#!/usr/bin/perl

use strict;
use warnings;

use File::Temp qw/tempfile/;
use POSIX qw/strftime/;
use Time::Local;

my $RIG_URL_PREFIX = 'http://metobs.ssec.wisc.edu/pub/cache/aoss/tower/ascii';


# MAIN -------------------------------------------------------------------------

my ($year, $month, $day) = process_arguments()                       or exit(1);
my $filename = fetch_weather_data($year, $month, $day)               or exit(1);
my $weather_ref = analyze_rig_data($filename)                        or exit(1);
save_data("temps-$year-$month-$day.txt", $weather_ref)               or exit(1);
exit 0;


# SUBROUTINES ------------------------------------------------------------------

# process_arguments() => (year, month, day)
#
# Examines the command-line arguments for a valid date string, and if found,
# returns its year, month, and day components.  Returns undef upon failure.

sub process_arguments {
    unless ((scalar(@ARGV) == 1) and ($ARGV[0] =~ /^(\d{4})-(\d\d)-(\d\d)$/)) {
        print "Run with a single date argument, YYYY-MM-DD\n";
        return;
    }
    return ($1, $2, $3);
}


# fetch_weather_data
#
# Downloads the RIG data file for the given date, if the downloaded file does
# not already exist.  Returns the name of the download file upon success (even
# if the downloaded file exists), or undef upon failure.

sub fetch_weather_data {
    my ($year, $month, $day) = @_;

    my $rig_filename = "rig_tower.$year-$month-$day.ascii";
    my $local_filename = "rig-$year-$month-$day.txt";
    if (-f $local_filename) {
        print "Already downloaded data for $year-$month-$day\n";
    } else {
        my $url = "$RIG_URL_PREFIX/$year/$month/$rig_filename";
        system('curl', '--fail', '--silent', '--output', $local_filename, $url);
        unless ($? == 0) {
            print "Failed to download '$url': $!\n";
            return;
        }
    }
    return $local_filename;
}


# analyze_rig_data(filename) => reference to hash
#
# Reads and analyzes RIG data from the given filename.  Returns a reference to a
# hash; see homework description for full details.  Returns undef upon failure.

sub analyze_rig_data {
    my $filename = shift;

    my $rig;
    unless (open($rig, '<', $filename)) {
        print "Could not open '$filename': $!\n";
        return;
    }

    my %weather;
    while (<$rig>) {
        my ($timestamp, $temperature) = extract_weather_data($_);
        my $hourstamp = timelocal(0, 0, (localtime($timestamp))[2..5]);
        if (exists $weather{$hourstamp}) {
            $weather{$hourstamp}{'min'} = $temperature if $temperature < $weather{$hourstamp}{'min'};
            $weather{$hourstamp}{'max'} = $temperature if $temperature > $weather{$hourstamp}{'max'};
        } else {
            $weather{$hourstamp} = { 'min' => $temperature, 'max' => $temperature };
        }
    }
    close($rig);

    return \%weather;
}


# extract_weather_data(line) => (timestamp, temperature)
#
# Analyzes a single line of AO&SS RIG data, extracts the time and temperature,
# converts both values to standardized units, and returns them.  Remember that
# the time in the RIG data is for UTC, not local time.  Also, the temperatures
# in the file are in Celsius, but the temperature part of the return value from
# this function should be in Fahrenheit.

sub extract_weather_data {
    my $line = shift;

    # WRITE THIS SUBROUTINE!

    return ($timestamp, $temperature_f);
}


# c2f(temperature_celsius) => temperature_fahrenheit
#
# Converts the given temperature from Celsius to Fahrenheit.  The argument is
# assumed to be valid, so no error checking is needed.  Returns the converted
# temperature.

sub c2f {
    # WRITE THIS SUBROUTINE!
}


# save_data(filename, reference to hash) => success?
#
# Builds up a single string of formatted weather data from the given hash, then
# uses write_file() to save the string to the given filename.  Does NOT replace
# an existing file at the given filename.  The reference refers to a hash; see
# homework description for full details.
#
# For each timestamp key, save one line of data:
#
#     DATE <tab> HOUR <tab> MIN TEMP <tab> MAX TEMP <newline>
#
# The date is in YYYY-MM-DD format, the hour is in 24-hour format (i.e., 0-23),
# and the temperatures have 3 digits of decimal precision.
#
# Returns true on success, false otherwise.

sub save_data {
    my ($filename, $weather_ref) = @_;

    # WRITE THIS SUBROUTINE!
}


# write_file(filename, contents) => status
#
# Writes the given string contents to the given file safely.  Returns true upon
# success or false otherwise.

sub write_file {
    my ($filename, $contents) = @_;

    # write new file
    my ($output_fh, $new_filename) = tempfile(DIR => '.', UNLINK => 0);  # croak() on error
    print $output_fh $contents or return;
    close($output_fh) or return;

    # backup old copy and save new
    if (-e $filename) {
        rename($filename, "$filename.BAK") or return;
    }
    rename($new_filename, $filename) or return;
    chmod 0644, $filename or return;
    return 1;
}