Remove numbers which are repeated several times in a row - groovy

Question

I have collection
def list = [4,1,1,1,3,5,1,1]
and I need to remove numbers which are repeated three times in a row. As a result I have to get an [4,3,5,1,1]. How to do this in groovy ?

Answer 1

This can be done by copying the list while ensuring the two previous elements are not the same as the one to be copied. If they are, drop the two previous elements, otherwise copy as normal.
This can be implemented with inject like this:
def list = [4,1,1,1,3,5,1,1]
def result = list.drop(2).inject(list.take(2)) { result, element ->
def prefixSize = result.size() - 2
if ([element] * 2 == result.drop(prefixSize)) {
result.take(prefixSize)
} else {
result + element
}
}
assert result == [4,3,5,1,1]

Answer 2

You can calculate the uniques size in the next three elements and drop them when they are 1:
def list = [4,1,1,1,3,5,1,1]
assert removeTriplets(list) == [4,3,5,1,1]
def removeTriplets(list) {
listCopy = [] + list
(list.size()-3).times { index ->
uniques = list[index..(index+2)].unique false
if (uniques.size() == 1)
listCopy = listCopy[0..(index-1)] + listCopy[(index+3)..-1]
}
listCopy
}

Answer 3

Another option is to use Run Length Encoding
First lets define a class which will hold our object and the number of times it occurs in a row:
class RleUnit {
def object
int runLength
RleUnit( object ) {
this( object, 1 )
}
RleUnit( object, int runLength ) {
this.object = object
this.runLength = runLength
}
RleUnit inc() {
new RleUnit( object, runLength + 1 )
}
String toString() { "$object($runLength)" }
}
We can then define a method which will encode a List into a List of RleUnit objects:
List<RleUnit> rleEncode( List list ) {
list.inject( [] ) { r, v ->
if( r && r[ -1 ].object == v ) {
r.take( r.size() - 1 ) << r[ -1 ].inc()
}
else {
r << new RleUnit( v )
}
}
}
And a method that takes a List of RleUnit objects, and unpacks it back to the original list:
List rleDecode( List<RleUnit> rle ) {
rle.inject( [] ) { r, v ->
r.addAll( [ v.object ] * v.runLength )
r
}
}
We can then encode the original list:
def list = [ 4, 1, 1, 1, 3, 5, 1, 1 ]
rle = rleEncode( list )
And filter this RleUnit list with the Groovy find method:
// remove all elements with a runLength of 3
noThrees = rle.findAll { it.runLength != 3 }
unpackNoThrees = rleDecode( noThrees )
assert unpackNoThrees == [ 4, 3, 5, 1, 1 ]
// remove all elements with a runLength of less than 3
threeOrMore = rle.findAll { it.runLength >= 3 }
unpackThreeOrMore = rleDecode( threeOrMore )
assert unpackThreeOrMore == [ 1, 1, 1 ]