const WORD_FONT_SIZE_PT = 14;
const WORD_LINE_SPACING = 1.5;

const A4_WIDTH_MM = 210;
const A4_HEIGHT_MM = 297;

const MARGIN_TOP_MM = 20;
const MARGIN_BOTTOM_MM = 20;
const MARGIN_LEFT_MM = 30;
const MARGIN_RIGHT_MM = 10;

const FIRST_LINE_INDENT_MM = 12.5;

const MM_PER_POINT = 0.3527777778;

const TEXT_WIDTH_MM = A4_WIDTH_MM - MARGIN_LEFT_MM - MARGIN_RIGHT_MM;
const TEXT_HEIGHT_MM = A4_HEIGHT_MM - MARGIN_TOP_MM - MARGIN_BOTTOM_MM;

const LINE_HEIGHT_MM = WORD_FONT_SIZE_PT * WORD_LINE_SPACING * MM_PER_POINT;
const LINES_PER_PAGE = Math.floor(TEXT_HEIGHT_MM / LINE_HEIGHT_MM);

const AVG_CHAR_WIDTH_EM = 0.58;
const AVG_CHAR_WIDTH_MM = WORD_FONT_SIZE_PT * AVG_CHAR_WIDTH_EM * MM_PER_POINT;

const CHARS_PER_LINE = Math.floor(TEXT_WIDTH_MM / AVG_CHAR_WIDTH_MM);
const AVG_PARAGRAPHS_PER_PAGE = 4; // average paragraph count across typical student papers
const INDENT_CHARS_LOSS = Math.round(
    (FIRST_LINE_INDENT_MM / AVG_CHAR_WIDTH_MM) * AVG_PARAGRAPHS_PER_PAGE,
);

const LAYOUT_FUDGE_FACTOR = 0.96;

export const CHARACTERS_PER_PAGE = Math.round(
    (LINES_PER_PAGE * CHARS_PER_LINE - INDENT_CHARS_LOSS) * LAYOUT_FUDGE_FACTOR,
);

export function pagesToCharacters(pages: number): number {
    if (!Number.isFinite(pages) || pages <= 0) return 0;
    return Math.max(0, Math.round(pages * CHARACTERS_PER_PAGE));
}

export function charactersToPages(characters: number): number {
    if (!Number.isFinite(characters) || characters <= 0) return 0;
    const raw = characters / CHARACTERS_PER_PAGE;
    return Math.max(0, Math.ceil(raw));
}