// @flow
/**
 * A collection of string matching algorithms built with React Select in mind.
 */

// Option type from React Select and similar libraries.
export type Option = {
    label?: string,
    value?: any,
};

type MapOfStrings = {[key: string]: string};


/**
 * Filters React Select options and sorts by similarity to a search filter.
 * Handles partial matches, eg. searching for "Waberg High" will find "Raoul
 * Wallenberg Traditional High School". Case insensitive. Ignores
 * non-alphanumeric characters.
 *
 * @param  options  An unfiltered list of Options.
 * @param? filter  A string to compare against Option labels.
 * @param? substitutions  Strings with multiple spellings or variations that we
 *           expect to match, eg. accented characters or abbreviated words.
 *
 * @return A filtered and sorted array of Options.
 */
export function filterOptions(
    options: Array<Option>,
    filter?: string,
    substitutions?: MapOfStrings,
): Array<Option> {
    // If the filter is blank, return the full list of Options.
    if (!filter) {
        return options;
    }

    const cleanFilter = cleanUpText(filter, substitutions);
    return options
        // Filter out undefined or null Options.
        .filter(({label, value}) => label != null && value != null)
        // Create a {score, Option} pair for each Option based on its label's
        // similarity to the filter text.
        .map(option => ({
            option: option,
            score: typeaheadSimilarity(
                cleanUpText(option.label, substitutions), cleanFilter),
        }))
        // Only include matches of the entire substring, with a slight
        // affordance for transposition or extra characters.
        .filter(pair => pair.score >= cleanFilter.length - 2)
        // Sort 'em by order of their score.
        .sort((a, b) => b.score - a.score)
        // …and grab the original Options back from their pairs.
        .map(pair => pair.option);
}


/**
 * Scores the similarity between two strings by returning the length of the
 * longest common subsequence. Intended for comparing strings of different
 * lengths; eg. when matching a typeahead search input with a school name.

 * Meant for use in an instant search box where results are being fetched
 * as a user is typing.
 *
 * @param  a  The longer string (though, we flip them if it's shorter).
 * @param  b  The shorter string, eg. a typeahead search input.
 *
 * @return The length of the longest common subsequence. Higher scores indicate
 *           closer matches.
 */
export function typeaheadSimilarity(a: string, b: string): number {
    const aLength = a.length;
    const bLength = b.length;
    const table = [];

    if (!aLength || !bLength) {
        return 0;
    }

    // Ensure `a` isn't shorter than `b`.
    if (aLength < bLength) {
        [a, b] = [b, a];
    }

    // Early exit if `a` includes `b`; these will be scored higher than any
    // other options with the same `b` (filter string), with a preference for
    // shorter `a` strings (option labels).
    if (a.indexOf(b) !== -1) {
        return bLength + 1 / aLength;
    }

    // Initialize the table axes:
    //
    //    0 0 0 0 ... bLength
    //    0
    //    0
    //
    //   ...
    //
    // aLength
    //
    for (let x = 0; x <= aLength; ++x) {
        table[x] = [0];
    }
    for (let y = 0; y <= bLength; ++y) {
        table[0][y] = 0;
    }

    // Populate the rest of the table with a dynamic programming algorithm.
    for (let x = 1; x <= aLength; ++x) {
        for (let y = 1; y <= bLength; ++y) {
            table[x][y] = a[x - 1] === b[y - 1] ?
                1 + table[x - 1][y - 1] :
                Math.max(table[x][y - 1], table[x - 1][y]);
        }
    }

    return table[aLength][bLength];
}


/**
 * Returns the Levenshtein distance between two strings.
 *
 * NOTE: The Jaro-Winkler distance also worked well and is slightly more
 * performant. Levenshtein seems to match more reliably, which is more
 * important here.
 *
 * @param  a  The first string for comparison.
 * @param  b  The second string for comparison.
 *
 * @return The Levenshtein distance, where lower distance indicates higher
 *           similarity.
 */
export function fullStringDistance(a: string, b: string): number {
    const aLength = a.length;
    const bLength = b.length;
    const table = [];

    if (!aLength) {
        return bLength;
    }
    if (!bLength) {
        return aLength;
    }

    // Initialize the table axes:
    //
    //    0 1 2 3 4 ... bLength
    //    1
    //    2
    //
    //   ...
    //
    // aLength
    //
    for (let x = 0; x <= aLength; ++x) {
        table[x] = [x];
    }
    for (let y = 0; y <= bLength; ++y) {
        table[0][y] = y;
    }

    // Populate the rest of the table with a dynamic programming algorithm.
    for (let x = 1; x <= aLength; ++x) {
        for (let y = 1; y <= bLength; ++y) {
            table[x][y] = a[x - 1] === b[y - 1] ?
                table[x - 1][y - 1] :
                1 + Math.min(
                    table[x - 1][y],       // Substitution,
                    table[x][y - 1],       // insertion,
                    table[x - 1][y - 1]);  // and deletion.
        }
    }

    return table[aLength][bLength];
}


/**
 * Apply string substitutions, remove non-alphanumeric characters, and convert
 * all letters to uppercase.
 *
 * eg. 'Scoil Bhríde Primary School' may become 'SCOILBHRIDEPRIMARYSCHOOL'.
 *
 * @param  input  An unsanitized input string.
 * @param  substitutions  Strings with multiple spellings or variations that we
 *          expect to match, for example accented characters or abbreviated
 *          words.
 *
 * @return The sanitized text.
 */
export function cleanUpText(
    input?: string,
    substitutions?: MapOfStrings,
): string {
    if (!input) {
        return '';
    }

    // Uppercase and remove all non-alphanumeric, non-accented characters.
    // Also remove underscores.
    input = input.toUpperCase().replace(/((?=[^\u00E0-\u00FC])\W)|_/g, '');

    if (!substitutions) {
        return input;
    }
    const safeSubstitutions: MapOfStrings = substitutions;  // For Flow.

    // Replace all strings in `safeSubstitutions` with their standardized
    // counterparts.
    return Object.keys(safeSubstitutions)
        .reduce((output, substitution) => {
            const unsubbed = new RegExp(substitution, 'g');
            return output.replace(unsubbed, safeSubstitutions[substitution]);
        }, input);
}