X-Git-Url: http://git.scottworley.com/inverse-tax/blobdiff_plain/a659a4f09cf859b2a63dc5c15f0d4589220c9a83..a5aef764d87ffe30b15f21b141b192dca8b04d0e:/tax.js

diff --git a/tax.js b/tax.js
index 7f32f74..10c44f3 100644
--- a/tax.js
+++ b/tax.js
@@ -1,5 +1,13 @@
 "use strict";
 
+function near(a, b, epsilon = 1e-6) {
+  return Math.abs(a - b) < epsilon;
+}
+
+function less_than_or_near(a, b, epsilon = 1e-6) {
+  return a < b || near(a, b, epsilon);
+}
+
 function parse_table(as_text) {
   function parse_line(line) {
     return line.trim().split(' ').filter(x => x !== '').map(x => parseFloat(x));
@@ -53,17 +61,14 @@ 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:
   //   net - b = m * gross
   //   (net - b) / m = gross
-  // 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 +87,14 @@ 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 (less_than_or_near(start, net) && less_than_or_near(net, end)) {
         return (net - b) / m;
       }
     }
+    return net;
   };
 }