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iterator.cr
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iterator.cr
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require "./enumerable"
# An `Iterator` allows processing sequences lazily, as opposed to `Enumerable` which processes
# sequences eagerly and produces an `Array` in most of its methods.
#
# As an example, let's compute the first three numbers in the range `1..10_000_000` that are even,
# multiplied by three. One way to do this is:
#
# ```
# (1..10_000_000).select(&.even?).map { |x| x * 3 }.first(3) # => [6, 12, 18]
# ```
#
# The above works, but creates many intermediate arrays: one for the *select* call,
# one for the *map* call and one for the *take* call. A more efficient way is to invoke
# `Range#each` without a block, which gives us an `Iterator` so we can process the operations
# lazily:
#
# ```
# (1..10_000_000).each.select(&.even?).map { |x| x * 3 }.first(3) # => #< Iterator(T)::First...
# ```
#
# `Iterator` redefines many of `Enumerable`'s method in a lazy way, returning iterators
# instead of arrays.
#
# At the end of the call chain we get back a new iterator: we need to consume it, either
# using `each` or `Enumerable#to_a`:
#
# ```
# (1..10_000_000).each.select(&.even?).map { |x| x * 3 }.first(3).to_a # => [6, 12, 18]
# ```
#
# To implement an `Iterator` you need to define a `next` method that must return the next
# element in the sequence or `Iterator::Stop::INSTANCE`, which signals the end of the sequence
# (you can invoke `stop` inside an iterator as a shortcut).
#
# Additionally, an `Iterator` can implement `rewind`, which must rewind the iterator to
# its initial state. This is needed to implement the `cycle` method.
#
# For example, this is an iterator that returns a sequence of `N` zeros:
#
# ```
# class Zeros
# include Iterator(Int32)
#
# def initialize(@size : Int32)
# @produced = 0
# end
#
# def next
# if @produced < @size
# @produced += 1
# 0
# else
# stop
# end
# end
#
# def rewind
# @produced = 0
# self
# end
# end
#
# zeros = Zeros.new(5)
# zeros.to_a # => [0, 0, 0, 0, 0]
#
# zeros.rewind
# zeros.first(3).to_a # => [0, 0, 0]
# ```
#
# The standard library provides iterators for many classes, like `Array`, `Hash`, `Range`, `String` and `IO`.
# Usually to get an iterator you invoke a method that would usually yield elements to a block,
# but without passing a block: `Array#each`, `Array#each_index`, `Hash#each`, `String#each_char`,
# `IO#each_line`, etc.
module Iterator(T)
include Enumerable(T)
# The class that signals that there are no more elements in an `Iterator`.
class Stop
INSTANCE = new
end
# `IteratorWrapper` eliminates some boilerplate when defining
# an `Iterator` that wraps another iterator.
#
# To use it, include this module in your iterator and make sure that the wrapped
# iterator is stored in the `@iterator` instance variable.
module IteratorWrapper
# Rewinds the wrapped iterator and returns `self`.
def rewind
@iterator.rewind
self
end
# Invokes `next` on the wrapped iterator and returns `stop` if
# the given value was a `Iterator::Stop`. Otherwise, returns the value.
macro wrapped_next
%value = @iterator.next
return stop if %value.is_a?(Stop)
%value
end
end
# Shortcut for `Iterator::Stop::INSTANCE`, to signal that there are no more elements in an iterator.
def stop
Iterator.stop
end
# ditto
def self.stop
Stop::INSTANCE
end
def self.of(element : T)
Singleton(T).new(element)
end
private struct Singleton(T)
include Iterator(T)
def initialize(@element : T)
end
def next
@element
end
def rewind
self
end
end
def self.of(&block : -> T)
SingletonProc(typeof(without_stop(&block))).new(block)
end
private def self.without_stop(&block : -> T)
e = block.call
raise "" if e.is_a?(Iterator::Stop)
e
end
private struct SingletonProc(T)
include Iterator(T)
def initialize(@proc : (-> (T | Iterator::Stop)) | (-> T))
end
def next
@proc.call
end
end
# Returns the next element in this iterator, or `Iterator::Stop::INSTANCE` if there
# are no more elements.
abstract def next
# Rewinds the iterator to its original state.
abstract def rewind
# Returns an iterator that returns elements from the original iterator until
# it is exhausted and then returns the elements of the second iterator.
#
# ```
# iter = (1..2).each.chain(('a'..'b').each)
# iter.next # => 1
# iter.next # => 2
# iter.next # => 'a'
# iter.next # => 'b'
# iter.next # => Iterator::Stop::INSTANCE
# ```
def chain(other : Iterator(U)) forall U
Chain(typeof(self), typeof(other), T, U).new(self, other)
end
private class Chain(I1, I2, T1, T2)
include Iterator(T1 | T2)
def initialize(@iterator1 : I1, @iterator2 : I2)
@iterator1_consumed = false
end
def next
if @iterator1_consumed
@iterator2.next
else
value = @iterator1.next
if value.is_a?(Stop)
@iterator1_consumed = true
value = @iterator2.next
end
value
end
end
def rewind
@iterator1.rewind
@iterator2.rewind
@iterator1_consumed = false
end
end
# Return an iterator that applies the given function to the element and then
# returns it unless it is `nil`. If the returned value would be `nil` it instead
# returns the next non `nil` value.
#
# ```
# iter = [1, nil, 2, nil].each.compact_map { |e| e.try &.*(2) }
# iter.next # => 2
# iter.next # => 4
# iter.next # => Iterator::Stop::INSTANCE
# ```
def compact_map(&func : T -> _)
CompactMap(typeof(self), T, typeof(func.call(first).not_nil!)).new(self, func)
end
private struct CompactMap(I, T, U)
include Iterator(U)
include IteratorWrapper
def initialize(@iterator : I, @func : T -> U?)
end
def next
while true
value = wrapped_next
mapped_value = @func.call(value)
return mapped_value unless mapped_value.is_a?(Nil)
end
end
end
# Returns an iterator that returns consecutive chunks of the size *n*.
#
# ```
# iter = (1..5).each.cons(3)
# iter.next # => [1, 2, 3]
# iter.next # => [2, 3, 4]
# iter.next # => [3, 4, 5]
# iter.next # => Iterator::Stop::INSTANCE
# ```
#
# By default, a new array is created and yielded for each consecutive when invoking `next`.
# * If *reuse* is given, the array can be reused
# * If *reuse* is an `Array`, this array will be reused
# * If *reuse* is truthy, the method will create a new array and reuse it.
#
# This can be used to prevent many memory allocations when each slice of
# interest is to be used in a read-only fashion.
def cons(n : Int, reuse = false)
raise ArgumentError.new "Invalid cons size: #{n}" if n <= 0
Cons(typeof(self), T, typeof(n)).new(self, n, reuse)
end
private struct Cons(I, T, N)
include Iterator(Array(T))
include IteratorWrapper
def initialize(@iterator : I, @n : N, reuse)
if reuse
if reuse.is_a?(Array)
@values = reuse
else
@values = Array(T).new(@n)
end
@reuse = true
else
@values = Array(T).new(@n)
@reuse = false
end
end
def next
loop do
elem = wrapped_next
@values << elem
@values.shift if @values.size > @n
break if @values.size == @n
end
if @reuse
@values
else
@values.dup
end
end
def rewind
@values.clear
super
end
end
# Returns an iterator that repeatedly returns the elements of the original
# iterator forever starting back at the beginning when the end was reached.
#
# ```
# iter = ["a", "b", "c"].each.cycle
# iter.next # => "a"
# iter.next # => "b"
# iter.next # => "c"
# iter.next # => "a"
# iter.next # => "b"
# iter.next # => "c"
# iter.next # => "a"
# # and so an and so on
# ```
def cycle
Cycle(typeof(self), T).new(self)
end
private struct Cycle(I, T)
include Iterator(T)
include IteratorWrapper
def initialize(@iterator : I)
end
def next
value = @iterator.next
if value.is_a?(Stop)
@iterator.rewind
@iterator.next
else
value
end
end
end
# Returns an iterator that repeatedly returns the elements of the original
# iterator starting back at the beginning when the end was reached,
# but only *n* times.
#
# ```
# iter = ["a", "b", "c"].each.cycle(2)
# iter.next # => "a"
# iter.next # => "b"
# iter.next # => "c"
# iter.next # => "a"
# iter.next # => "b"
# iter.next # => "c"
# iter.next # => Iterator::Stop::INSTANCE
# ```
def cycle(n : Int)
CycleN(typeof(self), T, typeof(n)).new(self, n)
end
private class CycleN(I, T, N)
include Iterator(T)
include IteratorWrapper
def initialize(@iterator : I, @n : N)
@count = 0
end
def next
return stop if @count >= @n
value = @iterator.next
if value.is_a?(Stop)
@count += 1
return stop if @count >= @n
@iterator.rewind
@iterator.next
else
value
end
end
def rewind
@count = 0
super
end
end
def each
self
end
# Calls the given block once for each element, passing that element
# as a parameter.
#
# ```
# iter = ["a", "b", "c"].each
# iter.each { |x| print x, " " } # Prints "a b c"
# ```
def each : Nil
while true
value = self.next
break if value.is_a?(Stop)
yield value
end
end
# Returns an iterator that then returns slices of *n* elements of the initial
# iterator.
#
# ```
# iter = (1..9).each.each_slice(3)
# iter.next # => [1, 2, 3]
# iter.next # => [4, 5, 6]
# iter.next # => [7, 8, 9]
# iter.next # => Iterator::Stop::INSTANCE
# ```
#
# By default, a new array is created and yielded for each consecutive when invoking `next`.
# * If *reuse* is given, the array can be reused
# * If *reuse* is an `Array`, this array will be reused
# * If *reuse* is truthy, the method will create a new array and reuse it.
#
# This can be used to prevent many memory allocations when each slice of
# interest is to be used in a read-only fashion.
def each_slice(n, reuse = false)
slice(n, reuse)
end
# Returns an iterator that flattens nested iterators and arrays into a single iterator
# whose type is the union of the simple types of all of the nested iterators and arrays
# (and their nested iterators and arrays, and so on).
#
# ```
# iter = [(1..2).each, ('a'..'b').each].each.flatten
# iter.next # => 1
# iter.next # => 2
# iter.next # => 'a'
# iter.next # => 'b'
# iter.next # => Iterator::Stop::INSTANCE
# ```
def flatten
Flatten(typeof(Flatten.iterator_type(self)), typeof(Flatten.element_type(self))).new(self)
end
private struct Flatten(I, T)
include Iterator(T)
@iterator : I
@stopped : Array(I)
@generators : Array(I)
def initialize(@iterator)
@generators = [@iterator]
@stopped = [] of I
end
def next
case value = @generators.last.next
when Iterator
@generators.push value
self.next
when Array
@generators.push value.each
self.next
when Stop
if @generators.size == 1
stop
else
@stopped << @generators.pop
self.next
end
else
value
end
end
def rewind
@generators.each &.rewind
@generators.clear
@generators << @iterator
@stopped.each &.rewind
@stopped.clear
self
end
def self.element_type(element)
case element
when Stop
raise ""
when Iterator
element_type(element.next)
when Array
element_type(element.each)
else
element
end
end
def self.iterator_type(iter)
case iter
when Iterator
iter || iterator_type iter.next
when Array
iterator_type iter.each
else
raise ""
end
end
end
# Returns a new iterator with the concatenated results of running the block
# (which is expected to return arrays or iterators)
# once for every element in the collection.
#
# ```
# iter = [1, 2, 3].each.flat_map { |x| [x, x] }
#
# iter.next # => 1
# iter.next # => 1
# iter.next # => 2
#
# iter = [1, 2, 3].each.flat_map { |x| [x, x].each }
#
# iter.to_a # => [1, 1, 2, 2, 3, 3]
# ```
def flat_map(&func : T -> Array(U) | Iterator(U) | U) forall U
FlatMap(typeof(self), U, typeof(FlatMap.iterator_type(self, func)), typeof(func)).new self, func
end
private class FlatMap(I0, T, I, F)
include Iterator(T)
include IteratorWrapper
@iterator : I0
@func : F
@nest_iterator : I?
@stopped : Array(I)
def initialize(@iterator, @func)
@nest_iterator = nil
@stopped = [] of I
end
def next
if iter = @nest_iterator
value = iter.next
if value.is_a?(Stop)
@stopped << iter
@nest_iterator = nil
self.next
else
value
end
else
case value = @func.call wrapped_next
when Array
@nest_iterator = value.each
self.next
when Iterator
@nest_iterator = value
self.next
else
value
end
end
end
def rewind
@nest_iterator.try &.rewind
@nest_iterator = nil
@stopped.each &.rewind
@stopped.clear
super
end
def self.iterator_type(iter, func)
value = iter.next
raise "" if value.is_a?(Stop)
case value = func.call value
when Array
value.each
when Iterator
value
else
raise ""
end
end
end
# Returns an iterator that chunks the iterator's elements in arrays of *size*
# filling up the remaining elements if no element remains with `nil` or a given
# optional parameter.
#
# ```
# iter = (1..3).each.in_groups_of(2)
# iter.next # => [1, 2]
# iter.next # => [3, nil]
# iter.next # => Iterator::Stop::INSTANCE
# ```
# ```
# iter = (1..3).each.in_groups_of(2, 'z')
# iter.next # => [1, 2]
# iter.next # => [3, 'z']
# iter.next # => Iterator::Stop::INSTANCE
# ```
#
# By default, a new array is created and yielded for each group.
# * If *reuse* is given, the array can be reused
# * If *reuse* is an `Array`, this array will be reused
# * If *reuse* is truthy, the method will create a new array and reuse it.
#
# This can be used to prevent many memory allocations when each slice of
# interest is to be used in a read-only fashion.
def in_groups_of(size : Int, filled_up_with = nil, reuse = false)
raise ArgumentError.new("Size must be positive") if size <= 0
InGroupsOf(typeof(self), T, typeof(size), typeof(filled_up_with)).new(self, size, filled_up_with, reuse)
end
private struct InGroupsOf(I, T, N, U)
include Iterator(Array(T | U))
include IteratorWrapper
@reuse : Array(T | U)?
def initialize(@iterator : I, @size : N, @filled_up_with : U, reuse)
if reuse
if reuse.is_a?(Array)
@reuse = reuse
else
@reuse = Array(T | U).new(@size)
end
else
@reuse = nil
end
end
def next
value = wrapped_next
if reuse = @reuse
reuse.clear
array = reuse
else
array = Array(T | U).new(@size)
end
array << value
(@size - 1).times do
new_value = @iterator.next
new_value = @filled_up_with if new_value.is_a?(Stop)
array << new_value
end
array
end
end
# Returns an iterator that applies the given block to the next element and
# returns the result.
#
# ```
# iter = [1, 2, 3].each.map &.*(2)
# iter.next # => 2
# iter.next # => 4
# iter.next # => 6
# iter.next # => Iterator::Stop::INSTANCE
# ```
def map(&func : T -> U) forall U
Map(typeof(self), T, U).new(self, func)
end
private struct Map(I, T, U)
include Iterator(U)
include IteratorWrapper
def initialize(@iterator : I, @func : T -> U)
end
def next
value = wrapped_next
@func.call(value)
end
end
# Returns an iterator that only returns elements for which the passed in
# block returns a falsey value.
#
# ```
# iter = [1, 2, 3].each.reject &.odd?
# iter.next # => 2
# iter.next # => Iterator::Stop::INSTANCE
# ```
def reject(&func : T -> U) forall U
Reject(typeof(self), T, U).new(self, func)
end
private struct Reject(I, T, B)
include Iterator(T)
include IteratorWrapper
def initialize(@iterator : I, @func : T -> B)
end
def next
while true
value = wrapped_next
unless @func.call(value)
return value
end
end
end
end
# Returns an iterator that only returns elements for which the passed
# in block returns a truthy value.
#
# ```
# iter = [1, 2, 3].each.select &.odd?
# iter.next # => 1
# iter.next # => 3
# iter.next # => Iterator::Stop::INSTANCE
# ```
def select(&func : T -> U) forall U
Select(typeof(self), T, U).new(self, func)
end
private struct Select(I, T, B)
include Iterator(T)
include IteratorWrapper
def initialize(@iterator : I, @func : T -> B)
end
def next
while true
value = wrapped_next
if @func.call(value)
return value
end
end
end
end
# Returns an iterator that skips the first *n* elements and only returns
# the elements after that.
#
# ```
# iter = (1..3).each.skip(2)
# iter.next # -> 3
# iter.next # -> Iterator::Stop::INSTANCE
# ```
def skip(n : Int)
raise ArgumentError.new "Attempted to skip negative size: #{n}" if n < 0
Skip(typeof(self), T, typeof(n)).new(self, n)
end
private class Skip(I, T, N)
include Iterator(T)
include IteratorWrapper
def initialize(@iterator : I, @n : N)
@original = @n
end
def next
while @n > 0
@n -= 1
wrapped_next
end
@iterator.next
end
def rewind
@n = @original
super
end
end
# Returns an iterator that only starts to return elements once the given block
# has returned falsey value for one element.
#
# ```
# iter = [1, 2, 3, 4, 0].each.skip_while { |i| i < 3 }
# iter.next # => 3
# iter.next # => 4
# iter.next # => 0
# iter.next # => Iterator::Stop::INSTANCE
# ```
def skip_while(&func : T -> U) forall U
SkipWhile(typeof(self), T, U).new(self, func)
end
private class SkipWhile(I, T, U)
include Iterator(T)
include IteratorWrapper
def initialize(@iterator : I, @func : T -> U)
@returned_false = false
end
def next
while true
value = wrapped_next
return value if @returned_false == true
unless @func.call(value)
@returned_false = true
return value
end
end
end
def rewind
@returned_false = false
super
end
end
# Alias of `each_slice`.
def slice(n : Int, reuse = false)
raise ArgumentError.new "Invalid slice size: #{n}" if n <= 0
Slice(typeof(self), T, typeof(n)).new(self, n, reuse)
end
private struct Slice(I, T, N)
include Iterator(Array(T))
include IteratorWrapper
@reuse : Array(T)?
def initialize(@iterator : I, @n : N, reuse)
if reuse
if reuse.is_a?(Array)
@reuse = reuse
else
@reuse = Array(T).new(@n)
end
else
@reuse = nil
end
end
def next
if reuse = @reuse
reuse.clear
values = reuse
else
values = Array(T).new(@n)
end
@n.times do
value = @iterator.next
break if value.is_a?(Stop)
values << value
end
if values.empty?
stop
else
values
end
end
end
# Returns an iterator that only returns every *n*th element, starting with the
# first.
#
# ```
# iter = (1..6).each.step(2)
# iter.next # => 1
# iter.next # => 3
# iter.next # => 5
# iter.next # => Iterator::Stop::INSTANCE
# ```
def step(n : Int)
Step(self, T, typeof(n)).new(self, n)
end
private struct Step(I, T, N)
include Iterator(T)
include IteratorWrapper
def initialize(@iterator : I, @by : N)
raise ArgumentError.new("n must be greater or equal 1") if @by < 1
end
def next
value = @iterator.next
return stop if value.is_a?(Stop)
(@by - 1).times do
@iterator.next
end
value
end
end
# Returns an iterator that only returns the first *n* elements of the
# initial iterator.
#
# ```
# iter = ["a", "b", "c"].each.first 2
# iter.next # => "a"
# iter.next # => "b"
# iter.next # => Iterator::Stop::INSTANCE
# ```
def first(n : Int)
raise ArgumentError.new "Attempted to take negative size: #{n}" if n < 0
First(typeof(self), T, typeof(n)).new(self, n)
end
private class First(I, T, N)
include Iterator(T)
include IteratorWrapper
def initialize(@iterator : I, @n : N)
@original = @n
end
def next
if @n > 0
@n -= 1
wrapped_next
else
stop
end
end
def rewind
@n = @original
super
end
end
# Returns an iterator that returns elements while the given block returns a
# truthy value.
#
# ```
# iter = (1..5).each.take_while { |i| i < 3 }
# iter.next # => 1
# iter.next # => 2
# iter.next # => Iterator::Stop::INSTANCE
# ```
def take_while(&func : T -> U) forall U
TakeWhile(typeof(self), T, U).new(self, func)
end
private class TakeWhile(I, T, U)
include Iterator(T)
include IteratorWrapper
def initialize(@iterator : I, @func : T -> U)
@returned_false = false
end
def next
return stop if @returned_false == true
value = wrapped_next
if @func.call(value)
value
else
@returned_false = true
stop
end
end
def rewind
@returned_false = false
super
end
end
# Returns an iterator that calls the given block with the next element of the
# iterator when calling `next`, still returning the original element.
#
# ```
# a = 0
# iter = (1..3).each.tap { |x| a += x }
# iter.next # => 1
# a # => 1
# iter.next # => 2
# a # => 3
# iter.next # => 3
# a # => 6
# iter.next # => Iterator::Stop::INSTANCE
# ```
def tap(&block : T ->)
Tap(typeof(self), T).new(self, block)
end
private struct Tap(I, T)
include Iterator(T)
include IteratorWrapper
def initialize(@iterator : I, @proc : T ->)
end
def next
value = wrapped_next
@proc.call(value)
value
end
end
# Returns an iterator that only returns unique values of the original
# iterator.
#
# ```
# iter = [1, 2, 1].each.uniq
# iter.next # => 1