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[inverse-tax] / tax.js

"use strict";

function parse_table(as_text) {
  function parse_line(line) {
    return line.trim().split(' ').filter(x => x !== '').map(x => parseFloat(x));
  }
  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) =>
    [start, i < table.length - 1 ? table[i+1][0] : Infinity, rate / 100.0]);
}

function sum(nums) {
  return nums.reduce((total, num) => total + num, 0);
}

function tax(table, income) {
  return sum(table.map(([start, end, rate]) =>
    Math.max(0, Math.min(income, end) - start) * rate));
}

function apply_deductible(table, deductible) {
  return table.map(([start, end, rate]) => [start + deductible, end + deductible, rate]);
}

function merge_tax_tables(t1, t2) {
  if (t1.length == 0) return t2;
  if (t2.length == 0) return t1;
  [start1, end1, rate1] = t1[0];
  [start2, end2, rate2] = t2[0];
  if (start1 == start2) {
    if (end1 == end2) {
      return [[start1, end1, rate1 + rate2]].concat(merge_tax_tables(t1.slice(1), t2.slice(1)));
    }
    if (end1 < end2) {
      return [[start1, end1, rate1 + rate2]].concat(merge_tax_tables(t1.slice(1), [[end1, end2, rate2]].concat(t2.slice(1))));
    }
    return merge_tax_tables(t2, t1);
  }
  if (start1 < start2) {
    if (end1 <= start2) {
      return [t1[0]].concat(merge_tax_tables(t1.slice(1), t2));
    }
    return [[start1, start2, rate1]].concat(merge_tax_tables([[start2, end1, rate1]].concat(t1.slice(1)), t2));
  }
  return merge_tax_tables(t2, t1);
}