import { hrpToPico } from "./hrp-pico";

/**
 * Encodes pico btc to human readable part amount. The hrp amount is defined
 * as the amount of bitcoin as a positive decimal integer with no leading 0s.
 * It is an optional number followed by an optional multiplier letter.
 *
 * The multipliers are:
 * m (milli)   0.001 bitcoin
 * u (micro)   0.000001 bitcoin
 * n (nano)    0.000000001 bitcoin
 * p (pico)    0.000000000001 bitcoin
 *
 * This means that 1msat is 10p and 1sat is 10n.
 */
export function encodePico(pico: string): string {
    if (!pico) return;
    const lsdIndex = _lsdIndex(pico);
    const hrpMultiplier = _minimumHrpMultiplier(lsdIndex);
    return (BigInt(pico) / hrpToPico(hrpMultiplier)).toString() + hrpMultiplier;
}

///////////////////////////////////////////////////////////////////////////////

/**
 * Finds the index of the least significant digit
 *
 * For example:
 *   if the value is 1002 pico, the first digit with a value is 0
 *   if the value is 1040 pico, the first digit with a value is 1
 *   if the value is 1000 pico, the first digit with a value is 3
 *
 * @return the index the least significant digit
 */
function _lsdIndex(msatStr: string): number {
    for (let i = msatStr.length - 1; i >= 0; i--) {
        if (Number(msatStr[i]) !== 0) return msatStr.length - i - 1;
    }
    throw new Error("Cannot encode 0");
}

/**
 * With the least significant digit we can determine the multiplier
 * that is required to miniimze the HRP.
 *
 * For instance
 *   if the value is 1002, we have a value of p
 *   if the value is 1000, we have a value of n
 *
 * @param lsdIndex
 * @return returns the string multiplier
 */
function _minimumHrpMultiplier(lsdIndex: number): string {
    if (lsdIndex < 3) return "p";
    else if (lsdIndex < 6) return "n";
    else if (lsdIndex < 9) return "u";
    else if (lsdIndex < 12) return "m";
    else return "";
}