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# For the outside of range values, I convert to log space (so 1/400 is
# just as far away from 1 as 400 is). I then treat the allowed range
# as a one standard deviation wide segment of a normal distribution,
# and use appropriate modifiers to make the result range from 0.001 to
# 0.1.
#
# The above formula was carefully chosen from thousands of
# possibilities, by picking things at random and scribbling them down
# on a piece of paper, then pouring sparkling apple cider all over and
# using the one that was still readable.
#
# Ok, not really. Just pretend that I went to that much trouble.
#
sub range_score {
    my ($self, $val, $unitName) = @_;
    my $ranges = $self->get_ranges();
    my $range = $ranges->{$unitName} || $ranges->{default};

    # Return 1 if it's in range
    if ($val >= $range->[0]) {
	if (! defined $range->[1] || ($val <= $range->[1])) {
	    return 1;
	}
    }

    $val = _sillylog($val);

    my $r0 = _sillylog($range->[0]);
    my $r1;
    if (defined $range->[1]) {
	$r1 = _sillylog($range->[1]);
    } else {
	$r1 = 4;
    }

    my $width = $r1 - $r0;
    my $mean = ($r0 + $r1) / 2;
    my $stddev = $width / 2;

    my $n = ($val - $mean) / $stddev; # Normalized value

    our $mulconst;
    $mulconst ||= 0.999 * exp(1/8);

    return 0.001 + $mulconst * exp(-$n**2/2);
}

# Infinity-free logarithm
sub _sillylog {
    my $x = shift;
    return log($x) if $x;
    return log(1e-50);
}

# pref_score : unitName -> score
#
# Maps a unit name (eg week) to a score. Higher scores are more likely
# to be chosen.
sub pref_score {
    my ($self, $unitName) = @_;
    my $prefs = $self->get_prefs();
    my $specific = $prefs->{$unitName};
    return defined($specific) ? $specific : $prefs->{default};
}

# get_prefs : void -> { unit name => score }
#
# Return a map of unit names to their score, higher scores meaning
# they're more likely to be chosen.
sub get_prefs {
    return { default => 0.1 };
}

sub get_ranges {
    return { default => [ 1, undef ] };
}

# render_unit : unit name x power -> descriptive string
#
# Return a rendering of the given unit name and a power to raise the
# unit to.
#
# Example: render_unit("weeks", 2) produces "weeks**2".
#
sub render_unit {
    my ($self, $name, $power, $options) = @_;
    if ($power == 1) {
	return $name;
    } else {
	return "$name**$power";
    }
}

# render : value x name x power -> descriptive string
#
# Return a rendering of the given value with the given units.
#
# Example: render(4.8, "weeks", -1) produces "4.8 weeks**-1".
#
sub render {
    my ($self, $val, $name, $power, $options) = @_;
    return sprintf("%.5g ",$val).$self->render_unit($name, $power, $options);
}

sub construct {
    return;
}

1;