Remove obsolete hash quality comparison

The projection test specifically required the rejected combine_hash implementation to outperform the restored combiner. Keep the functional chained-hashtable coverage while removing that incompatible A/B guard.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: NikolajBjorner

src/test/chashtable.cpp

@@ -21,8 +21,6 @@ Revision History:
 #include "util/hash.h"
 #include "util/util.h"
 #include <iostream>
-#include <cstdint>
-#include <vector>
 
 typedef chashtable<int, int_hash, default_eq<int> > int_table;
 typedef cmap<int, int, int_hash, default_eq<int> > int_map;
@@ -174,87 +172,6 @@ static void tst6() {
     });
 }
 
-static unsigned combine_hash_old(unsigned h1, unsigned h2) {
-    // Pre-change combine_hash implementation kept for A/B quality checks.
-    h2 -= h1; h2 ^= (h1 << 8);
-    h1 -= h2; h2 ^= (h1 << 16);
-    h2 -= h1; h2 ^= (h1 << 10);
-    return h2;
-}
-
-constexpr unsigned hash_compare_num_samples = 1u << 16;
-constexpr unsigned hash_compare_seed = 0x12345678u;
-constexpr unsigned hash_compare_alignment_shift = 12;
-
-struct hash_projection_stats_t {
-    unsigned m_occupied = 0;
-    uint64_t m_uniform_error = 0;
-};
-
-template<typename CombineHash>
-static hash_projection_stats_t get_projection_stats(CombineHash const& combine, unsigned bits, bool low_bits) {
-    // Project hashes to either a suffix (low bits) or prefix (high bits), then
-    // collect occupancy and a scaled squared-error uniformity score.
-    SASSERT(bits <= 16);
-    unsigned const num_buckets = 1u << bits;
-    unsigned const mask = num_buckets - 1;
-    int64_t const signed_num_samples = static_cast<int64_t>(hash_compare_num_samples);
-    int64_t const signed_num_buckets = static_cast<int64_t>(num_buckets);
-    std::vector<unsigned> counts(num_buckets, 0);
-    for (unsigned i = 0; i < hash_compare_num_samples; ++i) {
-        unsigned h = combine(i << hash_compare_alignment_shift, hash_compare_seed);
-        unsigned b = low_bits ? (h & mask) : (h >> (32 - bits));
-        counts[b]++;
-    }
-
-    hash_projection_stats_t stats;
-    for (unsigned c : counts) {
-        if (c != 0)
-            ++stats.m_occupied;
-        // Scaled squared deviation from ideal per-bucket load num_samples/num_buckets.
-        int64_t diff = static_cast<int64_t>(c) * signed_num_buckets - signed_num_samples;
-        stats.m_uniform_error += static_cast<uint64_t>(diff * diff);
-    }
-    return stats;
-}
-
-static void tst_combine_hash_low_bits() {
-    constexpr unsigned bits8 = 8;
-    constexpr unsigned bits16 = 16;
-
-    auto old_low8 = get_projection_stats(combine_hash_old, bits8, true);
-    auto new_low8 = get_projection_stats(combine_hash, bits8, true);
-    auto old_low16 = get_projection_stats(combine_hash_old, bits16, true);
-    auto new_low16 = get_projection_stats(combine_hash, bits16, true);
-    auto old_high8 = get_projection_stats(combine_hash_old, bits8, false);
-    auto new_high8 = get_projection_stats(combine_hash, bits8, false);
-    auto old_high16 = get_projection_stats(combine_hash_old, bits16, false);
-    auto new_high16 = get_projection_stats(combine_hash, bits16, false);
-
-    unsigned old_low8_collisions = hash_compare_num_samples - old_low8.m_occupied;
-    unsigned new_low8_collisions = hash_compare_num_samples - new_low8.m_occupied;
-    unsigned old_low16_collisions = hash_compare_num_samples - old_low16.m_occupied;
-    unsigned new_low16_collisions = hash_compare_num_samples - new_low16.m_occupied;
-
-    ENSURE(new_low8_collisions < old_low8_collisions);
-    ENSURE(new_low16_collisions < old_low16_collisions);
-    ENSURE(new_low8.m_uniform_error < old_low8.m_uniform_error);
-    ENSURE(new_low16.m_uniform_error < old_low16.m_uniform_error);
-
-    std::cout << "combine_hash old/new low8 collisions: "
-              << old_low8_collisions << "/" << new_low8_collisions << "\n";
-    std::cout << "combine_hash old/new low16 collisions: "
-              << old_low16_collisions << "/" << new_low16_collisions << "\n";
-    std::cout << "combine_hash old/new low8 uniform_error: "
-              << old_low8.m_uniform_error << "/" << new_low8.m_uniform_error << "\n";
-    std::cout << "combine_hash old/new low16 uniform_error: "
-              << old_low16.m_uniform_error << "/" << new_low16.m_uniform_error << "\n";
-    std::cout << "combine_hash old/new high8 uniform_error: "
-              << old_high8.m_uniform_error << "/" << new_high8.m_uniform_error << "\n";
-    std::cout << "combine_hash old/new high16 uniform_error: "
-              << old_high16.m_uniform_error << "/" << new_high16.m_uniform_error << "\n";
-}
-
 void tst_chashtable() {
     tst1();
     tst2();
@@ -263,6 +180,5 @@ void tst_chashtable() {
     tst4<dint_table>(1000,10);
     tst4<dint_table>(10000,10);
     tst4<int_table>(50000,1000);
-    tst_combine_hash_low_bits();
     tst5();
 }