diff --git a/csrc/faster_eval_api/faster_eval_api.cpp b/csrc/faster_eval_api/faster_eval_api.cpp
index 9700e36..4173dfd 100644
--- a/csrc/faster_eval_api/faster_eval_api.cpp
+++ b/csrc/faster_eval_api/faster_eval_api.cpp
@@ -54,6 +54,7 @@ namespace coco_eval
 
     m.def("deepcopy", &COCOeval::deepcopy, "COCOeval::deepcopy");
     m.def("_summarize", &COCOeval::_summarize, "COCOeval::_summarize");
+    m.def("calc_auc", &COCOeval::calc_auc, "COCOeval::calc_auc");
 
     pybind11::class_<COCOeval::Dataset>(m, "Dataset").def(pybind11::init<>())
     .def("append", &COCOeval::Dataset::append)
diff --git a/faster_coco_eval/core/faster_eval_api.py b/faster_coco_eval/core/faster_eval_api.py
index 44bb700..e23ddf3 100644
--- a/faster_coco_eval/core/faster_eval_api.py
+++ b/faster_coco_eval/core/faster_eval_api.py
@@ -3,6 +3,7 @@
 
 import logging
 import time
+from typing import List, Union
 
 import numpy as np
 
@@ -139,8 +140,10 @@ def accumulate(self):
     def math_matches(self):
         """For each ground truth, find the best matching detection and set the
         detection as matched."""
-        for gidx, ground_truth_matches in enumerate(self.ground_truth_matches):
-            gt_ids = self.ground_truth_orig_id[gidx]
+        for gidx, ground_truth_matches in enumerate(
+            self.ground_truth_matches[::-1]
+        ):
+            gt_ids = self.ground_truth_orig_id[-gidx - 1]
 
             for idx, dt_id in enumerate(ground_truth_matches):
                 if dt_id == 0:
@@ -152,6 +155,11 @@ def math_matches(self):
 
                 _gt_ann = self.cocoGt.anns[gt_id]
                 _dt_ann = self.cocoDt.anns[dt_id]
+
+                if self.params.useCats:
+                    if _gt_ann["category_id"] != _dt_ann["category_id"]:
+                        continue
+
                 _img_id = self.cocoGt.ann_img_map[gt_id]
                 _catId = _gt_ann["category_id"] if self.params.useCats else -1
 
@@ -173,6 +181,8 @@ def math_matches(self):
                     continue
 
                 iou = self.ious[(_img_id, _catId)][iou_dt_id, iou_gt_id]
+                if (iou <= 0) and self.params.useCats:
+                    continue
 
                 if not _gt_ann.get("matched", False):
                     _dt_ann["tp"] = True
@@ -211,8 +221,6 @@ def compute_mIoU(self) -> float:
                 ious.append(dt_ann["iou"])
         return sum(ious) / len(ious)
 
-    # TODO:
-    # Convert to C++
     def compute_mAUC(self) -> float:
         """Compute the mAUC metric."""
         aucs = []
@@ -290,26 +298,37 @@ def stats_as_dict(self):
             _label: float(self.all_stats[i]) for i, _label in enumerate(labels)
         }
 
-    # TODO:
-    # Convert to C++
     @staticmethod
-    def calc_auc(recall_list, precision_list):
-        """
-        Calculate area under precision recall curve
-        recall_list: list of recall values
-        precision_list: list of precision values
+    def calc_auc(
+        recall_list: Union[List[float], np.ndarray],
+        precision_list: Union[List[float], np.ndarray],
+        method: str = "c++",
+    ):
+        """Calculate area under precision recall curve.
+
+        Args:
+            recall_list (Union[List[float], np.ndarray]):
+                list of recall values
+            precision_list (Union[List[float], np.ndarray]):
+                list of precision values
+            method (str, optional): method to calculate auc. Defaults to "c++".
+
+        Returns:
+            float: area under precision recall curve
+
         """
         # https://towardsdatascience.com/how-to-efficiently-implement-area-under-precision-recall-curve-pr-auc-a85872fd7f14
-        # mrec = np.concatenate(([0.], recall_list, [1.]))
-        # mpre = np.concatenate(([0.], precision_list, [0.]))
-        mrec = recall_list
-        mpre = precision_list
+        if method == "c++":
+            return round(_C.calc_auc(recall_list, precision_list), 15)
+        else:
+            mrec = recall_list
+            mpre = precision_list
 
-        for i in range(mpre.size - 1, 0, -1):
-            mpre[i - 1] = np.maximum(mpre[i - 1], mpre[i])
+            for i in range(mpre.size - 1, 0, -1):
+                mpre[i - 1] = np.maximum(mpre[i - 1], mpre[i])
 
-        i = np.where(mrec[1:] != mrec[:-1])[0]
-        return np.sum((mrec[i + 1] - mrec[i]) * mpre[i + 1])
+            i = np.where(mrec[1:] != mrec[:-1])[0]
+            return np.sum((mrec[i + 1] - mrec[i]) * mpre[i + 1])
 
 
 # Reassignment, for smooth operation of pycocotools replacement
diff --git a/tests/basic.py b/tests/basic.py
index 5a6083c..ccd106f 100644
--- a/tests/basic.py
+++ b/tests/basic.py
@@ -3,6 +3,8 @@
 import os
 import unittest
 
+import numpy as np
+
 from faster_coco_eval import COCO, COCOeval_faster
 from faster_coco_eval.extra import PreviewResults
 
@@ -126,6 +128,20 @@ def test_confusion_matrix(self):
 
         self.assertEqual(result_cm, prepared_result)
 
+    def test_auc(self):
+        x = np.linspace(0, 0.55, 100)
+        y = np.linspace(0, 2, 100) + 0.1
+
+        cpp_auc = COCOeval_faster.calc_auc(x, y)
+        py_auc = COCOeval_faster.calc_auc(x, y, method="py")
+        # sklearn not in test space!
+        # from sklearn import metrics
+        # orig_auc = metrics.auc(x, y)
+        orig_auc = 1.1550000000000005
+
+        self.assertAlmostEqual(cpp_auc, orig_auc, places=8)
+        self.assertAlmostEqual(py_auc, orig_auc, places=8)
+
 
 if __name__ == "__main__":
     unittest.main()