f25-p0 fix: add back seeded HashFunction util, mod contention ratio test (#830)

f25-p0 fix: add back seeded HashFunction util

Author: songwdfu

src/include/primer/count_min_sketch.h

@@ -13,9 +13,12 @@
 #pragma once
 
 #include <cstdint>
+#include <functional>
 #include <utility>
 #include <vector>
 
+#include "common/util/hash_util.h"
+
 namespace bustub {
 
 template <typename KeyType>
@@ -79,6 +82,25 @@ class CountMinSketch {
   /** Dimensions of the count-min sketch matrix */
   const uint32_t width_;  // Number of buckets for each hash function
   const uint32_t depth_;  // Number of independent hash functions
+  /** Pre-computed hash functions for each row */
+  std::vector<std::function<size_t(const KeyType &)>> hash_functions_;
+
+  /** @fall2025 PLEASE DO NOT MODIFY THE FOLLOWING */
+  constexpr static size_t SEED_BASE = 15445;
+
+  /**
+   * @brief Seeded hash function generator
+   *
+   * @param seed Used for creating independent hash functions
+   * @return A function that maps items to column indices
+   */
+  inline auto HashFunction(size_t seed) -> std::function<size_t(const KeyType &)> {
+    return [seed, this](const KeyType &item) -> size_t {
+      auto h1 = std::hash<KeyType>{}(item);
+      auto h2 = bustub::HashUtil::CombineHashes(seed, SEED_BASE);
+      return bustub::HashUtil::CombineHashes(h1, h2) % width_;
+    };
+  }
 
   /** @todo (student) can add their data structures that support count-min sketch operations */
 };

src/primer/count_min_sketch.cpp

@@ -27,6 +27,13 @@ namespace bustub {
 template <typename KeyType>
 CountMinSketch<KeyType>::CountMinSketch(uint32_t width, uint32_t depth) : width_(width), depth_(depth) {
   /** @TODO(student) Implement this function! */
+
+  /** @fall2025 PLEASE DO NOT MODIFY THE FOLLOWING */
+  // Initialize seeded hash functions
+  hash_functions_.reserve(depth_);
+  for (size_t i = 0; i < depth_; i++) {
+    hash_functions_.push_back(this->HashFunction(i));
+  }
 }
 
 template <typename KeyType>

test/primer/count_min_sketch_test.cpp

@@ -721,7 +721,7 @@ TEST(CountMinSketchTest, ContentionRatioTest) {
   std::vector<size_t> time_ms_wo_mutex;
 
   // Two Threads inserting 10000 entries each
-  auto cms = CountMinSketch<int>(500, 15);
+  auto cms = CountMinSketch<int64_t>(500, 15);
   const int insert_iters = 10000;
   const int num_threads = 2;
 
@@ -760,6 +760,11 @@ TEST(CountMinSketchTest, ContentionRatioTest) {
     }
   }
 
+  // Check result correctness
+  for (int i = 0; i < 10; i++) {
+    ASSERT_EQ(cms.Count(i), 20000);
+  }
+
   std::cout << "<<< BEGIN" << std::endl;
   std::cout << "Multithreaded Insertion Time: ";
   double sum_1 = 0;