fourdjs/permute.js

// Utilities for generating sets of coordinates based on 
// permutations, even permutations and changes of sign.
// Based on https://www.qfbox.info/epermute


function pandita(a) {
	const n = a.length;
	for( let k = n - 2; k >= 0; k-- ) {
		if( a[k] < a[k + 1] ) {
			for( let l = n - 1; l >= 0; l-- ) {
				if( a[k] < a[l] ) {
					const tmp = a[k];
					a[k] = a[l];
					a[l] = tmp;
					const revtail = a.slice(k + 1);
					revtail.reverse();
					for( let i = 0; i < revtail.length; i++ ) {
						a[k + 1 + i] = revtail[i];
					}
					return Math.floor(revtail.length / 2) + 1;
				}
			}
			console.log("Shouldn't get here");
			process.exit();
		}
	}
	return false;
}

function permutations_old(a) {
	a.sort();
	const ps = [ [...a] ];
	let running = true;
	while( running ) {
		const s = pandita(a);
		if( s ) {
			ps.push([...a]);
		} else {
			running = false;
		}
	}
	return ps;
}

function permutations(a) {
	a.sort();
	const ps = [ [...a] ];
	let running = true;
	while( pandita(a) > 0 ) {
		ps.push([...a]);
	}
	return ps;
}


function permutations_even(a) {
	a.sort();
	let parity = 'even';
	const ps = [ [...a] ];
	let running = true;
	while( running ) {
		const s = pandita(a);
		if( s ) {
			if( parity === 'even' ) {
				if( s % 2 === 1 ) {
					parity = 'odd';
				}
			} else {
				if( s % 2 === 1 ) {
					parity = 'even';
				}
			}
			if( parity === 'even' ) {
				ps.push([...a]);
			}
		} else {
			running = false;
		}
	}
	return ps;
}

// for a given permutation, say [ 1, 1, 0, 0 ], return all
// of the valid changes of sign, so:
// [ [1, 1, 0, 0 ], [ -1, 1, 0, 0 ], [ 1, -1, 0, 0 ], [-1, -1, 0, 0 ]]
// ie don't do it on the zeros

function expand_sign(a, label) {
	const expanded = [];
	const exv = a.map((v) => v ? [ -v, v ] : [ 0 ]);
	for( const xv of exv[0] ) {
		for( const yv of exv[1] ) {
			for( const zv of exv[2] ) {
				for( const wv of exv[3] ) {
					expanded.push({label: label, x: xv, y:yv, z:zv, w:wv});
				}
			}
		}
	}
	return expanded;
}


export function coordinates(a, label, even=false) {
	const ps = even ? permutations_even(a) : permutations(a);
	const coords = [];
	for( const p of ps ) {
		const expanded = expand_sign(p, label);
		coords.push(...expanded);
	}
	return coords;
}