# src/loader.js

    // @flow

    import * as BufferLayout from 'buffer-layout';

    import {Account} from './account';
    import {PublicKey} from './publickey';
    import {NUM_TICKS_PER_SECOND} from './timing';
    import {Transaction, PACKET_DATA_SIZE} from './transaction';
    import {sendAndConfirmTransaction} from './util/send-and-confirm-transaction';
    import {sleep} from './util/sleep';
    import type {Connection} from './connection';
    import {SystemProgram} from './system-program';

    /**
     * Program loader interface
     */
    export class Loader {
      /**
       * Amount of program data placed in each load Transaction
       */
      static get chunkSize(): number {
        // Keep program chunks under PACKET_DATA_SIZE, leaving enough room for the
        // rest of the Transaction fields
        //
        // TODO: replace 300 with a proper constant for the size of the other
        // Transaction fields
        return PACKET_DATA_SIZE - 300;
      }

      /**
       * Loads a generic program
       *
       * @param connection The connection to use
       * @param payer System account that pays to load the program
       * @param program Account to load the program into
       * @param programId Public key that identifies the loader
       * @param data Program octets
       */
      static async load(
        connection: Connection,
        payer: Account,
        program: Account,
        programId: PublicKey,
        data: Array<number>,
      ): Promise<PublicKey> {
        {
          const transaction = SystemProgram.createAccount(
            payer.publicKey,
            program.publicKey,
            1,
            data.length,
            programId,
          );
          await sendAndConfirmTransaction(connection, transaction, payer);
        }

        const dataLayout = BufferLayout.struct([
          BufferLayout.u32('instruction'),
          BufferLayout.u32('offset'),
          BufferLayout.u32('bytesLength'),
          BufferLayout.u32('bytesLengthPadding'),
          BufferLayout.seq(
            BufferLayout.u8('byte'),
            BufferLayout.offset(BufferLayout.u32(), -8),
            'bytes',
          ),
        ]);

        const chunkSize = Loader.chunkSize;
        let offset = 0;
        let array = data;
        let transactions = [];
        while (array.length > 0) {
          const bytes = array.slice(0, chunkSize);
          const data = Buffer.alloc(chunkSize + 16);
          dataLayout.encode(
            {
              instruction: 0, // Load instruction
              offset,
              bytes,
            },
            data,
          );

          const transaction = new Transaction().add({
            keys: [{pubkey: program.publicKey, isSigner: true, isDebitable: true}],
            programId,
            data,
          });
          transactions.push(
            sendAndConfirmTransaction(connection, transaction, payer, program),
          );

          // Delay ~1 tick between write transactions in an attempt to reduce AccountInUse errors
          // since all the write transactions modify the same program account
          await sleep(1000 / NUM_TICKS_PER_SECOND);

          // Run up to 8 Loads in parallel to prevent too many parallel transactions from
          // getting rejected with AccountInUse.
          //
          // TODO: 8 was selected empirically and should probably be revisited
          if (transactions.length === 8) {
            await Promise.all(transactions);
            transactions = [];
          }

          offset += chunkSize;
          array = array.slice(chunkSize);
        }
        await Promise.all(transactions);

        // Finalize the account loaded with program data for execution
        {
          const dataLayout = BufferLayout.struct([BufferLayout.u32('instruction')]);

          const data = Buffer.alloc(dataLayout.span);
          dataLayout.encode(
            {
              instruction: 1, // Finalize instruction
            },
            data,
          );

          const transaction = new Transaction().add({
            keys: [{pubkey: program.publicKey, isSigner: true, isDebitable: true}],
            programId,
            data,
          });
          await sendAndConfirmTransaction(connection, transaction, payer, program);
        }
        return program.publicKey;
      }
    }