diff --git a/src/util/file-tree-spec.ts b/src/util/file-tree-spec.ts
new file mode 100644
index 0000000..36bfb9e
--- /dev/null
+++ b/src/util/file-tree-spec.ts
@@ -0,0 +1,93 @@
+import assert from 'assert'
+import { promises as fs } from 'fs'
+import hasha from 'hasha'
+import path from 'path'
+import YAML from 'yaml'
+
+import { yamlStringifyNoFold } from './yaml'
+
+// FileTreeSpec is a data structure for verifying and comparing all files in a directory.
+// FileTreeSpec maps file path to:
+// - SHA-256 hash if file is a regular file
+// - DIR_SPEC_PLACEHOLDER value if file is a directory
+//
+// Other file types are not supported.
+export type FileTreeSpec = Map<string, string>
+
+export const DIR_SPEC_PLACEHOLDER = 'dir'
+
+export async function getFileTreeSpec(dir: string) {
+  let res: FileTreeSpec = new Map<string, string>()
+  await getFileTreeSpecStep(dir, dir, res)
+  return res
+}
+
+export function fileTreeSpecToYaml(fth: FileTreeSpec) {
+  return yamlStringifyNoFold(new Map([...fth.entries()].sort()))
+}
+
+export function parseFileTreeSpecYaml(yamlStr: string) {
+  return YAML.parse(yamlStr, { mapAsMap: true }) as FileTreeSpec
+}
+
+async function getFileTreeSpecStep(baseDir: string, dir: string, dst: FileTreeSpec) {
+  let dirents = await fs.readdir(dir, { withFileTypes: true })
+  let promises: Promise<void>[] = []
+  for (const dirent of dirents) {
+    let dePath = path.join(dir, dirent.name)
+    if (dirent.isDirectory()) {
+      dst.set(path.relative(baseDir, dePath), DIR_SPEC_PLACEHOLDER)
+      promises.push(getFileTreeSpecStep(baseDir, dePath, dst))
+    } else if (dirent.isFile()) {
+      promises.push(hashFile(baseDir, dePath, dst))
+    } else {
+      throw new Error('unexpected Dirent type ' + dirent)
+    }
+  }
+
+  await Promise.all(promises)
+}
+
+async function hashFile(baseDir: string, filePath: string, dst: FileTreeSpec) {
+  let hash = await hasha.fromFile(filePath, { algorithm: 'sha256' })
+  dst.set(path.relative(baseDir, filePath), hash)
+}
+
+export class FileTreeComparison {
+  constructor(readonly a: FileTreeSpec, readonly b: FileTreeSpec) {}
+
+  // present in B, but missing in A
+  readonly newEntries = new Map<string, string>()
+  // present in A, but missing in B
+  readonly missingEntries = new Map<string, string>()
+  readonly changedEntries: string[] = []
+
+  numDiffs() {
+    return this.newEntries.size + this.missingEntries.size + this.changedEntries.length
+  }
+
+  static async get(a: FileTreeSpec, b: FileTreeSpec) {
+    let allPaths = new Set([...a.keys(), ...b.keys()])
+
+    let res = new FileTreeComparison(a, b)
+
+    for (let entry of allPaths) {
+      let valA = a.get(entry)
+      let valB = b.get(entry)
+
+      if (valA === valB) {
+        continue
+      }
+      if (valA === undefined) {
+        assert(valB !== undefined)
+        res.newEntries.set(entry, valB)
+      } else if (valB === undefined) {
+        res.missingEntries.set(entry, valA)
+      } else {
+        res.changedEntries.push(entry)
+      }
+    }
+
+    return res
+  }
+}