// The 4-decimal amount rule from README.md's Display Consistency section, and
// the one exception to it, in one place. Three call sites had grown their own
// copy of the truncation — the history and balance lists
// (`src/shared/transactions.js`), the approval screen's ERC-20 amount line
// (`src/popup/views/approval.js`) and its Uniswap swap detail lines
// (`src/shared/uniswap.js`) — and a fix applied to one of them left the other
// two showing a different number for the same value.
//
// The two functions below are the two policies, not two implementations of
// one: summary lists truncate, and the screens that state what is being
// authorized truncate with a floor. Keeping them adjacent is the point, so a
// change to the rule cannot reach one screen and miss another.

// Truncate to exactly four decimal places. Truncation, never rounding: an
// amount must never be displayed as larger than it is, so 0.99999 stays
// 0.9999.
function truncateAmount(val) {
    const parts = val.split(".");
    if (parts.length === 1) return val + ".0000";
    return parts[0] + "." + (parts[1] + "0000").slice(0, 4);
}

// The same rule, plus the invariant the approval and confirmation screens
// hold: a nonzero amount never renders as zero. Truncating to four decimals
// does exactly that to an amount below 0.0001 — one base unit of an 18-decimal
// token, 500 of an 8-decimal one — and a real transfer or allowance then reads
// as "nothing is being moved" on the screen whose whole job is to say what is
// being authorized.
//
// When the truncated string carries no significant digit and the value does,
// the amount is extended to its first significant digit instead. It stays in
// token units, the same unit as the symbol printed beside it. A genuine zero
// still renders 0.0000, and anything at or above the floor is untouched.
function truncateAmountNeverZero(val) {
    const truncated = truncateAmount(val);
    // Tests the whole truncated string, integer part included: 1.00005 has a
    // significant digit already and stays 1.0000.
    if (/[1-9]/.test(truncated)) return truncated;
    const parts = val.split(".");
    if (parts.length === 1) return truncated;
    const sig = parts[1].search(/[1-9]/);
    if (sig === -1) return truncated;
    return parts[0] + "." + parts[1].slice(0, sig + 1);
}

module.exports = { truncateAmount, truncateAmountNeverZero };