web/src/components/Markdown.tsx

import { useMemo } from "react";

/**
 * Lightweight markdown renderer for LLM output.
 * Handles: code blocks, inline code, bold, italic, headers, links, lists, horizontal rules.
 * NOT a full CommonMark parser — optimized for typical assistant message patterns.
 */
export function Markdown({ content, highlightTerms }: { content: string; highlightTerms?: string[] }) {
  const blocks = useMemo(() => parseBlocks(content), [content]);

  return (
    <div className="text-sm text-foreground leading-relaxed space-y-2">
      {blocks.map((block, i) => (
        <Block key={i} block={block} highlightTerms={highlightTerms} />
      ))}
    </div>
  );
}

/* ------------------------------------------------------------------ */
/*  Types                                                              */
/* ------------------------------------------------------------------ */

type BlockNode =
  | { type: "code"; lang: string; content: string }
  | { type: "heading"; level: number; content: string }
  | { type: "hr" }
  | { type: "list"; ordered: boolean; items: string[] }
  | { type: "paragraph"; content: string };

/* ------------------------------------------------------------------ */
/*  Block parser                                                       */
/* ------------------------------------------------------------------ */

function parseBlocks(text: string): BlockNode[] {
  const lines = text.split("\n");
  const blocks: BlockNode[] = [];
  let i = 0;

  while (i < lines.length) {
    const line = lines[i];

    // Fenced code block
    const fenceMatch = line.match(/^```(\w*)/);
    if (fenceMatch) {
      const lang = fenceMatch[1] || "";
      const codeLines: string[] = [];
      i++;
      while (i < lines.length && !lines[i].startsWith("```")) {
        codeLines.push(lines[i]);
        i++;
      }
      i++; // skip closing ```
      blocks.push({ type: "code", lang, content: codeLines.join("\n") });
      continue;
    }

    // Heading
    const headingMatch = line.match(/^(#{1,4})\s+(.+)/);
    if (headingMatch) {
      blocks.push({ type: "heading", level: headingMatch[1].length, content: headingMatch[2] });
      i++;
      continue;
    }

    // Horizontal rule
    if (/^[-*_]{3,}\s*$/.test(line)) {
      blocks.push({ type: "hr" });
      i++;
      continue;
    }

    // Unordered list
    if (/^[-*+]\s/.test(line)) {
      const items: string[] = [];
      while (i < lines.length && /^[-*+]\s/.test(lines[i])) {
        items.push(lines[i].replace(/^[-*+]\s/, ""));
        i++;
      }
      blocks.push({ type: "list", ordered: false, items });
      continue;
    }

    // Ordered list
    if (/^\d+[.)]\s/.test(line)) {
      const items: string[] = [];
      while (i < lines.length && /^\d+[.)]\s/.test(lines[i])) {
        items.push(lines[i].replace(/^\d+[.)]\s/, ""));
        i++;
      }
      blocks.push({ type: "list", ordered: true, items });
      continue;
    }

    // Empty line
    if (line.trim() === "") {
      i++;
      continue;
    }

    // Paragraph — collect consecutive non-empty, non-special lines
    const paraLines: string[] = [];
    while (
      i < lines.length &&
      lines[i].trim() !== "" &&
      !lines[i].match(/^```/) &&
      !lines[i].match(/^#{1,4}\s/) &&
      !lines[i].match(/^[-*+]\s/) &&
      !lines[i].match(/^\d+[.)]\s/) &&
      !lines[i].match(/^[-*_]{3,}\s*$/)
    ) {
      paraLines.push(lines[i]);
      i++;
    }
    if (paraLines.length > 0) {
      blocks.push({ type: "paragraph", content: paraLines.join("\n") });
    }
  }

  return blocks;
}

/* ------------------------------------------------------------------ */
/*  Block renderer                                                     */
/* ------------------------------------------------------------------ */

function Block({ block, highlightTerms }: { block: BlockNode; highlightTerms?: string[] }) {
  switch (block.type) {
    case "code":
      return (
        <pre className="bg-secondary/60 border border-border px-3 py-2.5 text-xs font-mono leading-relaxed overflow-x-auto">
          <code>{block.content}</code>
        </pre>
      );

    case "heading": {
      const Tag = `h${Math.min(block.level, 4)}` as "h1" | "h2" | "h3" | "h4";
      const sizes: Record<string, string> = {
        h1: "text-base font-bold",
        h2: "text-sm font-bold",
        h3: "text-sm font-semibold",
        h4: "text-sm font-medium",
      };
      return <Tag className={sizes[Tag]}><InlineContent text={block.content} highlightTerms={highlightTerms} /></Tag>;
    }

    case "hr":
      return <hr className="border-border" />;

    case "list": {
      const Tag = block.ordered ? "ol" : "ul";
      return (
        <Tag className={`space-y-0.5 ${block.ordered ? "list-decimal" : "list-disc"} pl-5 text-sm`}>
          {block.items.map((item, i) => (
            <li key={i}><InlineContent text={item} highlightTerms={highlightTerms} /></li>
          ))}
        </Tag>
      );
    }

    case "paragraph":
      return <p><InlineContent text={block.content} highlightTerms={highlightTerms} /></p>;
  }
}

/* ------------------------------------------------------------------ */
/*  Inline parser + renderer                                           */
/* ------------------------------------------------------------------ */

type InlineNode =
  | { type: "text"; content: string }
  | { type: "code"; content: string }
  | { type: "bold"; content: string }
  | { type: "italic"; content: string }
  | { type: "link"; text: string; href: string }
  | { type: "br" };

function parseInline(text: string): InlineNode[] {
  const nodes: InlineNode[] = [];
  // Pattern priority: code > link > bold > italic > bare URL > line break
  const pattern = /(`[^`]+`)|(\[([^\]]+)\]\(([^)]+)\))|(\*\*([^*]+)\*\*)|(\*([^*]+)\*)|(\bhttps?:\/\/[^\s<>)\]]+)|(\n)/g;
  let lastIndex = 0;
  let match: RegExpExecArray | null;

  while ((match = pattern.exec(text)) !== null) {
    if (match.index > lastIndex) {
      nodes.push({ type: "text", content: text.slice(lastIndex, match.index) });
    }

    if (match[1]) {
      // Inline code
      nodes.push({ type: "code", content: match[1].slice(1, -1) });
    } else if (match[2]) {
      // [text](url) link
      nodes.push({ type: "link", text: match[3], href: match[4] });
    } else if (match[5]) {
      // **bold**
      nodes.push({ type: "bold", content: match[6] });
    } else if (match[7]) {
      // *italic*
      nodes.push({ type: "italic", content: match[8] });
    } else if (match[9]) {
      // Bare URL
      nodes.push({ type: "link", text: match[9], href: match[9] });
    } else if (match[10]) {
      // Line break within paragraph
      nodes.push({ type: "br" });
    }

    lastIndex = match.index + match[0].length;
  }

  if (lastIndex < text.length) {
    nodes.push({ type: "text", content: text.slice(lastIndex) });
  }

  return nodes;
}

function InlineContent({ text, highlightTerms }: { text: string; highlightTerms?: string[] }) {
  const nodes = useMemo(() => parseInline(text), [text]);

  return (
    <>
      {nodes.map((node, i) => {
        switch (node.type) {
          case "text":
            return <HighlightedText key={i} text={node.content} terms={highlightTerms} />;
          case "code":
            return (
              <code key={i} className="bg-secondary/60 px-1.5 py-0.5 text-xs font-mono text-primary/90">
                {node.content}
              </code>
            );
          case "bold":
            return <strong key={i} className="font-semibold"><HighlightedText text={node.content} terms={highlightTerms} /></strong>;
          case "italic":
            return <em key={i}><HighlightedText text={node.content} terms={highlightTerms} /></em>;
          case "link":
            return (
              <a
                key={i}
                href={node.href}
                target="_blank"
                rel="noreferrer"
                className="text-primary underline underline-offset-2 decoration-primary/30 hover:decoration-primary/60 transition-colors"
              >
                {node.text}
              </a>
            );
          case "br":
            return <br key={i} />;
        }
      })}
    </>
  );
}

/** Highlight search terms within a plain text string. */
function HighlightedText({ text, terms }: { text: string; terms?: string[] }) {
  if (!terms || terms.length === 0) return <>{text}</>;

  // Build a regex that matches any of the search terms (case-insensitive)
  const escaped = terms.map((t) => t.replace(/[.*+?^${}()|[\]\\]/g, "\\$&"));
  const regex = new RegExp(`(${escaped.join("|")})`, "gi");
  const parts = text.split(regex);

  return (
    <>
      {parts.map((part, i) =>
        regex.test(part) ? (
          <mark key={i} className="bg-warning/30 text-warning px-0.5">{part}</mark>
        ) : (
          <span key={i}>{part}</span>
        )
      )}
    </>
  );
}