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function draw_roulette(ctx, w, h, datalist){
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var radius = h / 2 - 5;
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var center = new Array(w / 2, h / 2);
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var init = 0;
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var total = datalist.reduce((acc, value) => acc + value, init);
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var lastend = 0;
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var offset = Math.PI / 2;
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for (i=0; i < datalist.length; i++){
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var part = datalist[i];
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ctx.beginPath();
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ctx.fillStyle = colist[i];
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ctx.moveTo(center[0], center[1]);
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var arcsector = Math.PI * (2 * part / total);
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ctx.arc(center[0], center[1], radius, lastend - offset, lastend + arcsector - offset, false);
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ctx.lineTo(center[0], center[1]);
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ctx.fill();
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ctx.closePath();
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lastend += arcsector;
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}
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}
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