Rust's str::rsplitn in TypeScript

How to use

Install via yarn add locutus and import: import { rsplitn } from 'locutus/rust/str/rsplitn'.

Or with CommonJS: const { rsplitn } = require('locutus/rust/str/rsplitn')

Use a bundler that supports tree-shaking so you only ship the functions you actually use. Vite, webpack, Rollup, and Parcel all handle this. For server-side use this is less of a concern.

Examples

These examples are extracted from test cases that automatically verify our functions against their native counterparts.

#	code	expected result
1	rsplitn(3, ' ', 'Mary had a little lamb')	['lamb', 'little', 'Mary had a']
2	rsplitn(2, '::', 'lion::tiger::leopard')	['leopard', 'lion::tiger']
3	rsplitn(3, 'X', 'lionXXtigerXleopard')	['leopard', 'tiger', 'lionX']

Notes

• Reverse-order variant of splitn.

Here's what our current TypeScript equivalent to Rust's str::rsplitn looks like.

TSTypeScript JSJavaScript TSStandalone TS JSStandalone JS

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export function rsplitn(n: number, delimiter: string, str: string): string[] {
  //      discuss at: https://locutus.io/rust/rsplitn/
  // parity verified: Rust 1.85
  //     original by: Kevin van Zonneveld (https://kvz.io)
  //          note 1: Reverse-order variant of splitn.
  //       example 1: rsplitn(3, ' ', 'Mary had a little lamb')
  //       returns 1: ['lamb', 'little', 'Mary had a']
  //       example 2: rsplitn(2, '::', 'lion::tiger::leopard')
  //       returns 2: ['leopard', 'lion::tiger']
  //       example 3: rsplitn(3, 'X', 'lionXXtigerXleopard')
  //       returns 3: ['leopard', 'tiger', 'lionX']

  const limit = Math.trunc(Number(n))
  if (!Number.isFinite(limit) || limit <= 0) {
    return []
  }

  const source = String(str)
  const value = String(delimiter)
  if (limit === 1) {
    return [source]
  }

  if (value === '') {
    const parts = ['', ...Array.from(source), ''].reverse()
    if (parts.length <= limit) {
      return parts
    }
    return [
      ...parts.slice(0, limit - 1),
      parts
        .slice(limit - 1)
        .reverse()
        .join(''),
    ]
  }

  const out: string[] = []
  let remainder = source

  for (let i = 0; i < limit - 1; i += 1) {
    const index = remainder.lastIndexOf(value)
    if (index < 0) {
      break
    }

    out.push(remainder.slice(index + value.length))
    remainder = remainder.slice(0, index)
  }

  out.push(remainder)
  return out
}

export function rsplitn(n, delimiter, str) {
  //      discuss at: https://locutus.io/rust/rsplitn/
  // parity verified: Rust 1.85
  //     original by: Kevin van Zonneveld (https://kvz.io)
  //          note 1: Reverse-order variant of splitn.
  //       example 1: rsplitn(3, ' ', 'Mary had a little lamb')
  //       returns 1: ['lamb', 'little', 'Mary had a']
  //       example 2: rsplitn(2, '::', 'lion::tiger::leopard')
  //       returns 2: ['leopard', 'lion::tiger']
  //       example 3: rsplitn(3, 'X', 'lionXXtigerXleopard')
  //       returns 3: ['leopard', 'tiger', 'lionX']

  const limit = Math.trunc(Number(n))
  if (!Number.isFinite(limit) || limit <= 0) {
    return []
  }

  const source = String(str)
  const value = String(delimiter)
  if (limit === 1) {
    return [source]
  }

  if (value === '') {
    const parts = ['', ...Array.from(source), ''].reverse()
    if (parts.length <= limit) {
      return parts
    }
    return [
      ...parts.slice(0, limit - 1),
      parts
        .slice(limit - 1)
        .reverse()
        .join(''),
    ]
  }

  const out = []
  let remainder = source

  for (let i = 0; i < limit - 1; i += 1) {
    const index = remainder.lastIndexOf(value)
    if (index < 0) {
      break
    }

    out.push(remainder.slice(index + value.length))
    remainder = remainder.slice(0, index)
  }

  out.push(remainder)
  return out
}

Improve this function

Locutus is a community effort following The McDonald's Theory: we ship first iterations, hoping others will improve them. If you see something that could be better, we'd love your contribution.

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