#!/usr/bin/env perl
# TODO:
# implement -s and -r flags
# implement --timeout flag in main loop
# create --help / -h flag and possibly 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
# 
# Usage
# listen [flags] [file(s) to watch] [command(s) to run on file changes]
# 
# Flags
# -a if all of the files have changed, required if multiple files
# -o 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 immediately run exec command, then start listening
# --timeout=[time] change timeout time in whole seconds (DEFAULT 1 SECOND)

# GLOBALS
local $| = 1;
my $ARGC = scalar @ARGV;
my $TIMEOUT = 1;
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 $CHECKSUM_COMMAND = "sha1sum";

sub flags {
  if ($ARGC == 0) {
    print "listen: To what?\n";
    exit 2;
  }

  my $current_arg = 0;

  foreach $arg (@ARGV) {
    if ($arg =~ m/-[aos]/) {
      if ($arg =~ m/a/) {
        $ALL_FLAG = "def";
      } 
      if ($arg =~ m/o/) {
        $ANY_FLAG = "def";
      }
      if ($arg =~ m/s/) {
        $CHECKSUM_FLAG = "def";
      }
    } elsif ($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 = $timeout_split[1];
        print "timeout is now $TIMEOUT seconds\n";
      } else {
        print "timeout flag invalid\n";
        exit 2;
      }
    } else {
      last;
    }
    $current_arg++;
  }

  $START_LISTEN = $current_arg;
}

sub get_exec {
  if ($START_LISTEN + 1 == $ARGC) {
    print "listen: Not enough arguments\n";
    exit 3;
  }

  $EXEC_LISTEN = $ARGV[-1];
}

sub require_flags {
  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";
  }
}

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;
    }
    if (not -r $ARGV[$i]) {
      print "listen: $ARGV[$i]: Permission denied (warning)\n";
    }
  }
}

sub start {
  print "This shell command will run:\n";
  print "  $EXEC_LISTEN\n";

  if ($MULTI_FILES) {
    if ($ALL_FLAG) {
      print "When all the files below have been modified:\n";
      for (my $i = $START_LISTEN; $i < $EXEC_POSITION; $i++) {
        print "  $ARGV[$i]\n";
      }
    } elsif ($ANY_FLAG) {
      print "When any of the files below have been modified:\n";
      for (my $i = $START_LISTEN; $i < $EXEC_POSITION; $i++) {
        print "  $ARGV[$i]\n";
      }
    }
  } else {
    print "When this file has been modified:\n";
    print "  $ARGV[$START_LISTEN]\n";
  }

  print "Starting now...\n";

  my $previous_epoch = (stat($ARGV[$START_LISTEN]))[9];
  my $current_epoch = (stat($ARGV[$START_LISTEN]))[9];
  while (1) {
    $current_epoch = (stat($ARGV[$START_LISTEN]))[9];
    if ($current_epoch > $previous_epoch) {
      print "--- File \"$ARGV[$START_LISTEN]\" modified. Running command... ---\n";
      system $EXEC_LISTEN;
      my $status = $? >> 8;
      if (int($status) != 0) {
        print "--- WARNING: Exit code is $status. Returned to listen... ---\n";
      } else {
        print "--- Returned to listen... ---\n";
      }

      $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();