#!/usr/bin/env perl
# TODO:
# possibly create a man page
#
# listen: a simple configurable build system
# created by Bryson Steck, @brysonsteck on GitHub
# free and open source under the GPL Version 3
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <https://www.gnu.org/licenses/>.
# 
# Usage
# listen [flags] [file(s) to watch] [command(s) to run on file changes]
# 
# Flags
# -a, --all if all of the files have changed, required if multiple files
# -o, --any if any one of the files have changed, required if multiple files
# -s to use checksum on file change instead of when the file was modified
# -r, --run immediately run exec command, then start listening
# --timeout=[time] change timeout time in whole seconds (DEFAULT 1 SECOND)
# --help prints help

# GLOBALS
local $| = 1;
my $VERSION = "0.1.0";
my $ARGC = scalar @ARGV;
my $TIMEOUT = undef;
my $START_LISTEN = 0;
my $EXEC_POSITION = $ARGC - 1;
my $EXEC_LISTEN = undef;
my $ALL_FLAG = undef;
my $ANY_FLAG = undef;
my $MULTI_FILES = undef;
my $CHECKSUM_FLAG = undef;
my $RUN_FLAG = undef;
my $black = "\033[0;90m";
my $nocolor = "\033[0m";

# comment the line below if you use Windows
my $CHECKSUM_COMMAND = "/usr/bin/env cksum";
# uncomment the line below if you use Windows
#my $CHECKSUM_COMMAND = "C:\Windows\System32\certutil.exe -hashfile";

# check for flags if any
sub flags {
  # quit if no args
  if ($ARGC == 0) {
    print "listen: To what?\n";
    exit 2;
  }

  my $current_arg = 0;

  foreach $arg (@ARGV) {
    # shorthand args
    if ($arg =~ m/^-[aosrhv]+$/) {
      if ($arg =~ m/a/) {
        $ALL_FLAG = "def";
      } 
      if ($arg =~ m/o/) {
        $ANY_FLAG = "def";
      }
      if ($arg =~ m/s/) {
        $CHECKSUM_FLAG = "def";
      }
      if ($arg =~ m/r/) {
        $RUN_FLAG = "def";
      }
      if ($arg =~ m/h/) {
        print "\nlisten v$VERSION - a simple automation system\n";
        print "Copyright 2022 Bryson Steck\n";
        print "Free and open source under the GNU General Public License v3.\n";
        print "Run 'listen --license' to view the license and warranty disclaimer.\n\n";
        print "usage: listen [-v | --version] [-h | --help] [-a | --all]\n";
        print "              [-o | --any] [-s] [-r | --run] FILE1 [FILE2 ...]\n";
        print "              COMMAND\n\n";
        print "informational flags:\n";
        print "  -h | --help    -> Print this message\n";
        print "  -v | --version -> Print the version of listen\n";
        print "  --license      -> Print the license/warranty disclaimer\n";
        print "                    (GNU General Public License v3)\n\n";
        print "multiple file flags:\n";
        print "  -a | --all     -> Run COMMAND if ALL of the files have been modified\n";
        print "  -o | --any     -> Run COMMAND if ANY of the files have been modified\n\n";
        print "other flags:\n";
        print "  -r | --run     -> Run COMMAND before starting listen\n";
        print "  -s             -> Check for file modification based on cksum\n";
        print "                    (as opposed to the files' modified timestamp)\n\n";
        exit 1;
      }
      if ($arg =~ m/v/) {
        print "listen v$VERSION\n";
      }
    # longhand args
    } elsif ($arg =~ m/--timeout=/) {
      my @timeout_split = split /\=/, $arg;
      if (scalar @timeout_split == 1) {
        print "timeout flag invalid\n";
        exit 2;
      } elsif (int($timeout_split[1]) > 0) {
        $TIMEOUT = int($timeout_split[1]);
        print "timeout is now $TIMEOUT seconds\n";
      } else {
        print "timeout flag invalid\n";
        exit 2;
      }
    } elsif ($arg =~ m/--help/) {
      print "\nlisten v$VERSION - a simple automation system\n";
      print "Copyright 2022 Bryson Steck\n";
      print "Free and open source under the GNU General Public License v3.\n";
      print "Run 'listen --license' to view the license and warranty disclaimer.\n\n";
      print "usage: listen [-v | --version] [-h | --help] [-a | --all]\n";
      print "              [-o | --any] [-s] [-r | --run] FILE1 [FILE2 ...]\n";
      print "              COMMAND\n\n";
      print "informational flags:\n";
      print "  -h | --help    -> Print this message\n";
      print "  -v | --version -> Print the version of listen\n";
      print "  --license      -> Print the license/warranty disclaimer\n";
      print "                    (GNU General Public License v3)\n\n";
      print "multiple file flags:\n";
      print "  -a | --all     -> Run COMMAND if ALL of the files have been modified\n";
      print "  -o | --any     -> Run COMMAND if ANY of the files have been modified\n\n";
      print "other flags:\n";
      print "  -r | --run     -> Run COMMAND before starting listen\n";
      print "  -s             -> Check for file modification based on cksum\n";
      print "                    (as opposed to the files' modified timestamp)\n\n";
      exit 1;
    } elsif ($arg =~ m/--version/) {
      print "listen v$VERSION\n";
      exit 1;
    } elsif ($arg =~ m/--license/) {
      print "listen is free and open source under the GNU GPL Version 3.0.\n\n";
      print "This program is free software: you can redistribute it and/or modify\n";
      print "it under the terms of the GNU General Public License as published by\n";
      print "the Free Software Foundation, either version 3 of the License, or\n";
      print "(at your option) any later version.\n\n";
      print "This program is distributed in the hope that it will be useful,\n";
      print "but WITHOUT ANY WARRANTY; without even the implied warranty of\n";
      print "MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the\n";
      print "GNU General Public License for more details.\n\n";
      print "You should have received a copy of the GNU General Public License\n";
      print "along with this program.  If not, see <https://www.gnu.org/licenses/>.\n";
      exit 1;
    } elsif ($arg =~ m/--all/) {
      $ALL_FLAG = "def";
    } elsif ($arg =~ m/--any/) {
      $ANY_FLAG = "def";
    } elsif ($arg =~ m/--run/) {
      $RUN_FLAG = "def";
    # at this point, either it is an unknown flag or the start of the filename(s)
    } elsif ($arg =~ m/^-[^aosrhv]+$/) {
      print "listen: Unknown flag: $arg\n";
      exit 2;
    } elsif ($arg =~ m/^--[a-z]/) {
      print "listen: Unknown flag: $arg\n";
      exit 2;
    } else {
      last;
    }
    $current_arg++;
  }

  # set index of the start of file(s)
  $START_LISTEN = $current_arg;
}

# get last argument as executable
sub get_exec {
  # return early if there is no executable
  if ($START_LISTEN + 1 == $ARGC) {
    print "listen: Executable missing\n";
    exit 3;
  }
  
  # set executable index
  $EXEC_LISTEN = $ARGV[-1];
}

# check flags are compatible with each other
sub require_flags {
  # if there is multiple files, check if both or no flags are found
  if ($EXEC_POSITION - $START_LISTEN != 1) {
    if (!$ALL_FLAG and !$ANY_FLAG) {
      print "listen: Either -a or -o must be specified with multiple files to watch\n";
      exit 4;
    } elsif ($ALL_FLAG and $ANY_FLAG) {
      print "listen: You must specify either -a or -o for multiple files, not both\n";
      exit 4;
    }

    $MULTI_FILES = "def";
  # otherwise if there are flags found for a singular file, quit
  } else {
    if ($ALL_FLAG) {
      print "listen: Invalid argument -a for singular file\n";
      exit 4;
    } elsif ($ANY_FLAG) {
      print "listen: Invalid argument -o for singular file\n";
      exit 4;
    }
  }
  # if checksum is wanted, make sure timeout is not present
  if ($CHECKSUM_FLAG) {
    if ($TIMEOUT) {
      print "listen: Timeout cannot be used in checksum mode\n";
      exit 4;
    }
  }
}

# check file(s) if they exist and they are accessable
sub check_files {
  for (my $i = $START_LISTEN; $i < $EXEC_POSITION; $i++) {
    if (not -e $ARGV[$i]) {
      print "listen: $ARGV[$i]: No such file or directory\n";
      exit 5;
    } elsif (not -r $ARGV[$i]) {
      # being unable to read a file does not mean the executable will fail
      # continue anyway, but warn the user
      print "listen: $ARGV[$i]: Permission denied (warning)\n";
    }
  }
}

# return the checksum/time modified
sub diff {
  my @return;

  # if checksum wanted, append output to return
  if ($CHECKSUM_FLAG) {
    for (my $i = $START_LISTEN; $i < $EXEC_POSITION; $i++) {
      push @return, `$CHECKSUM_COMMAND $ARGV[$i]`;
    }
  # else, append epoch
  } else {
    for (my $i = $START_LISTEN; $i < $EXEC_POSITION; $i++) {
      push @return, (stat($ARGV[$i]))[9];
    }
  }

  return @return;
}

# main subroutine
sub start {
  # if underscore is present in command, replace it with file listening to
  # only if singular file being listened to
  if (!$MULTI_FILES) {
    if ($EXEC_LISTEN =~ m/ _ /) {
      $EXEC_LISTEN =~ s/ _ / $ARGV[$START_LISTEN] /g;
    } elsif ($EXEC_LISTEN =~ m/ _/) {
      $EXEC_LISTEN =~ s/ _/ $ARGV[$START_LISTEN] /g;
    } elsif ($EXEC_LISTEN =~ m/_ /) {
      $EXEC_LISTEN =~ s/_ / $ARGV[$START_LISTEN] /g;
    }
  }

  # print starting information
  print "$black& listen $VERSION\n";
  print "& This program is free software, and comes with ABSOLUTELY NO WARRANTY.\n";
  print "& Run 'listen --license' for details.\n&\n";
  print "& This shell command will run:\n";
  print "&   $EXEC_LISTEN\n";

  # add additional text if in checksum mode
  my $checksum_text = "";
  my $modified_text = "been modified:";
  if ($CHECKSUM_FLAG) {
    $checksum_text = "the checksum of ";
    $modified_text = "changed:";
  }

  # print different things with multiple files depending on flag
  if ($MULTI_FILES) {
    if ($ALL_FLAG) {
      print "& When ${checksum_text}all the files below have $modified_text\n";
      for (my $i = $START_LISTEN; $i < $EXEC_POSITION; $i++) {
        print "&   $ARGV[$i]\n";
      }
    } elsif ($ANY_FLAG) {
      print "& When ${checksum_text}any of the files below have $modified_text\n";
      for (my $i = $START_LISTEN; $i < $EXEC_POSITION; $i++) {
        print "&   $ARGV[$i]\n";
      }
    }
  # otherwise if singular, always print this
  } else {
    print "& When ${checksum_text}this file has $modified_text\n";
    print "&   $ARGV[$START_LISTEN]\n";
  }

  # run command immediately if flag is specified, then start listening
  if ($RUN_FLAG) {
    print "& Running now, then starting listen...${nocolor}\n";
    system $EXEC_LISTEN;
    my $status = $? >> 8;
    if (int($status) != 0) {
      print "& WARNING: Exit code is $status. Returned to listen...$nocolor\n";
    } else {
      print "& Returned to listen...$nocolor\n";
    }
  # otherwise, start listening
  } else {
    print "& Starting now...$nocolor\n";
  }

  # start by initializing comparison arrays
  my @previous_epoch = diff();
  my @current_epoch = diff();

  # main loop
  while (1) {
    my $run = undef;
    my $file_changed = undef;
    my @modified_files;

    # initialize modified_files array, only used if all flag is present
    for (my $i = $START_LISTEN; $i < $EXEC_POSITION; $i++) {
      push @modified_files, "no";
    }

    # get current modified info
    @current_epoch = diff();

    # logic for multiple files
    if ($MULTI_FILES) {
      # check if all files are modified
      if ($ALL_FLAG) {
        my $counter = 0;

        # if output differs, mark as modified
        foreach $modified (@modified_files) {
          if (@current_epoch[$counter] != @previous_epoch[$counter]) {
            if ($TIMEOUT) {
              if (!($current_epoch[$counter] - $previous_epoch[$counter] <= $TIMEOUT)) {
                $modified = "yes";
              }
            } else {
              $modified = "yes";
            }
          }
          $counter++;
        }
        # if one file is not modified, do not run
        foreach $modified (@modified_files) {
          $run = 'def';
          if ($modified !~ "yes") {
            $run = undef;
            last;
          }
        }
      # check if any of the files are modified
      } else {
        my $counter = 0;

        # if output differs, run command
        foreach (@current_epoch) {
          if ($current_epoch[$counter] != $previous_epoch[$counter]) {
            if ($TIMEOUT) {
              if (!($current_epoch[$counter] - $previous_epoch[$counter] <= $TIMEOUT)) {
                $run = "def";
                $file_changed = $counter;
                last;
              }
            } else {
              $run = "def";
              $file_changed = $counter;
              last;
            }
          }
          $counter++;
        }
      }
    # logic for singular files
    } else {
      if ($current_epoch[0] != $previous_epoch[0]) {
        if ($TIMEOUT) {
          if (!($current_epoch[0] - $previous_epoch[0] <= $TIMEOUT)) {
            $run = "def";
            $file_changed = 0;
          }
        } else {
          $run = "def";
          $file_changed = 0;
        }
      }
    }

    # if logic above allows the executable to run
    if ($run) {
      # change output if multiple files are present
      if ($MULTI_FILES) {
        if ($ANY_FLAG) {
          print "$black& File \"$ARGV[$file_changed + 1]\" modified. Running command...$nocolor\n";
        } elsif ($ALL_FLAG) {
          print "$black& All files have been modified. Running command...$nocolor\n";
        }
      } else {
        print "$black& File \"$ARGV[$START_LISTEN]\" modified. Running command...$nocolor\n";
      }

      # run command
      system $EXEC_LISTEN;
      # catch return code
      my $status = $? >> 8;
      if (int($status) != 0) {
        print "$black& WARNING: Exit code is $status. Returned to listen...$nocolor\n";
      } else {
        print "$black& Returned to listen...$nocolor\n";
      }

      # overwrite current as previous
      @previous_epoch = @current_epoch;
    }

    sleep(1);
  }
}

# ---- START OF SCRIPT ----
# start by checking flags
flags();

# get executable to run on trigger
get_exec();

# check if additional flags are needed 
# because multiple files are being listened to
require_flags();

# check if file(s) exist
check_files();

# start listen
start();