"use strict";

const AWS = require("aws-sdk")
const Uuid = require("uuid/v4");

const MAX_BATCH_COUNT = 5;
const MAX_BATCH_SIZE = 25;

// Stream our S3 object (in this case, a CSV file) and write it to DynamoDB
((region, bucket, key, tableName) => {

  AWS.config.setPromisesDependency(require("bluebird"));
  AWS.config.update({ region });

  const s3 = new AWS.S3();
  const ddb = new AWS.DynamoDB();

  let count = 0;
  let batches = [];

  const s3Params = {
    Bucket: bucket,
    Key: key
  }

  const csvStream = require("fast-csv").fromStream(
    s3.getObject(s3Params).createReadStream(),
    { headers: true }
  );

  csvStream.on("data", async (row) => {
    // If we have no batches or our current batch is full, add one
    if (shouldWrite(batches, MAX_BATCH_COUNT, MAX_BATCH_SIZE)) {
      csvStream.pause();
      await writeToDynamo(ddb);
      csvStream.resume();
      batches = [];
    }

    addItem(row, tableName);
    count++;
  }).on("end", _ => {
    console.log(`Done! Processed ${count} records`);
  });

  /**
   * Writes our batches to DynamoDB
   */
  const writeToDynamo = async (dynamo) =>
    Promise.all(batches.map(batch => {
      return dynamo.batchWriteItem(batch).promise();
    })).then(_ => {
      // Drain our set of batches
      batches = [];
      console.log("Total records uploaded so far: " + count);
    }).catch(err => {
      console.error(err);
    });

  /**
    * Determines if it"s time to write to DynamoDB
    *
    * @param {*} batches Our set of batches
    * @param {*} MAX_BATCH_COUNT The max number of batches in a set
    * @param {*} MAX_BATCH_SIZE The capacity of a batch
    */
  const shouldWrite = (batches, MAX_BATCH_COUNT, MAX_BATCH_SIZE) =>
    batches.length == MAX_BATCH_COUNT && batchFull(batches[batches.length - 1], MAX_BATCH_SIZE);

  /**
    * Determines if our batch is full
    * @param {*} batch Given batch
    * @param {*} MAX_BATCH_SIZE The capacity of a batch
    */
  const batchFull = (batch, MAX_BATCH_SIZE) => batch.RequestItems[tableName].length == MAX_BATCH_SIZE;

  /**
    * Adds an item to our tail batch
    * @param {*} data The object to add to our batch
    */
  const addItem = (csvRowData, tableName) => {
    if (batches.length == 0 || (batchFull(batches[batches.length - 1], MAX_BATCH_SIZE) && batches.length < MAX_BATCH_COUNT)) {
      batches.push(
        {
          RequestItems: {
            [tableName]: []
          }
        }
      );
    }

    /** Modify to fit your CSV/Dynamo schema accordingly */
    let item = {
      PutRequest: {
        Item: {
          "id": {
            "S": Uuid()
          },
          "foo": {
            "S": "foo",
          },
          "bar": {
            "S": "bar",
          },
        }
      }
    }

    batches[batches.length - 1].RequestItems[tableName].push(item);
  }
})(
  "<region>",
  "<bucket>",
  "<csv_s3_path>",
  "<table_name>"
);