"""Column coordinate conversion utilities."""


def col_to_letter(col: int) -> str:
    """Convert 0-based column index to Excel letter.

    0 → A, 1 → B, ..., 25 → Z, 26 → AA, 27 → AB, ...
    """
    result = ''
    col += 1
    while col > 0:
        col -= 1
        result = chr(col % 26 + ord('A')) + result
        col //= 26
    return result


def letter_to_col(letter: str) -> int:
    """Convert Excel column letter to 0-based index.

    A → 0, B → 1, ..., Z → 25, AA → 26, ...
    """
    result = 0
    for ch in letter.upper():
        result = result * 26 + (ord(ch) - ord('A') + 1)
    return result - 1


def cell_ref_to_rc(ref: str) -> tuple[int, int]:
    """Convert Excel cell reference (e.g. 'A1', 'BC42') to (row, col).

    Returns 0-based (row, col).
    """
    col_letters = []
    row_digits = []
    for ch in ref:
        if ch.isalpha():
            col_letters.append(ch)
        else:
            row_digits.append(ch)
    col = letter_to_col(''.join(col_letters))
    row = int(''.join(row_digits)) - 1  # 1-based → 0-based
    return row, col


def rc_to_cell_ref(row: int, col: int) -> str:
    """Convert 0-based (row, col) to Excel cell reference.

    (0, 0) → 'A1', (1, 2) → 'C2', ...
    """
    return col_to_letter(col) + str(row + 1)