Angle difference between two bearings

Angle difference between two bearings is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

Finding the angle between two bearings is often confusing.

Task

We need to find the angle which is the result of the subtraction b2-b1, where b1 and b2 are both bearings. The result must be expressed in the range +180 to -180 degrees. Compute the angle for the following pairs:

• 20 degrees (b1) and 45 degrees (b2)
• -45 and 45
• -85 and 90
• -95 and 90
• -45 and 125
• -45 and 145

C++

<lang javascript>#include <cmath>

1. include <iostream>

using namespace std;

double getDifference(double b1, double b2) { double val = std::min((b1-b2)<0?b1-b2+360:b1-b2, (b2-b1)<0?b2-b1+360:b2-b1); if (val > M_PI) val -= 2*M_PI; return val; }

int main() { const double deg2rad = M_PI/180.0; const double rad2deg = 180.0/M_PI; cout << getDifference(20.0*deg2rad, 45.0*deg2rad)*rad2deg << endl; cout << getDifference(-45.0*deg2rad, 45.0*deg2rad)*rad2deg << endl; cout << getDifference(-85.0*deg2rad, 90.0*deg2rad)*rad2deg << endl; cout << getDifference(-95.0*deg2rad, 90.0*deg2rad)*rad2deg << endl; cout << getDifference(-45.0*deg2rad, 125.0*deg2rad)*rad2deg << endl; cout << getDifference(-45.0*deg2rad, 145.0*deg2rad)*rad2deg << endl; return 0; } </lang>

Output:

25
90
175
-175
170
-170

JavaScript

This approach should be reliable but it is also very inefficient.

<lang javascript>function relativeBearing(b1Rad, b2Rad) { b1y = Math.cos(b1Rad); b1x = Math.sin(b1Rad); b2y = Math.cos(b2Rad); b2x = Math.sin(b2Rad); crossp = b1y * b2x - b2y * b1x; dotp = b1x * b2x + b1y * b2y; if(crossp > 0.) return Math.acos(dotp); return -Math.acos(dotp); }

function test() { var deg2rad = 3.14159265/180.0; var rad2deg = 180.0/3.14159265; return relativeBearing(20.0*deg2rad, 45.0*deg2rad)*rad2deg+"\n" +relativeBearing(-45.0*deg2rad, 45.0*deg2rad)*rad2deg+"\n" +relativeBearing(-85.0*deg2rad, 90.0*deg2rad)*rad2deg+"\n" +relativeBearing(-95.0*deg2rad, 90.0*deg2rad)*rad2deg+"\n" +relativeBearing(-45.0*deg2rad, 125.0*deg2rad)*rad2deg+"\n" +relativeBearing(-45.0*deg2rad, 145.0*deg2rad)*rad2deg+"\n"; }</lang>

Output:

25.000000000000004
90
174.99999999999997
-175.00000041135993
170.00000000000003
-170.00000041135996

Perl 6

Works with: Rakudo version 2016.11

<lang perl6>sub infix:<∠> (Int $b1, Int $b2) {

  (my $b = ($b2 - $b1 + 720) % 360) > 180 ?? $b - 360 !! $b;

}

1. TESTING

for 20, 45,

  -45, 45,
  -85, 90,
  -95, 90,
  -45, 125,
  -45, 145
 -> $b1, $b2 { say "$b1 ∠ $b2 = ", $b1 ∠ $b2 }</lang>

Output:

20 ∠ 45 = 25
20 ∠ 45 = 25
-45 ∠ 45 = 90
-85 ∠ 90 = 175
-95 ∠ 90 = -175
-45 ∠ 125 = 170
-45 ∠ 145 = -170