Compare different_approaches to build ranges of consecutive items https://x.com/chgeuer/status/1768027992691781757

consecutive_ranges.exs

Mix.install([
  {:benchee, "~> 1.3"}
])

defmodule TailRecursive do
  defstruct from: nil, to: nil

  def identify([], _), do: []
  def identify([item], _), do: %__MODULE__{from: item, to: item}
  def identify(v, is_next) when is_list(v) do
    v
    |> Enum.sort()
    |> Enum.dedup()
    |> impl([{}], is_next)
  end

  defp impl([], a, _), do: reverse(a, [])
  defp impl([current | tail], [{from, to} | acc], is_next) do
    if is_next.(to, current) do
      impl(tail, [{from, current} | acc], is_next)
    else
      impl(tail, [{current, current}, {from, to} | acc], is_next)
    end
  end

  defp impl([current | tail], [{} | acc], is_next) do
    impl(tail, [{current, current} | acc], is_next)
  end

  defp reverse([], a), do: a
  defp reverse([{from, to} | tail], a) do
    reverse(tail, [%__MODULE__{from: from, to: to} | a])
  end
end

defmodule TailSinglePass do
  defstruct from: nil, to: nil

  def identify([], _), do: []
  def identify([item], _), do: %__MODULE__{from: item, to: item}
  def identify(v, is_next) when is_list(v) do
    v
    |> Enum.sort(:desc)
    |> then(fn [max | rest] -> impl(rest, [{max, max}], is_next) end)
  end

  def impl([], [{from, to} | acc], _), do: [%__MODULE__{from: from, to: to} | acc]
  def impl([item, item | rest], acc, is_next), do: impl([item | rest], acc, is_next)
  def impl([previous | tail], [{from, to} | acc], is_next) do
    if is_next.(previous, from) do
      impl(tail, [{previous, to} | acc], is_next)
    else
      impl(tail, [{previous, previous}, %__MODULE__{from: from, to: to} | acc], is_next)
    end
  end
end

defmodule RecursiveEnumerate do
  defstruct from: nil, to: nil
  
  def identify([], _), do: []
  def identify([item], _), do: %__MODULE__{from: item, to: item}
  def identify(v, is_next) when is_list(v) do
    v
    |> Enum.sort()
    |> Enum.dedup()
    |> impl(is_next)
  end

  defp impl([], _), do: []
  defp impl([x], _), do: [%__MODULE__{from: x, to: x}]
  defp impl([current | tail], is_next) do
    {range, rest} =
      %__MODULE__{from: current, to: current}
      |> enumerate_sequence(tail, is_next)
    
    [range | impl(rest, is_next)]
  end

  defp enumerate_sequence(range, [], _), do: {range, []}
  defp enumerate_sequence(range, [next | tail], is_next) do
    if is_next.(range.to, next) do
      enumerate_sequence(%__MODULE__{range | to: next}, tail, is_next)
    else
      {range, [next | tail]}
    end
  end
end

defmodule Recursive do
  defstruct from: nil, to: nil

  def identify([], _), do: []
  def identify([item], _), do: %__MODULE__{from: item, to: item}
  def identify(v, is_next) do
    [item | tail] =
      v
      |> Enum.sort()
      |> Enum.dedup()
    
    impl({item, item}, tail, is_next)
  end

  defp impl({from, to}, [], _), do: [%__MODULE__{from: from, to: to}]
  defp impl({from, to}, [next | tail], is_next) do
    if is_next.(to, next) do
      impl({from, next}, tail, is_next)
    else
      [%__MODULE__{from: from, to: to} | impl({next, next}, tail, is_next)]
    end
  end
end

defmodule ZipReduce do
  defstruct from: nil, to: nil

  def identify([], _), do: []
  def identify([item], _), do: %__MODULE__{from: item, to: item}
  def identify(v, is_next) when is_list(v) and is_function(is_next, 2) do
    previous_items =
      v
      |> Enum.sort()
      |> Enum.dedup()

    reduce_fn = fn
      previous, current, [{from, _} = range | tail] ->
        if is_next.(previous, current) do
          [{from, current} | tail]
        else
          [{current, current}, range | tail]
        end
    end

    [head | next_items] = previous_items

    previous_items
    |> Enum.zip_reduce(next_items, [{head, head}], reduce_fn)
    |> reverse([])
  end

  defp reverse([], a), do: a
  defp reverse([{from, to} | tail], a) do
    reverse(tail, [%__MODULE__{from: from, to: to} | a])
  end
end

defmodule IsNext do
  def integer(previous, next), do: previous + 1 == next
end


ExUnit.start()

defmodule SequenceTest do
  use ExUnit.Case

  def original_data do
    [
      -1, 0, 1, 2, 3, 4, 5, 6, 7,
      1999, 2000, 2001,
      2010, 2011, 2012,
      2014,
      2020, 2021, 2022, 2023, 2024
    ]
  end

  def generate_wide_random_data(range) do
    for _ <- range, do: Enum.random(-1_000_000..1_000_000)
  end

  def generate_narrow_random_data(range) do
    for _ <- range, do: Enum.random(1..1000)
  end
  
  test "simple case" do
    data = original_data()

    tail_results = identify(data, TailRecursive)
    tail_single_pass_results = identify(data, TailSinglePass)
    recursive_enum_results = identify(data, RecursiveEnumerate)
    recursive_results = identify(data, Recursive)
    zip_results = identify(data, ZipReduce)
    
    assert tail_results == tail_single_pass_results
    assert tail_results == recursive_enum_results
    assert tail_results == recursive_results
    assert tail_results == zip_results
  end

  test "benchmark" do
    data = generate_wide_random_data(1..1_000_000)
    
    {tail_time, tail_results} = measure_time(data, TailRecursive)
    {tail_single_pass_time, tail_single_pass_results} = measure_time(data, TailSinglePass)
    {enumerate_time, enumerate_results} = measure_time(data, RecursiveEnumerate)
    {recursive_time, recursive_results} = measure_time(data, Recursive)
    {zip_time, zip_results} = measure_time(data, ZipReduce)
    
    IO.inspect(%{
      TailRecursive => tail_time,
      TailSinglePass => tail_single_pass_time,
      RecursiveEnumerate => enumerate_time,
      Recursive => recursive_time,
      ZipReduce => zip_time
    }, label: "Example time (seconds)")

    assert tail_results == tail_single_pass_results
    assert tail_results == enumerate_results
    assert tail_results == recursive_results
    assert tail_results == zip_results
  end

  defp identify(data, module) do
    data
    |> module.identify(&IsNext.integer/2)
    |> Enum.map(&{&1.from, &1.to})
  end

  defp measure_time(data, module) do
    {time, results} = :timer.tc(fn -> identify(data, module) end)
    
    {time / 1_000_000, results}
  end

  def benchmark_with_benchee do
    modules = [TailRecursive, TailSinglePass, RecursiveEnumerate, Recursive, ZipReduce]
    cases =
      for module <- modules, into: %{} do
        {
          inspect(module),
          fn data -> module.identify(data, &IsNext.integer/2) end
        }
      end
    
    Benchee.run(
      cases,
      inputs: %{
        "100 items, narrow" => generate_narrow_random_data(1..1_00),
        "100 items, wide" => generate_wide_random_data(1..1_00),
        "10000 items, narrow" => generate_narrow_random_data(1..1_000),
        "10000 items, wide" => generate_wide_random_data(1..1_000),
        "1000000 items, narrow" => generate_narrow_random_data(1..1_000_000),
        "1000000 items, wide" => generate_wide_random_data(1..1_000_000),
        "original" => original_data()
      },
      time: 15,
      memory_time: 2
    )
  end
end

ExUnit.run()

SequenceTest.benchmark_with_benchee()

dedup_benchmark.exs

range = 1..1_000_000
data = for _ <- range, do: Enum.random(range)

uniq_task = Task.async(fn -> :timer.tc(fn -> data |> Enum.sort() |> Enum.uniq() end) end)
dedup_task = Task.async(fn -> :timer.tc(fn -> data |> Enum.sort() |> Enum.dedup() end) end)

{time_uniq, uniq_results} = Task.await(uniq_task)
{time_dedup, dedup_results} = Task.await(dedup_task)

results_match = uniq_results == dedup_results

dbg({results_match, time_uniq, time_dedup})

outputs.txt

##### With input 100 items, narrow #####
Name                         ips        average  deviation         median         99th %
TailSinglePass          462.80 K        2.16 μs   ±772.78%           2 μs        3.13 μs
TailRecursive           407.14 K        2.46 μs   ±714.89%        2.29 μs        4.58 μs
ZipReduce               385.00 K        2.60 μs   ±717.94%        2.38 μs        5.50 μs
Recursive               360.37 K        2.77 μs   ±670.92%        2.63 μs        3.63 μs
RecursiveEnumerate      320.11 K        3.12 μs   ±637.17%        2.96 μs        4.08 μs

Comparison: 
TailSinglePass          462.80 K
TailRecursive           407.14 K - 1.14x slower +0.30 μs
ZipReduce               385.00 K - 1.20x slower +0.44 μs
Recursive               360.37 K - 1.28x slower +0.61 μs
RecursiveEnumerate      320.11 K - 1.45x slower +0.96 μs

Memory usage statistics:

Name                  Memory usage
TailSinglePass            16.91 KB
TailRecursive             18.52 KB - 1.10x memory usage +1.61 KB
ZipReduce                 18.57 KB - 1.10x memory usage +1.66 KB
Recursive                 17.04 KB - 1.01x memory usage +0.125 KB
RecursiveEnumerate        17.27 KB - 1.02x memory usage +0.36 KB

**All measurements for memory usage were the same**

##### With input 100 items, wide #####
Name                         ips        average  deviation         median         99th %
TailSinglePass          443.42 K        2.26 μs   ±838.09%        2.08 μs        3.04 μs
TailRecursive           383.72 K        2.61 μs   ±712.32%        2.42 μs        3.75 μs
ZipReduce               361.39 K        2.77 μs   ±646.03%        2.58 μs        5.58 μs
Recursive               343.84 K        2.91 μs   ±649.72%        2.75 μs           5 μs
RecursiveEnumerate      316.60 K        3.16 μs   ±543.40%           3 μs        4.46 μs

Comparison: 
TailSinglePass          443.42 K
TailRecursive           383.72 K - 1.16x slower +0.35 μs
ZipReduce               361.39 K - 1.23x slower +0.51 μs
Recursive               343.84 K - 1.29x slower +0.65 μs
RecursiveEnumerate      316.60 K - 1.40x slower +0.90 μs

Memory usage statistics:

Name                  Memory usage
TailSinglePass            19.61 KB
TailRecursive             21.08 KB - 1.07x memory usage +1.47 KB
ZipReduce                 21.13 KB - 1.08x memory usage +1.52 KB
Recursive                 19.52 KB - 1.00x memory usage -0.09375 KB
RecursiveEnumerate        19.49 KB - 0.99x memory usage -0.11719 KB

**All measurements for memory usage were the same**

##### With input 10000 items, narrow #####
Name                         ips        average  deviation         median         99th %
TailSinglePass           37.24 K       26.86 μs    ±23.54%       25.21 μs       39.13 μs
TailRecursive            34.72 K       28.80 μs    ±24.17%       27.29 μs       42.79 μs
Recursive                34.63 K       28.88 μs    ±24.64%       27.58 μs       39.54 μs
ZipReduce                34.55 K       28.94 μs    ±20.17%       27.33 μs       43.00 μs
RecursiveEnumerate       28.19 K       35.47 μs   ±244.41%       33.67 μs       49.83 μs

Comparison: 
TailSinglePass           37.24 K
TailRecursive            34.72 K - 1.07x slower +1.94 μs
Recursive                34.63 K - 1.08x slower +2.02 μs
ZipReduce                34.55 K - 1.08x slower +2.09 μs
RecursiveEnumerate       28.19 K - 1.32x slower +8.61 μs

Memory usage statistics:

Name                  Memory usage
TailSinglePass           170.29 KB
TailRecursive            164.55 KB - 0.97x memory usage -5.73438 KB
Recursive                154.66 KB - 0.91x memory usage -15.62500 KB
ZipReduce                164.60 KB - 0.97x memory usage -5.68750 KB
RecursiveEnumerate       163.97 KB - 0.96x memory usage -6.32031 KB

**All measurements for memory usage were the same**

##### With input 10000 items, wide #####
Name                         ips        average  deviation         median         99th %
TailSinglePass           35.06 K       28.52 μs    ±35.76%       25.88 μs       63.42 μs
TailRecursive            31.49 K       31.75 μs    ±46.01%       29.38 μs       63.50 μs
ZipReduce                30.78 K       32.49 μs    ±25.12%       30.21 μs       64.13 μs
Recursive                28.05 K       35.65 μs    ±21.57%       33.58 μs       56.54 μs
RecursiveEnumerate       25.79 K       38.78 μs    ±20.00%       36.92 μs       60.00 μs

Comparison: 
TailSinglePass           35.06 K
TailRecursive            31.49 K - 1.11x slower +3.23 μs
ZipReduce                30.78 K - 1.14x slower +3.97 μs
Recursive                28.05 K - 1.25x slower +7.13 μs
RecursiveEnumerate       25.79 K - 1.36x slower +10.26 μs

Memory usage statistics:

Name                  Memory usage
TailSinglePass           215.77 KB
TailRecursive            226.83 KB - 1.05x memory usage +11.06 KB
ZipReduce                226.88 KB - 1.05x memory usage +11.11 KB
Recursive                211.20 KB - 0.98x memory usage -4.56250 KB
RecursiveEnumerate       211.18 KB - 0.98x memory usage -4.58594 KB

**All measurements for memory usage were the same**

##### With input 1000000 items, narrow #####
Name                         ips        average  deviation         median         99th %
Recursive                   9.20      108.75 ms    ±16.74%       99.03 ms      152.69 ms
ZipReduce                   8.83      113.29 ms    ±17.67%      103.79 ms      164.12 ms
RecursiveEnumerate          8.81      113.56 ms    ±15.98%      101.69 ms      155.48 ms
TailRecursive               8.79      113.71 ms    ±17.59%      102.30 ms      155.65 ms
TailSinglePass              8.28      120.71 ms     ±9.65%      118.69 ms      160.41 ms

Comparison: 
Recursive                   9.20
ZipReduce                   8.83 - 1.04x slower +4.54 ms
RecursiveEnumerate          8.81 - 1.04x slower +4.82 ms
TailRecursive               8.79 - 1.05x slower +4.97 ms
TailSinglePass              8.28 - 1.11x slower +11.97 ms

Memory usage statistics:

Name                  Memory usage
Recursive                203.13 MB
ZipReduce                203.15 MB - 1.00x memory usage +0.0153 MB
RecursiveEnumerate       203.16 MB - 1.00x memory usage +0.0229 MB
TailRecursive            203.15 MB - 1.00x memory usage +0.0153 MB
TailSinglePass           232.26 MB - 1.14x memory usage +29.12 MB

**All measurements for memory usage were the same**

##### With input 1000000 items, wide #####
Name                         ips        average  deviation         median         99th %
TailSinglePass              6.99      143.04 ms    ±13.87%      135.46 ms      199.42 ms
TailRecursive               6.95      143.84 ms     ±7.94%      139.10 ms      191.68 ms
ZipReduce                   6.91      144.70 ms     ±6.22%      141.84 ms      170.99 ms
RecursiveEnumerate          6.51      153.52 ms     ±7.12%      151.90 ms      194.96 ms
Recursive                   6.44      155.18 ms    ±13.19%      146.74 ms      200.63 ms

Comparison: 
TailSinglePass              6.99
TailRecursive               6.95 - 1.01x slower +0.80 ms
ZipReduce                   6.91 - 1.01x slower +1.66 ms
RecursiveEnumerate          6.51 - 1.07x slower +10.48 ms
Recursive                   6.44 - 1.08x slower +12.14 ms

Memory usage statistics:

Name                  Memory usage
TailSinglePass           295.71 MB
TailRecursive            301.19 MB - 1.02x memory usage +5.48 MB
ZipReduce                301.19 MB - 1.02x memory usage +5.48 MB
RecursiveEnumerate       296.28 MB - 1.00x memory usage +0.56 MB
Recursive                289.18 MB - 0.98x memory usage -6.53860 MB

**All measurements for memory usage were the same**

##### With input original #####
Name                         ips        average  deviation         median         99th %
TailSinglePass            2.67 M      374.10 ns ±14947.37%         250 ns         417 ns
Recursive                 2.51 M      398.23 ns  ±9032.98%         292 ns        1416 ns
TailRecursive             2.21 M      453.48 ns ±10881.95%         292 ns        1459 ns
ZipReduce                 1.87 M      535.64 ns  ±8249.00%         334 ns        1500 ns
RecursiveEnumerate        1.61 M      620.82 ns  ±4047.50%         500 ns        1583 ns

Comparison: 
TailSinglePass            2.67 M
Recursive                 2.51 M - 1.06x slower +24.13 ns
TailRecursive             2.21 M - 1.21x slower +79.38 ns
ZipReduce                 1.87 M - 1.43x slower +161.54 ns
RecursiveEnumerate        1.61 M - 1.66x slower +246.72 ns

Memory usage statistics:

Name                  Memory usage
TailSinglePass             2.15 KB
Recursive                  2.15 KB - 1.00x memory usage +0 KB
TailRecursive              2.48 KB - 1.15x memory usage +0.33 KB
ZipReduce                  2.52 KB - 1.17x memory usage +0.38 KB
RecursiveEnumerate         2.52 KB - 1.17x memory usage +0.38 KB

**All measurements for memory usage were the same**

Hi @chgeuer

Thank you! Have added extra approach and tested it. I wanted to measure how different approaches work for this task. Also wanted to verify if tail-recursive approach is much faster than recursive one.

Originally (in the Twitter post) the algorithm used Enum.uniq/1, (internally uniq_list/3). Recursive algorithm used Enum.dedup/1, (which is single tail-recursive pass + reverse).

Then both tail-recursive and recursive algorithms migrated to Enum.dedup/1, but recursive algorithm was still faster. It iterated through the list only once (I do not count initial calls to Enum.sort/1 and Enum.dedup/1. Tail-recursive algorithm needed 3 passes - create ranges, reverse the result, and wrap the results to a struct.

I removed this overhead in the next iteration, by combining Enum.map/2 and Enum.reverse/1 into a single pass of ConsecutiveSequence.reverse/2 in this comment. Now tail-recursive approach started to work much faster than recursive one. It used only 2 passes instead of 3 (generate ranges and map + reverse).

I looked into different other approaches to speed up the algorithm. Let's see what do they use:

1. TailRecursive - Slightly optimised version of original post.
2. TailSinglePass - Tries to do only one tail-recursive pass. It uses Enum.sort/2 function, so the input to impl/3 will be in descending order. This allows us to omit final reverse/2 at all. We already sort the data, so why not to leverage this. I have also removed a call to Enum.dedup/1, because added deduplication to the impl/3 function. It seems this approach is the fastest in most cases (except the case when we have very large number of items, but the distribution is narrow).
3. RecursiveEnumerate - uses single pass on the list. It finds new range in the impl/2 function. Then it traverses this range, and gathers all items into a single range item via enumerate_sequence/2.
4. Recursive - Single pass on the list. It uses a head of accumulator to check if impl/3 can extend the range.
5. ZipReduce - Uses idea to compare a list to its shifted copy with Enum.zip_reduce/4. Needs 2 passes - to build ranges and to reverse them.

Of course there could be more ways to combine ideas from these approaches, to gain more speed, but it is would be more difficult to maintain them in future. The results are pretty similar, especially on large number of items.