Add problem_list api.

Fix  AC/Total count bug.

problem/views/oj.py

@@ -1,8 +1,46 @@
+from django.db.models import Q, Count
 from utils.api import APIView
 
-from ..models import ProblemTag
+from ..models import ProblemTag, Problem
+from ..serializers import ProblemSerializer, TagSerializer
 
 
 class ProblemTagAPI(APIView):
     def get(self, request):
-        return self.success([item.name for item in ProblemTag.objects.all().order_by("id")])
+        tags = ProblemTag.objects.annotate(problem_number=Count("problem"))\
+            .filter(problem_number__gt=0).order_by("-problem_number")
+        return self.success(TagSerializer(tags, many=True).data)
+
+
+class ProblemAPI(APIView):
+    def get(self, request):
+        # 问题详情页
+        problem_id = request.GET.get("id")
+        if problem_id:
+            try:
+                problem = Problem.objects.get(id=problem_id)
+                return self.success(ProblemSerializer(problem).data)
+            except Problem.DoesNotExist:
+                return self.error("Problem does not exist")
+
+        problems = Problem.objects.filter(visible=True)
+        # 按照标签筛选
+        tag_text = request.GET.get("tag", None)
+        if tag_text:
+            try:
+                tag = ProblemTag.objects.get(name=tag_text)
+            except ProblemTag.DoesNotExist:
+                return self.error("The Tag does not exist.")
+            problems = tag.problem_set.all().filter(visible=True)
+
+        # 搜索的情况
+        keyword = request.GET.get("keyword", "").strip()
+        if keyword:
+            problems = problems.filter(Q(title__contains=keyword) | Q(description__contains=keyword))
+
+        # 难度筛选
+        difficulty_rank = request.GET.get('difficulty', None)
+        if difficulty_rank:
+            problems = problems.filter(difficulty=difficulty_rank)
+
+        return self.success(self.paginate_data(request, problems, ProblemSerializer))