Justifies text to a certain width (usually for console text or source code text)

justify.js

// Get width
const width = parseInt(process.argv[2] || '80');

// Read STDIN
let buffer = Buffer.alloc(0);

process.stdin.resume();
process.stdin.on('data', function(data) {
	buffer = Buffer.concat([buffer, data]);
});

const resultLines = [];

process.stdin.on('end', function() {
	process.stdin.pause();

	const str = buffer.toString();
	const paragraphs = str.split(/\n\n*/g); // Don't know why (\r?\n)+ doesn't work.
	const title = paragraphs.shift();
	center(title);
	paragraphs.forEach(p => split(p));

	console.log(resultLines.map(l => l.replace(/\s*$/, '')).join('\n').replace(/[\s\r\n]*$/, ''));
});

function center(str) {
	const padding = Math.floor((width - str.length) / 2);
	resultLines.push(' '.repeat(padding) + str, '');
}

function split(str) {
	const [_, initialIndent, rawLine] = str.match(/^(\s*(?:[a-z]+\.\s+)?)(.*)$/);
	const words = rawLine.split(/\s+/g);

	const indent = initialIndent.replace('\t', '    ').length - 1;

	const lines = [];

	const whiteIndent = initialIndent.replace(/./g, ' ');

	let lineLength = indent;
	let lastLine = [];

	words.forEach(word => {
		if (lineLength + word.length + 1 > width) {
			lines.push([lineLength, lastLine]);
			lineLength = indent;
			lastLine = [];
		}

		lastLine.push(word);
		lineLength += word.length + 1;
	});

	let indentString = initialIndent;

	for (const [length, line] of lines) {
		const leftover = width - length;
		let content;
		let curLength = length;
		let cur = 0;
		const hit = new Set();
		let count = 0;
		while (curLength < width) {
			do {
				cur += Math.floor((Math.random() * 2) + (line.length / leftover));
				cur = cur % line.length;
			} while (cur === 0
			         || (hit.has(cur)
			         && ++count < 100));

			hit.add(cur);
			line[cur] = ' ' + line[cur];
			++curLength;
		}

		content = line.join(' ');

		resultLines.push(indentString + content);

		indentString = whiteIndent;
	}

	if (lastLine.length > 0) {
		resultLines.push(indentString + lastLine.join(' '));
	}

	resultLines.push('');
}