Add Business Tax

[inverse-tax] / tax.test.js

diff --git a/tax.test.js b/tax.test.js
index 125cc2091cccd1587c27d9687e10271864f20066..5080001436529b89c94c58dc4fbe90ef1f89e98a 100644 (file)
--- a/tax.test.js
+++ b/tax.test.js
@@ -6,17 +6,13 @@ function test(description, f) {
   f();
 }
 
-function near(a, b, epsilon = 1e-6) {
-  return Math.abs(a - b) < epsilon;
-}
-
 function rand(limit = 99) {
   return Math.round(Math.random() * limit);
 }
 
 function make_random_tax_table() {
   function make_random_table(min_threshold) {
-    if (rand(2)) return [];
+    if (rand(4) == 0) return [];
     const start = min_threshold === undefined ? rand() : min_threshold;
     const end = start + 1 + rand();
     return [[start, rand()]].concat(make_random_table(end));
@@ -59,4 +55,16 @@ test("invert", () => {
   assert.strictEqual(invert([[10, Infinity, .1]])(19), 20);
   assert.strictEqual(invert([[10, Infinity, .1]])( 9),  9);
   assert.strictEqual(invert([[0, 100, .1], [100, Infinity, .2]])(170), 200);
+  assert.strictEqual(invert([[0, 100, .1], [100, Infinity, .2]])(112), 127.5);
+  assert.strictEqual(invert([[6, 90, 0.75], [90, Infinity, 0.12]])(27), 90);
+
+  for (var i = 0; i < 1000; i++) {
+    const t = make_random_tax_table();
+    const inverted_t = invert(t);
+    for (var j = 0; j < 20; j++) {
+      const net = rand(250);
+      const gross = inverted_t(net);
+      assert.ok(near(net, gross - tax(t, gross)));
+    }
+  }
 });