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diff --git a/tests/run-coverage-file.el b/tests/run-coverage-file.el
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+;;; run-coverage-file.el --- Undercover setup for per-file coverage runs -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Loaded by `make coverage' before each test file runs, BEFORE
+;; pearl.el is loaded.  Instrumenting must happen first so the
+;; subsequent load picks up the instrumented source.
+;;
+;; Coverage data is merged across per-file invocations into a single
+;; simplecov JSON at .coverage/simplecov.json (under the project root).
+
+;;; Code:
+
+(unless (require 'undercover nil t)
+  (message "")
+  (message "ERROR: undercover not installed.")
+  (message "Run 'make setup' to install development dependencies.")
+  (message "")
+  (kill-emacs 1))
+
+;; Resolve project root from this file's location so undercover patterns
+;; and the report-file path don't depend on default-directory at load time.
+(defvar run-coverage--project-root
+  (file-name-directory
+   (directory-file-name
+    (file-name-directory (or load-file-name buffer-file-name))))
+  "Absolute path to the pearl project root.")
+
+;; Force coverage collection in non-CI environments.  Must be set after
+;; loading undercover because the library's top-level form
+;; `(setq undercover-force-coverage (getenv "UNDERCOVER_FORCE"))' would
+;; otherwise overwrite the value.
+(setq undercover-force-coverage t)
+
+;; Local runs emit simplecov for whatever local tooling wants it.  CI sets
+;; CI=true (GitHub Actions does this automatically), so we emit a coveralls
+;; JSON instead and leave it on disk for the upload action to pick up.
+;; The `undercover' macro splices each configuration list into `(list ,@it)',
+;; which evaluates the elements.  Wildcard strings have to stay atoms — using
+;; `(:files ...)' form lets us evaluate `expand-file-name' to an absolute path.
+(undercover (:files (expand-file-name "pearl.el" run-coverage--project-root))
+            (:report-format (if (getenv "CI") 'coveralls 'simplecov))
+            (:report-file (expand-file-name
+                           (if (getenv "CI")
+                               ".coverage/coveralls.json"
+                             ".coverage/simplecov.json")
+                           run-coverage--project-root))
+            (:merge-report t)
+            (:send-report nil))
+
+;;; run-coverage-file.el ends here