水道料金（上水道・下水道・消費税）

<?php
declare(strict_types=1);

use BcMath\Number;

/**
 * 水道料金。
 * v2: 十進（BcMath\Number）で計算し、四捨五入は RoundingMode::HalfAwayFromZero で明示する
 * （float だと 15 × 16.9 が 253.49999999999997 になる）。
 * @return array{water:int, sewer:int, subtotal:int, tax:int, total:int}
 */
function yen(Number $x): int
{
    return (int) $x->round(0, RoundingMode::HalfAwayFromZero)->value;
}

function calcBill(string $usage, string $waterBase, string $waterUnit, string $sewerBase, string $sewerUnit, string $tax): array
{
    $u = new Number($usage);
    $water = yen(new Number($waterBase) + $u * new Number($waterUnit));
    $sewer = yen(new Number($sewerBase) + $u * new Number($sewerUnit));
    $subtotal = $water + $sewer;
    $t = $tax === 'incl' ? yen(new Number($subtotal) * 10 / 100) : 0;
    return ['water' => $water, 'sewer' => $sewer, 'subtotal' => $subtotal, 'tax' => $t, 'total' => $subtotal + $t];
}

<?php
declare(strict_types=1);

/**
 * 水道料金。
 * v1: よくある書き方（float と round）。
 * @return array{water:int, sewer:int, subtotal:int, tax:int, total:int}
 */
function calcBill(string $usage, string $waterBase, string $waterUnit, string $sewerBase, string $sewerUnit, string $tax): array
{
    $u = (float) $usage;
    $water = (int) round((float) $waterBase + $u * (float) $waterUnit);
    $sewer = (int) round((float) $sewerBase + $u * (float) $sewerUnit);
    $subtotal = $water + $sewer;
    $t = $tax === 'incl' ? (int) round($subtotal * 0.1) : 0;
    return ['water' => $water, 'sewer' => $sewer, 'subtotal' => $subtotal, 'tax' => $t, 'total' => $subtotal + $t];
}

"""水道料金。

v2: decimal で計算し、四捨五入は ROUND_HALF_UP で明示する
（float だと 15 × 16.9 が 253.49999999999997 になり、round は偶数丸め）。
"""
from decimal import ROUND_HALF_UP, Decimal


def _yen(x: Decimal) -> int:
    return int(x.quantize(Decimal(1), rounding=ROUND_HALF_UP))


def calc_bill(usage: str, water_base: str, water_unit: str, sewer_base: str, sewer_unit: str, tax: str) -> dict:
    u = Decimal(usage)
    water = _yen(Decimal(water_base) + u * Decimal(water_unit))
    sewer = _yen(Decimal(sewer_base) + u * Decimal(sewer_unit))
    subtotal = water + sewer
    t = _yen(Decimal(subtotal) * 10 / 100) if tax == "incl" else 0
    return {"water": water, "sewer": sewer, "subtotal": subtotal, "tax": t, "total": subtotal + t}

"""水道料金。

v1: よくある書き方（float と round）。
"""


def calc_bill(usage: str, water_base: str, water_unit: str, sewer_base: str, sewer_unit: str, tax: str) -> dict:
    u = float(usage)
    water = round(float(water_base) + u * float(water_unit))
    sewer = round(float(sewer_base) + u * float(sewer_unit))
    subtotal = water + sewer
    t = round(subtotal * 0.1) if tax == "incl" else 0
    return {"water": water, "sewer": sewer, "subtotal": subtotal, "tax": t, "total": subtotal + t}

package main

import (
	"fmt"
	"math/big"
)

// Output は水道料金の内訳（円）。
type Output struct {
	Water    int64 `json:"water"`
	Sewer    int64 `json:"sewer"`
	Subtotal int64 `json:"subtotal"`
	Tax      int64 `json:"tax"`
	Total    int64 `json:"total"`
}

// Input は水道料金の入力。
type Input struct {
	Usage     string `json:"usage"`
	WaterBase string `json:"water_base"`
	WaterUnit string `json:"water_unit"`
	SewerBase string `json:"sewer_base"`
	SewerUnit string `json:"sewer_unit"`
	Tax       string `json:"tax"`
}

func rat(s string) (*big.Rat, error) {
	r, ok := new(big.Rat).SetString(s)
	if !ok {
		return nil, fmt.Errorf("invalid decimal: %q", s)
	}
	return r, nil
}

// yen は 0 以上の有理数を円に四捨五入する（0.5 ちょうどは繰り上げ）。
func yen(x *big.Rat) int64 {
	v := new(big.Rat).Add(x, big.NewRat(1, 2))
	return new(big.Int).Quo(v.Num(), v.Denom()).Int64()
}

func fee(base, usage, unit string) (int64, error) {
	b, err := rat(base)
	if err != nil {
		return 0, err
	}
	u, err := rat(usage)
	if err != nil {
		return 0, err
	}
	p, err := rat(unit)
	if err != nil {
		return 0, err
	}
	return yen(new(big.Rat).Add(b, new(big.Rat).Mul(u, p))), nil
}

// CalcBill は水道料金を求める。
// v2: math/big.Rat で正確に計算し、四捨五入は自前で行う（float64 だと 15 × 16.9 が 253.49999999999997 になる）。
func CalcBill(in Input) (Output, error) {
	water, err := fee(in.WaterBase, in.Usage, in.WaterUnit)
	if err != nil {
		return Output{}, err
	}
	sewer, err := fee(in.SewerBase, in.Usage, in.SewerUnit)
	if err != nil {
		return Output{}, err
	}
	subtotal := water + sewer
	var tax int64
	if in.Tax == "incl" {
		tax = yen(big.NewRat(subtotal*10, 100))
	}
	return Output{water, sewer, subtotal, tax, subtotal + tax}, nil
}

package main

import (
	"math"
	"strconv"
)

// Output は水道料金の内訳（円）。
type Output struct {
	Water    int64 `json:"water"`
	Sewer    int64 `json:"sewer"`
	Subtotal int64 `json:"subtotal"`
	Tax      int64 `json:"tax"`
	Total    int64 `json:"total"`
}

// Input は水道料金の入力。
type Input struct {
	Usage     string `json:"usage"`
	WaterBase string `json:"water_base"`
	WaterUnit string `json:"water_unit"`
	SewerBase string `json:"sewer_base"`
	SewerUnit string `json:"sewer_unit"`
	Tax       string `json:"tax"`
}

func f(s string) float64 { v, _ := strconv.ParseFloat(s, 64); return v }

// CalcBill は水道料金を求める。
// v1: よくある書き方（float64 と math.Round）。
func CalcBill(in Input) (Output, error) {
	u := f(in.Usage)
	water := int64(math.Round(f(in.WaterBase) + u*f(in.WaterUnit)))
	sewer := int64(math.Round(f(in.SewerBase) + u*f(in.SewerUnit)))
	subtotal := water + sewer
	var tax int64
	if in.Tax == "incl" {
		tax = int64(math.Round(float64(subtotal) * 0.1))
	}
	return Output{water, sewer, subtotal, tax, subtotal + tax}, nil
}

// 水道料金。
// v2: BigInt の分数で正確に計算し、四捨五入も自前で行う（Number だと 15 × 16.9 が 253.49999999999997 になる）。

/** "16.9" → { n: 169n, d: 10n } */
function dec(s) {
  const m = /^(\d+)(?:\.(\d+))?$/.exec(s);
  if (!m) throw new RangeError(`invalid decimal: ${s}`);
  const frac = m[2] ?? "";
  return { n: BigInt(m[1] + frac), d: 10n ** BigInt(frac.length) };
}

/** base + usage × unit を円に四捨五入（すべて 0 以上） */
function fee(base, usage, unit) {
  const b = dec(base), u = dec(usage), p = dec(unit);
  const den = b.d * u.d * p.d;
  const num = b.n * u.d * p.d + u.n * p.n * b.d;
  return (2n * num + den) / (2n * den);
}

export function calcBill({ usage, water_base, water_unit, sewer_base, sewer_unit, tax }) {
  const water = fee(water_base, usage, water_unit);
  const sewer = fee(sewer_base, usage, sewer_unit);
  const subtotal = water + sewer;
  const t = tax === "incl" ? (2n * subtotal * 10n + 100n) / 200n : 0n; // subtotal × 10/100 を四捨五入
  return { water: Number(water), sewer: Number(sewer), subtotal: Number(subtotal), tax: Number(t), total: Number(subtotal + t) };
}

// 水道料金。
// v1: よくある書き方（Number と Math.round）。

export function calcBill({ usage, water_base, water_unit, sewer_base, sewer_unit, tax }) {
  const u = Number(usage);
  const water = Math.round(Number(water_base) + u * Number(water_unit));
  const sewer = Math.round(Number(sewer_base) + u * Number(sewer_unit));
  const subtotal = water + sewer;
  const t = tax === "incl" ? Math.round(subtotal * 0.1) : 0;
  return { water, sewer, subtotal, tax: t, total: subtotal + t };
}