ForHosting KIT · Developer Utilities

Fenwick tree index calculator

A Fenwick tree, also called a binary indexed tree, answers prefix sums and point updates in logarithmic time by storing partial ranges whose lengths are powers of two.

● BetaFree · in your browser

Run — free How it works & API

Use it from WebAPIEmailTelegramApp soon

Every navigation step depends on one bit trick: the lowest set bit of the current 1-based index, written lsb equals i bitwise-and negative i in two's complement. Adding that lsb walks the update chain; subtracting it walks the prefix-query chain. Students, competitive programmers, and library authors need those parent and next indices without re-deriving the identities under exam pressure. This Fenwick tree index calculator accepts a positive index i and optionally a tree size n, returns the lsb, the one-step update next and query parent, binary forms for teaching, and when n is set the full update path and query path used by classic BIT code. The pure deterministic logic matches free browser use and paid API calls byte for byte.

How to use it

Enter your values in the form above. The tool checks them before calculating and shows the result on the same page.

Check your inputs

Use the labels and units shown next to each field. If something is missing or outside the allowed range, the page points to the field to fix.

Use it again or automate it

Use the browser tool for individual checks and the API when you need the same capability in an automated workflow.

What you can do with it

Get an answer now

Enter one set of values and see the result without building a spreadsheet or script.

Compare scenarios

Change one value at a time and rerun the calculation to understand what affects the result.

Automate repeated work

Use the API when the same calculation needs to run inside your product or workflow.

FAQ

How do I use this capability?

Complete the fields above and run it on this page. The form highlights anything that needs attention.

For developers — API access

Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.

API endpoint

POSThttps://api.kit.forhosting.com/dev/fenwick-index

Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.

Call it from your stack

curl -X POST https://api.kit.forhosting.com/dev/fenwick-index \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"index":13}'

const res = await fetch("https://api.kit.forhosting.com/dev/fenwick-index", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "index": 13
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/dev/fenwick-index",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "index": 13
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/dev/fenwick-index", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"index":13}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"index":13}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/dev/fenwick-index", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "index": 13
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "dev.fenwick_index",
  "status": "queued",
  "_links": {
    "result": "/tasks/tsk_…/result"
  }
}

The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.

Pricing

Per request$0.002

Published price — no tokens, no invented credits. A failed task is never charged.

Errors

HTTP	Code	Meaning
`401`	`unauthorized`	Missing or invalid API key.
`402`	`insufficient_balance`	Your balance doesn't cover the task price.
`404`	`unknown_type`	That task type doesn't exist.
`429`	`rate_limited`	Too many requests. Use the webhook instead of polling.

Read the full KIT documentation →