X-Git-Url: http://git.scottworley.com/inverse-tax/blobdiff_plain/53d6ad337ca89bb5ee82e1ce28fdcf18aec00568..272ad45ce0d6bd3fd425a02523e0b6b4feb47dee:/tax.js

diff --git a/tax.js b/tax.js
index 0d94200..2687387 100644
--- a/tax.js
+++ b/tax.js
@@ -7,11 +7,15 @@ function parse_table(as_text) {
   return as_text.trim().split('\n').map(parse_line);
 }
 
-function parse_tax_table(as_text) {
-  return parse_table(as_text).map(([start, rate], i, table) =>
+function tax_table_from_table(table) {
+  return table.map(([start, rate], i, table) =>
     [start, i < table.length - 1 ? table[i+1][0] : Infinity, rate / 100.0]);
 }
 
+function parse_tax_table(as_text) {
+  return tax_table_from_table(parse_table(as_text));
+}
+
 function sum(nums) {
   return nums.reduce((total, num) => total + num, 0);
 }
@@ -54,10 +58,9 @@ function invert(table) {
   // 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]));
@@ -76,13 +79,14 @@ 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;
   };
 }