to find if any word exists in *every* line in input

repeating_words.groovy

/**
 * Problem: to find if any word exists in *every* line in input
 */

def input =  """This is big line
This is another big big line
There is something common here
"""

/*  APPROACH ONE : order of O(n)
    1. store each unique word in line, in a map against its count 
    2. all the words with occurences = line count are the ones we are looking for
*/

def word_count = [:]             // store word -> occurence map here
def max_count=0, line_count=0    //

input.eachLine { line, n->
    def unique =  [] as HashSet
    line.split(" ").each { unique << it }       // O(n)
    unique.each { word->                        // O(n)
        def count = word_count[word]            // O(1)
        word_count[word]= (count) ? count+1 : 1 // O(1)
    }
    line_count++
}

word_count.each {word,count-> // complexity O(n)
    if (count==line_count) println "*$word* appears in every line"
}


/*  APPROACH TWO : order of O(n) (faster and consumes less memory)
    1. store 1st line's uniq words in set (lets call it 'repeating')
    2. from next line on, only move forward with those words in 'repeating' that also exists in this line 
    3. may abort loop if 'repeating' becomes empy
*/


def repeating = null // would be hashset

for (line in input.split(/\n/)){
    def this_line =  [] as HashSet
    line.split(" ").each {this_line << it } // O(n)

    if (repeating==null) repeating = this_line
    else repeating.retainAll(this_line)     // O(n) as contains(find) is O(1) in HashSet becuase it uses map internally
    
    if (repeating.size()==0) {
        println "no repeating element found"
        break // no further process required
    }
}

if (repeating.size()) println "$repeating appears in every line"