package cos418_hw1_1

import (
	"bufio"
	"io"
	"strconv"
	"sync"
	"os"
)
// Sum numbers from channel `nums` and output sum to `out`.
// You should only output to `out` once.
// Do NOT modify function signature.
func sumWorker(nums chan int, out chan int) {
	// TODO: implement me
	// HINT: use for loop over `nums`
	sum := 0
    for num := range nums {
        sum += num
    }
    out <- sum
	close(out)
}

// Read integers from the file `fileName` and return sum of all values.
// This function must launch `num` go routines running
// `sumWorker` to find the sum of the values concurrently.
// You should use `checkError` to handle potential errors.
// Do NOT modify function signature.
func sum(num int, fileName string) int {
	// TODO: implement me
	// HINT: use `readInts` and `sumWorkers`
	// HINT: used buffered channels for splitting numbers between workers
	file, err := os.Open(fileName)
    checkError(err)
    defer file.Close()
    numbers, err := readInts(file)
    checkError(err)
	chunk := len(numbers) / num
    input := make(chan int, num)
    output := make(chan int)
	var wg sync.WaitGroup
	for i := 0; i < num; i++ {
        start := i * chunk
        end := (i + 1) * chunk
        if i == num-1 {
            end = len(numbers)
        }

        wg.Add(1)
        go func(start, end int) {
            defer wg.Done()
            partialSum := 0
            for j := start; j < end; j++ {
                partialSum += numbers[j]
            }
            input <- partialSum
        }(start, end)
    }
    go func() {
        wg.Wait()
        close(input)

    }()
	go sumWorker(input, output)
	
	return <-output
}


// Read a list of integers separated by whitespace from `r`.
// Return the integers successfully read with no error, or
// an empty slice of integers and the error that occurred.
// Do NOT modify this function.
func readInts(r io.Reader) ([]int, error) {
	scanner := bufio.NewScanner(r)
	scanner.Split(bufio.ScanWords)
	var elems []int
	for scanner.Scan() {
		val, err := strconv.Atoi(scanner.Text())
		if err != nil {
			return elems, err
		}
		elems = append(elems, val)
	}
	return elems, nil
}