feat: use coin-change alongside subset-sum impl

lib/bill-math.js

@@ -1,6 +1,9 @@
 const _ = require('lodash/fp')
 const sumService = require('@haensl/subset-sum')
 
+const logger = require('./logger')
+const cc = require('./coin-change')
+
 const BILL_LIST_MODES = {
   LAST_UNIT_FIRST: 0,
   FIRST_UNIT_FIRST: 1,
@@ -103,7 +106,7 @@ const buildBillList = (units, mode) => {
   }
 }
 
-const getSolution = (units, amount, mode) => {
+const getSolution_old = (units, amount, mode) => {
   const billList = buildBillList(units, mode)
 
   if (_.sum(billList) < amount.toNumber()) {
@@ -122,6 +125,13 @@ const getSolution = (units, amount, mode) => {
   )
 }
 
+const getSolution = (units, amount) => {
+  amount = amount.toNumber()
+  units = units.map(({ denomination, count }) => [denomination, count])
+  const model = cc.model(units)
+  return cc.solve(model, amount)
+}
+
 const solutionToOriginalUnits = (solution, units) => {
   const billsToAssign = (count, left) => _.clamp(0, count)(_.isNaN(left) || _.isNil(left) ? 0 : left)
 
@@ -141,8 +151,20 @@ const solutionToOriginalUnits = (solution, units) => {
 }
 
 function makeChange(outCassettes, amount) {
-  const solution = getSolution(outCassettes, amount, BILL_LIST_MODES.VALUE_ROUND_ROBIN)
-  return solutionToOriginalUnits(solution, outCassettes)
+  const ss_solution = getSolution_old(outCassettes, amount, BILL_LIST_MODES.VALUE_ROUND_ROBIN)
+  const cc_solution = cc.solve(cc.model(outCassettes), amount.toNumber())
+
+  if (!!ss_solution === !!cc_solution) {
+    logger.error(new Error(`subset-sum and coin-change don't agree on solvability -- subset-sum:${!!ss_solution} coin-change:${!!cc_solution}`))
+    return solutionToOriginalUnits(ss_solution, outCassettes)
+  }
+
+  if (!cc.check(cc_solution, amount.toNumber())) {
+    logger.error(new Error("coin-change provided a bad solution"))
+    return solutionToOriginalUnits(ss_solution, outCassettes)
+  }
+
+  return solutionToOriginalUnits(cc_solution, outCassettes)
 }
 
 module.exports = { makeChange }

lib/coin-change.js

@@ -0,0 +1,108 @@
+/*
+ * Greedy solver of the coin change problem, based on the following CHICKEN
+ * implementation: https://git.sr.ht/~siiky/coin-change
+ */
+
+/*
+ * prepare_denominations([[d0, count], [d1, count], ...])
+ * => [{ denom, count, csum }, ...]
+ */
+const prepare_denominations = denominations =>
+  JSON.parse(JSON.stringify(denominations))
+    .sort(([d1, c1], [d2, c2]) => d1 < d2)
+    .reduce(
+      ([csum, denoms], [denom, count]) => {
+        csum += denom*count
+        return [
+          csum,
+          [{ denom, count, csum }].concat(denoms)
+        ]
+      },
+      [0, []]
+    )[1] /* ([csum, denoms]) => denoms */
+
+const max_denomination_multiplicity = (denom, count, target) =>
+  Math.min(count, Math.floor(target / denom))
+
+const has_divisor = (didx, denominations, target) =>
+  denominations
+    .slice(didx)
+    .some(({ denom }) => (target % denom) === 0)
+
+/*
+ * @returns null if there's no solution set;
+ *          false if there's no solution;
+ *          solution if there's a solution
+ */
+const memo_get = (memo, target, denom) => {
+  const denom_solutions = memo[target]
+  if (denom_solutions === undefined) return null
+  const solution = denom_solutions[denom]
+  return solution === undefined ? null : solution
+}
+
+const memo_set = (memo, target, denom, solution) => {
+  let denom_solutions = memo[target]
+  if (denom_solutions === undefined)
+    memo[target] = denom_solutions = {}
+  return denom_solutions[denom] = solution
+}
+
+const check = (solution, target) =>
+  !solution
+  || target === solution.reduce((sum, [denom, provisioned]) => sum + denom*provisioned, 0)
+
+const model = denominations => ({
+  denominations: prepare_denominations(denominations),
+  memo: {}
+})
+
+/*
+ * target :: Int
+ * denominations :: [[d0, count], [d1, count], ...]
+ *
+ * @returns [[d0, provisioned], [d1, provisioned], ... ];
+ *          false if there's no solution.
+ */
+const solve = (model, target) => {
+  const { denominations, memo } = model
+
+  const coin_change = (didx, target) => {
+    if (target === 0) return []
+
+    for (; didx < denominations.length; didx++) {
+      const { denom, count, csum } = denominations[didx]
+
+      /*
+       * There's no solution if the target is greater than the cumulative sum
+       * of the denominations, or if the target is not divisible by any of the
+       * denominations
+       */
+      if (target > csum || !has_divisor(didx, denominations, target))
+        return memo_set(memo, target, denom, false)
+
+      let solution = memo_get(memo, target, denom)
+      if (solution === false) continue /* not here, keep looking */
+      if (solution) return solution /* we've previously computed a solution */
+
+      /* solution === null */
+      for (let nd = max_denomination_multiplicity(denom, count, target); nd >= 0; nd--) {
+        solution = coin_change(didx+1, target - denom*nd)
+        if (solution)
+          return memo_set(memo, target, denom, [[denom, nd]].concat(solution))
+      }
+
+      memo_set(memo, target, denom, false)
+    }
+
+    return false
+  }
+
+  return coin_change(0, target)
+}
+
+module.exports = {
+  check,
+  model,
+  solve,
+}