]> git.scottworley.com Git - inverse-tax/blame - tax.js
Test with larger tables
[inverse-tax] / tax.js
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1"use strict";
2
3function parse_table(as_text) {
4 function parse_line(line) {
5 return line.trim().split(' ').filter(x => x !== '').map(x => parseFloat(x));
6 }
7 return as_text.trim().split('\n').map(parse_line);
8}
9
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10function tax_table_from_table(table) {
11 return table.map(([start, rate], i, table) =>
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12 [start, i < table.length - 1 ? table[i+1][0] : Infinity, rate / 100.0]);
13}
14
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15function parse_tax_table(as_text) {
16 return tax_table_from_table(parse_table(as_text));
17}
18
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19function sum(nums) {
20 return nums.reduce((total, num) => total + num, 0);
21}
22
23function tax(table, income) {
24 return sum(table.map(([start, end, rate]) =>
25 Math.max(0, Math.min(income, end) - start) * rate));
26}
27
28function apply_deductible(table, deductible) {
29 return table.map(([start, end, rate]) => [start + deductible, end + deductible, rate]);
30}
31
32function merge_tax_tables(t1, t2) {
33 if (t1.length == 0) return t2;
34 if (t2.length == 0) return t1;
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35 const [start1, end1, rate1] = t1[0];
36 const [start2, end2, rate2] = t2[0];
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37 if (start1 == start2) {
38 if (end1 == end2) {
39 return [[start1, end1, rate1 + rate2]].concat(merge_tax_tables(t1.slice(1), t2.slice(1)));
40 }
41 if (end1 < end2) {
42 return [[start1, end1, rate1 + rate2]].concat(merge_tax_tables(t1.slice(1), [[end1, end2, rate2]].concat(t2.slice(1))));
43 }
44 return merge_tax_tables(t2, t1);
45 }
46 if (start1 < start2) {
47 if (end1 <= start2) {
48 return [t1[0]].concat(merge_tax_tables(t1.slice(1), t2));
49 }
50 return [[start1, start2, rate1]].concat(merge_tax_tables([[start2, end1, rate1]].concat(t1.slice(1)), t2));
51 }
52 return merge_tax_tables(t2, t1);
53}
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54
55function invert(table) {
56 // Here we solve
57 // net = m * gross + b
58 // for gross:
59 // net - b = m * gross
60 // (net - b) / m = gross
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61
62 const ms = table.map(([start, end, rate]) => 1 - rate);
b442070b 63 const full_brackets = [0].concat(table.map(([start, end, rate]) => (end - start) * rate)).slice(0, table.length);
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64 function sum_lower_brackets(remaining_brackets, acc = 0) {
65 if (remaining_brackets.length == 0) return [];
66 return [acc + remaining_brackets[0]].concat(sum_lower_brackets(remaining_brackets.slice(1), acc + remaining_brackets[0]));
67 }
68 const bs = sum_lower_brackets(full_brackets).map((lower_brackets, i) => {
69 const [start, end, rate] = table[i];
70 // Finding b:
71 // net = gross - lower_brackets - rate * (gross - start)
72 // net = gross - lower_brackets - rate * gross + rate * start
73 // net = gross - rate * gross - lower_brackets + rate * start
74 // net = (1 - rate) * gross - lower_brackets + rate * start
75 // net = m * gross - lower_brackets + rate * start
76 // \_____________________________/ - here is b
77 return rate * start - lower_brackets;
78 });
79 const inverse_table = table.map(([start, end, rate], i) => {
80 const m = ms[i];
81 const b = bs[i];
b442070b 82 return [m * start + b, m * end + b, m, b];
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83 });
84 return function(net) {
85 for (const [start, end, m, b] of inverse_table) {
68251a4c 86 if (start <= net && net <= end) {
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87 return (net - b) / m;
88 }
89 }
49188cbe 90 return net;
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91 };
92}