From: Scott Worley <scottworley@scottworley.com>
Date: Thu, 23 May 2019 21:38:38 +0000 (-0700)
Subject: Calculate inverse bounds correctly
X-Git-Tag: v1.0~12
X-Git-Url: http://git.scottworley.com/inverse-tax/commitdiff_plain/b442070b44917301d40c5a6cbf996898175908be?ds=sidebyside

Calculate inverse bounds correctly
---

diff --git a/tax.js b/tax.js
index d9bd34c..cd0027a 100644
--- a/tax.js
+++ b/tax.js
@@ -61,7 +61,7 @@ function invert(table) {
   // and the calculate the inverse's bounds
 
   const ms = table.map(([start, end, rate]) => 1 - rate);
-  const full_brackets = [[0]].concat(table.map(([start, end, rate]) => (end - start) * rate)).slice(0, table.length);
+  const full_brackets = [0].concat(table.map(([start, end, rate]) => (end - start) * rate)).slice(0, table.length);
   function sum_lower_brackets(remaining_brackets, acc = 0) {
     if (remaining_brackets.length == 0) return [];
     return [acc + remaining_brackets[0]].concat(sum_lower_brackets(remaining_brackets.slice(1), acc + remaining_brackets[0]));
@@ -80,7 +80,7 @@ function invert(table) {
   const inverse_table = table.map(([start, end, rate], i) => {
     const m = ms[i];
     const b = bs[i];
-    return [(start - b) / m, (end - b) / m, m, b];
+    return [m * start + b, m * end + b, m, b];
   });
   return function(net) {
     for (const [start, end, m, b] of inverse_table) {
diff --git a/tax.test.js b/tax.test.js
index 125cc20..27767fe 100644
--- a/tax.test.js
+++ b/tax.test.js
@@ -59,4 +59,5 @@ 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);
 });