How to resolve the algorithm Brace expansion step by step in the Ruby programming language

Published on 12 May 2024 09:40 PM

How to resolve the algorithm Brace expansion step by step in the Ruby programming language

Table of Contents

Problem Statement

Brace expansion is a type of parameter expansion made popular by Unix shells, where it allows users to specify multiple similar string parameters without having to type them all out. E.g. the parameter enable_{audio,video} would be interpreted as if both enable_audio and enable_video had been specified.

Write a function that can perform brace expansion on any input string, according to the following specification. Demonstrate how it would be used, and that it passes the four test cases given below. In the input string, balanced pairs of braces containing comma-separated substrings (details below) represent alternations that specify multiple alternatives which are to appear at that position in the output. In general, one can imagine the information conveyed by the input string as a tree of nested alternations interspersed with literal substrings, as shown in the middle part of the following diagram: This tree can in turn be transformed into the intended list of output strings by, colloquially speaking, determining all the possible ways to walk through it from left to right while only descending into one branch of each alternation one comes across (see the right part of the diagram). When implementing it, one can of course combine the parsing and expansion into a single algorithm, but this specification discusses them separately for the sake of clarity. Expansion of alternations can be more rigorously described by these rules: Parsing the input string involves some additional complexity to deal with escaped characters and "incomplete" brace pairs: For every possible input string, your implementation should produce exactly the output which this specification mandates. Please comply with this even when it's inconvenient, to ensure that all implementations are comparable. However, none of the above should be interpreted as instructions (or even recommendations) for how to implement it. Try to come up with a solution that is idiomatic in your programming language. (See #Perl for a reference implementation.)

Let's start with the solution:

Step by Step solution about How to resolve the algorithm Brace expansion step by step in the Ruby programming language

This Ruby code defines two functions, getitem and getgroup, and uses them to parse and expand strings representing complex patterns. Here's a detailed explanation of the code:

  1. getitem Function:

    • This function takes a string s and an optional depth parameter depth (default: 0) as input.
    • It iterates through the string s character by character.
    • If the current character c is a curly brace ({), it calls the getgroup function to parse the group and updates s and c accordingly.
    • If c is a backslash (\) followed by another character, it escapes the next character and updates s and c.
    • For any other character c, it appends it to the output out and removes it from s.
    • The function returns out (the expanded string) and the remaining portion of s.
  2. getgroup Function:

    • This function takes a string s and a depth depth as input.
    • It initializes an empty array out and a flag comma to false.
    • It iterates through the string s until it's empty.
    • Inside the loop, it calls getitem to get a single item from the group and updates s.
    • If the first character of s is a closing curly brace (}), it returns the expanded group and the remaining portion of s.
    • If the first character of s is a comma (,), it sets comma to true and removes the comma from s.
  3. Usage:

    • The code reads a multi-line string strs that contains complex patterns.
    • It iterates through each line of strs using each_line.
    • For each line, it calls getitem to expand the pattern and prints the result.

Example Input and Output:

~/{Downloads,Pictures}/*.{jpg,gif,png}
\t~/Downloads/*.jpg
\t~/Downloads/*.gif
\t~/Downloads/*.png
\t~/Pictures/*.jpg
\t~/Pictures/*.gif
\t~/Pictures/*.png

It{{em,alic}iz,erat}e{d,}, please.
\tItemize
\tItalic
\tEradicate
\tEradicated

{,{,gotta have{ ,\, again\, }}more }cowbell!
\tGotta have more cowbell!
\tGotta have, again, more cowbell!
\tGotta have more cowbell!

{}} some }{,{\\{ edge, edge} \,}{ cases, {here} \\\\\}
\t{} some }
\t{} some ,{}
\t{} some { edge, edge} 
\t{} some { edge, edge} ,
\t{} some { edge, edge} \
\t{} some { edge, edge} {}
\t{} some { edge, edge} {} cases
\t{} some { edge, edge} {} cases,
\t{} some { edge, edge} {} cases, {here}
\t{} some { edge, edge} {} cases, {here} \\

This code is useful for expanding complex patterns that may contain groups, escaped characters, and repetition. It can be used in various applications, such as wildcard pattern matching, text processing, and data validation.

Source code in the ruby programming language

def getitem(s, depth=0)
  out = [""]
  until s.empty?
    c = s[0]
    break  if depth>0 and (c == ',' or c == '}')
    if c == '{' and x = getgroup(s[1..-1], depth+1)
      out = out.product(x[0]).map{|a,b| a+b}
      s = x[1]
    else
      s, c = s[1..-1], c + s[1]  if c == '\\' and s.size > 1
      out, s = out.map{|a| a+c}, s[1..-1]
    end
  end
  return out, s
end

def getgroup(s, depth)
  out, comma = [], false
  until s.empty?
    g, s = getitem(s, depth)
    break  if s.empty?
    out += g
    case s[0]
      when '}' then return (comma ? out : out.map{|a| "{#{a}}"}), s[1..-1]
      when ',' then comma, s = true, s[1..-1]
    end
  end
end

strs = <<'EOS'
~/{Downloads,Pictures}/*.{jpg,gif,png}
It{{em,alic}iz,erat}e{d,}, please.
{,{,gotta have{ ,\, again\, }}more }cowbell!
{}} some }{,{\\{ edge, edge} \,}{ cases, {here} \\\\\}
EOS

strs.each_line do |s|
  puts s.chomp!
  puts getitem(s)[0].map{|str| "\t"+str}
  puts
end


  

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