mdiff.js

/**
 * mdiff.js
 *
 * Used to produce colorful and informative diffs for comparison of generated
 * Markdown.  Unlike the built-in diffs used in python or node.js assert libraries,
 * is actually designed to be effective for long, single-line comparisons.
 *
 * Based on diffing library difflib, a js port of the python library.
 *
 * The sole exported function diff_strings(string_0, string_1) returns a pretty-printed
 * unicode string containing their diff.
 */

const _ = require('underscore');
const difflib = require('difflib');

function apply_color(input_string, changes) {
    let previous_index = 0;
    let processed_string = input_string.slice(0, 2);
    input_string = input_string.slice(2);

    const formatter = {
        delete: (string) => { return "\u001b[31m" + string + "\u001b[0m"; },
        insert: (string) => { return "\u001b[32m" + string + "\u001b[0m"; },
        replace: (string) => { return "\u001b[33m" + string + "\u001b[0m"; },
    };
    changes.forEach((change) => {
        if (formatter.hasOwnProperty(change.tag)) {
            processed_string += input_string.slice(previous_index, change.beginning_index);
            processed_string += formatter[change.tag](
                input_string.slice(change.beginning_index, change.ending_index)
            );
            previous_index = change.ending_index;
        }
    });

    processed_string += input_string.slice(previous_index);
    return processed_string;
}

/**
 * The library difflib produces diffs that look as follows:
 *
 * - <p>upgrade! yes</p>
 * ?    ^^     -
 * + <p>downgrade yes.</p>
 * ?    ^^^^         +
 *
 * The purpose of this function is to facilitate converting these diffs into
 * colored versions, where the question-mark lines are removed, replaced with
 * directions to add appropriate color to the lines that they annotate.
 */
function parse_questionmark_line(questionmark_line) {
    let current_sequence = "";  // Either "^", "-", "+", or ""
    let beginning_index = 0;
    let index = 0;

    const changes_list = [];
    const aliases = {
        "^": "replace",
        "+": "insert",
        "-": "delete",
    };
    const add_change = () => {
        if (current_sequence) {
            changes_list.push({
                tag: aliases[current_sequence],
                beginning_index,
                ending_index: index,
            });
            current_sequence = "";
        }
    };

    questionmark_line = questionmark_line.slice(2).trimRight("\n");

    for (const character of questionmark_line) {
        if (aliases.hasOwnProperty(character)) {
            if (current_sequence !== character) {
                add_change();
                current_sequence = character;
                beginning_index = index;
            }
        } else {
            add_change();
        }
        index += 1;
    }

    // In case we have a "change" involving the last character on a line
    // e.g. a string such as "? ^^  -- ++++"
    add_change();

    return changes_list;
}

function diff_strings(string_0, string_1) {
    let output_lines = [];
    let ndiff_output = "";
    let changes_list = [];

    ndiff_output = difflib.ndiff(string_0.split("\n"), string_1.split("\n"));

    ndiff_output.forEach((line) => {
        if (line.startsWith("+")) {
            output_lines.push(line);
        } else if (line.startsWith("-")) {
            output_lines.push(line);
        } else if (line.startsWith("?")) {
            changes_list = parse_questionmark_line(line);
            output_lines[output_lines.length - 1] = apply_color(
                output_lines[output_lines.length - 1], changes_list);
        } else {
            output_lines.push(line);
        }
    });

    const emphasize_codes = (string) => {
        return "\u001b[34m" + string.slice(0, 1) + "\u001b[0m" + string.slice(1);
    };
    output_lines = _.map(output_lines, emphasize_codes);

    return output_lines.join("\n");
}

module.exports = { diff_strings };

// Simple CLI for this module
// Only run this code if called as a command-line utility
if (require.main === module) {
    // First two args are just "node" and "mdiff.js"
    const argv = require('minimist')(process.argv.slice(2));

    if (_.has(argv, "help")) {
        console.log(process.argv[0] + " " + process.argv[1] +
            " [ --help ]" +
            " string_0" +
            " string_1" +
            "\n" +
            "Where string_0 and string_1 are the strings to be diffed"
        );
    }

    const output = diff_strings(argv._[0], argv._[1]);
    console.log(output);
}