package com.example.export;

import java.nio.charset.StandardCharsets;
import java.text.Normalizer;
import java.text.NumberFormat;
import java.util.Arrays;
import java.util.List;
import java.util.Locale;

/**
 * One widget's data as a downloadable CSV table: the labels a widget is
 * currently showing plus the numbers behind them, already resolved against the
 * bundle — this class never sees a translation key, only display text.
 * <p>
 * The dialect is the one German spreadsheet software expects: {@code ;} as the
 * separator (a decimal comma makes {@code ,} unusable), CRLF line breaks, and a
 * UTF-8 BOM in {@link #toBytes(Locale)} so Excel reads the umlauts. Numbers are
 * formatted for the same {@link Locale} the widget renders in, so the file
 * carries the values the user sees, not their machine representation.
 */
public record CsvExport(String categoryHeader, String valueHeader,
                        List<String> categories, List<Double> values) {

    private static final String SEPARATOR = ";";
    private static final String LINE_BREAK = "\r\n";

    /** UTF-8 byte order mark — without it Excel reads the file as ANSI. */
    private static final String BOM = "";

    public CsvExport {
        categories = List.copyOf(categories);
        values = List.copyOf(values);
        if (categories.size() != values.size()) {
            throw new IllegalArgumentException(
                    "each value needs a category: %d values, %d categories"
                            .formatted(values.size(), categories.size()));
        }
    }

    /** A header row plus one row per data point, in the widget's own order. */
    public String toCsv(Locale locale) {
        NumberFormat numbers = NumberFormat.getNumberInstance(locale);
        numbers.setGroupingUsed(false);
        numbers.setMaximumFractionDigits(2);

        StringBuilder csv = new StringBuilder();
        row(csv, quote(categoryHeader), quote(valueHeader));
        for (int i = 0; i < categories.size(); i++) {
            row(csv, quote(categories.get(i)), numbers.format(values.get(i)));
        }
        return csv.toString();
    }

    public byte[] toBytes(Locale locale) {
        return (BOM + toCsv(locale)).getBytes(StandardCharsets.UTF_8);
    }

    private static void row(StringBuilder csv, String... fields) {
        csv.append(String.join(SEPARATOR, fields)).append(LINE_BREAK);
    }

    /** Quotes a field the way RFC 4180 does, doubling embedded quotes. Only
     *  where it is needed, so a plain label stays readable in a text editor. */
    private static String quote(String field) {
        if (field.contains(SEPARATOR) || field.contains("\"")
                || field.contains("\n") || field.contains("\r")) {
            return '"' + field.replace("\"", "\"\"") + '"';
        }
        return field;
    }

    /**
     * A file name built from what the user picked the export from — widget
     * title and reporting period, say — so a downloads folder full of exports
     * is still readable. Parts are slugified and joined with {@code -};
     * {@code "Umsatz-Entwicklung"} and {@code "Halbjahr"} become
     * {@code umsatz-entwicklung-halbjahr.csv}.
     */
    public static String fileName(String... parts) {
        String name = Arrays.stream(parts).map(CsvExport::slug)
                .filter(part -> !part.isEmpty())
                .reduce((a, b) -> a + "-" + b)
                .orElse("export");
        return name + ".csv";
    }

    /** Lowercase ASCII, dashes for everything else: umlauts are decomposed and
     *  their accents dropped, {@code ß} spelled out — {@link Normalizer} has no
     *  decomposition for it. */
    private static String slug(String text) {
        String ascii = Normalizer.normalize(text.toLowerCase(Locale.ROOT).replace("ß", "ss"),
                        Normalizer.Form.NFD)
                .replaceAll("\\p{M}", "");
        return ascii.replaceAll("[^a-z0-9]+", "-").replaceAll("^-|-$", "");
    }
}