package main

import (
	"fmt"
	"math/rand"
	"sort"
	"sync"
	"time"
)

func mergeSortChannelsInternal(data []float64, sorted chan<- float64) {
	defer close(sorted)

	if len(data) <= 10000 {
		sortedData := make([]float64, len(data))
		copy(sortedData, data)
		mergeSortSequential(sortedData)
		for _, x := range sortedData {
			sorted <- x
		}
	} else {
		mid := len(data) / 2
		left := data[:mid]
		right := data[mid:]

		sortedLeft := make(chan float64, 1000)
		sortedRight := make(chan float64, 1000)

		go mergeSortChannelsInternal(left, sortedLeft)
		go mergeSortChannelsInternal(right, sortedRight)
		mergeChannels(sortedLeft, sortedRight, sorted)
	}
}

func mergeChannels(a, b <-chan float64, out chan<- float64) {
	var okA, okB bool
	var valA, valB float64

	for {
		if !okA {
			valA, okA = <-a
		}

		if !okB {
			valB, okB = <-b
		}

		if !okA && !okB {
			break
		}

		if !okA {
			out <- valB
			okB = false
			continue
		}

		if !okB {
			out <- valA
			okA = false
			continue
		}

		if valA < valB {
			out <- valA
			okA = false
		} else {
			out <- valB
			okB = false
		}
	}
}

func mergeSortChannels(data []float64) {
	sorted := make(chan float64, 1000)
	go mergeSortChannelsInternal(data, sorted)

	sortedData := make([]float64, len(data))
	i := 0
	for x := range sorted {
		sortedData[i] = x
		i++
	}

	copy(data, sortedData)
}

func mergeSortParallel(data []float64) {
	if len(data) <= 10000 {
		mergeSortSequential(data)
		return
	}

	mid := len(data) / 2
	left := data[:mid]
	right := data[mid:]

	var wg sync.WaitGroup
	wg.Add(2)

	go func() { mergeSortParallel(left); wg.Done() }()
	go func() { mergeSortParallel(right); wg.Done() }()

	wg.Wait()

	copy(data, merge(left, right))
}

func mergeSortSequential(data []float64) {
	if len(data) <= 1 {
		return
	}

	mid := len(data) / 2
	left := data[:mid]
	right := data[mid:]

	mergeSortSequential(left)
	mergeSortSequential(right)

	copy(data, merge(left, right))
}

func merge(left, right []float64) []float64 {
	result := make([]float64, 0, len(left)+len(right))
	i, j := 0, 0

	for i < len(left) && j < len(right) {
		if left[i] < right[j] {
			result = append(result, left[i])
			i++
		} else {
			result = append(result, right[j])
			j++
		}
	}

	result = append(result, left[i:]...)
	result = append(result, right[j:]...)

	return result
}

func main() {

	//1. DATA CREATION
	data := make([]float64, 1000*1000*20) //*1000*20)

	for i := range data {
		data[i] = rand.Float64() * 100 // Random floats between 0 and 100
	}

	expected := make([]float64, len(data))
	copy(expected, data)
	sort.Float64s(expected)

	//2. SORTING
	start := time.Now()

	//mergeSortSequential(data)
	//mergeSortParallel(data)
	mergeSortChannels(data)

	elapsed := time.Since(start)
	fmt.Printf("MergeSort took %s\n", elapsed)

	//3. VERIFICATION
	if sort.Float64sAreSorted(data) {
		fmt.Println("Data is sorted correctly")
	} else {
		fmt.Println("Data is NOT sorted correctly")
	}

	if len(data) != len(expected) {
		fmt.Println("Data and expected slices have different lengths")
		return
	}

	for i := range data {
		if data[i] != expected[i] {
			fmt.Println("Data and expected slices do not match")
			return
		}
	}

	fmt.Println("Data and expected slices match")
}