Koch curve: Difference between revisions

=={{header|Prolog}}==

{{works with|SWI Prolog}}

Produces an SVG file showing a Koch snowflake.

<lang prolog>main:-

write_koch_snowflake('koch_snowflake.svg').

write_koch_snowflake(File):-

open(File, write, Stream),

koch_snowflake(Stream, 600, 5),

close(Stream).

koch_snowflake(Stream, Size, N):-

format(Stream, "<svg xmlns='http://www.w3.org/2000/svg' width='~d' height='~d'>\n",

[Size, Size]),

write(Stream, "<rect width='100%' height='100%' fill='black'/>\n"),

write(Stream, "<path stroke-width='1' stroke='white' fill='none' d='"),

Sqrt3_2 = 0.86602540378444,

Length is Size * Sqrt3_2 * 0.95,

X0 is (Size - Length)/2,

Y0 is Size/2 - Length * Sqrt3_2/3,

X1 is X0 + Length/2,

Y1 is Y0 + Length * Sqrt3_2,

X2 is X0 + Length,

format(Stream, 'M ~g,~g ', [X0, Y0]),

koch_curve(Stream, X0, Y0, X1, Y1, N),

koch_curve(Stream, X1, Y1, X2, Y0, N),

koch_curve(Stream, X2, Y0, X0, Y0, N),

write(Stream, "z'/>\n</svg>\n").

koch_curve(Stream, _, _, X1, Y1, 0):-

!,

format(Stream, 'L ~g,~g\n', [X1, Y1]).

koch_curve(Stream, X0, Y0, X1, Y1, N):-

N > 0,

Sqrt3_2 = 0.86602540378444,

N1 is N - 1,

Dx is X1 - X0,

Dy is Y1 - Y0,

X2 is X0 + Dx/3,

Y2 is Y0 + Dy/3,

X3 is X0 + Dx/2 - Dy * Sqrt3_2/3,

Y3 is Y0 + Dy/2 + Dx * Sqrt3_2/3,

X4 is X0 + 2 * Dx/3,

Y4 is Y0 + 2 * Dy/3,

koch_curve(Stream, X0, Y0, X2, Y2, N1),

koch_curve(Stream, X2, Y2, X3, Y3, N1),

koch_curve(Stream, X3, Y3, X4, Y4, N1),

koch_curve(Stream, X4, Y4, X1, Y1, N1).</lang>