From 6c360fcafda0ac43aebadb6b731e2bfd4a8ed47c Mon Sep 17 00:00:00 2001
From: Mike Lynch <m.lynch@sydney.edu.au>
Date: Fri, 3 Nov 2023 12:57:40 +1100
Subject: [PATCH] Start of code to build tetrahedra from 600-cell

---
 layer600cell.js | 81 +++++++++++++++++++++++++++++++++++++++++++++++++
 polytopes.js    |  4 +--
 2 files changed, 83 insertions(+), 2 deletions(-)
 create mode 100644 layer600cell.js

diff --git a/layer600cell.js b/layer600cell.js
new file mode 100644
index 0000000..4c2716e
--- /dev/null
+++ b/layer600cell.js
@@ -0,0 +1,81 @@
+
+
+import * as POLYTOPES from './polytopes.js';
+
+// face detection for the 600-cell
+
+
+export function nodes_links(links, nodeid) {
+	return links.filter((l) => l.source === nodeid || l.target === nodeid);
+}
+
+
+export function linked(links, n1, n2) {
+	const ls = nodes_links(nodes_links(links, n1), n2);
+	if( ls.length ) {
+		return ls[0]
+	} else {
+		return false;
+	}
+}
+
+
+function fingerprint(ids) {
+	const sids = [...ids];
+	sids.sort();
+	return sids.join(',');
+}
+
+
+export function make_600cell() {
+	const nodes = POLYTOPES.make_600cell_vertices();
+	const links = POLYTOPES.auto_detect_edges(nodes, 12);
+	return {
+		nodes: nodes,
+		links: links
+	}
+}
+
+export function link_to_tetras(nodes, links, link) {
+	const n1 = link.source;
+	const n2 = link.target;
+	const nl1 = nodes_links(links, n1).filter((l) => l.id !== link.id);
+	const nl2 = nodes_links(links, n2).filter((l) => l.id !== link.id);
+	const p1 = new Set();
+	const p = new Set();
+	for( const nl of nl1 ) {
+		if( nl.source !== n1 ) {
+			p1.add(nl.source);
+		}
+		if( nl.target !== n1 ) {
+			p1.add(nl.target);
+		}
+	}
+	for( const nl of nl2 ) {
+		if( nl.source !== n2 && p1.has(nl.source) ) {
+			p.add(nl.source);
+		}
+		if( nl.target !== n2 && p1.has(nl.target) ) {
+			p.add(nl.target);
+		}
+	}
+	const lp = Array.from(p);
+	const seen = {};
+	const tetras = [];
+	for( const p1 of lp ) {
+		for( const p2 of lp ) {
+			if( p1 != p2 ) {
+				if( linked(links, p1, p2) ) {
+					const fp = fingerprint([n1, n2, p1, p2]);
+					if( !seen[fp] ) {
+						seen[fp] = true;
+						tetras.push([n1, n2, p1, p2])
+					}
+				}
+			}
+		}
+	}
+	return tetras;
+}
+
+
diff --git a/polytopes.js b/polytopes.js
index aceeb25..202187f 100644
--- a/polytopes.js
+++ b/polytopes.js
@@ -22,7 +22,7 @@ function dist2(n1, n2) {
 	return (n1.x - n2.x) ** 2 + (n1.y - n2.y) ** 2 + (n1.z - n2.z) ** 2 + (n1.w - n2.w) ** 2;
 }
 
-function auto_detect_edges(nodes, neighbours, debug=false) {
+export function auto_detect_edges(nodes, neighbours, debug=false) {
 	const seen = {};
 	const nnodes = nodes.length;
 	const links = [];
@@ -560,7 +560,7 @@ function map_coord(i, coords, values) {
 }
 
 
-function make_600cell_vertices() {
+export function make_600cell_vertices() {
 	const coords = {
 		0: '0',
 		1: '1',