text nodes** — never `dangerouslySetInnerHTML`
* (SPEC §9). Pass `renderContent` to plug in a syntax highlighter; that is
* explicitly your trust boundary.
*
* ## States
*
* - **Changes** — the normal table.
* - **No changes** — an explicit message, not a blank box.
* - **Refused** — when a guard trips, an honest explanation that distinguishes
* "too large" (`truncatedReason: "size"`) from "too different"
* (`"complexity"`), plus a "Diff anyway" button that retries with the
* guards off. That approval is scoped to the inputs that earned it: change
* the documents and the guards are back on.
*
* @example
* ```tsx
* import { DiffViewer } from "@usefy/diff-viewer";
* import "@usefy/diff-viewer/styles.css";
*
* <DiffViewer oldText={before} newText={after} view="split" />
* ```
*
* @example
* ```tsx
* // Bring your own highlighter — the seam hands you plain strings.
* <DiffViewer
* oldText={before}
* newText={after}
* renderContent={({ text, segments }) =>
* segments ? segments.map((s, i) => <Token key={i} {...s} />) : <Token text={text} />
* }
* />
* ```
*/
export function DiffViewer({
oldText = "",
newText = "",
diff,
view = "split",
showLineNumbers = true,
wrap = false,
virtualizeThreshold = 200,
rowHeight = 22,
defaultExpandAll = false,
renderContent,
labels,
theme = "system",
classNames = {},
className,
onExpand,
...diffOptions
}: DiffViewerProps): ReactNode {
const isDark = useResolvedDark(theme);
// The option object is spread from props, so memoise on the resolved
// values rather than on an identity that changes every render.
const resolved = resolveOptions(diffOptions);
const optionsKey = JSON.stringify(resolved);
/**
* A token that changes whenever the *inputs* change.
*
* "Diff anyway" is approval for one specific pair of documents, so it is
* stored as the token it was granted for rather than as a bare boolean.
* A bare boolean stayed `true` across a document swap, and the next
* refused pair would then be diffed with every guard set to `Infinity` —
* re-opening the multi-second synchronous freeze the cost guard exists to
* prevent, inside `useMemo`, during render. Comparing tokens resolves
* `forced` back to `false` in the *same* render the inputs change, before
* anything is computed.
*/
const inputToken = useMemo(() => ({}), [oldText, newText, diff, optionsKey]);
const [forcedFor, setForcedFor] = useState<object | null>(null);
const forced = forcedFor === inputToken;
const model = useMemo(() => {
if (diff) return diff;
const options: DiffOptions = forced
? { ...diffOptions, maxLines: Infinity, maxBytes: Infinity, maxEditDistance: Infinity }
: diffOptions;
return computeDiff(oldText, newText, options);
// `optionsKey` stands in for the spread options object.
// eslint-disable-next-line react-hooks/exhaustive-deps
}, [diff, oldText, newText, optionsKey, forced]);
/**
* Expansion state, held together with the model it belongs to.
*
* Keys are `hunkIndex:side`, which say nothing about *which document* they
* came from — so carrying the map across a model change made a fresh diff
* render already-expanded, and a stale `top` larger than the new hidden
* run drove the remaining count negative and made the gap vanish entirely.
* Storing the model alongside makes a mismatch impossible to miss, with no
* effect and no extra render.
*/
const [reveal, setReveal] = useState<{
model: DiffResult | null;
map: Record<string, Revealed>;
}>({ model: null, map: NO_REVEALS });
const revealed = reveal.model === model ? reveal.map : NO_REVEALS;
const text = { ...DEFAULT_LABELS, ...labels };
const gapLabels: GapLabels = {
expandUp: text.expandUp,
expandDown: text.expandDown,
expandAll: text.expandAll,
ariaExpandUp: text.ariaExpandUp,
ariaExpandDown: text.ariaExpandDown,
ariaExpandAll: text.ariaExpandAll,
hiddenLines: text.hiddenLines,
};
const rowLabels = { added: text.added, removed: text.removed };
/**
* Reveal part of a collapsed region.
*
* Everything is derived from the state the clicked gap actually rendered
* with — nothing is computed inside the state updater. StrictMode invokes
* updaters twice, so an updater that also decided how many lines to report
* would double-fire `onExpand`; keeping the updater a pure merge makes
* that impossible by construction.
*
* `remaining` is always ≥ 1 here: `Gap` renders nothing when there is
* nothing left to reveal, so there is no button to click.
*/
const handleExpand = useCallback(
(
hunkIndex: number,
side: "before" | "after",
current: Revealed,
remaining: number,
direction: ExpandDirection,
) => {
const key = regionKey(hunkIndex, side);
const step = Math.min(resolved.expandStep, remaining);
const lines = direction === "all" ? remaining : step;
const next: Revealed =
direction === "all"
? { top: current.top + remaining, bottom: current.bottom }
: direction === "up"
? { top: current.top + step, bottom: current.bottom }
: { top: current.top, bottom: current.bottom + step };
setReveal((previous) => ({
model,
map: { ...(previous.model === model ? previous.map : NO_REVEALS), [key]: next },
}));
onExpand?.(hunkIndex, lines);
},
[model, onExpand, resolved.expandStep],
);
const columnCount = view === "split" ? (showLineNumbers ? 6 : 4) : showLineNumbers ? 4 : 2;
const rootClass = cx(
styles.root,
isDark && styles.dark,
theme === "light" && styles.light,
wrap && styles.wrap,
classNames.root,
className,
);
/* ---------------------------------------------------------------------- */
/* The render plan */
/* ---------------------------------------------------------------------- */
/**
* Flatten the model plus the current expansion state into the exact list
* of rows to draw. Built even for the guarded states (where it is empty)
* so the hook order never changes.
*/
const rows = useMemo<RenderRow[]>(() => {
if (model.truncated) return [];
const out: RenderRow[] = [];
const pushLines = (lines: readonly DiffLine[], keyPrefix: string) => {
if (view === "split") {
alignRows(lines).forEach((row, index) =>
out.push({ kind: "split", key: `${keyPrefix}-${index}`, row }),
);
} else {
lines.forEach((line, index) =>
out.push({ kind: "unified", key: `${keyPrefix}-${index}`, line }),
);
}
};
const pushRegion = (hidden: DiffLine[], hunkIndex: number, side: "before" | "after") => {
if (hidden.length === 0) return;
const key = regionKey(hunkIndex, side);
const state = defaultExpandAll
? { top: hidden.length, bottom: 0 }
: (revealed[key] ?? NOTHING_REVEALED);
const remaining = Math.max(0, hidden.length - state.top - state.bottom);
if (state.top > 0) pushLines(hidden.slice(0, state.top), `${key}-top`);
if (remaining > 0) {
out.push({ kind: "gap", key, hunkIndex, side, hidden: remaining, state });
}
if (state.bottom > 0) {
pushLines(hidden.slice(hidden.length - state.bottom), `${key}-bottom`);
}
};
model.hunks.forEach((hunk, hunkIndex) => {
pushRegion(hunk.hiddenBefore, hunkIndex, "before");
pushLines(hunk.lines, `hunk-${hunkIndex}`);
pushRegion(hunk.hiddenAfter, hunkIndex, "after");
});
return out;
}, [model, view, revealed, defaultExpandAll]);
const scrollerRef = useRef<HTMLDivElement | null>(null);
const overflowing = useOverflowing(scrollerRef, [rows, view, showLineNumbers, wrap]);
/* ---------------------------------------------------------------------- */
/* Virtualization (SPEC §3.3, resolved decision #6) */
/* ---------------------------------------------------------------------- */
const safeRowHeight = Math.max(1, toPositive(rowHeight, 22));
const threshold = Math.max(0, toPositive(virtualizeThreshold, 200, true));
// `wrap` makes rows variable-height, which fixed-height windowing cannot
// model — measured virtualization is a different, much larger problem.
const wantsVirtual = rows.length > threshold;
const virtualEnabled = wantsVirtual && !wrap;
const rowWindow = useVirtualRows({
enabled: virtualEnabled,
total: rows.length,
rowHeight: safeRowHeight,
scrollRef: scrollerRef,
});
useEffect(() => {
if (wrap && wantsVirtual && typeof console !== "undefined") {
console.warn(
`[@usefy/diff-viewer] wrap is on, so virtualization is disabled and all ${rows.length} rows are in the DOM. ` +
"Wrapped rows have no fixed height, which fixed-height windowing cannot model (SPEC resolved decision #6). " +
"Turn off wrap for large diffs.",
);
}
}, [wrap, wantsVirtual, rows.length]);
/**
* Keep the keyboard somewhere sensible when the focused control is
* windowed out — but never yank focus a user has deliberately moved away.
*
* Scrolling an expander out of the window unmounts it, and the browser
* drops focus to `<body>`; catching that in the scroll container keeps a
* keyboard user in the diff with arrow-key scrolling intact.
*
* The trap this avoids: a stale "focus was inside" flag would let an
* *unrelated* resize (which also moves the window) steal focus off `<body>`
* even though the user chose to leave. The discriminator is
* `isConnected` — when a user tabs or clicks away the element stays in the
* DOM, and `focusout` clears the ref; when windowing removes it the element
* is already gone, so the ref survives for the window effect to act on.
*/
const focusedInside = useRef<HTMLElement | null>(null);
useEffect(() => {
const node = scrollerRef.current;
if (!node) return;
const remember = (e: FocusEvent) => {
focusedInside.current = e.target as HTMLElement;
};
const forget = (e: FocusEvent) => {
// Still in the DOM ⇒ a real focus move, not an unmount ⇒ don't chase it.
Eif ((e.target as HTMLElement).isConnected) focusedInside.current = null;
};
node.addEventListener("focusin", remember);
node.addEventListener("focusout", forget);
return () => {
node.removeEventListener("focusin", remember);
node.removeEventListener("focusout", forget);
};
}, []);
useEffect(() => {
const node = scrollerRef.current;
const focused = focusedInside.current;
if (!node || !focused) return;
// Restore focus only when the element that held it was actually windowed
// out (removed from the DOM) and focus landed on <body> as a result.
Eif (
!focused.isConnected &&
typeof document !== "undefined" &&
document.activeElement === document.body
) {
node.focus({ preventScroll: true });
}
}, [rowWindow.start, rowWindow.end]);
const visibleRows = rowWindow.virtualized
? rows.slice(rowWindow.start, rowWindow.end)
: rows;
/* ---------------------------------------------------------------------- */
/* Guarded and empty states */
/* ---------------------------------------------------------------------- */
if (model.truncated) {
const canRetry = !diff;
return (
<div className={rootClass} data-usefy-diff-state="truncated">
<div className={styles.notice} role="status">
<strong className={styles.noticeTitle}>
{model.truncatedReason === "complexity" ? text.tooDifferent : text.tooLarge}
</strong>
{canRetry && (
<div className={styles.noticeAction}>
<button
type="button"
className={styles.gapButton}
onClick={() => setForcedFor(inputToken)}
>
{text.diffAnyway}
</button>
</div>
)}
</div>
</div>
);
}
if (model.hunks.length === 0) {
return (
<div className={rootClass} data-usefy-diff-state="unchanged">
<div className={styles.notice} role="status">
<strong className={styles.noticeTitle}>{text.noChanges}</strong>
{text.noChangesDetail}
</div>
</div>
);
}
/* ---------------------------------------------------------------------- */
/* The diff itself */
/* ---------------------------------------------------------------------- */
return (
<div className={rootClass} data-usefy-diff-state="diff" data-usefy-diff-view={view}>
<div className={cx(styles.header, classNames.header)}>
{labels?.stats ? (
labels.stats(model.stats)
) : (
<span className={styles.stats}>
<span className={styles.statAdded}>+{model.stats.added}</span>
<span className={styles.statRemoved}>−{model.stats.removed}</span>
<span className={styles.statUnchanged}>{model.stats.unchanged} unchanged</span>
</span>
)}
</div>
<div
// The height bound is driven by whether windowing is WANTED, not by
// whether it has happened yet. Gating it on the outcome deadlocks:
// an unbounded scroller grows to the full 20 000-row height, so it
// always "fits" its own content, so the window never narrows, so the
// bound is never applied. Intent first, measurement second.
className={cx(styles.scroller, virtualEnabled && styles.virtualized)}
ref={scrollerRef}
style={{ "--usefy-diff-row-height": `${safeRowHeight}px` } as React.CSSProperties}
data-usefy-diff-virtualized={rowWindow.virtualized ? "true" : undefined}
// A scrollable region is only reachable by keyboard if it can take
// focus; applied whenever there is actually something to scroll.
{...(overflowing || virtualEnabled
? { tabIndex: 0, role: "group", "aria-label": text.scrollRegionLabel }
: {})}
>
{/* aria-rowcount is the TRUE total so assistive tech never sees the
window as the whole document. */}
<table className={styles.table} aria-rowcount={rows.length}>
<caption className={styles.srOnly}>
{text.tableLabel}: {model.stats.added} added, {model.stats.removed} removed,{" "}
{model.stats.unchanged} unchanged.
</caption>
{/* Explicit column widths — the first row is a collapsed-region
row spanning every column and must not dictate the layout. */}
<colgroup>
{view === "split" ? (
<>
{showLineNumbers && <col className={styles.colNumber} />}
<col className={styles.colMarker} />
<col className={styles.colContent} />
{showLineNumbers && <col className={styles.colNumber} />}
<col className={styles.colMarker} />
<col className={styles.colContent} />
</>
) : (
<>
{showLineNumbers && (
<>
<col className={styles.colNumber} />
<col className={styles.colNumber} />
</>
)}
<col className={styles.colMarker} />
<col className={styles.colContent} />
</>
)}
</colgroup>
<tbody>
{/* Spacers stand in for the rows that are not in the DOM, so the
scrollbar reflects the whole document rather than the
window. */}
{rowWindow.paddingTop > 0 && (
<tr aria-hidden="true" className={styles.spacer}>
<td colSpan={columnCount} style={{ height: rowWindow.paddingTop }} />
</tr>
)}
{visibleRows.map((entry, offset) => {
const index = rowWindow.start + offset;
const ariaRowIndex = index + 1;
if (entry.kind === "gap") {
return (
<Gap
key={entry.key}
ariaRowIndex={ariaRowIndex}
hidden={entry.hidden}
columnCount={columnCount}
styles={styles}
classNames={classNames}
labels={gapLabels}
expandStep={resolved.expandStep}
onExpand={(direction) =>
handleExpand(
entry.hunkIndex,
entry.side,
entry.state,
entry.hidden,
direction,
)
}
/>
);
}
if (entry.kind === "split") {
return (
<SplitRowView
key={entry.key}
ariaRowIndex={ariaRowIndex}
row={entry.row}
showLineNumbers={showLineNumbers}
styles={styles}
classNames={classNames}
labels={rowLabels}
renderContent={renderContent}
/>
);
}
return (
<UnifiedRowView
key={entry.key}
ariaRowIndex={ariaRowIndex}
line={entry.line}
showLineNumbers={showLineNumbers}
styles={styles}
classNames={classNames}
labels={rowLabels}
renderContent={renderContent}
/>
);
})}
{rowWindow.paddingBottom > 0 && (
<tr aria-hidden="true" className={styles.spacer}>
<td colSpan={columnCount} style={{ height: rowWindow.paddingBottom }} />
</tr>
)}
</tbody>
</table>
</div>
</div>
);
}
|