From 0d99c33c4edbb712dbd0a04c244b6f483b93c197 Mon Sep 17 00:00:00 2001
From: nuno maduro <enunomaduro@gmail.com>
Date: Mon, 20 Apr 2026 13:16:59 -0700
Subject: [PATCH] wip

---
 src/Plugins/Tia.php                |  44 ++++++---
 src/Plugins/Tia/CoverageMerger.php | 149 +++++++++++++++++++++++++++++
 src/Support/Coverage.php           |   6 ++
 3 files changed, 187 insertions(+), 12 deletions(-)
 create mode 100644 src/Plugins/Tia/CoverageMerger.php

diff --git a/src/Plugins/Tia.php b/src/Plugins/Tia.php
index a2d6ea5b..348fa1e1 100644
--- a/src/Plugins/Tia.php
+++ b/src/Plugins/Tia.php
@@ -85,6 +85,21 @@ final class Tia implements AddsOutput, HandlesArguments, Terminable
 
     private const string AFFECTED_FILE = 'tia-affected.json';
 
+    /**
+     * Cache file holding PHPUnit's `CodeCoverage` object from the last
+     * `--tia --coverage` run. When the next run replays most tests from
+     * the TIA graph, only the affected tests produce fresh coverage; the
+     * rest is merged in from this cache so the report stays complete.
+     */
+    private const string COVERAGE_CACHE_FILE = 'tia-coverage.php';
+
+    /**
+     * Marker file dropped by `Tia` to tell `Support\Coverage` to apply the
+     * merge. Absent on plain `--coverage` runs so non-TIA usage keeps its
+     * current (narrow) behaviour.
+     */
+    private const string COVERAGE_MARKER_FILE = 'tia-coverage.marker';
+
     private const string WORKER_PREFIX = 'tia-worker-';
 
     private const string WORKER_RESULTS_PREFIX = 'tia-worker-results-';
@@ -196,6 +211,16 @@ final class Tia implements AddsOutput, HandlesArguments, Terminable
         return self::tempDir().DIRECTORY_SEPARATOR.self::WORKER_RESULTS_PREFIX.'*.json';
     }
 
+    public static function coverageCachePath(): string
+    {
+        return self::tempDir().DIRECTORY_SEPARATOR.self::COVERAGE_CACHE_FILE;
+    }
+
+    public static function coverageMarkerPath(): string
+    {
+        return self::tempDir().DIRECTORY_SEPARATOR.self::COVERAGE_MARKER_FILE;
+    }
+
     /**
      * True when TIA is piggybacking on PHPUnit's own coverage driver. Toggled
      * in `handleArguments` whenever `--tia` runs alongside `--coverage` so
@@ -505,20 +530,15 @@ final class Tia implements AddsOutput, HandlesArguments, Terminable
             }
         }
 
-        // Force record mode whenever `--coverage` is active. Replay short-
-        // circuits tests via cached results, which would make their code
-        // paths invisible to PHPUnit's coverage driver and tank the report.
-        // A `--tia --coverage` run is the one the user wants FULL coverage
-        // from — we just harvest graph edges alongside, to feed future
-        // `--tia` (no `--coverage`) runs.
-        if ($graph instanceof Graph && ! $this->piggybackCoverage) {
-            return $this->enterReplayMode($graph, $projectRoot, $arguments);
+        // Drop the marker so `Support\Coverage::report()` knows to merge the
+        // current (narrow) coverage with the cached full-run snapshot. Plain
+        // `--coverage` runs don't drop it, so their behaviour is untouched.
+        if ($this->piggybackCoverage) {
+            @file_put_contents(self::coverageMarkerPath(), '');
         }
 
-        if ($graph instanceof Graph && $this->piggybackCoverage) {
-            $this->output->writeln(
-                '  <fg=cyan>TIA</> `--coverage` active — running full suite and refreshing graph.',
-            );
+        if ($graph instanceof Graph) {
+            return $this->enterReplayMode($graph, $projectRoot, $arguments);
         }
 
         return $this->enterRecordMode($projectRoot, $arguments);
diff --git a/src/Plugins/Tia/CoverageMerger.php b/src/Plugins/Tia/CoverageMerger.php
new file mode 100644
index 00000000..3094c8bc
--- /dev/null
+++ b/src/Plugins/Tia/CoverageMerger.php
@@ -0,0 +1,149 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Pest\Plugins\Tia;
+
+use Pest\Plugins\Tia;
+use SebastianBergmann\CodeCoverage\CodeCoverage;
+use Throwable;
+
+/**
+ * Merges the current run's PHPUnit coverage into a cached full-suite
+ * snapshot so `--tia --coverage` can produce a complete report after
+ * executing only the affected tests.
+ *
+ * Invoked from `Pest\Support\Coverage::report()` right before the coverage
+ * file is consumed. A marker file dropped by the `Tia` plugin gates the
+ * behaviour — plain `--coverage` runs (no `--tia`) leave the marker absent
+ * and therefore keep their existing semantics.
+ *
+ * Algorithm
+ * ---------
+ * The PHPUnit coverage PHP file unserialises to a `CodeCoverage` object.
+ * Its `ProcessedCodeCoverageData` stores, per source file, per line, the
+ * list of test IDs that covered that line. We:
+ *
+ *   1. Load the cached snapshot (from a previous `--tia --coverage` run).
+ *   2. Strip every test id that re-ran this time from the cached map —
+ *      the tests that ran now are the ones whose attribution is fresh.
+ *   3. Merge the current run into the stripped cached snapshot via
+ *      `CodeCoverage::merge()`.
+ *   4. Write the merged result back to the report path (so Pest's report
+ *      generator sees the full suite) and to the cache path (for the
+ *      next invocation).
+ *   5. Remove the marker so subsequent plain `--coverage` runs are
+ *      untouched.
+ *
+ * If no cache exists yet (first `--tia --coverage` run on this machine)
+ * we simply save the current file as the cache — nothing to merge yet.
+ *
+ * @internal
+ */
+final class CoverageMerger
+{
+    public static function applyIfMarked(string $reportPath): void
+    {
+        $markerPath = Tia::coverageMarkerPath();
+
+        if (! is_file($markerPath)) {
+            return;
+        }
+
+        @unlink($markerPath);
+
+        $cachePath = Tia::coverageCachePath();
+
+        if (! is_file($cachePath)) {
+            // First `--tia --coverage` run: nothing cached yet, the current
+            // report is the full suite itself. Save it verbatim so the next
+            // run has a snapshot to merge against.
+            @copy($reportPath, $cachePath);
+
+            return;
+        }
+
+        try {
+            /** @var CodeCoverage $cached */
+            $cached = require $cachePath;
+
+            /** @var CodeCoverage $current */
+            $current = require $reportPath;
+        } catch (Throwable) {
+            // Corrupt cache or unreadable report — fall back to the plain
+            // PHPUnit behaviour (the existing `require $reportPath` in the
+            // caller still runs against the untouched file).
+            return;
+        }
+
+        self::stripCurrentTestsFromCached($cached, $current);
+
+        $cached->merge($current);
+
+        // Serialise the merged object back using PHPUnit's own "return
+        // expression" PHP format. Using `var_export` on the serialised
+        // payload keeps the file self-contained and independent of
+        // PHPUnit's internal exporter — the reader only needs to
+        // `require` it back.
+        $serialised = "<?php return unserialize(".var_export(serialize($cached), true).");\n";
+
+        @file_put_contents($reportPath, $serialised);
+        @file_put_contents($cachePath, $serialised);
+    }
+
+    /**
+     * Removes from `$cached`'s per-line test attribution any test id that
+     * appears in `$current`. Those tests just ran, so the fresh slice is
+     * authoritative — keeping stale attribution in the cache would claim
+     * a test still covers a line it no longer touches.
+     */
+    private static function stripCurrentTestsFromCached(CodeCoverage $cached, CodeCoverage $current): void
+    {
+        $currentIds = self::collectTestIds($current);
+
+        if ($currentIds === []) {
+            return;
+        }
+
+        $cachedData = $cached->getData();
+        $lineCoverage = $cachedData->lineCoverage();
+
+        foreach ($lineCoverage as $file => $lines) {
+            foreach ($lines as $line => $ids) {
+                if ($ids === null || $ids === []) {
+                    continue;
+                }
+
+                $filtered = array_values(array_diff($ids, $currentIds));
+
+                if ($filtered !== $ids) {
+                    $lineCoverage[$file][$line] = $filtered;
+                }
+            }
+        }
+
+        $cachedData->setLineCoverage($lineCoverage);
+    }
+
+    /**
+     * @return array<int, string>
+     */
+    private static function collectTestIds(CodeCoverage $coverage): array
+    {
+        $ids = [];
+
+        foreach ($coverage->getData()->lineCoverage() as $lines) {
+            foreach ($lines as $hits) {
+                if ($hits === null) {
+                    continue;
+                }
+
+                foreach ($hits as $id) {
+                    $ids[$id] = true;
+                }
+            }
+        }
+
+        return array_keys($ids);
+    }
+}
diff --git a/src/Support/Coverage.php b/src/Support/Coverage.php
index a36dd8ed..6845ad4e 100644
--- a/src/Support/Coverage.php
+++ b/src/Support/Coverage.php
@@ -88,6 +88,12 @@ final class Coverage
             throw ShouldNotHappen::fromMessage(sprintf('Coverage not found in path: %s.', $reportPath));
         }
 
+        // If TIA's marker is present, this run executed only the affected
+        // tests. Merge their fresh coverage slice into the cached full-run
+        // snapshot (stored by the previous `--tia --coverage` pass) so the
+        // report reflects the entire suite, not just what re-ran.
+        \Pest\Plugins\Tia\CoverageMerger::applyIfMarked($reportPath);
+
         /** @var CodeCoverage $codeCoverage */
         $codeCoverage = require $reportPath;
         unlink($reportPath);