\datethis @*Intro. This program computes the up-up-or-down-down permutation of $\{1,2,\ldots,2n-1\}$ that corresponds to a given $2\times n$ whirlpool permutation. The latter permutation appears on the command line, as a permutation of $\{0,1,\ldots,2n-1\}$, with 0 in the bottom left corner. (I've made no attempt to be efficient.) (But I didn't go out of my way to be inefficient.) (Apologies for doing this hurriedly.) @d maxn 100 @c #include #include int a[2*maxn]; int used[2*maxn]; int answer[2*maxn]; main(int argc,char*argv[]) { register int i,j,k,n,nn,t,saven; @; for (;n>1;n--) @; @; } @ @= if (argc<5 || ((argc&1)==0)) { fprintf(stderr,"Usage: %s a11 a12 ... a1n 0 a22 ... a2n\n", argv[0]); exit(-1); } nn=argc-1, n=saven=nn/2; if (n>maxn) { fprintf(stderr,"Recompile me: This program has maxn=%d!\n", maxn); exit(-99); } for (k=0;k=nn) { fprintf(stderr,"Matrix entry %d' out of range!\n", a[k]); exit(-3); } if (used[a[k]]) { fprintf(stderr,"Duplicate matrix entry %d'!\n", a[k]); exit(-4); } used[a[k]]=1; } if (a[n]) { fprintf(stderr,"Matrix entry a21 should be zero, not %d!\n", a[n]); exit(-5); } @; @ @= for (k=n+1;k; exit(-6); } } @ @= { register int t,nnp; nnp=n+n-2; answer[nnp+1]=a[0],answer[nnp]=a[1]; for (k=1;kanswer[nnp+1]) t--; a[k-1]=t-1; t=a[k+saven]; if (t>answer[nnp+1]) t--; a[k+saven-1]=t-1; } for (t=nnp-a[saven],k=0;k= n=saven; answer[1]=1; for (k=0;k=0;k-=2) { t=answer[k+1]; for (j=1;j<=t;j++) if (used[j]) t++; answer[k+1]=t; t=answer[k]; for (j=1;j<=t;j++) if (used[j]) t++; answer[k]=t; used[t]=used[answer[k+1]]=1; } for (k=nn-1;k;k--) printf(" %d", answer[k]); printf("\n"); @*Index.