tools/ui/src/lib/utils/latex-protection.ts

import {
	CODE_BLOCK_PLACEHOLDER_REGEXP,
	CODE_BLOCK_REGEXP,
	LATEX_BACKSLASH,
	LATEX_BLOCKQUOTE_PREFIX_REGEXP,
	LATEX_CURRENCY_DOLLAR_REGEXP,
	LATEX_CURRENCY_ESCAPE,
	LATEX_DIGIT_REGEXP,
	LATEX_DISPLAY_BLOCK_REGEXP,
	LATEX_DISPLAY_CLOSE,
	LATEX_DISPLAY_CONVERT_REGEXP,
	LATEX_DISPLAY_DELIMITER,
	LATEX_DISPLAY_OPEN,
	LATEX_INLINE_CLOSE,
	LATEX_INLINE_CONVERT_REGEXP,
	LATEX_INLINE_DELIMITER,
	LATEX_INLINE_OPEN,
	LATEX_LINEBREAK_REGEXP,
	LATEX_MATH_AND_CODE_PATTERN,
	LATEX_MHCHEM_CE,
	LATEX_MHCHEM_PU,
	LATEX_NEIGHBOR_CHAR_REGEXP,
	LATEX_NON_WHITESPACE_REGEXP,
	LATEX_PLACEHOLDER_REGEXP,
	LATEX_PROTECT_REGEXP,
	LATEX_TRIGGER_REGEXP,
	MHCHEM_PATTERN_MAP,
	NEWLINE
} from '$lib/constants';

/**
 * Replaces inline LaTeX expressions enclosed in `$...$` with placeholders, avoiding dollar signs
 * that appear to be part of monetary values or identifiers.
 *
 * This function processes the input line by line and skips `$` sequences that are likely
 * part of money amounts (e.g., `$5`, `$100.99`) or code-like tokens (e.g., `var$`, `$var`).
 * Valid LaTeX inline math is replaced with a placeholder like `<<LATEX_0>>`, and the
 * actual LaTeX content is stored in the provided `latexExpressions` array.
 *
 * @param content - The input text potentially containing LaTeX expressions.
 * @param latexExpressions - An array used to collect extracted LaTeX expressions.
 * @returns The processed string with LaTeX replaced by placeholders.
 */
export function maskInlineLaTeX(content: string, latexExpressions: string[]): string {
	if (!content.includes(LATEX_INLINE_DELIMITER)) {
		return content;
	}

	return content
		.split(NEWLINE)
		.map((line) => {
			if (line.indexOf(LATEX_INLINE_DELIMITER) == -1) {
				return line;
			}

			let processedLine = '';
			let currentPosition = 0;

			while (currentPosition < line.length) {
				const openDollarIndex = line.indexOf(LATEX_INLINE_DELIMITER, currentPosition);

				if (openDollarIndex == -1) {
					processedLine += line.slice(currentPosition);

					break;
				}

				// Is there a next $-sign?
				const closeDollarIndex = line.indexOf(LATEX_INLINE_DELIMITER, openDollarIndex + 1);

				if (closeDollarIndex == -1) {
					processedLine += line.slice(currentPosition);

					break;
				}

				const charBeforeOpen = openDollarIndex > 0 ? line[openDollarIndex - 1] : '';
				const charAfterOpen = line[openDollarIndex + 1];
				const charBeforeClose =
					openDollarIndex + 1 < closeDollarIndex ? line[closeDollarIndex - 1] : '';
				const charAfterClose = closeDollarIndex + 1 < line.length ? line[closeDollarIndex + 1] : '';

				let shouldSkipAsNonLatex = false;

				if (closeDollarIndex == currentPosition + 1) {
					// No content
					shouldSkipAsNonLatex = true;
				}

				if (LATEX_NEIGHBOR_CHAR_REGEXP.test(charBeforeOpen)) {
					// Character, digit, $, _ or - before first '$', no TeX.
					shouldSkipAsNonLatex = true;
				}

				if (
					LATEX_DIGIT_REGEXP.test(charAfterOpen) &&
					(LATEX_NEIGHBOR_CHAR_REGEXP.test(charAfterClose) || ' ' == charBeforeClose)
				) {
					// First $ seems to belong to an amount.
					shouldSkipAsNonLatex = true;
				}

				if (shouldSkipAsNonLatex) {
					processedLine += line.slice(currentPosition, openDollarIndex + 1);
					currentPosition = openDollarIndex + 1;

					continue;
				}

				// Treat as LaTeX
				processedLine += line.slice(currentPosition, openDollarIndex);
				const latexContent = line.slice(openDollarIndex, closeDollarIndex + 1);

				latexExpressions.push(latexContent);
				processedLine += `<<LATEX_${latexExpressions.length - 1}>>`;
				currentPosition = closeDollarIndex + 1;
			}

			return processedLine;
		})
		.join(NEWLINE);
}

function escapeBrackets(text: string): string {
	return text.replace(
		LATEX_MATH_AND_CODE_PATTERN,
		(
			match: string,
			codeBlock: string | undefined,
			squareBracket: string | undefined,
			roundBracket: string | undefined
		): string => {
			if (codeBlock != null) {
				return codeBlock;
			} else if (squareBracket != null) {
				return `${LATEX_DISPLAY_DELIMITER}${squareBracket}${LATEX_DISPLAY_DELIMITER}`;
			} else if (roundBracket != null) {
				return `${LATEX_INLINE_DELIMITER}${roundBracket}${LATEX_INLINE_DELIMITER}`;
			}

			return match;
		}
	);
}

// Escape $\\ce{...} → $\\ce{...} but with proper handling
function escapeMhchem(text: string): string {
	return MHCHEM_PATTERN_MAP.reduce((result, [pattern, replacement]) => {
		return result.replace(pattern, replacement);
	}, text);
}

const doEscapeMhchem = false;
/**
 * Preprocesses markdown content to safely handle LaTeX math expressions while protecting
 * against false positives (e.g., dollar amounts like $5.99) and ensuring proper rendering.
 *
 * This function:
 * - Protects code blocks (```) and inline code (`...`)
 * - Safeguards block and inline LaTeX: \(...\), \[...\], $$...$$, and selective $...$
 * - Escapes standalone dollar signs before numbers (e.g., $5 → \$5) to prevent misinterpretation
 * - Restores protected LaTeX and code blocks after processing
 * - Converts \(...\) → $...$ and \[...\] → $$...$$ for compatibility with math renderers
 * - Applies additional escaping for brackets and mhchem syntax if needed
 *
 * @param content - The raw text (e.g., markdown) that may contain LaTeX or code blocks.
 * @returns The preprocessed string with properly escaped and normalized LaTeX.
 *
 * @example
 * preprocessLaTeX("Price: $10. The equation is \\(x^2\\).")
 * // → "Price: $10. The equation is $x^2$."
 */
/** Bounded cache for preprocessLaTeX results. */
const LATEX_CACHE_MAX_SIZE = 64;
const latexCache = new Map<string, string>();

export function preprocessLaTeX(content: string): string {
	// See also:
	// https://github.com/danny-avila/LibreChat/blob/main/client/src/utils/latex.ts

	// Memoize on the input string. During streaming the prefix before an
	// incomplete code block stays the same across multiple tokens, so the
	// full protect/restore pipeline would re-run unnecessarily.
	const cached = latexCache.get(content);

	if (cached !== undefined) return cached;

	// Save original before the function mutates `content` through steps 0-8
	const originalContent = content;

	// Every step below keys off a `$` or a backslash escape (\[ \] \( \) \ce{ \pu{).
	// With neither present the protect/restore passes round-trip the input
	// unchanged, so skip them: the step 2 scan is O(n^2) in line length and costs
	// ~90ms on a 26KB single-line message that contains no math at all. This
	// matters during streaming, where the whole message is reprocessed per frame.
	if (!LATEX_TRIGGER_REGEXP.test(content)) {
		if (latexCache.size >= LATEX_CACHE_MAX_SIZE) {
			latexCache.delete(latexCache.keys().next().value!);
		}

		latexCache.set(originalContent, content);

		return content;
	}

	// Step 0: Temporarily remove blockquote markers (>) to process LaTeX correctly
	// Store the structure so we can restore it later
	const blockquoteMarkers: Map<number, string> = new Map();
	const lines = content.split(NEWLINE);
	const processedLines = lines.map((line, index) => {
		const match = line.match(LATEX_BLOCKQUOTE_PREFIX_REGEXP);

		if (match) {
			blockquoteMarkers.set(index, match[1]);

			return line.slice(match[1].length);
		}

		return line;
	});

	content = processedLines.join(NEWLINE);

	// Step 1: Protect code blocks
	const codeBlocks: string[] = [];

	content = content.replace(CODE_BLOCK_REGEXP, (match) => {
		codeBlocks.push(match);

		return `<<CODE_BLOCK_${codeBlocks.length - 1}>>`;
	});

	// Step 2: Protect existing LaTeX expressions
	const latexExpressions: string[] = [];

	// Match \S...\[...\] and protect them and insert a line-break.
	// Guarded: with no `\[` present this pattern still probes every start offset,
	// expanding `.*?` to the end of each line before failing - O(n^2) for nothing.
	if (content.includes(LATEX_DISPLAY_OPEN)) {
		content = content.replace(LATEX_DISPLAY_BLOCK_REGEXP, (match, group1, group2, group3) => {
			// Check if there are characters following the formula (display-formula in a table-cell?)
			if (group1.endsWith(LATEX_BACKSLASH)) {
				return match; // Backslash before \[, do nothing.
			}

			const hasSuffix = LATEX_NON_WHITESPACE_REGEXP.test(group3);

			let optBreak;

			if (hasSuffix) {
				latexExpressions.push(`${LATEX_INLINE_OPEN}${group2.trim()}${LATEX_INLINE_CLOSE}`); // Convert into inline.
				optBreak = '';
			} else {
				latexExpressions.push(`${LATEX_DISPLAY_OPEN}${group2}${LATEX_DISPLAY_CLOSE}`);
				optBreak = NEWLINE;
			}

			return `${group1}${optBreak}<<LATEX_${latexExpressions.length - 1}>>${optBreak}${group3}`;
		});
	}

	// Match \(...\), \[...\], $$...$$ and protect them
	content = content.replace(LATEX_PROTECT_REGEXP, (match) => {
		latexExpressions.push(match);

		return `<<LATEX_${latexExpressions.length - 1}>>`;
	});

	// Protect inline $...$ but NOT if it looks like money (e.g., $10, $3.99)
	content = maskInlineLaTeX(content, latexExpressions);

	// Step 3: Escape standalone $ before digits (currency like $5 → \$5)
	// (Now that inline math is protected, this will only escape dollars not already protected)
	content = content.replace(LATEX_CURRENCY_DOLLAR_REGEXP, LATEX_CURRENCY_ESCAPE);

	// Step 4: Restore protected LaTeX expressions (they are valid)
	content = content.replace(LATEX_PLACEHOLDER_REGEXP, (_, index) => {
		let expr = latexExpressions[parseInt(index)];

		const match = expr.match(LATEX_LINEBREAK_REGEXP);

		if (match) {
			// Katex: The $$-delimiters should be in their own line
			// if there are \\-line-breaks.
			const formula = match[1];
			const prefix = formula.startsWith(NEWLINE) ? '' : NEWLINE;
			const suffix = formula.endsWith(NEWLINE) ? '' : NEWLINE;

			expr = LATEX_DISPLAY_DELIMITER + prefix + formula + suffix + LATEX_DISPLAY_DELIMITER;
		}

		return expr;
	});

	// Step 5: Apply additional escaping functions (brackets and mhchem)
	// This must happen BEFORE restoring code blocks to avoid affecting code content
	content = escapeBrackets(content);

	if (doEscapeMhchem && (content.includes(LATEX_MHCHEM_CE) || content.includes(LATEX_MHCHEM_PU))) {
		content = escapeMhchem(content);
	}

	// Step 6: Convert remaining \(...\) → $...$, \[...\] → $$...$$
	// This must happen BEFORE restoring code blocks to avoid affecting code content
	content = content
		// Using the look‑behind pattern `(?<!\\)` we skip matches
		// that are preceded by a backslash, e.g.
		// `Definitions\\(also called macros)` (title of chapter 20 in The TeXbook).
		.replace(LATEX_INLINE_CONVERT_REGEXP, (_, formula: string) => {
			return `${LATEX_INLINE_DELIMITER}${formula}${LATEX_INLINE_DELIMITER}`;
		}) // inline
		.replace(
			// Using the look‑behind pattern `(?<!\\)` we skip matches
			// that are preceded by a backslash, e.g. `\\[4pt]`.
			LATEX_DISPLAY_CONVERT_REGEXP, // display, see also PR #16599
			(_, formula: string) => {
				return `${LATEX_DISPLAY_DELIMITER}${formula}${LATEX_DISPLAY_DELIMITER}`;
			}
		);

	// Step 7: Restore code blocks
	// This happens AFTER all LaTeX conversions to preserve code content
	content = content.replace(CODE_BLOCK_PLACEHOLDER_REGEXP, (_, index) => {
		return codeBlocks[parseInt(index)];
	});

	// Step 8: Restore blockquote markers
	if (blockquoteMarkers.size > 0) {
		const finalLines = content.split(NEWLINE);
		const restoredLines = finalLines.map((line, index) => {
			const marker = blockquoteMarkers.get(index);

			return marker ? marker + line : line;
		});

		content = restoredLines.join(NEWLINE);
	}

	if (latexCache.size >= LATEX_CACHE_MAX_SIZE) {
		latexCache.delete(latexCache.keys().next().value!);
	}

	latexCache.set(originalContent, content);

	return content;
}