68 lines
1.2 KiB
JavaScript
68 lines
1.2 KiB
JavaScript
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// Utilities for generating sets of coordinates based on
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// permutations, even permutations and changes of sign.
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// Based on https://www.qfbox.info/epermute
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function pandita(a) {
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const n = a.length;
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for( let k = n - 2; k >= 0; k-- ) {
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if( a[k] < a[k + 1] ) {
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for( let l = n - 1; l >= 0; l-- ) {
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if( a[k] < a[l] ) {
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const tmp = a[k];
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a[k] = a[l];
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a[l] = tmp;
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const revtail = a.slice(k + 1);
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revtail.reverse();
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for( let i = 0; i < revtail.length; i++ ) {
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a[k + 1 + i] = revtail[i];
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}
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return Math.floor(revtail.length / 2) + 1;
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}
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}
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console.log("Shouldn't get here");
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process.exit();
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}
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}
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return false;
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}
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function permutations(a) {
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a.sort();
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const ps = [ [...a] ];
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while( pandita(a) ) {
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ps.push([...a]);
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}
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return ps;
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}
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function permutations_even(a) {
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a.sort();
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let parity = 'even';
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const ps = [ [...a] ];
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let running = true;
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while( running ) {
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const s = pandita(a);
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if( s ) {
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if( parity === 'even' ) {
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if( s % 2 === 1 ) {
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parity = 'odd';
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}
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} else {
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if( s % 2 === 1 ) {
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parity = 'even';
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}
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}
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if( parity === 'even' ) {
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ps.push([...a]);
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}
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} else {
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running = false;
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}
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}
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return ps;
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}
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