package main

import (
	"bufio"
	"cmp"
	_ "embed"
	"flag"
	"fmt"
	"io"
	"math/rand/v2"
	"os"
	"slices"
	"strings"
)

var (
	wordList = flag.String("w", "", "use `wordlist` for N-grams")
	numWords = flag.Int("n", 1, "generate `n` words")
)

type choice struct {
	r         rune
	cumWeight int
}

func pick(choices []choice) rune {
	if len(choices) == 0 {
		panic("pick: nothing to choose from")
	}
	n, _ := slices.BinarySearchFunc(choices, rand.IntN(choices[len(choices)-1].cumWeight)+1, func(c choice, n int) int {
		return cmp.Compare(c.cumWeight, n)
	})
	return choices[n].r
}

type biGram struct {
	a, b rune
}

func buildNGrams(r io.Reader) (map[biGram][]choice, error) {
	type triGram struct {
		a, b, c rune
	}
	triGrams := make(map[triGram]int)
	total := 0
	scanner := bufio.NewScanner(r)
	for scanner.Scan() {
		line := scanner.Text()
		var a, b rune = -1, -1
		for _, r := range line {
			triGrams[triGram{a, b, r}]++
			total++
			a, b = b, r
		}
		triGrams[triGram{a, b, -1}]++
	}
	if err := scanner.Err(); err != nil {
		return nil, err
	}
	triGramsKeys := make([]triGram, 0, len(triGrams))
	for t := range triGrams {
		triGramsKeys = append(triGramsKeys, t)
	}
	slices.SortFunc(triGramsKeys, func(x, y triGram) int {
		return cmp.Or(
			cmp.Compare(x.a, y.a),
			cmp.Compare(x.b, y.b),
			cmp.Compare(x.c, y.c))
	})
	biGrams := make(map[biGram][]choice)
	for _, t := range triGramsKeys {
		choices := biGrams[biGram{t.a, t.b}]
		currentCumTotal := 0
		if len(choices) > 0 {
			currentCumTotal = choices[len(choices)-1].cumWeight
		}
		biGrams[biGram{t.a, t.b}] = append(choices, choice{t.c, currentCumTotal + triGrams[t]})
	}
	return biGrams, nil
}

//go:embed words.txt
var defaultWordList string

func run() error {
	var wordListReader io.Reader
	if *wordList != "" {
		var err error
		wordListReader, err = os.Open(*wordList)
		if err != nil {
			return err
		}
	} else {
		wordListReader = strings.NewReader(defaultWordList)
	}
	nGrams, err := buildNGrams(wordListReader)
	if err != nil {
		return err
	}
	w := bufio.NewWriter(os.Stdout)
	defer w.Flush()
	for i := 0; *numWords < 0 || i < *numWords; i++ {
		var a, b rune = -1, -1
		for {
			c := pick(nGrams[biGram{a, b}])
			if c == -1 {
				break
			}
			w.WriteRune(c)
			a, b = b, c
		}
		w.WriteByte('\n')
	}
	return nil
}

func main() {
	flag.Parse()
	if err := run(); err != nil {
		fmt.Fprintf(os.Stderr, "words: %s\n", err)
		os.Exit(1)
	}
}