sub LCS
{
    my $a = shift;                  # array ref
    my $b = shift;                  # array ref or hash ref
    my $matchVector = _longestCommonSubsequence( $a, $b, 0, @_ );
    my @retval;
    my $i;
    for ( $i = 0 ; $i <= $#$matchVector ; $i++ )
    {
        if ( defined( $matchVector->[$i] ) )
        {
            push ( @retval, $a->[$i] );
        }
    }
    return wantarray ? @retval : \@retval;
}

sub LCS_length
{
    my $a = shift;                          # array ref
    my $b = shift;                          # array ref or hash ref
    return _longestCommonSubsequence( $a, $b, 1, @_ );
}

sub LCSidx
{
    my $a= shift @_;
    my $b= shift @_;
    my $match= _longestCommonSubsequence( $a, $b, 0, @_ );
    my @am= grep defined $match->[$_], 0..$#$match;
    my @bm= @{$match}[@am];
    return \@am, \@bm;
}

sub compact_diff
{
    my $a= shift @_;
    my $b= shift @_;
    my( $am, $bm )= LCSidx( $a, $b, @_ );
    my @cdiff;
    my( $ai, $bi )= ( 0, 0 );
    push @cdiff, $ai, $bi;
    while( 1 ) {
        while(  @$am  &&  $ai == $am->[0]  &&  $bi == $bm->[0]  ) {
            shift @$am;
            shift @$bm;
            ++$ai, ++$bi;
        }
        push @cdiff, $ai, $bi;
        last   if  ! @$am;
        $ai = $am->[0];
        $bi = $bm->[0];
        push @cdiff, $ai, $bi;
    }
    push @cdiff, 0+@$a, 0+@$b
        if  $ai < @$a || $bi < @$b;
    return wantarray ? @cdiff : \@cdiff;
}

sub diff
{
    my $a      = shift;    # array ref
    my $b      = shift;    # array ref
    my $retval = [];
    my $hunk   = [];
    my $discard = sub {
        push @$hunk, [ '-', $_[0], $a->[ $_[0] ] ];
    };
    my $add = sub {
        push @$hunk, [ '+', $_[1], $b->[ $_[1] ] ];
    };
    my $match = sub {
        push @$retval, $hunk
            if 0 < @$hunk;
        $hunk = []
    };
    traverse_sequences( $a, $b,
        { MATCH => $match, DISCARD_A => $discard, DISCARD_B => $add }, @_ );
    &$match();
    return wantarray ? @$retval : $retval;
}

sub sdiff
{
    my $a      = shift;    # array ref
    my $b      = shift;    # array ref
    my $retval = [];
    my $discard = sub { push ( @$retval, [ '-', $a->[ $_[0] ], "" ] ) };
    my $add = sub { push ( @$retval, [ '+', "", $b->[ $_[1] ] ] ) };
    my $change = sub {
        push ( @$retval, [ 'c', $a->[ $_[0] ], $b->[ $_[1] ] ] );
    };
    my $match = sub {
        push ( @$retval, [ 'u', $a->[ $_[0] ], $b->[ $_[1] ] ] );
    };
    traverse_balanced(
        $a,
        $b,
        {
            MATCH     => $match,
            DISCARD_A => $discard,
            DISCARD_B => $add,
            CHANGE    => $change,
        },
        @_
    );
    return wantarray ? @$retval : $retval;
}

########################################
my $Root= __PACKAGE__;
package Algorithm::Diff::_impl;
use strict;

sub _Idx()  { 0 } # $me->[_Idx]: Ref to array of hunk indices
            # 1   # $me->[1]: Ref to first sequence
            # 2   # $me->[2]: Ref to second sequence
sub _End()  { 3 } # $me->[_End]: Diff between forward and reverse pos
sub _Same() { 4 } # $me->[_Same]: 1 if pos 1 contains unchanged items
sub _Base() { 5 } # $me->[_Base]: Added to range's min and max
sub _Pos()  { 6 } # $me->[_Pos]: Which hunk is currently selected
sub _Off()  { 7 } # $me->[_Off]: Offset into _Idx for current position
sub _Min() { -2 } # Added to _Off to get min instead of max+1

sub Die
{
    require Carp;
    Carp::confess( @_ );
}

sub _ChkPos
{
    my( $me )= @_;
    return   if  $me->[_Pos];
    my $meth= ( caller(1) )[3];
    Die( "Called $meth on 'reset' object" );
}

sub _ChkSeq
{
    my( $me, $seq )= @_;
    return $seq + $me->[_Off]
        if  1 == $seq  ||  2 == $seq;
    my $meth= ( caller(1) )[3];
    Die( "$meth: Invalid sequence number ($seq); must be 1 or 2" );
}

sub getObjPkg
{
    my( $us )= @_;
    return ref $us   if  ref $us;
    return $us . "::_obj";
}

sub new
{
    my( $us, $seq1, $seq2, $opts ) = @_;
    my @args;
    for( $opts->{keyGen} ) {
        push @args, $_   if  $_;
    }
    for( $opts->{keyGenArgs} ) {
        push @args, @$_   if  $_;
    }
    my $cdif= Algorithm::Diff::compact_diff( $seq1, $seq2, @args );
    my $same= 1;
    if(  0 == $cdif->[2]  &&  0 == $cdif->[3]  ) {
        $same= 0;
        splice @$cdif, 0, 2;
    }
    my @obj= ( $cdif, $seq1, $seq2 );
    $obj[_End] = (1+@$cdif)/2;
    $obj[_Same] = $same;
    $obj[_Base] = 0;
    my $me = bless \@obj, $us->getObjPkg();
    $me->Reset( 0 );
    return $me;
}

sub Reset
{
    my( $me, $pos )= @_;
    $pos= int( $pos || 0 );
    $pos += $me->[_End]
        if  $pos < 0;
    $pos= 0
        if  $pos < 0  ||  $me->[_End] <= $pos;
    $me->[_Pos]= $pos || !1;
    $me->[_Off]= 2*$pos - 1;
    return $me;
}

sub Base
{
    my( $me, $base )= @_;
    my $oldBase= $me->[_Base];
    $me->[_Base]= 0+$base   if  defined $base;
    return $oldBase;
}

sub Copy
{
    my( $me, $pos, $base )= @_;
    my @obj= @$me;
    my $you= bless \@obj, ref($me);
    $you->Reset( $pos )   if  defined $pos;
    $you->Base( $base );
    return $you;
}

sub Next {
    my( $me, $steps )= @_;
    $steps= 1   if  ! defined $steps;
    if( $steps ) {
        my $pos= $me->[_Pos];
        my $new= $pos + $steps;
        $new= 0   if  $pos  &&  $new < 0;
        $me->Reset( $new )
    }
    return $me->[_Pos];
}

sub Prev {
    my( $me, $steps )= @_;
    $steps= 1   if  ! defined $steps;
    my $pos= $me->Next(-$steps);
    $pos -= $me->[_End]   if  $pos;
    return $pos;
}

sub Diff {
    my( $me )= @_;
    $me->_ChkPos();
    return 0   if  $me->[_Same] == ( 1 & $me->[_Pos] );
    my $ret= 0;
    my $off= $me->[_Off];
    for my $seq ( 1, 2 ) {
        $ret |= $seq
            if  $me->[_Idx][ $off + $seq + _Min ]
            <   $me->[_Idx][ $off + $seq ];
    }
    return $ret;
}

sub Min {
    my( $me, $seq, $base )= @_;
    $me->_ChkPos();
    my $off= $me->_ChkSeq($seq);
    $base= $me->[_Base] if !defined $base;
    return $base + $me->[_Idx][ $off + _Min ];
}

sub Max {
    my( $me, $seq, $base )= @_;
    $me->_ChkPos();
    my $off= $me->_ChkSeq($seq);
    $base= $me->[_Base] if !defined $base;
    return $base + $me->[_Idx][ $off ] -1;
}

sub Range {
    my( $me, $seq, $base )= @_;
    $me->_ChkPos();
    my $off = $me->_ChkSeq($seq);
    if( !wantarray ) {
        return  $me->[_Idx][ $off ]
            -   $me->[_Idx][ $off + _Min ];
    }
    $base= $me->[_Base] if !defined $base;
    return  ( $base + $me->[_Idx][ $off + _Min ] )
        ..  ( $base + $me->[_Idx][ $off ] - 1 );
}

sub Items {
    my( $me, $seq )= @_;
    $me->_ChkPos();
    my $off = $me->_ChkSeq($seq);
    if( !wantarray ) {
        return  $me->[_Idx][ $off ]
            -   $me->[_Idx][ $off + _Min ];
    }
    return
        @{$me->[$seq]}[
                $me->[_Idx][ $off + _Min ]
            ..  ( $me->[_Idx][ $off ] - 1 )
        ];
}

sub Same {
    my( $me )= @_;
    $me->_ChkPos();
    return wantarray ? () : 0
        if  $me->[_Same] != ( 1 & $me->[_Pos] );
    return $me->Items(1);
}

my %getName;
BEGIN {
    %getName= (
        same => \&Same,
        diff => \&Diff,
        base => \&Base,
        min  => \&Min,
        max  => \&Max,
        range=> \&Range,
        items=> \&Items, # same thing
    );
}

sub Get
{
    my $me= shift @_;
    $me->_ChkPos();
    my @value;
    for my $arg (  @_  ) {
        for my $word (  split ' ', $arg  ) {
            my $meth;
            if(     $word !~ /^(-?\d+)?([a-zA-Z]+)([12])?$/
                ||  not  $meth= $getName{ lc $2 }
            ) {
                Die( $Root, ", Get: Invalid request ($word)" );
            }
            my( $base, $name, $seq )= ( $1, $2, $3 );
            push @value, scalar(
                4 == length($name)
                    ? $meth->( $me )
                    : $meth->( $me, $seq, $base )
            );
        }
    }
    if(  wantarray  ) {
        return @value;
    } elsif(  1 == @value  ) {
        return $value[0];
    }
    Die( 0+@value, " values requested from ",
        $Root, "'s Get in scalar context" );
}


my $Obj= getObjPkg($Root);
no strict 'refs';

for my $meth (  qw( new getObjPkg )  ) {
    *{$Root."::".$meth} = \&{$meth};
    *{$Obj ."::".$meth} = \&{$meth};
}
for my $meth (  qw(
    Next Prev Reset Copy Base Diff
    Same Items Range Min Max Get
    _ChkPos _ChkSeq
)  ) {
    *{$Obj."::".$meth} = \&{$meth};
}

1;
__END__

=head1 NAME

Algorithm::Diff - Compute `intelligent' differences between two files / lists

=head1 SYNOPSIS

    require Algorithm::Diff;

    # This example produces traditional 'diff' output:

    my $diff = Algorithm::Diff->new( \@seq1, \@seq2 );

    $diff->Base( 1 );   # Return line numbers, not indices
    while(  $diff->Next()  ) {
        next   if  $diff->Same();
        my $sep = '';
        if(  ! $diff->Items(2)  ) {
            sprintf "%d,%dd%d\n",
                $diff->Get(qw( Min1 Max1 Max2 ));
        } elsif(  ! $diff->Items(1)  ) {
            sprintf "%da%d,%d\n",
                $diff->Get(qw( Max1 Min2 Max2 ));
        } else {
            $sep = "---\n";
            sprintf "%d,%dc%d,%d\n",
                $diff->Get(qw( Min1 Max1 Min2 Max2 ));
        }
        print "< $_"   for  $diff->Items(1);
        print $sep;
        print "> $_"   for  $diff->Items(2);
    }


    # Alternate interfaces:

    use Algorithm::Diff qw(
        LCS LCS_length LCSidx
        diff sdiff compact_diff
        traverse_sequences traverse_balanced );

    @lcs    = LCS( \@seq1, \@seq2 );
    $lcsref = LCS( \@seq1, \@seq2 );
    $count  = LCS_length( \@seq1, \@seq2 );

    ( $seq1idxref, $seq2idxref ) = LCSidx( \@seq1, \@seq2 );


    # Complicated interfaces:

    @diffs  = diff( \@seq1, \@seq2 );

    @sdiffs = sdiff( \@seq1, \@seq2 );

    @cdiffs = compact_diff( \@seq1, \@seq2 );

    traverse_sequences(
        \@seq1,
        \@seq2,
        {   MATCH     => \&callback1,
            DISCARD_A => \&callback2,
            DISCARD_B => \&callback3,
        },
        \&key_generator,
        @extra_args,
    );

    traverse_balanced(
        \@seq1,
        \@seq2,
        {   MATCH     => \&callback1,