UI

[inverse-tax] / tax.js

diff --git a/tax.js b/tax.js
index 7f32f74717a01386caaa937ead3f1d8443b3f610..f3d2406cce017d7fbb7b9056d2be507b7b3f5990 100644 (file)
--- a/tax.js
+++ b/tax.js
@@ -53,8 +53,6 @@ function merge_tax_tables(t1, t2) {
 }
 
 function invert(table) {
-  if (table.length == 0) return x => x;
-
   // Here we solve
   //   net = m * gross + b
   // for gross:
@@ -63,7 +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]));
@@ -82,13 +80,14 @@ 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) {
-      if (start < net && net < end) {
+      if (start <= net && net <= end) {
         return (net - b) / m;
       }
     }
+    return net;
   };
 }