defmodule Markov do
@moduledoc """
Markov-chain-based trained text generator implementation.
Next token prediction uses two previous tokens.
"""
defstruct links: %{[:start, :start] => %{end: 1}},
sanitize_tokens: false,
shift: false
# Conditionally sanitizes a token list
@spec cond_sanitize_tokens([any()], %Markov{}) :: [any()]
defp cond_sanitize_tokens(tokens, chain) do
if chain.sanitize_tokens do
tokens |> Enum.map(&Markov.TextUtil.sanitize_token/1)
else tokens end
end
# Conditionally shifts probabilities
@spec cond_shift_probs(%{[any()] => any()}, %Markov{}) :: %{[any()] => any()}
def cond_shift_probs(links, %Markov{shift: shift}) do
if shift and map_size(links) >= 2 do
# sort links by their probability
links = links
|> Enum.into([])
|> Enum.sort(fn {_, foo}, {_, bar} -> foo > bar end)
# choose the peak
peak = max(1, length(links) * 0.10) |> floor()
{_, peak_prob} = Enum.at(links, peak)
# determine by how much the first most probable path
# is more likely than the peak
{_, first_prob} = Enum.at(links, 0)
ratio = min(first_prob / peak_prob, 10)
# https://www.desmos.com/calculator/mq3qjg8zpm
# fun fact: this is literally the first time ever in my
# 8 year long programming career that i actually had to
# use "real" maths
exp = 1.7
links = for i <- 0 .. length(links)-1 do
{k, _} = Enum.at(links, i)
{k, v} = cond do
# massively dampen probabilities before the peak
i < peak ->
offset = (first_prob / (ratio ** exp))
coeff = (peak_prob - (peak_prob * ratio / (ratio ** exp + 1))) / (peak ** ratio)
{k, (coeff * (i ** ratio)) + offset}
# leave the peak as is
i == peak ->
{k, peak_prob}
# gradual fall-off
i > peak ->
len = length(links)
coeff = peak_prob / ((len - peak - 1) ** (1 / ratio))
{k, coeff * ((-i + len - 1) ** (1 / ratio))}
end
# last step: rounding
{k, round(v)}
end
links |> Enum.into(%{})
else links end
end
@doc """
Trains `chain` using `text` or a list of `tokens`.
Returns the modified chain.
## Example
chain = %Markov{}
|> Markov.train("hello, world!")
|> Markov.train("example string number two")
|> Markov.train("hello, Elixir!")
|> Markov.train("fourth string")
chain = %Markov{}
|> Markov.train(["individual tokens", :can_be, 'arbitrary terms'])
"""
@spec train(%Markov{}, String.t() | [any()]) :: %Markov{}
def train(%Markov{}=chain, text) when is_binary(text) do
tokens = String.split(text)
train(chain, tokens)
end
def train(%Markov{}=chain, tokens) when is_list(tokens) do
# add start and end tokens
tokens = [:start, :start] ++ tokens ++ [:end]
# adjust link weights
new_links = Enum.reduce Markov.ListUtil.ttuples(tokens), chain.links, fn {first, second, third}, acc ->
from = [first, second] |> cond_sanitize_tokens(chain)
to = third
links_from = acc[from]
links_from = if links_from == nil do %{} else links_from end
if links_from[to] == nil do
Map.put(acc, from, Map.put(links_from, to, 1))
else
Map.put(acc, from, Map.put(links_from, to, links_from[to] + 1))
end
end
# forcefully break the start -> end link
new_links = Map.put(new_links, [:start, :start], Map.delete(new_links[[:start, :start]], :end))
chain = %{chain | links: new_links}
chain
end
@doc """
Removes a `token` from all generation paths `chain` could produce.
Returns the modifier chain
## Example
iex> %Markov{} |>
...> Markov.train("a b c") |>
...> Markov.forget_token("b") |>
...> Markov.generate_text()
"a"
"""
@spec forget_token(%Markov{}, any()) :: %Markov{}
def forget_token(%Markov{}=chain, token) do
# sanitize the token
token = if chain.sanitize_tokens do
token |> Markov.TextUtil.sanitize_token
else token end
# remove links that point to the token
%{chain | links: chain.links |> Enum.map(fn
{[_, _]=k, v} ->
{k, Enum.filter(v, fn {k, _} -> k != token end) |> Enum.into(%{})}
{k, v} -> {k, v}
end) |> Enum.into(%{})
# terminate states that point nowhere
|> Enum.map(fn
{k, %{}=map} when map_size(map) == 0 ->
{k, %{end: 1}}
{k, v} -> {k, v}
end) |> Enum.into(%{})}
end
@doc """
Predicts the next state of a `chain` assuming `current` state.
Note: current state conists of two tokens.
Returns the next predicted state.
## Example
iex> %Markov{} |> Markov.train("1 2 3 4 5") |> Markov.next_state(["2", "3"])
"4"
iex> %Markov{} |> Markov.train("1 2") |> Markov.next_state([:start, :start])
"1"
iex> %Markov{} |> Markov.train([:a, :b, :c]) |> Markov.next_state([:a, :b])
:c
"""
@spec next_state(%Markov{}, any()) :: any()
def next_state(%Markov{}=chain, current) do
# sanitize state
current = current |> cond_sanitize_tokens(chain)
# get links from current state
# (enforce constant order by converting to proplist)
links = chain.links[current]
|> cond_shift_probs(chain)
|> Enum.into([])
# do the magic
sum = Enum.unzip(links)
|> Tuple.to_list
|> List.last
|> Enum.sum
:rand.uniform(sum) - 1 |> probabilistic_select(links, sum)
end
@doc """
Generates a list of tokens using the `chain`
Optionally prepends `acc` to it and assumes the previous
two states were `[state1, state2]=state`. The amount of
the resulting token list is limited by `limit`.
Returns the generated list.
## Example
iex> %Markov{} |> Markov.train([:a, :b, :c]) |> Markov.generate_tokens()
[:a, :b, :c]
iex> %Markov{} |> Markov.train([:a, :b, :c]) |>
...> Markov.generate_tokens([], [:a, :b])
[:c]
"""
@spec generate_tokens(%Markov{}, [any()], [any()], integer()) :: String.t()
def generate_tokens(%Markov{}=chain, acc \\ [], state \\ [:start, :start], limit \\ 100) do
# iterate through states until :end
new_state = next_state(chain, state)
if new_state == :end or limit <= 0 do
acc
else
generate_tokens(chain, acc ++ [new_state], [state |> Enum.at(1), new_state], limit - 1)
end
end
@doc """
Generates a string of text using the `chain`
Optionally assumes the previous two states were `[state1, state2]=state`.
Returns the generated text.
## Example
iex> %Markov{} |> Markov.train("hello, world!") |> Markov.generate_text()
"hello, world!"
iex> %Markov{} |> Markov.train("hello, world!") |>
...> Markov.generate_text([:start, "hello,"])
"world!"
"""
@spec generate_text(%Markov{}, [any()]) :: String.t()
def generate_text(%Markov{}=chain, state \\ [:start, :start]) do
generate_tokens(chain, [], state) |> Enum.join(" ")
end
@spec probabilistic_select(integer(), list({any(), integer()}), integer(), integer()) :: any()
defp probabilistic_select(number, [{name, add} | tail]=_choices, sum, acc \\ 0) do
if (number >= acc) and (number < acc + add) do
name
else
probabilistic_select(number, tail, sum, acc + add)
end
end
@doc """
Enables token sanitization on a `chain`.
When this mode is enabled, the chain doesn't understand the difference similar textual tokens.
This mode can't be disabled once it has been enabled.
Returns the modified chain.
"""
@spec enable_token_sanitization(%Markov{}) :: %Markov{}
def enable_token_sanitization(%Markov{}=chain) do
sanitize = fn t -> t |> Enum.map(&Markov.TextUtil.sanitize_token/1) end
find_similar_states = fn [_,_]=state ->
state = state |> sanitize.()
chain.links |> Map.keys |> Enum.filter(fn s ->
s |> sanitize.() == state
end)
end
combine_states = fn states ->
states |> Enum.reduce(%{}, fn state, acc ->
Map.merge(acc, state, fn _, v1, v2 -> v1 + v2 end)
end)
end
{new_links, _} = chain.links |> Map.keys |> Enum.reduce({%{}, []} , fn k, {map, ignore} ->
sanitized = sanitize.(k)
unless sanitized in ignore do
similar = find_similar_states.(k) # also includes this one
combined = similar |> Enum.map(fn k -> Map.get(chain.links, k) end) |> combine_states.()
map = map |> Map.put(sanitized, combined)
{map, ignore ++ [sanitized]}
else
{map, ignore}
end
end)
%{chain | links: new_links, sanitize_tokens: true}
end
end
