/*
README

SatoshiDice has a new address bc1quykuahxrjx6d3h6ga4rkyg0hl5e59tcthqyhw6.
Send up to .2 BTC to this address and have a 50% chance to 1.98x your money.
If you win, the money will be sent to the change address and if there is no change address, the first input address.
Your winnings will be 1.98x your original bet minus 0.00001 btc for fees.

The provably fair number is generated from blockhash of the tx + next blockhash + next blockhash + next blockhash + txid.
The following script will generate the a number between 0 and 999999. 500000-999999 means you WIN!

Details:
To generate the provably fair number, get the blockhash of the block that includes your transaction, concatenate with the next 3 block's blockhash concatenate with your txid
Take the string generated and hash it with SHA-256.
Take the first 5 digits of the SHA-256 and convert to decimal from base 16 Hexidecimal. If this number is less than 1,000,000 then this is the generated number. If not, take the next 5 and so on until the generated number under 1 million.

Provably Fair:
Because no one can predict the next blockhash without 50% of the mining power, it is safe that its generated randomly
In order to cheat, one must mine the block that includes the gambling transaction and the next 3 block before broadcasting the gambling transaction. This is impossible to do.
*/
//Copy paste the commented line below into the console with your transaction id that you want to roll
//getBlockInfo('5bc79366fc8e21105ad022c09e9a79834172eea3bf75d4a63750bc43d43faad0');
let main = 'https://api.blockcypher.com/v1/btc/main';
function getBlockInfo(txid){
  fetch(main + '/txs/' + txid).then((txdata) => txdata.json()).then(function(data){
    fetch(main + '/blocks/' + (parseInt(data.block_height) + 1) + '?txstart=1&limit=1').then((txdata2) => txdata2.json()).then(function(data2){
      fetch(main + '/blocks/' + (parseInt(data.block_height) + 2) + '?txstart=1&limit=1').then((txdata3) => txdata3.json()).then(function(data3){
        setTimeout(function (){ //To stop API limits
          fetch(main + '/blocks/' + (parseInt(data.block_height) + 3) + '?txstart=1&limit=1').then((txdata4) => txdata4.json()).then(function(data4){
            provableRandomNum(data.block_hash + data2.hash + data3.hash + data4.hash, txid);
          });
        }, 1000);
      });
    });
  });
}

async function sha256(message) {
  const msgBuffer = new TextEncoder('utf-8').encode(message);
  const hashBuffer = await crypto.subtle.digest('SHA-256', msgBuffer);
  const hashArray = Array.from(new Uint8Array(hashBuffer));
  const hashHex = hashArray.map(b => ('00' + b.toString(16)).slice(-2)).join('');
  return hashHex;
}

async function provableRandomNum(blockhash, txid) {
  sha256(blockhash + txid).then(function(res) {
    for (let i = 0; i < 64 / 5; i++) {
      //transform hash into blocks of 5 and convert it to int from hex
      let result = parseInt(res.substring(i * 5, 5), 16);
      if (result < 1000000){
          console.log(result);
          return;
      }
    }
  });
}