inventory.reorder-point

Reorder point: demand over the supplier lead time, rounded up to whole units, plus safety stock.

1.0.0 · published 2026-10-03 by charlie · Anterra

Pinned by 13 tests, run in TypeScript, Python and Rust.

What it does

The stock level at which to place a replenishment order:

reorder point = lead-time demand + safety stock lead-time demand = demand x leadTimeDays / demandPeriodDays, rounded up

For example

reorder_point(70, 7, 10, 20) → 120 70 a week over a 10-day lead time, plus 20 safety stock
reorder_point(1,000, 30, 7, 50) → 284 1000 a month over 7 days is 233.33, rounded up to 234, plus 50
reorder_point(2, 7, 2, 0) → 1 a fraction under a half still rounds up: 2 a week over 2 days is 0.57, so 1

The function

The same function in TypeScript, Python and Rust, pinned by the same tests. Pick your language; the choice follows you around the registry.

def reorder_point(demand: int, demand_period_days: int, lead_time_days: int, safety_stock: int) -> int

demand	int	units used in one demand period, e.g. 1000 a month
demand_period_days	int	days in that period: 1, 7, 30, 365; lets demand per day be fractional without floats
lead_time_days	int	days from placing an order to having the stock available
safety_stock	int	buffer in units, e.g. from inventory.safety-stock
returns	int	reorder when stock on hand plus on order falls to this many units

Your code names it in one line, in the file that uses it

from fune.inventory.reorder_point import reorder_point  # inventory.reorder-point@^1

impl/python.py · 22 lines · open · raw

Imports name this capability’s declared dependencies, which fune builds next to it in your project; each one links to its page.

from .math_round_div import round_div  ← from math.round-div ^1.0.0 · built alongside by fune


def _whole_units(name: str, value: int) -> None:
    if isinstance(value, bool) or not isinstance(value, int) or value < 0:
        raise ValueError("%s must be a whole number, not negative, received %r" % (name, value))


def reorder_point(demand: int, demand_period_days: int, lead_time_days: int, safety_stock: int) -> int:
    """Lead-time demand, rounded up to whole units, plus safety stock.

    Rounding up rather than to nearest: 233.33 units of lead-time demand needs
    234 in hand, or the last third of a unit is a stock-out.
    """
    _whole_units("demand", demand)
    _whole_units("leadTimeDays", lead_time_days)
    _whole_units("safetyStock", safety_stock)
    if isinstance(demand_period_days, bool) or not isinstance(demand_period_days, int) or demand_period_days <= 0:
        raise ValueError(
            "demandPeriodDays must be a whole number of days, at least 1, received %r" % (demand_period_days,)
        )
    return round_div(demand * lead_time_days, demand_period_days, "up") + safety_stock

Install

fune build

With that line in your source, in a Python project (language python in fune.project), fune build resolves it and its 1 dependency, pins them in fune.lock, downloads only the Python package of each, and builds the code above into your project’s .fune/build, one readable file per capability with a header linking back here. Or pin a range in fune.project and build in one step:

fune add inventory.reorder-point

Download for Python inventory.reorder-point-1.0.0-python.fune · 5,773 bytes sha256 c95f39ef4a96d69d828d795b14b578331c8ebd5a4c723100075d00c3e39a2432

The manifest, vectors and README with only the Python implementation. Install it without the registry with fune add ./inventory.reorder-point-1.0.0-python.fune, or fetch it from a terminal with fune pull inventory.reorder-point@1.0.0:python.

The whole function, every language, is one file too: inventory.reorder-point-1.0.0.fune, 8,362 bytes, sha256 04b058af629a2ff2051b887513bb1b28d44cd180350dc6a5fb183a40448df0e0. It installs into a project of any language.

Customise it in your app

The seams this capability offers. Put a marker directly above a function of your own and fune build wires it into the built code; the package on the registry is not changed, the built file’s header lists it under CUSTOMISED, and fune hooks lists every hook in the project. How hooks work.

before — your function gets the arguments and returns them, changed or not, or throws to refuse the call.

# fune: before inventory.reorder-point

after — your function gets the result and the arguments, and returns the final result.

# fune: after inventory.reorder-point

replace — inside this capability’s code only, calls to a dependency go to your function, with the same signature. Other capabilities that use it are unaffected; write in * to replace it everywhere.

# fune: replace math.round-div in inventory.reorder-point

step — your function runs at a numbered point inside the function’s body, receives the in-scope values it names as parameters, and may return replacements. List the points with fune show inventory.reorder-point --steps.

# fune: step inventory.reorder-point after <n|label>

Tests

A version published now needs at least 8 tests for every function, and one that expects the error for each function that throws; the registry refuses it otherwise. fune verify --all runs each case in TypeScript, Python and Rust, and a project runs them again with fune verify. This page lists the cases; it does not run them. The exact JSON is vectors.json.

Case	Arguments		Expected
70 a week over a 10-day lead time, plus 20 safety stock	70, 7, 10, 20	→	120
1000 a month over 7 days is 233.33, rounded up to 234, plus 50	1,000, 30, 7, 50	→	284
a fraction under a half still rounds up: 2 a week over 2 days is 0.57, so 1	2, 7, 2, 0	→	1
an exact half rounds up: 15 per 2 days over 1 day is 7.5, so 8	15, 2, 1, 0	→	8
a yearly figure: 3650 a year over 14 days is exactly 140	3,650, 365, 14, 0	→	140
a daily rate: 10 a day over 5 days, no safety stock	10, 1, 5, 0	→	50
zero lead time leaves only the safety stock	1,000, 30, 0, 25	→	25
zero demand leaves only the safety stock	0, 30, 14, 25	→	25
a demand period of zero days is an error	1,000, 0, 7, 0	→	error: demandPeriodDays must be a whole number of days, at least 1
negative demand is an error	-10, 7, 7, 0	→	error: demand must be a whole number

Show the other 3 tests

Case	Arguments		Expected
negative lead time is an error	10, 7, -1, 0	→	error: leadTimeDays must be a whole number
negative safety stock is an error	10, 7, 7, -5	→	error: safetyStock must be a whole number
a fractional lead time is an error: express it in the period's days	10, 7, 2.5, 0	→	error: leadTimeDays must be a whole number

More from the author

Demand is given per period (units a week, a month, a year) with the period's length in days, so a daily rate of 33.3 units (1000 a month) needs no float: the division happens once, exactly, with `math.round-div`.

**Lead-time demand is rounded up**, never to nearest. 1000 a month over a 7-day lead time is 233.33 units; ordering at 233 plans to run out a third of a unit early. Safety stock is already whole units and is added unchanged.

Compare the result with stock on hand **plus stock already on order**, or an order placed yesterday triggers another today.

Zero lead time or zero demand leaves just the safety stock. Negative values and a demand period of zero days are errors.

Source: the standard textbook definition, e.g. APICS Dictionary ("order point") and Silver, Pyke and Thomas, *Inventory and Production Management in Supply Chains*, 4th ed., ch. 6.

Files

Path	Bytes
README.md	1,082
impl/python.py	1,066
impl/rust.rs	1,475
impl/typescript.ts	986
vectors.json	1,600