#! /usr/bin/perl -w

# Here is another way to do it, using a trigonometry 'toolkit'
# approach and a different interface. Sides are $a, $b, and $c, and
# corresponding opposing angles are $A, $B, and $C. Note that
# pathological cases will break the program (e.g., using two angles
# whose sum is greater than 180); more robust error checking is
# required.

use strict;

test();
main();

sub main {
  print "Enter an angle (in degrees) and its opposing side:\n";
  print 'Angle: ';
  chomp(my $A = <STDIN>);
  print 'Side : ';
  chomp(my $a = <STDIN>);
  print q/Enter 'a' if you have an angle, 's' if you have a side: /;
 GET_WHAT:
  chomp(my $what = lc <STDIN>);
  if ($what eq 'a') {
    print 'Other angle: ';
    chomp(my $B = <STDIN>);
    my $C = 180 - $A - $B;
    my $b = aAB_to_b($a, $A, $B);
    my $c = aAB_to_b($a, $A, $C);
    printf 'The other two sides are %.2f and %.2f, '
        . "and the other angle is %.2f\n",
        $b, $c, $C;
  }
  elsif ($what eq 's') {
    print 'Other side: ';
    chomp(my $b = <STDIN>);
    my $B = aAb_to_B($a, $A, $b);
    my $C = 180 - $A - $B;
    my $c = aAB_to_b($a, $A, $C);
    printf 'The other side is %.2f, '
        . "and the other two angles are %.2f and %.2f\n",
        $c, $B, $C;
  }
  else {
    print q/Please enter 'a' or 's': /;
    goto GET_WHAT;
  }
}

sub aAb_to_B {
  my ($a, $A, $b) = @_;
  return rad2deg(asin($b * sin(deg2rad($A)) / $a));
}

sub aAB_to_b {
  my ($a, $A, $B) = @_;
  return sin(deg2rad($B)) * $a / sin(deg2rad($A));
}

sub asin {
  my $sine = shift;
  return undef unless $sine >= -1 && $sine <= 1;
  return atan2($sine, sqrt(1 - $sine * $sine));
}

use constant PI => 4 * atan2(1, 1);

sub deg2rad { $_[0] / 180 * PI  }
sub rad2deg { $_[0] / PI  * 180 }

sub test {
  # More tests would be welcome.
  expect('aAB_to_b', 10, 5, 30, 90);
  expect('aAb_to_B', 90, 5, 30, 10);
}

sub expect {
  my ($sub, $exp, @args) = @_;
  no strict 'refs';
  my $got = $sub->(@args);
  return if abs($got - $exp) < 0.0001;
  local $" = ', ';
  printf "Expected $sub(@args) to give $exp, but got %.2f\n", $got;
}
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