use strict;
use warnings;
use Test::More;
use File::Temp qw(tempdir);
use Data::DisjointSet::Shared;

# A creator killed between ftruncate() and header init leaves a full-size,
# all-zero (magic==0) file.  new() must recover it instead of bricking the path,
# but must never clobber a valid or foreign file.

my $dir = tempdir(CLEANUP => 1);
my $p   = "$dir/recover.dsu";
my $N   = 16;

# Learn the on-disk size for this geometry.
{ my $d = Data::DisjointSet::Shared->new($p, $N); }
my $total = -s $p;
unlink $p;

# 1. Recovery: an abandoned all-zero file of exactly $total bytes is re-initialized.
{
    open my $f, '>', $p or die $!; truncate $f, $total or die $!; close $f;
    is(-s $p, $total, "abandoned file is $total bytes (a killed creator's ftruncate)");
    my $d = eval { Data::DisjointSet::Shared->new($p, $N) };
    ok($d, "new() recovers an abandoned mid-init file instead of bricking") or diag $@;
  SKIP: {
        skip "no handle", 5 unless $d;
        is($d->find(0), 0, 'recovered set: element 0 is its own singleton root');
        is($d->find(5), 5, 'recovered set: element 5 is its own singleton root');
        is($d->num_sets, $N, 'recovered set starts with N singleton sets (no stale unions)');
        my $m = $d->union(0, 5);
        is($m, 1, 'recovered set is usable: union(0,5) newly merges');
        is($d->find(0), $d->find(5), 'recovered set is usable: find confirms the union');
    }
    undef $d; unlink $p;
}

# 2. No clobber: a file with nonzero (foreign) magic still errors.
{
    open my $f, '>', $p or die $!; print $f "XXXX"; truncate $f, $total or die $!; close $f;
    my $d = eval { Data::DisjointSet::Shared->new($p, $N) };
    ok(!$d, "new() refuses a foreign nonzero-magic file (no clobber)");
    like($@, qr/invalid/i, "  ... reporting an invalid file");
    undef $d; unlink $p;
}

# 3. No recovery for the wrong size: magic==0 but size != total still errors.
{
    open my $f, '>', $p or die $!; truncate $f, $total + 8 or die $!; close $f;
    my $d = eval { Data::DisjointSet::Shared->new($p, $N) };
    ok(!$d, "new() refuses an uninitialized file of the wrong size");
    undef $d; unlink $p;
}

# 4. A valid file is attached, never re-initialized (its data survives).
{
    my $a = Data::DisjointSet::Shared->new($p, $N); $a->union(2, 3); undef $a;
    my $r = Data::DisjointSet::Shared->new($p, $N);
    is($r->find(2), $r->find(3), 'reopening a valid file preserves its data');
    is($r->num_sets, $N - 1, 'reopening a valid file preserves num_sets (not re-initialized)');
    undef $r; unlink $p;
}

# 5. A magic==0 file of the right size but with NON-zero data (not a fresh
#    ftruncate) is NOT recovered -- recovery only re-inits a provably-empty file.
{
    open my $f, '>', $p or die $!; truncate $f, $total or die $!; close $f;
    open $f, '+<', $p or die $!; seek $f, $total - 1, 0; print $f "\x01"; close $f;
    my $d = eval { Data::DisjointSet::Shared->new($p, $N) };
    ok(!$d, "new() refuses a magic==0 file that is not all-zero (no clobber of real data)");
    undef $d; unlink $p;
}

done_testing;
