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#!/usr/bin/env perl
use strict;
use warnings;
use feature qw{ say signatures state };
no warnings qw{ experimental };
use List::Util qw{min};
my @input;
push @input, [qw{ kitten sitting }];
push @input, [qw{ sunday monday }];
push @input, [qw{ slight lights }];
push @input, [qw{ fed feed }];
push @input, [qw{ test test }];
push @input, [qw{ test text }];
for my $x (@input) {
edit_distance( $x->@* );
}
# I first found the Levenshtein distance when poking in the center of
# perlbrew. This is how it knows, when you type `perlbrew xeec` to
# suggest you try `perlbrew exec` instead. This gives us the first
# part, the number of changes you'd need to get from S1 to S2. I thought
# about but never implemented it as a kind-of 404 page for endpoints:
# you look like you're looking for "index" but typed "idnex", for example.
# What we don't get from editdist is WHICH changes those would be.
# therefore, it's a half-solution for this problem.
sub edit_distance ( $s1, $s2 ) {
editdist( $s1, $s2 );
}
# let's try to make this a whole solution
sub editdist ( $s1, $s2 ) {
my @s1 = split //, $s1;
my @s2 = split //, $s2;
my @d;
$d[$_][0] = $_ for ( 0 .. @s1 );
$d[0][$_] = $_ for ( 0 .. @s2 );
# this creates a two-dimensional array that starts like this:
# [0,1,2,3,4,5,6]
# [1, , , , , , ]
# [2, , , , , , ]
# [3, , , , , , ]
# [4, , , , , , ]
# [5, , , , , , ]
# which gets filled in iteratively in the nested loops below
for my $i ( 1 .. @s1 ) {
for my $j ( 1 .. @s2 ) {
# Let's understand this. For a particular i and j position
# if the two agree, D[i][j] equals D[-i][-j]
# if they don't however, we find the value above
# the value before and the one above and before, find
# the lowest, and add one.
# this means that $d[-1][-1] would have the total
$d[$i][$j] = (
$s1[ $i - 1 ] eq $s2[ $j - 1 ]
? $d[ $i - 1 ][ $j - 1 ]
: 1 + min(
$d[ $i - 1 ][$j],
$d[$i][ $j - 1 ],
$d[ $i - 1 ][ $j - 1 ]
)
);
}
}
print <<"END";
Input: S1: $s1
S2: $s2
Change Count: $d[-1][-1]
END
my @operations = find_changes( \@d, \@s1, \@s2 );
my $c = 1;
for my $operation ( reverse @operations ) {
say qq{ Operation $c: $operation};
$c++;
}
# returns the last column of the last row, which SHOULD
# be the min changes.
return $d[-1][-1];
}
# d = 2-dimensional array, result of LD
# s1 = array of first input
# s2 = array of second input
# i = current row
# j = current column
sub find_changes ( $d, $s1, $s2, $i = -1, $j = -1 ) {
# -1 means implicit end of array, which gets turned
# into explicit end of array
$i = $i == -1 ? -1 + scalar $d->@* : $i;
$j = $j == -1 ? -1 + scalar $d->[-1]->@* : $j;
# zero means that there are no more changes
return if $d->[$i][$j] == 0;
my $v = $d->[$i][$j];
my $v1 = $d->[ $i - 1 ][ $j - 1 ];
my $v2 = $d->[$i][ $j - 1 ];
my $v3 = $d->[ $i - 1 ][$j];
my @output;
if (0) {
# The impossible situation we never planned for.
# I LIKE to put an if ( false ) statement first,
# so it's easy to just move the elsifs around
# should I decide or discern that I have the order
# wrong.
}
elsif ( $v1 == $v - 1 ) {
my $c1 = $s1->[ $i - 1 ];
my $c2 = $s2->[ $j - 1 ];
push @output, qq{replace '$c1' with '$c2'};
push @output, find_changes( $d, $s1, $s2, $i - 1, $j - 1 );
}
elsif ( $v2 == $v - 1 ) {
my $c1 = $s1->[ $i - 1 ];
my $c2 = $s2->[ $j - 1 ];
if ( $j == scalar $s2->@* ) {
push @output, qq{insert '$c2' at the end};
}
else { push @output, qq{insert '$c2'}; }
push @output, find_changes( $d, $s1, $s2, $i, $j - 1 );
}
elsif ( $v3 == $v - 1 ) {
my $c1 = $s1->[ $i - 1 ];
my $c2 = $s2->[ $j - 1 ];
push @output, qq{remove '$c1' from beginning} if $i == 1;
push @output, qq{remove '$c1'} if $i != 1;
push @output, find_changes( $d, $s1, $s2, $i - 1, $j );
}
elsif ( $v1 == $v ) {
push @output, find_changes( $d, $s1, $s2, $i - 1, $j - 1 );
}
elsif ( $v2 == $v ) {
push @output, find_changes( $d, $s1, $s2, $i, $j - 1 );
}
elsif ( $v3 == $v ) {
push @output, find_changes( $d, $s1, $s2, $i - 1, $j );
}
return @output;
}
# -------------------------------------------------------------------
# straight copy of Wikipedia's "Levenshtein Distance"
sub levenshtein_distance {
my ( $f, $g ) = @_;
my @a = split //, $f;
my @b = split //, $g;
# There is an extra row and column in the matrix. This is the
# distance from the empty string to a substring of the target.
my @d;
$d[$_][0] = $_ for ( 0 .. @a );
$d[0][$_] = $_ for ( 0 .. @b );
for my $i ( 1 .. @a ) {
for my $j ( 1 .. @b ) {
$d[$i][$j] = (
$a[ $i - 1 ] eq $b[ $j - 1 ]
? $d[ $i - 1 ][ $j - 1 ]
: 1 + min(
$d[ $i - 1 ][$j],
$d[$i][ $j - 1 ],
$d[ $i - 1 ][ $j - 1 ]
)
);
}
}
return $d[@a][@b];
}
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