feat: modularize flattener tool into separate components with improved project root detection (#417)

tools/flattener/discovery.js

@@ -0,0 +1,70 @@
+const path = require("node:path");
+const { execFile } = require("node:child_process");
+const { promisify } = require("node:util");
+const { glob } = require("glob");
+const { loadIgnore } = require("./ignoreRules.js");
+
+const pExecFile = promisify(execFile);
+
+async function isGitRepo(rootDir) {
+  try {
+    const { stdout } = await pExecFile("git", [
+      "rev-parse",
+      "--is-inside-work-tree",
+    ], { cwd: rootDir });
+    return String(stdout || "").toString().trim() === "true";
+  } catch {
+    return false;
+  }
+}
+
+async function gitListFiles(rootDir) {
+  try {
+    const { stdout } = await pExecFile("git", [
+      "ls-files",
+      "-co",
+      "--exclude-standard",
+    ], { cwd: rootDir });
+    return String(stdout || "")
+      .split(/\r?\n/)
+      .map((s) => s.trim())
+      .filter(Boolean);
+  } catch {
+    return [];
+  }
+}
+
+/**
+ * Discover files under rootDir.
+ * - Prefer git ls-files when available for speed/correctness
+ * - Fallback to glob and apply unified ignore rules
+ * @param {string} rootDir
+ * @param {object} [options]
+ * @param {boolean} [options.preferGit=true]
+ * @returns {Promise<string[]>} absolute file paths
+ */
+async function discoverFiles(rootDir, options = {}) {
+  const { preferGit = true } = options;
+  const { filter } = await loadIgnore(rootDir);
+
+  // Try git first
+  if (preferGit && await isGitRepo(rootDir)) {
+    const relFiles = await gitListFiles(rootDir);
+    const filteredRel = relFiles.filter((p) => filter(p));
+    return filteredRel.map((p) => path.resolve(rootDir, p));
+  }
+
+  // Glob fallback
+  const globbed = await glob("**/*", {
+    cwd: rootDir,
+    nodir: true,
+    dot: true,
+    follow: false,
+  });
+  const filteredRel = globbed.filter((p) => filter(p));
+  return filteredRel.map((p) => path.resolve(rootDir, p));
+}
+
+module.exports = {
+  discoverFiles,
+};