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C++20 [Minimal] Static Perfect Hash library
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compile-time cpp20 perfect-hash simd

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// <!-- // The MIT License (MIT) // // Copyright (c) 2024 Kris Jusiak kris@jusiak.net // // Permission is hereby granted, free of charge, to any person obtaining a copy // of this software and associated documentation files (the "Software"), to deal // in the Software without restriction, including without limitation the rights // to use, copy, modify, merge, publish, distribute, sublicense, and/or sell // copies of the Software, and to permit persons to whom the Software is // furnished to do so, subject to the following conditions: // // The above copyright notice and this permission notice shall be included in all // copies or substantial portions of the Software. // // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR // IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, // FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE // AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE // SOFTWARE. //

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// --> Overview / Examples / API / FAQ

MPH: [Minimal] Static perfect hash library

https://en.wikipedia.org/wiki/Perfect_hash_function

MIT Licence Version Build Try it online

Use case

A static perfect hash function maps a set of keys known in advance to a set of values with no collisions.

Features

Requirements

Overview

Hello world (https://godbolt.org/z/dzd6o3Pxo)

enum class color { red, green, blue };

constexpr auto colors = std::array{
  std::pair{"red"sv, color::red},
  std::pair{"green"sv, color::green},
  std::pair{"blue"sv, color::blue},
};

static_assert(color::green == mph::lookup<colors>("green"));
static_assert(color::red   == mph::lookup<colors>("red"));
static_assert(color::blue  == mph::lookup<colors>("blue"));

std::print("{}", mph::lookup<colors>("green"sv)); // prints 1

mph::lookup assumes only valid input and returns mapped value direclty.

static_assert(not mph::find<colors>("unknown"));
static_assert(mph::find<colors>("green"));
static_assert(mph::find<colors>("red"));
static_assert(mph::find<colors>("blue"));

std::print("{}", *mph::find<colors>("green"sv)); // prints 1

mph::find doesnt assume valid input and returns optional of mapped value.

Performance (https://godbolt.org/z/rqYj9a1cr)

int lookup(int id) {
  static constexpr std::array ids{
    std::pair{54u, 91u},
    std::pair{64u, 324u},
    std::pair{91u, 234u},
  };
  return mph::lookup<ids>(id);
}
lookup: // g++ -DNDEBUG -std=c++20 -O3
  imull   $1275516394, %edi, %eax
  shrl    $23, %eax
  movl    $24029728, %ecx
  shrxl   %eax, %ecx, %eax
  andl    $511, %eax
  retq

Performance (https://godbolt.org/z/vv6W4nGfb)

int lookup(int id) {
  static constexpr std::array ids{
    std::pair{54u, 91u},
    std::pair{324u, 54u},
    std::pair{64u, 324u},
    std::pair{234u, 64u},
    std::pair{91u, 234u},
  };
  return mph::lookup<ids>(id);
}
lookup: // g++ -DNDEBUG -std=c++20 -O3
  andl    $7, %edi
  leaq    lookup(%rip), %rax
  movl    (%rax,%rdi,4), %eax
  retq

lookup:
 .long   324
 .long   0
 .long   64
 .long   234
 .long   54
 .long   0
 .long   91

Performance (https://godbolt.org/z/qMzxKK4sd)

int find(int id) {
  static constexpr std::array ids{
    std::pair{27629, 1},
    std::pair{6280, 2},
    // 1..128 pairs...
    std::pair{33691, 128},
  };
  return *mph::find<ids>(id);
}
find: // g++ -DNDEBUG -std=c++20 -O3 -mbmi2 -mavx512f
  vpbroadcastd    %edi, %zmm0
  shll            $4, %edi
  movzbl          %dil, %ecx
  leaq            find
  vpcmpeqd        (%rdx,%rcx,4), %zmm0, %k0
  kmovw           %k0, %esi
  kortestw        %k0, %k0
  rep             bsfq %rax, %rax
  movl            $64, %eax
  addl            %eax, %ecx
  xorl            %eax, %eax
  testw           %si, %si
  cmovnel         1024(%rdx,%rcx,4), %eax
  vzeroupper
  retq

find:
  ... // see godbolt

Performance (https://godbolt.org/z/KaKzf7Pax)

int find(std::span<const char, 8> str) {
  static constexpr auto symbols = std::array{
    std::pair{"AMZN    "sv, 1},
    std::pair{"AAPL    "sv, 2},
    std::pair{"GOOGL   "sv, 3},
    std::pair{"META    "sv, 4},
    std::pair{"MSFT    "sv, 5},
    std::pair{"NVDA    "sv, 6},
    std::pair{"TSLA    "sv, 7},
  };
  return *mph::find<symbols>(str);
}
find: // g++ -DNDEBUG -std=c++20 -O3 -mbmi2
  movq    8(%rsi), %rax
  movl    $1031, %ecx
  leaq    find(%rip), %rdx
  xorl    %esi, %esi
  movq    (%rax), %rax
  pextq   %rcx, %rax, %rcx
  shll    $4, %ecx
  cmpq    (%rcx,%rdx), %rax
  movzbl  8(%rcx,%rdx), %eax
  cmovnel %esi, %eax
  retq

find:
  ... // see godbolt

Performance (https://godbolt.org/z/fdMPsYWjE)

int find(std::string_view str) {
  using std::literals::operator""sv;
  // values assigned from 0..N-1
  static constexpr std::array symbols{
    "BTC "sv, "ETH "sv, "BNB "sv,
    "SOL "sv, "XRP "sv, "DOGE"sv,
    "TON "sv, "ADA "sv, "SHIB"sv,
    "AVAX"sv, "LINK"sv, "BCH "sv,
  };
  return *mph::find<symbols>(str);
}
find: // g++ -DNDEBUG -std=c++20 -O3 -mbmi2
  shll    $3, %edi
  bzhil   %edi, (%rsi), %eax
  movl    $789, %ecx
  pextl   %ecx, %eax, %ecx
  leaq    find(%rip), %rdx
  xorl    %esi, %esi
  cmpl    (%rdx,%rcx,8), %eax
  movzbl  4(%rdx,%rcx,8), %eax
  cmovnel %esi, %eax
  retq

find:
  ... // see godbolt

Examples

Benchmarks (https://github.com/qlibs/mph/tree/benchmark)

clang++ -std=c++20 -O3 -DNDEBUG -mbmi2 benchmark.cpp

| ns/op |           op/s | err% |total | benchmark
|------:|---------------:|-----:|-----:|:----------
| 12.25 |  81,602,449.70 | 0.3% | 0.15 | `random_strings_5_len_4.std.map`
|  5.56 | 179,750,906.50 | 0.2% | 0.07 | `random_strings_5_len_4.std.unordered_map`
|  9.17 | 109,096,850.98 | 0.2% | 0.11 | `random_strings_5_len_4.boost.unordered_map`
| 13.48 |  74,210,250.54 | 0.3% | 0.16 | `random_strings_5_len_4.boost.flat_map`
|  7.70 | 129,942,965.18 | 0.3% | 0.09 | `random_strings_5_len_4.gperf`
|  1.61 | 621,532,188.81 | 0.1% | 0.02 | `random_strings_5_len_4.mph`
| 14.66 |  68,218,086.71 | 0.8% | 0.18 | `random_strings_5_len_8.std.map`
| 13.45 |  74,365,239.56 | 0.2% | 0.16 | `random_strings_5_len_8.std.unordered_map`
|  9.68 | 103,355,605.09 | 0.2% | 0.12 | `random_strings_5_len_8.boost.unordered_map`
| 16.00 |  62,517,180.19 | 0.4% | 0.19 | `random_strings_5_len_8.boost.flat_map`
|  7.70 | 129,809,356.36 | 0.3% | 0.09 | `random_strings_5_len_8.gperf`
|  1.58 | 633,084,194.24 | 0.1% | 0.02 | `random_strings_5_len_8.mph`
| 17.21 |  58,109,576.87 | 0.3% | 0.21 | `random_strings_6_len_2_5.std.map`
| 15.28 |  65,461,167.99 | 0.2% | 0.18 | `random_strings_6_len_2_5.std.unordered_map`
| 12.21 |  81,931,391.20 | 0.4% | 0.15 | `random_strings_6_len_2_5.boost.unordered_map`
| 17.15 |  58,323,741.08 | 0.5% | 0.21 | `random_strings_6_len_2_5.boost.flat_map`
|  7.94 | 125,883,197.55 | 0.5% | 0.09 | `random_strings_6_len_2_5.gperf`
|  6.05 | 165,239,616.00 | 0.5% | 0.07 | `random_strings_6_len_2_5.mph`
| 31.61 |  31,631,402.94 | 0.2% | 0.38 | `random_strings_100_len_8.std.map`
| 15.32 |  65,280,594.09 | 0.2% | 0.18 | `random_strings_100_len_8.std.unordered_map`
| 17.13 |  58,383,850.20 | 0.3% | 0.20 | `random_strings_100_len_8.boost.unordered_map`
| 31.42 |  31,822,519.67 | 0.2% | 0.38 | `random_strings_100_len_8.boost.flat_map`
|  8.04 | 124,397,773.85 | 0.2% | 0.10 | `random_strings_100_len_8.gperf`
|  1.58 | 632,813,481.73 | 0.1% | 0.02 | `random_strings_100_len_8.mph`
| 32.62 |  30,656,015.03 | 0.3% | 0.39 | `random_strings_100_len_1_8.std.map`
| 19.34 |  51,697,107.73 | 0.5% | 0.23 | `random_strings_100_len_1_8.std.unordered_map`
| 19.51 |  51,254,525.17 | 0.3% | 0.23 | `random_strings_100_len_1_8.boost.unordered_map`
| 33.58 |  29,780,574.17 | 0.6% | 0.40 | `random_strings_100_len_1_8.boost.flat_map`
| 13.06 |  76,577,037.07 | 0.7% | 0.16 | `random_strings_100_len_1_8.gperf`
|  6.02 | 166,100,665.07 | 0.2% | 0.07 | `random_strings_100_len_1_8.mph`
|  1.28 | 778,723,795.75 | 0.1% | 0.02 | `random_uints_5.mph`

g++ -std=c++20 -O3 -DNDEBUG -mbmi2 benchmark.cpp

| ns/op |           op/s | err% |total | benchmark
|------:|---------------:|-----:|-----:|:----------
| 12.28 |  81,460,330.38 | 0.9% | 0.15 | `random_strings_5_len_4.std.map`
|  5.29 | 188,967,241.90 | 0.3% | 0.06 | `random_strings_5_len_4.std.unordered_map`
|  9.69 | 103,163,192.67 | 0.2% | 0.12 | `random_strings_5_len_4.boost.unordered_map`
| 13.56 |  73,756,333.08 | 0.4% | 0.16 | `random_strings_5_len_4.boost.flat_map`
|  7.69 | 130,055,662.66 | 0.6% | 0.09 | `random_strings_5_len_4.gperf`
|  1.39 | 718,910,252.82 | 0.1% | 0.02 | `random_strings_5_len_4.mph`
| 14.26 |  70,103,007.82 | 2.4% | 0.17 | `random_strings_5_len_8.std.map`
| 13.36 |  74,871,047.51 | 0.4% | 0.16 | `random_strings_5_len_8.std.unordered_map`
|  9.82 | 101,802,074.00 | 0.3% | 0.12 | `random_strings_5_len_8.boost.unordered_map`
| 15.97 |  62,621,571.95 | 0.3% | 0.19 | `random_strings_5_len_8.boost.flat_map`
|  7.92 | 126,265,206.30 | 0.3% | 0.09 | `random_strings_5_len_8.gperf`
|  1.40 | 713,596,376.62 | 0.4% | 0.02 | `random_strings_5_len_8.mph`
| 15.98 |  62,576,142.34 | 0.5% | 0.19 | `random_strings_6_len_2_5.std.map`
| 17.56 |  56,957,868.12 | 0.5% | 0.21 | `random_strings_6_len_2_5.std.unordered_map`
| 11.68 |  85,637,378.45 | 0.3% | 0.14 | `random_strings_6_len_2_5.boost.unordered_map`
| 17.25 |  57,965,732.68 | 0.6% | 0.21 | `random_strings_6_len_2_5.boost.flat_map`
|  9.13 | 109,580,632.48 | 0.7% | 0.11 | `random_strings_6_len_2_5.gperf`
|  7.17 | 139,563,745.72 | 0.4% | 0.09 | `random_strings_6_len_2_5.mph`
| 30.20 |  33,117,522.76 | 0.7% | 0.36 | `random_strings_100_len_8.std.map`
| 15.01 |  66,627,962.89 | 0.4% | 0.18 | `random_strings_100_len_8.std.unordered_map`
| 16.79 |  59,559,414.60 | 0.6% | 0.20 | `random_strings_100_len_8.boost.unordered_map`
| 31.36 |  31,884,629.57 | 0.8% | 0.38 | `random_strings_100_len_8.boost.flat_map`
|  7.75 | 128,973,947.61 | 0.7% | 0.09 | `random_strings_100_len_8.gperf`
|  1.50 | 667,041,673.54 | 0.1% | 0.02 | `random_strings_100_len_8.mph`
| 30.92 |  32,340,612.08 | 0.4% | 0.37 | `random_strings_100_len_1_8.std.map`
| 25.35 |  39,450,222.09 | 0.4% | 0.30 | `random_strings_100_len_1_8.std.unordered_map`
| 19.76 |  50,609,820.90 | 0.2% | 0.24 | `random_strings_100_len_1_8.boost.unordered_map`
| 32.39 |  30,878,018.77 | 0.6% | 0.39 | `random_strings_100_len_1_8.boost.flat_map`
| 11.20 |  89,270,687.92 | 0.2% | 0.13 | `random_strings_100_len_1_8.gperf`
|  7.17 | 139,471,159.67 | 0.5% | 0.09 | `random_strings_100_len_1_8.mph`
|  1.93 | 519,047,110.39 | 0.3% | 0.02 | `random_uints_5.mph`

Benchmarks (https://qlibs.github.io/mph/perfect_hashing)

Benchmark Benchmark

API

namespace mph {
/**
 * Static [minimal] perfect hash lookup function
 * @tparam entries constexpr array of keys or key/value pairs
 */
template<const auto& entries>
inline constexpr auto lookup = [](const auto& key) {
  if constexpr(constexpr lookup$magic_lut<entries> lookup{}; lookup) {
    return lookup(key);
  } else {
    return lookup$pext<entries>(key);
  }
};

/**
 * Static perfect hash find function
 * @tparam entries constexpr array of keys or key/value pairs
 */
template<const auto& entries>
inline constexpr auto find =
  []<u8 probability = 50u>(const auto& key, const auto& unknown = {}) -> optional {
    if constexpr (entries.size() == 0u) {
      return unknown;
    } else if constexpr (entries.size() <= 64u) {
      return find$pext<entries>.operator()<probability>(key, unknown);
    } else {
      constexpr auto bucket_size = simd_size_v<key_type, simd_abi::native<key_type>>;
      return find$simd<entries, bucket_size>.operator()<probability>(key, unknown);
    }
  };
} // namespace mph

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