From 30c5ec526a97d10a8c6cfaa9f8cd0222a69d3659 Mon Sep 17 00:00:00 2001
From: Andrew Belt <andrewpbelt@gmail.com>
Date: Thu, 19 Sep 2019 16:47:23 -0400
Subject: [PATCH] Add factorial router example.

---
 examples/factorial_router.js | 49 ++++++++++++++++++++++++++++++++++++
 1 file changed, 49 insertions(+)
 create mode 100644 examples/factorial_router.js

diff --git a/examples/factorial_router.js b/examples/factorial_router.js
new file mode 100644
index 0000000..4740184
--- /dev/null
+++ b/examples/factorial_router.js
@@ -0,0 +1,49 @@
+/**
+Factorial router
+by Andrew Belt
+
+There are 120 ways to route 5 inputs into 5 outputs.
+This uses input 6 as a CV control to choose the particular permutation.
+The CV is split into 120 intervals, each corresponding to a permutation of the inputs 1 through 5.
+*/
+
+function permutation(arr, k) {
+	var n = arr.length
+	// Clone array
+	arr = arr.slice(0)
+	// Compute n!
+	var factorial = 1
+	for (var i = 2; i <= n; i++) {
+		factorial *= i
+	}
+
+	// Build permutation array by selecting the j'th element from the remaining elements until all are chosen.
+	var perm = []
+	for (var i = n; i >= 1; i--) {
+		factorial /= i
+		var j = Math.floor(k / factorial)
+		k %= factorial
+		var el = arr[j]
+		arr.splice(j, 1)
+		perm.push(el)
+	}
+
+	return perm
+}
+
+config.bufferSize = 16
+
+function process(block) {
+	// Get factorial index from input 6
+	var k = Math.floor(block.inputs[5][0] / 10 * 120)
+	k = Math.min(Math.max(k, 0), 120 - 1)
+	// Get index set permutation
+	var perm = permutation([1, 2, 3, 4, 5], k)
+	display(perm.join(", "))
+	for (var i = 0; i < 5; i++) {
+		// Permute input i
+		for (var j = 0; j < block.bufferSize; j++) {
+			block.outputs[i][j] = block.inputs[perm[i] - 1][j]
+		}
+	}
+}