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# mbostock/.block

Last active November 28, 2019 00:39
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Prim’s Algorithm V
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 redirect: https://observablehq.com/@mbostock/prims-algorithm-v

Another variation of color-cycling a spanning tree generated by Prim’s algorithm. Here the periodicity of the color scale by tree depth is varied over time, alternating between emphasis of micro and macro structure. This idea was suggested in a Hacker News comment.

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 !function(){function t(t){return function(e,i){e=d3.hsl(e),i=d3.hsl(i);var r=(e.h+120)*a,h=(i.h+120)*a-r,s=e.s,l=i.s-s,o=e.l,u=i.l-o;return isNaN(l)&&(l=0,s=isNaN(s)?i.s:s),isNaN(h)&&(h=0,r=isNaN(r)?i.h:r),function(a){var e=r+h*a,i=Math.pow(o+u*a,t),c=(s+l*a)*i*(1-i);return"#"+n(i+c*(-.14861*Math.cos(e)+1.78277*Math.sin(e)))+n(i+c*(-.29227*Math.cos(e)-.90649*Math.sin(e)))+n(i+c*1.97294*Math.cos(e))}}}function n(t){var n=(t=0>=t?0:t>=1?255:0|255*t).toString(16);return 16>t?"0"+n:n}var a=Math.PI/180;d3.scale.cubehelix=function(){return d3.scale.linear().range([d3.hsl(300,.5,0),d3.hsl(-240,.5,1)]).interpolate(d3.interpolateCubehelix)},d3.interpolateCubehelix=t(1),d3.interpolateCubehelix.gamma=t}();
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 var N = 1 << 0, S = 1 << 1, W = 1 << 2, E = 1 << 3; self.addEventListener("message", function(event) { postMessage(generateMaze(event.data.width, event.data.height)); }); function generateMaze(cellWidth, cellHeight) { var cells = new Array(cellWidth * cellHeight), frontier = minHeap(function(a, b) { return a.weight - b.weight; }), startIndex = (cellHeight - 1) * cellWidth; cells[startIndex] = 0; frontier.push({index: startIndex, direction: N, weight: Math.random()}); frontier.push({index: startIndex, direction: E, weight: Math.random()}); while ((edge = frontier.pop()) != null) { var edge, i0 = edge.index, d0 = edge.direction, i1 = i0 + (d0 === N ? -cellWidth : d0 === S ? cellWidth : d0 === W ? -1 : +1), x0 = i0 % cellWidth, y0 = i0 / cellWidth | 0, x1, y1, d1, open = cells[i1] == null; // opposite not yet part of the maze if (d0 === N) x1 = x0, y1 = y0 - 1, d1 = S; else if (d0 === S) x1 = x0, y1 = y0 + 1, d1 = N; else if (d0 === W) x1 = x0 - 1, y1 = y0, d1 = E; else x1 = x0 + 1, y1 = y0, d1 = W; if (open) { cells[i0] |= d0, cells[i1] |= d1; if (y1 > 0 && cells[i1 - cellWidth] == null) frontier.push({index: i1, direction: N, weight: Math.random()}); if (y1 < cellHeight - 1 && cells[i1 + cellWidth] == null) frontier.push({index: i1, direction: S, weight: Math.random()}); if (x1 > 0 && cells[i1 - 1] == null) frontier.push({index: i1, direction: W, weight: Math.random()}); if (x1 < cellWidth - 1 && cells[i1 + 1] == null) frontier.push({index: i1, direction: E, weight: Math.random()}); } } return cells; } function minHeap(compare) { var heap = {}, array = [], size = 0; heap.empty = function() { return !size; }; heap.push = function(value) { up(array[size] = value, size++); return size; }; heap.pop = function() { if (size <= 0) return; var removed = array[0], value; if (--size > 0) value = array[size], down(array[0] = value, 0); return removed; }; function up(value, i) { while (i > 0) { var j = ((i + 1) >> 1) - 1, parent = array[j]; if (compare(value, parent) >= 0) break; array[i] = parent; array[i = j] = value; } } function down(value, i) { while (true) { var r = (i + 1) << 1, l = r - 1, j = i, child = array[j]; if (l < size && compare(array[l], child) < 0) child = array[j = l]; if (r < size && compare(array[r], child) < 0) child = array[j = r]; if (j === i) break; array[i] = child; array[i = j] = value; } } return heap; }
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