feat(frontend): in-book text search core (TDD)

frontend/src/lib/search.ts

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+import { chapterText, type TxtChapter } from "./chapters";
+
+export interface SearchHit {
+  ch: number;
+  /** 命中在章内文本的起点 */
+  pos: number;
+  snippet: string;
+  matchStart: number; // snippet 内
+  matchLen: number;
+}
+
+const CTX = 20; // snippet 前后文
+
+/** 全书线性扫描（文本已分章在内存,量级足够;大小写不敏感,不折叠全半角）。 */
+export function searchChapters(text: string, chapters: TxtChapter[], q: string, limit = 500): SearchHit[] {
+  const needle = q.trim().toLowerCase();
+  if (!needle) return [];
+  const bodies = chapters.map((_, ci) => chapterText(text, chapters, ci).toLowerCase());
+  const hits: SearchHit[] = [];
+  for (let ci = 0; ci < chapters.length && hits.length < limit; ci++) {
+    const body = bodies[ci];
+    let from = 0;
+    for (;;) {
+      const at = body.indexOf(needle, from);
+      if (at < 0 || hits.length >= limit) break;
+      const raw = chapterText(text, chapters, ci);
+      const s = Math.max(0, at - CTX);
+      const e = Math.min(raw.length, at + needle.length + CTX);
+      hits.push({
+        ch: ci,
+        pos: at,
+        snippet: (s > 0 ? "…" : "") + raw.slice(s, e) + (e < raw.length ? "…" : ""),
+        matchStart: at - s + (s > 0 ? 1 : 0),
+        matchLen: needle.length,
+      });
+      from = at + needle.length;
+    }
+  }
+  return hits;
+}
+
+export function splitHighlight(snippet: string, matchStart: number, matchLen: number) {
+  return {
+    before: snippet.slice(0, matchStart),
+    match: snippet.slice(matchStart, matchStart + matchLen),
+    after: snippet.slice(matchStart + matchLen),
+  };
+}

frontend/test/search.test.ts

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+import { describe, expect, it } from "vitest";
+import { searchChapters, splitHighlight } from "../src/lib/search";
+import { splitChapters } from "../src/lib/chapters";
+
+const TEXT = ["第一章 起风", "风来了又走。", "第二章 落雨", "雨点敲窗，风声相伴。", "风止"].join("\n");
+
+describe("searchChapters", () => {
+  const chs = splitChapters(TEXT);
+  it("finds case-insensitive hits with chapter index and position", () => {
+    const hits = searchChapters(TEXT, chs, "风");
+    expect(hits.length).toBeGreaterThanOrEqual(3);
+    expect(hits[0].ch).toBe(0);
+    expect(hits.every((h) => h.snippet.includes("风"))).toBe(true);
+  });
+  it("returns empty for no match / empty query", () => {
+    expect(searchChapters(TEXT, chs, "不存在")).toEqual([]);
+    expect(searchChapters(TEXT, chs, "  ")).toEqual([]);
+  });
+  it("caps at limit", () => {
+    expect(searchChapters(TEXT, chs, "风", 2)).toHaveLength(2);
+  });
+});
+
+describe("splitHighlight", () => {
+  it("splits snippet into before/match/after", () => {
+    expect(splitHighlight("abc风def", 3, 1)).toEqual({ before: "abc", match: "风", after: "def" });
+  });
+});