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ProjectEuler/pe/p220.html
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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="utf-8" />
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<meta name="author" content="Colin Hughes" />
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<title>Problem 220 - Project Euler</title>
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<h2>Heighway Dragon</h2><div class="info" style="cursor:help;width:200px;margin-bottom:10px;"><h3>Problem 220</h3><span style="width:300px;color:#666;">Published on Saturday, 6th December 2008, 09:00 am; Solved by 1410</span></div>
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<div class="problem_content" role="problem">
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<p>Let <b><i>D</i></b><sub>0</sub> be the two-letter string "Fa". For n<img src='images/symbol_ge.gif' width='10' height='12' alt='≥' border='0' style='vertical-align:middle;' />1, derive <b><i>D</i></b><sub>n</sub> from <b><i>D</i></b><sub>n-1</sub> by the string-rewriting rules:</p>
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<p style="margin-left:40px;">"a" <img src='images/symbol_maps.gif' width='15' height='7' alt='→' border='0' style='vertical-align:middle;' /> "aRbFR"<br />
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"b" <img src='images/symbol_maps.gif' width='15' height='7' alt='→' border='0' style='vertical-align:middle;' /> "LFaLb"</p>
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<p>Thus, <b><i>D</i></b><sub>0</sub> = "Fa", <b><i>D</i></b><sub>1</sub> = "FaRbFR", <b><i>D</i></b><sub>2</sub> = "FaRbFRRLFaLbFR", and so on.</p>
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<p>These strings can be interpreted as instructions to a computer graphics program, with "F" meaning "draw forward one unit", "L" meaning "turn left 90 degrees", "R" meaning "turn right 90 degrees", and "a" and "b" being ignored. The initial position of the computer cursor is (0,0), pointing up towards (0,1).</p>
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<p>Then <b><i>D</i></b><sub>n</sub> is an exotic drawing known as the <i>Heighway Dragon</i> of order <i>n</i>. For example, <b><i>D</i></b><sub>10</sub> is shown below; counting each "F" as one step, the highlighted spot at (18,16) is the position reached after 500 steps.</p>
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<img src="project/images/p_220.gif" alt="" /></div>
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<p>What is the position of the cursor after 10<sup>12</sup> steps in <b><i>D</i></b><sub>50</sub> ?<br />
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Give your answer in the form <i>x</i>,<i>y</i> with no spaces.</p>
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