feat: 상품 랭킹 정보 조회 로직 구현

* Redis ZSET으로부터 상품의 랭킹 정보를 조회합니다.

- 순위(rank)
- 점수(score)

Author: Kimjipang

apps/commerce-api/src/main/java/com/loopers/application/product/RankingRedisReader.java

@@ -0,0 +1,40 @@
+package com.loopers.application.product;
+
+import lombok.RequiredArgsConstructor;
+import org.springframework.data.redis.core.RedisCallback;
+import org.springframework.data.redis.core.StringRedisTemplate;
+import org.springframework.data.redis.serializer.RedisSerializer;
+import org.springframework.stereotype.Component;
+
+import java.util.List;
+
+@Component
+@RequiredArgsConstructor
+public class RankingRedisReader {
+    private final StringRedisTemplate redisTemplate;
+
+    public RankingInfo getDailyRanking(String date, Long productId) {
+        String key = "ranking:all:" + date;
+        String member = String.valueOf(productId);
+
+        RedisSerializer<String> serializer = redisTemplate.getStringSerializer();
+        byte[] keyBytes = serializer.serialize(key);
+        byte[] memberBytes = serializer.serialize(member);
+
+        @SuppressWarnings("unchecked")
+        List<Object> results = (List<Object>) redisTemplate.executePipelined((RedisCallback<Object>) connection -> {
+            connection.zScore(keyBytes, memberBytes);   // -> Double (or null)
+            connection.zRevRank(keyBytes, memberBytes); // -> Long   (or null) 0-base
+            connection.zCard(keyBytes);                 // -> Long
+            return null;
+        });
+
+        Double score = (Double) results.get(0);
+        Long revRank0 = (Long) results.get(1);
+        Long total = (Long) results.get(2);
+
+        Integer rank = (revRank0 == null) ? null : Math.toIntExact(revRank0 + 1);
+
+        return new RankingInfo(date, score, rank, total);
+    }
+}