// binary manualsort asks you at the CLI which of two things you like
// better, repeatedly, and pukes up a list sorted accordingly.
package main

import (
	"bufio"
	"fmt"
	"log"
	"math/rand"
	"os"
	"slices"
	"strings"
	"sync"
	"time"
)

// handwritten merge sort because it minimizes comparisons, which are
// the expensive part when asking a human for every comparison.
// quicksort's "in-place" behavior isn't necessary.
func merge(a, b []string, less func(string, string) bool) []string {
	if len(a) == 0 {
		return b
	}
	if len(b) == 0 {
		return a
	}
	ret := make([]string, 0, len(a)+len(b))
	for len(a) > 0 && len(b) > 0 {
		if less(a[0], b[0]) {
			ret = append(ret, a[0])
			a = a[1:]
		} else {
			ret = append(ret, b[0])
			b = b[1:]
		}
	}
	if len(a) > 0 {
		ret = append(ret, a...)
	}
	if len(b) > 0 {
		ret = append(ret, b...)
	}
	return ret
}

var (
	stdinReader *bufio.Reader
	truthy      = []string{
		"y", "yes", "1", "t", "true", "aye", "left", "up", "top", "first", "p",
	}
	falsey = []string{
		"n", "no", "0", "2", "f", "false", "nay", "right", "down", "bottom", "second", "nil",
	}
)

type Question struct {
	First  string
	Second string
	Reply  chan<- bool
}

func (q *Question) Ask() {
	q.Reply <- better(q.First, q.Second)
}

func (q *Question) Push(qc chan<- *Question) bool {
	c := make(chan bool, 1)
	q.Reply = c
	qc <- q
	return <-c
}

func betterLoop(qq <-chan *Question) {
	var pool []*Question
	drain := time.After(100 * time.Millisecond)
	for {
		select {
		case q, ok := <-qq:
			if !ok {
				qq = nil
				continue
			}
			pool = append(pool, q)
		case <-drain:
			rand.Shuffle(len(pool), func(i, j int) { pool[i], pool[j] = pool[j], pool[i] })
			for _, q := range pool {
				q.Ask()
			}
			pool = pool[:0]
			if qq == nil {
				return
			}
			drain = time.After(100 * time.Millisecond)
		}
	}
}

func better(first, second string) bool {
	for {
		fmt.Println()
		fmt.Println("Is")
		fmt.Println(first)
		fmt.Println("better than")
		fmt.Println(second)
		fmt.Print("? > ")
		reply, err := stdinReader.ReadString('\n')
		if err != nil && len(reply) == 0 {
			log.Fatalf("I/O error: %v", err)
		}
		reply = strings.TrimSpace(reply)
		reply = strings.ToLower(reply)
		if slices.Contains(truthy, reply) {
			return true
		}
		if slices.Contains(falsey, reply) {
			return false
		}
		fmt.Printf("Huh? I didn't understand %q. Answer \"y\" or \"n\".\n", reply)
	}
}

func parallelBetter(first, second string, qq chan<- *Question) bool {
	q := Question{
		First:  first,
		Second: second,
	}
	return q.Push(qq)
}

func mergeSort(items []string) []string {
	if len(items) <= 1 {
		return items
	}
	midpoint := len(items) / 2
	return merge(mergeSort(items[:midpoint]), mergeSort(items[midpoint:]), better)
}

func parallelMergeSort(questions chan<- *Question, items []string) []string {
	if len(items) <= 1 {
		return items
	}
	midpoint := len(items) / 2
	var wg sync.WaitGroup
	wg.Add(2)
	var r1, r2 []string
	go func() {
		r1 = parallelMergeSort(questions, items[:midpoint])
		wg.Done()
	}()
	go func() {
		r2 = parallelMergeSort(questions, (items[midpoint:]))
		wg.Done()
	}()
	wg.Wait()
	return merge(r1, r2, func(a, b string) bool {
		return parallelBetter(a, b, questions)
	})
}

func main() {
	if len(os.Args) != 3 {
		log.Fatal("wrong number of args, expected 2. Usage: manualsort <item list file> <output file path>")
	}
	ifile, err := os.Open(os.Args[1])
	if err != nil {
		log.Fatalf("can't open %q: %v.", os.Args[1], err)
	}
	ofile, err := os.OpenFile(os.Args[2], os.O_CREATE|os.O_EXCL, 0600)
	if err != nil {
		log.Fatalf("can't exclusively create %q: %v.", os.Args[2], err)
	}
	stdinReader = bufio.NewReader(os.Stdin)

	var items []string
	scanner := bufio.NewScanner(ifile)
	for scanner.Scan() {
		if t := strings.TrimSpace(scanner.Text()); len(t) > 0 {
			items = append(items, t)
		}
	}
	if err := scanner.Err(); err != nil {
		log.Fatalf("can't read %q: %v", os.Args[1], err)
	}

	rand.Shuffle(len(items), func(i, j int) {
		items[i], items[j] = items[j], items[i]
	})
	qchan := make(chan *Question, len(items))
	go betterLoop(qchan)
	sorted := parallelMergeSort(qchan, items)
	close(qchan)

	fmt.Println("Sorted. Saving...")
	obuf := bufio.NewWriter(ofile)
	for i, s := range sorted {
		_, err := obuf.WriteString(fmt.Sprintf("%6d %s\n", i+1, s))
		if err != nil {
			log.Printf("error saving %d (%s): %v\n", i+1, s, err)
		}
	}
	obuf.Flush()
	ofile.Sync()
	ofile.Close()
}