package Test::MockFile::Light;

use strict;

use Carp;

# ---

my %file;

sub define_file_mocker {
	my ($name, $content) = @_;

	$content ||= '';

	$file{$name} = $content;
}

sub undefine_file_mocker {
	my ($name, $content) = @_;

	delete $file{$name};
}

# ---

sub import {
	my ($class, %arg) = @_;

	my $package = caller;

	_export_functions_to($package);

	my $module = defined $arg{module} ? $arg{module} : $package;

	_override_open_builtin_for($module);
}

# ---

sub _export_functions_to {
	my ($package) = @_;

	no strict 'refs';

	*{"$package\::define_file_mocker"} = \&define_file_mocker;

	*{"$package\::undefine_file_mocker"} = \&undefine_file_mocker;
}

# ---

sub _override_open_builtin_for {
	my ($module) = @_;

	my $mode_and_name_parser = qr/
		^
		\s*
		(|<|>|>>|\+<|\+>|\+>>)	# the mode
		\s*
		(\w.*?)					# the file name
		\s*
		$
	/x;

	no strict 'refs';

	*{"$module\::open"} = sub (\[*$]$;$) {
		my ($fh, $mode, $name) = @_;

		$name ||= '';

		my $compound = "$mode $name";

		if ($compound =~ $mode_and_name_parser) {
			$mode = $1 || '<';
			$name = $2;
		}
		else {
			croak 'Unexpected open() parameters for file mocking';
		}

		if ($mode eq '<' && ! defined $file{$name}) {
			$! = 2;

			return 0;
		}

		return open $$fh, $mode, \$file{$name};
	};
}

# ---

1;


##</code><code>##

package Foo::Bar;

sub process {
	my ($class, $file_name) = @_;

	my $sum = 0;

	open my $fh, $file_name;
	while (<$fh>) {
		chomp;
		$sum += $_;
	}
	close $fh;

	return $sum;
}

1;

##</code><code>##

use Test::More tests => 3;
use Test::MockFile::Light module => 'Foo::Bar';

use_ok('Foo::Bar');

my $file_name = 'data.txt';
my $content = join("\n", 1..5);

define_file_mocker($file_name, $content);

can_ok(Foo::Bar, 'process');

is(Foo::Bar->process($file_name), 15, 'unexpected sum');