This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
\\Ellermann's clockwise square spiral, first step (0,0) -> (0,1)
y=vector(10000);L=0;d=1;n=0;
for(r=1, 100, d=-d; k=floor(r/2)*d; for(j=1, L++, y[n++]=k); forstep(j=k-d, -floor((r+1)/2)*d+d, -d, y[n++]=j));
x=vector(10100);L=1;d=-1;n=0;
for(r=1, 100, d=-d; k=floor(r/2)*d; for(j=1, L++, x[n++]=-k); forstep(j=k-d, -floor((r+1)/2)*d+d, -d, x[n++]=-j));
\\ Position in spiral
findpos(i,j)={my(size=(2*max(abs(i),abs(j))+1)^2);forstep(k=size,1,-1, if(i==x[k]&&j==y[k], return(k)))};
atan2(y,x)=if(x>0,atan(y/x),if(x==0,if(y>0,Pi/2,-Pi/2),if(y>=0,atan(y/x)+Pi,atan(y/x)-Pi)));
angle(v,w)=atan2(v[1]*w[2]-v[2]*w[1],v[1]*w[1]+v[2]*w[2]);
move=[2,1;1,2;-1,2;-2,1;-2,-1;-1,-2;1,-2;2,-1]; \\ 8 Knight moves
m=6;b=matrix(2*m+1, 2*m+1, i, j, 0);setb(pos)={b[pos[1]+m+1, pos[2]+m+1]=1};
getb(pos)=b[pos[1]+m+1, pos[2]+m+1];
inring(n, p)=!(abs(p[1])angmin, jmin=j; angmin=adiff;jlast=j)))));if(jmin>0,p+=move[jmin,];setb(p);););p+=move[jlast,];setb(p)); \\ Hugo Pfoertner, May 11 2019
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