test_hash.cpp
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#include "crucible/core.hpp"
#include "crucible/test.hpp"
#include <cstdint>
namespace test_hash
{
CRUCIBLE_TEST_SCENARIO(hash_short_strings)
{
// A list of 256 random dictionary words
auto const all_values {
crucible::make_heap_buffer<crucible::string_view>({
"physoclist", "Gordyaean", "Panionian", "infirm", "gigerium", "gymnosporous", "discredit",
"angulated", "onus", "pharmacopsychology", "brutalize", "variolization", "unmindfulness",
"woodwall", "cryingly", "phytolithological", "convenience", "tregadyne", "arecoline", "reunite",
"horsefoot", "spatted", "nannoplankton", "Noam", "Tabellariaceae", "spaework", "align", "tell",
"preworthy", "photopography", "scleranth", "ungainly", "cumene", "nosotrophy", "barb",
"semiduplex", "Lapageria", "hyperimmunity", "tritely", "oversusceptible", "trilabe",
"sufficientness", "valethood", "Malebranchism", "Paumari", "preliterally", "slobbery",
"refringence", "lam", "preindulge", "agitative", "cheirography", "Mephistopheleanly", "ladkin",
"turnhall", "sunburntness", "metaplasm", "Madonna", "pollable", "mootman", "cheddaring",
"pontic", "hypsoisotherm", "libretto", "granulation", "sarsenet", "megaerg",
"adrenocorticotropic", "Soohong", "gris", "Eucommia", "wrathfulness", "woodruff",
"corrigibleness", "enthraldom", "logeum", "unreeving", "archdetective", "Maskins",
"pancreatalgia", "miseffect", "Norseland", "ureteropyosis", "Coniothyrium", "pteropaedes",
"Ctenophora", "enterocele", "was", "superambitious", "bridgebuilding", "triglochin", "glider",
"exaltative", "drivescrew", "hypophloeous", "deboistness", "declensionally", "constringe",
"uncontrovertibly", "testaceology", "Kate", "phrenosinic", "Venezuelan", "syllogistics",
"megacerine", "speech", "unordinariness", "fooling", "reannexation", "blackfire", "sloosh",
"tracheolar", "callidness", "zokor", "telemeteorographic", "basial", "pteridophilism",
"wristfall", "unproportionedness", "amidoketone", "thermantic", "Kshatriyahood", "Sertularia",
"neurataxia", "Calite", "thaumoscopic", "pruning", "unalleviably", "kingless",
"irreconcilability", "Claude", "Sphaerocarpus", "parasitophobia", "nebbuck", "kinnikinnick",
"dockization", "Abrahamite", "repurchase", "Homerian", "charlatanically", "polderboy",
"saurischian", "cardiorrhaphy", "apoplectoid", "mankin", "allotee", "aminate", "notal",
"Hypnaceae", "identism", "understring", "Anaxagorean", "titty", "salvability",
"hydrophthalmus", "esponton", "brassily", "Cocanucos", "slugging", "palet", "spontaneous",
"cation", "superagency", "challie", "Mirach", "cinematographic", "alexipyretic",
"corticosterone", "semihyperbolical", "mewer", "chivalresque", "nights", "subequally", "knotty",
"nonvariable", "ambrosine", "Sphenopteris", "Adai", "meteorite", "tomjohn", "Yokuts",
"Franklin", "unjudgelike", "violoncellist", "abolitionist", "festerment", "Novemberish",
"unicostate", "noncirculation", "claught", "scattered", "curatial", "sweetberry", "unbragged",
"Llanberisslate", "birational", "thurible", "thetically", "cipolin", "prestress", "tailpipe",
"thiuram", "participable", "redolence", "lobstering", "feveret", "microsublimation",
"choreutic", "tagrag", "platitudinization", "solipedous", "Alopiidae", "Hildebrandist",
"silaginoid", "bore", "festucine", "thymocyte", "blacker", "gib", "hydrargyrism",
"unapplicableness", "unintelligibly", "Mister", "rentaller", "undermine", "Emesidae",
"coxcombhood", "squidge", "postponement", "magnify", "stapelia", "diathermotherapy", "chutney",
"stupefactive", "holler", "vociferous", "visie", "scrounger", "homoerotic", "synchronized",
"puddinghouse", "acuclosure", "zoophytical", "unprovide", "noncustomary", "twitch",
"nonauthentication", "frow", "klop", "unfeigningly", "encampment", "preregistration",
"mimeographic", "delphacid", "kilter", "remarry"
})
};
// Hash the words in the word list
auto const all_hashes {
crucible::iterate(all_values)
.map([](crucible::string_view const s) {
return crucible::hash(s);
}).materialize<crucible::heap_buffer<std::uint64_t>>()
};
// Same number of buckets as keys, to simulate a really bad load factor
auto buckets { crucible::make_heap_buffer<std::size_t>(all_values.size()) };
// Simulate a hash table by just counting how many items would go into each bucket, assuming we
// store an item at hash(item) modulo (number of buckets)
for (std::size_t i { 0 }; i < all_hashes.size(); ++i) {
#if 0 // Debugging information
CRUCIBLE_PRINT("hash({}) = {} ({} mod {})", all_values[i], all_hashes[i], all_hashes[i] % buckets.size(), buckets.size());
#endif
auto const j { all_hashes[i] % buckets.size() };
++buckets[j];
#if 0 // Debugging information
if (buckets[j] > 1) {
CRUCIBLE_PRINT("collision at {}", all_values[i]);
}
#endif
}
// A very simple conservative test: given that we have the same number of buckets as keys, we
// want to have <50% of the populated buckets (iem count > 0) have a collision (item count > 1).
//
// There are much more sophisticated tests that can check for stricter guarantees (one worth
// looking into is the chi-squared test), but this test is more about verifying that the API
// works and produces more-or-less evenly distributed hashes (i.e., hash tables won't just utterly
// break) than it is about proving anything in particular about the hash function.
double const populated_bucket_count {
crucible::iterate(buckets)
.fold([](double const result, std::size_t const item_count) {
return result + (item_count > 0 ? 1.0 : 0.0);
}, 0.0)
};
double const collision_bucket_count {
crucible::iterate(buckets)
.fold([](double const result, std::size_t const item_count) {
return result + (item_count > 1 ? 1.0 : 0.0);
}, 0.0)
};
#if 0 // Debugging information
double const load { all_hashes.size() / static_cast<double>(buckets.size()) };
double const ratio { collision_bucket_count / populated_bucket_count };
CRUCIBLE_PRINT("buckets: {} collisions: {} populated: {} ratio: {} load: {}", buckets.size(), collision_bucket_count, populated_bucket_count, ratio, load);
#endif
CRUCIBLE_ASSERT_LT(collision_bucket_count / populated_bucket_count, 0.5);
}
}
CRUCIBLE_TEST_MAIN