stream

{}[https://github.com/monora/stream/actions/workflows/ruby.yml]

= Extended External Iterators (forward and backward)

== Description

Module +Stream+ defines an interface for {external iterators}[https://wiki.c2.com/?ExternalIterator]. A stream can be seen as an iterator on a sequence of objects +x1,...,xn+. The state of the stream is uniquely determined by the following methods:

• at_beginning?
• at_end?
• current
• peek

State changes are done with the following operations:

• set_to_begin
• set_to_end
• forward
• backward

With the help of the method +current_edge+ the state of a stream +s+ can be exactly defined

s.current_edge == [s.current, s.peek]

If +s+ a stream on [x1,...,xn]. Consider the edges [xi,xi+1] i=1,...,n and [x0,x1] and [xn,xn+1] (x0 and xn+1 are helper elements to define the boundary conditions). Then if +s+ is non empty, the following conditions must be true:

s.at_beginning? <=> s.current_edge == [x0,x1] s.at_end? <=> s.current_edge == [xn,xn+1] s.isEmpty? <=> s.at_beginning? && s.at_end? <=> s.current_edge == [x0,x1] <=> n = 0 s.set_to_end => s.at_end? s.set_to_begin => s.at_beginning?

If 0 <= i < n and s.current_edge == [xi, xi+1] , then:

[s.forward, s.current_edge] == [xi+1, [xi+1, xi+2]]

If 1 <= i < n and s.current_edge == [xi, xi+1] , then:

[s.backward, s.current_edge] == [xi, [xi-1, xi]]

The result of peek is the same as of forward without changing state. The result of current is the same as of backward without changing state.

Module +Stream+ includes +Enumerable+ implementing +each+ in the obvious way.

Not every stream needs to implement +backward+ and +at_beginning?+ thus being not reversable. If they are reversable peek can easily be implemented using +forward+ and +backward+, as is done in module +Stream+. If a stream is not reversable all derived streams provided by the stream module (filter, mapping, concatenation) can be used anyway. Explicit or implicit (via peek or current) uses of backward would throw a +NotImplementedError+.

Classes implementing the stream interface must implement the following methods:

• basic_forward

• basic_backward

• at_end?

• at_beginning?

The methods +set_to_end+ and +set_to_begin+ are by default implemented as:

set_to_end : until at_end?; do basic_forward end set_to_begin : until at_beginning?; do basic_backward end

The methods +forward+ and +backward+ are by default implemented as:

forward: raise EndOfStreamException if at_end?; basic_forward. backward: raise EndOfStreamException if at_beginning?; basic_backward

Thus subclasses must only implement four methods. Efficiency sometimes demands better implementations.

There are several concrete classes implementing the stream interface:

• +Stream::EmptyStream+ (boring)
• +Stream::CollectionStream+ created by the method +Array#create_stream+
• +Stream::FilteredStream+ created by the method +Stream#filtered+
• +Stream::ReversedStream+ created by the method +Stream#reverse+
• +Stream::ConcatenatedStream+ created by the method +Stream#concatenate+
• +Stream::ImplicitStream+ using closures for the basic methods to implement

== Installation

gem install stream

or download the latest sources from the git repository https://github.com/monora/stream.

== Examples

=== Iterate over three streams

g = ('a'..'f').create_stream h = (1..10).create_stream i = (10..20).create_stream

until g.at_end? || h.at_end? || i.at_end? p [g.forward, h.forward, i.forward] end

Output:

["a", 1, 10] ["b", 2, 11] ["c", 3, 12] ["d", 4, 13] ["e", 5, 14] ["f", 6, 15]

=== Concatenate file streams

def filestream fname Stream::ImplicitStream.new { |s| f = open(fname) s.at_end_proc = proc {f.eof?} s.forward_proc = proc {f.readline} # Need not implement backward moving to use the framework } end

(filestream("/etc/passwd") + ('a'..'f').create_stream + filestream("/etc/group")).each do |l| puts l end

=== Two filtered collection streams concatenated and reversed

def newstream; (1..6).create_stream; end s = newstream.filtered { |x| x % 2 == 0 } + newstream.filtered { |x| x % 2 != 0 } s = s.reverse puts "Contents : #{s.to_a.join ' '}" puts "At end? : #{s.at_end?}" puts "At beginning? : #{s.at_beginning?}" puts "2xBackwards : #{s.backward} #{s.backward}" puts "Forward : #{s.forward}" puts "Peek : #{s.peek}" puts "Current : #{s.current}" puts "set_to_begin : Peek=#{s.set_to_begin;s.peek}"

Output:

Contents : 5 3 1 6 4 2 At end? : true At beginning? : false 2xBackwards : 2 4 Forward : 4 Peek : 2 Current : 4 set_to_begin : Peek=5

=== An infinite stream (do not use +set_to_end+!)

def randomStream Stream::ImplicitStream.new { |s| s.set_to_begin_proc = proc {srand 1234} s.at_end_proc = proc {false} s.forward_proc = proc {rand} } end s = randomStream.filtered { |x| x >= 0.5 }.collect { |x| sprintf("%5.2f",x*100) } puts "5 random numbers: #{(1..5).collect {|x| s.forward}}\n"

Output:

5 random numbers: ["62.21", "78.54", "78.00", "80.19", "95.81"]

== License

Author:: Horst Duchene License:: Copyright (c) 2001, 2013, 2016, 2020, 2022 Horst Duchene (Released under the same license as Ruby (see LICENSE))