is-atomic.js

import regexSource from './regex-source.js'

const closingCharacters = {
	'(': `)`,
	'[': `]`,
}

export default function isAtomic(regex) {
	const string = regexSource(regex)

	return /^\w$/.test(string) || enclosedByTopLevelCharacters(string)
}

function enclosedByTopLevelCharacters(string) {
	const openingCharacter = string[0]
	const closingCharacter = closingCharacters[openingCharacter]


	const closedByAppropriateCharacter = closingCharacter !== undefined
		&& string[string.length - 1] === closingCharacter


	if (!closedByAppropriateCharacter) {
		return false
	}

	return !isClosedBeforeEndOfString(string, openingCharacter, closingCharacter)
}


function isClosedBeforeEndOfString(string, openingCharacter, closingCharacter) {
	let depth = 0

	for (let characterIndex = 0; characterIndex < string.length - 1; ++characterIndex) {
		depth = calculateNewDepth(depth, openingCharacter, closingCharacter, string[characterIndex])
		if (depth === 0) {
			return true
		}
	}

	return false
}

function calculateNewDepth(previousDepth, openingCharacter, closingCharacter, character) {
	if (character === openingCharacter) {
		return previousDepth + 1
	} else if (character === closingCharacter) {
		return previousDepth - 1
	} else {
		return previousDepth
	}
}