#
# A line tangent to two circles. Between zero and four solutions.
#

#
# Given a C coefficent of line Ax + By + C = 0, compute A and B and
# constructs a line from this geometry.
#
MakeLineFromC = function( Lc, X0, Y0, R0, X1, Y1, R1 ): La: Lb: Det: Pt1: Pt2:
    Det = X0 * Y1 - X1 * Y0:
    La = ( Y1 * (R0 - Lc) - Y0 * (R1 - Lc) ) / Det:
    Lb = ( X0 * (R1 - Lc) - X1 * (R0 - Lc) ) / Det:
    if ( abs( La ) > abs( Lb ),
	Pt1 = ctlpt( E2, (-Lc - Lb * 10) / La,                10 ):
	Pt2 = ctlpt( E2, (-Lc + Lb * 10) / La,               -10 ),
	Pt1 = ctlpt( E2,                   10, (-Lc - La * 10) / Lb ):
	Pt2 = ctlpt( E2,                  -10, (-Lc + La * 10) / Lb ) ):
    return = Pt1 + Pt2;

#
# Compute the C coefficient of the line Ax + By + C = 0, and make lines.
#
LineTanCircCircAux = function( X0, Y0, R0, X1, Y1, R1 ):
    Qa: Qb: Qc: A: B: C: Dsc:
    Qa = sqr( Y1 - Y0 ) + sqr( X0 - X1 ):
    Qb = -2 * ( Y1 - Y0 ) * ( Y1 * R0 - Y0 * R1 ) -
	  2 * ( X0 - X1 ) * ( X0 * R1 - X1 * R0 ):
    Qc = sqr( Y1 * R0 - Y0 * R1 ) +
	 sqr( X0 * R1 - X1 * R0 ) -
	 sqr( X0 * Y1 - X1 * Y0 ):
    Dsc = sqr( Qb ) - 4 * Qa * Qc:
    if ( Dsc < 0,
	 return = nil(),
         return = list( MakeLineFromC( ( -Qb + sqrt( Dsc ) ) / ( 2 * Qa ),
				       X0, Y0, R0, X1, Y1, R1 ),
			MakeLineFromC( ( -Qb - sqrt( Dsc ) ) / ( 2 * Qa ),
				       X0, Y0, R0, X1, Y1, R1 ) ) );

#
# Compute all the possible lines Ax + By + C = 0 thru given two circles.
#
LineTanCircCirc = function( X0, Y0, R0, X1, Y1, R1 ): Circs:
    Circs = list( circle( vector( X0, Y0, 0.0 ), R0 ),
		  circle( vector( X1, Y1, 0.0 ), R1 ) ):
    color( Circs, red ):
    return = Circs +
	     LineTanCircCircAux( X0, Y0,  R0, X1, Y1,  R1 ) +
	     LineTanCircCircAux( X0, Y0,  R0, X1, Y1, -R1 ) +
	     LineTanCircCircAux( X0, Y0, -R0, X1, Y1, -R1 ) +
	     LineTanCircCircAux( X0, Y0, -R0, X1, Y1,  R1 );

#
# We do not deal with the generated cases here (i.e. Y0 = Y1 or X0 = X1).
#

save_mat = view_mat;
view_mat = sc( 0.25 );
viewobj( view_mat );

All = LineTanCircCirc( -2, 0.5, 1.5, 2, 0.25, 1 );
interact( All );

All = LineTanCircCirc( -1, 0.15, 1.5, 0.5, -0.5, 1.25 );
interact( All );

All = LineTanCircCirc( -0.5, 0.5, 2, 0, 0.25, 1 );
interact( All );

view_mat = save_mat;