Fix/delta_t should be non-negative in history (#121)

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "FSRS-Optimizer"
-version = "5.0.0"
+version = "5.0.1"
 readme = "README.md"
 dependencies = [
     "matplotlib>=3.7.0",

src/fsrs_optimizer/fsrs_optimizer.py

@@ -691,7 +691,7 @@ def cum_concat(x):
             return list(accumulate(x))
 
         t_history_list = df.groupby("card_id", group_keys=False)["delta_t"].apply(
-            lambda x: cum_concat([[int(i)] for i in x])
+            lambda x: cum_concat([[int(max(0, i))] for i in x])
         )
         df["t_history"] = [
             ",".join(map(str, item[:-1]))
@@ -1227,7 +1227,9 @@ def preview_sequence(self, test_rating_sequence: str, requestRetention: float):
                     (
                         f"{ivl}d"
                         if ivl < 30
-                        else f"{ivl / 30:.1f}m" if ivl < 365 else f"{ivl / 365:.1f}y"
+                        else f"{ivl / 30:.1f}m"
+                        if ivl < 365
+                        else f"{ivl / 365:.1f}y"
                     )
                     for ivl in map(int, t_history.split(","))
                 ]