numerica/packages/core/src/exponent.ts

/**
 * Valid values that can be converted to exponential notation.
 */
export type ValidValue = string | number | bigint;

/**
 * Options to use when converting a number to exponential notation.
 */
export interface NumberToExponentialOptions {
	/**
	 * The decimal point character to use. Defaults to ".".
	 */
	decimalPoint?: string;
	/**
	 * The grouping symbol to use. Defaults to ",".
	 */
	groupingSymbol?: string;
	/**
	 * The exponent character to use. Defaults to "e".
	 */
	exponentDelimiter?: string;
}

/**
 * Default decimal point character.
 */
const DEFAULT_DECIMAL_POINT = '.' as const;

/**
 * Default grouping symbol.
 */
const DEFAULT_GROUPING_SYMBOL = ',' as const;

/**
 * Default exponent character.
 */
const DEFAULT_EXPONENT_DELIMITER = 'e' as const;

/**
 * Default positive symbol.
 */
const DEFAULT_POSITIVE_SYMBOL = '+' as const;

/**
 * Default negative symbol.
 */
const DEFAULT_NEGATIVE_SYMBOL = '-' as const;

/**
 * Forces a value to have a decimal point.
 * @param value - The value to force a decimal point on.
 * @param decimalPoint - The decimal point character to use.
 * @returns string The value with a decimal point.
 */
const forceDecimalPoint = (value: string, decimalPoint: string) => (
	value.includes(decimalPoint)
		? value
		: `${value}${decimalPoint}0`
);

/**
 * Error thrown when a value is not in exponential notation.
 */
export class InvalidFormatError extends TypeError {
	constructor(value: string) {
		super(`Value must be in exponential notation. Received: ${value}`);
	}
}

/**
 * Error thrown when a value is not a string, number, or bigint.
 */
export class InvalidValueTypeError extends TypeError {
	constructor(value: unknown) {
		super(`Value must be a string, number, or bigint. Received: ${typeof value}`);
	}
}

/**
 * Forces a number to have a sign.
 * @param value - The value to force a sign on.
 * @returns string The value with a sign.
 */
const forceNumberSign = (value: number | bigint) => {
	const isExponentNegative = value < 0;
	const exponentValueAbs = isExponentNegative ? -value : value;
	const exponentSign = isExponentNegative ? DEFAULT_NEGATIVE_SYMBOL : DEFAULT_POSITIVE_SYMBOL;
	return `${exponentSign}${exponentValueAbs}`;
};

interface ExponentialComponents {
	integer: string;
	fractional: string;
	exponent: string;
}

/**
 * Extracts the integer, fractional, and exponent components of a string in exponential notation.
 * @param value - The string value to extract components from.
 * @param options - Options to use when extracting components.
 * @returns ExponentialComponents The extracted components.
 */
export const extractExponentialComponents = (
	value: string,
	options = {} as NumberToExponentialOptions,
): ExponentialComponents => {
	const {
		decimalPoint = DEFAULT_DECIMAL_POINT,
		groupingSymbol = DEFAULT_GROUPING_SYMBOL,
		exponentDelimiter = DEFAULT_EXPONENT_DELIMITER,
	} = options;
	const valueWithoutGroupingSymbols = value.replace(new RegExp(`${groupingSymbol}`, 'g'), '');
	const exponentDelimiterIndex = valueWithoutGroupingSymbols.indexOf(exponentDelimiter);

	if (exponentDelimiterIndex < 0) {
		// We force the value to have decimal point so that we can extract the integer and fractional
		// components.
		const stringValueWithDecimal = forceDecimalPoint(valueWithoutGroupingSymbols, decimalPoint);
		const [integerRaw, fractionalRaw] = stringValueWithDecimal.split(decimalPoint);
		const integer = integerRaw.replace(/^0+/g, '');
		const fractional = fractionalRaw.replace(/0+$/g, '');
		const exponentValue = BigInt(integer.length - 1);
		return {
			integer,
			exponent: forceNumberSign(exponentValue),
			fractional,
		};
	}

	if (exponentDelimiterIndex !== valueWithoutGroupingSymbols.lastIndexOf(exponentDelimiter)) {
		throw new InvalidFormatError(value);
	}

	const [base, exponentRaw] = valueWithoutGroupingSymbols.split(exponentDelimiter);
	const [integerRaw, fractionalRaw = ''] = base.split(decimalPoint);
	const integerWithoutZeroes = integerRaw.replace(/^0+/g, '');
	const integer = integerWithoutZeroes[0] ?? '0';
	const extraIntegerDigits = integerWithoutZeroes.slice(1);
	const fractional = `${extraIntegerDigits}${fractionalRaw.replace(/0+$/g, '')}`;
	const exponentValue = BigInt(exponentRaw) + BigInt(extraIntegerDigits.length);
	return {
		integer,
		exponent: forceNumberSign(exponentValue),
		fractional,
	};
};

/**
 * Converts a numeric value to a string in exponential notation. Supports numbers of all types.
 * @param value - The value to convert.
 * @param options - Options to use when extracting components.
 * @returns string The value in exponential notation.
 */
export const numberToExponential = (
	value: ValidValue,
	options = {} as NumberToExponentialOptions,
): string => {
	const valueRaw = value as unknown;

	if (typeof valueRaw === 'bigint' || typeof valueRaw === 'number') {
		return numberToExponential(valueRaw.toString(), options);
	}

	if (typeof valueRaw !== 'string') {
		throw new InvalidValueTypeError(valueRaw);
	}

	if (valueRaw.startsWith(DEFAULT_NEGATIVE_SYMBOL)) {
		return `${DEFAULT_NEGATIVE_SYMBOL}${numberToExponential(valueRaw.slice(DEFAULT_NEGATIVE_SYMBOL.length), options)}}`;
	}

	const {
		decimalPoint = DEFAULT_DECIMAL_POINT,
		groupingSymbol	= DEFAULT_GROUPING_SYMBOL,
		exponentDelimiter = DEFAULT_EXPONENT_DELIMITER,
	} = options;

	// Remove invalid characters.
	// We also remove the grouping symbols in case they come from human input.
	const stringValue = valueRaw
		.replace(new RegExp(`${groupingSymbol}`, 'g'), '')
		.toLowerCase()
		.replace(/\s/g, '');

	const {
		integer,
		fractional,
		exponent,
	} = extractExponentialComponents(stringValue, options);

	const significantDigits = `${integer}${fractional}`;
	if (significantDigits.length === 0) {
		// We copy the behavior from `Number.prototype.toExponential` here.
		return `0${exponentDelimiter}${DEFAULT_POSITIVE_SYMBOL}0`;
	}

	const significandInteger = significantDigits[0];
	const significandFractional = significantDigits.slice(1).replace(/0+$/g, '');

	if (significandFractional.length === 0) {
		return `${significandInteger}${exponentDelimiter}${exponent}`;
	}

	return `${significandInteger}${decimalPoint}${significandFractional}${exponentDelimiter}${exponent}`;
};