2 files changed, 85 insertions, 0 deletions

diff --git a/internal/journal/read.go b/internal/journal/read.go
index 3de15d2..de06422 100644
--- a/internal/journal/read.go
+++ b/internal/journal/read.go
@@ -43,6 +43,42 @@ type Run struct {
 	Dirs   []string
 	Counts map[string]int // action -> count of status "ok"
 	Undone bool           // a later run reversed this one
+	UndoOf string         // for an undo run, the run it reverses; "" otherwise
+}
+
+// ReversedKey identifies one reversal an undo run carried out: the file's
+// directory and name, the undo action and the path it started from - enough
+// to tell which step of the original run it reversed.
+type ReversedKey struct {
+	Dir, File, Action, Src string
+}
+
+// ReversedSteps counts, for runID, every reversal that earlier undo runs of
+// it completed ("ok" undo- entries of runs whose run-start says they undo
+// runID), so a later undo of the same run can offer only what is left
+// (review M10). An undo run's own unparsable lines are skipped; a missing
+// reversal is then offered again, where its own checks refuse it if it had
+// in fact happened.
+func ReversedSteps(path, runID string) (map[ReversedKey]int, error) {
+	lines, err := readLines(path)
+	if err != nil {
+		return nil, err
+	}
+	undoRuns := map[string]bool{}
+	for _, line := range lines {
+		if e, ok := parseLine(line); ok && e.Action == "run-start" && e.Detail == UndoOf(runID) {
+			undoRuns[e.Run] = true
+		}
+	}
+	out := map[ReversedKey]int{}
+	for _, line := range lines {
+		e, ok := parseLine(line)
+		if !ok || !undoRuns[e.Run] || e.Status != "ok" || !strings.HasPrefix(e.Action, "undo-") {
+			continue
+		}
+		out[ReversedKey{Dir: e.Dir, File: e.File, Action: e.Action, Src: e.Src}]++
+	}
+	return out, nil
 }
 
 // Entries returns every entry belonging to runID, in file order. A line
@@ -198,6 +234,7 @@ func Runs(path string, n int) ([]Run, error) {
 	sort.SliceStable(runs, func(i, j int) bool { return runs[i].Start.After(runs[j].Start) })
 	for i := range runs {
 		runs[i].Undone = undoes[runs[i].ID]
+		runs[i].UndoOf = pendingUndo[runs[i].ID]
 	}
 
 	if n > 0 && n < len(runs) {
diff --git a/internal/journal/read_test.go b/internal/journal/read_test.go
index 2f0f40f..2b46bfb 100644
--- a/internal/journal/read_test.go
+++ b/internal/journal/read_test.go
@@ -625,3 +625,51 @@ func TestEntriesFailsClosedOnMissingRunStart(t *testing.T) {
 		t.Errorf("entries = %+v", got)
 	}
 }
+
+// TestReversedStepsCountsEveryUndoOfARun: the steps earlier undo runs of a
+// run already reversed - only "ok" undo entries of runs that undo it - so
+// a later undo of the same run can offer just what is left (review M10).
+// Runs also names the run an undo run reversed.
+func TestReversedStepsCountsEveryUndoOfARun(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "krino.log")
+	w, err := Open(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	at := time.Date(2026, 9, 11, 9, 0, 0, 0, time.UTC)
+	for _, e := range []Entry{
+		{Time: at, Run: "R", Action: "run-start", Status: "ok"},
+		{Time: at, Run: "R", Dir: "dl", File: "a.pdf", Step: 1, Action: "move", Status: "ok", Src: "/dl/a.pdf", Dst: "/w/a.pdf"},
+		{Time: at, Run: "R", Action: "run-end", Status: "ok"},
+		{Time: at.Add(time.Minute), Run: "U1", Action: "run-start", Status: "ok", Detail: UndoOf("R")},
+		{Time: at.Add(time.Minute), Run: "U1", Dir: "dl", File: "a.pdf", Step: 1, Action: "undo-move", Status: "ok", Src: "/w/a.pdf", Dst: "/dl/a.pdf"},
+		{Time: at.Add(time.Minute), Run: "U1", Dir: "dl", File: "b.pdf", Step: 1, Action: "undo-rename", Status: "failed", Src: "/dl/r-b.pdf", Dst: "/dl/b.pdf"},
+		{Time: at.Add(time.Minute), Run: "U1", Action: "run-end", Status: "ok"},
+		{Time: at.Add(2 * time.Minute), Run: "U2", Action: "run-start", Status: "ok", Detail: UndoOf("OTHER")},
+		{Time: at.Add(2 * time.Minute), Run: "U2", Dir: "dl", File: "a.pdf", Step: 1, Action: "undo-move", Status: "ok", Src: "/w/a.pdf", Dst: "/dl/a.pdf"},
+		{Time: at.Add(2 * time.Minute), Run: "U2", Action: "run-end", Status: "ok"},
+	} {
+		if err := w.Append(e); err != nil {
+			t.Fatal(err)
+		}
+	}
+	w.Close()
+	got, err := ReversedSteps(path, "R")
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := map[ReversedKey]int{{Dir: "dl", File: "a.pdf", Action: "undo-move", Src: "/w/a.pdf"}: 1}
+	if len(got) != len(want) || got[ReversedKey{Dir: "dl", File: "a.pdf", Action: "undo-move", Src: "/w/a.pdf"}] != 1 {
+		t.Errorf("ReversedSteps = %v, want %v", got, want)
+	}
+	runs, err := Runs(path, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, r := range runs {
+		want := map[string]string{"R": "", "U1": "R", "U2": "OTHER"}[r.ID]
+		if r.UndoOf != want {
+			t.Errorf("run %s: UndoOf = %q, want %q", r.ID, r.UndoOf, want)
+		}
+	}
+}