package simulation

import (
	"math/big"
	"math/rand"
	"time"

	sdk "github.com/cosmos/cosmos-sdk/types"
)

const (
	letterBytes   = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
	letterIdxBits = 6                    // 6 bits to represent a letter index
	letterIdxMask = 1<<letterIdxBits - 1 // All 1-bits, as many as letterIdxBits
	letterIdxMax  = 63 / letterIdxBits   // # of letter indices fitting in 63 bits
)

// shamelessly copied from
// https://stackoverflow.com/questions/22892120/how-to-generate-a-random-string-of-a-fixed-length-in-golang#31832326
// Generate a random string of a particular length
func RandStringOfLength(r *rand.Rand, n int) string {
	b := make([]byte, n)
	// A src.Int63() generates 63 random bits, enough for letterIdxMax characters!
	for i, cache, remain := n-1, r.Int63(), letterIdxMax; i >= 0; {
		if remain == 0 {
			cache, remain = r.Int63(), letterIdxMax
		}
		if idx := int(cache & letterIdxMask); idx < len(letterBytes) {
			b[i] = letterBytes[idx]
			i--
		}
		cache >>= letterIdxBits
		remain--
	}
	return string(b)
}

// Generate a random amount
// Note: The range of RandomAmount includes max, and is, in fact, biased to return max as well as 0.
func RandomAmount(r *rand.Rand, max sdk.Int) sdk.Int {
	var randInt = big.NewInt(0)
	switch r.Intn(10) {
	case 0:
		// randInt = big.NewInt(0)
	case 1:
		randInt = max.BigInt()
	default: // NOTE: there are 10 total cases.
		randInt = big.NewInt(0).Rand(r, max.BigInt()) // up to max - 1
	}
	return sdk.NewIntFromBigInt(randInt)
}

// RandomDecAmount generates a random decimal amount
// Note: The range of RandomDecAmount includes max, and is, in fact, biased to return max as well as 0.
func RandomDecAmount(r *rand.Rand, max sdk.Dec) sdk.Dec {
	var randInt = big.NewInt(0)
	switch r.Intn(10) {
	case 0:
		// randInt = big.NewInt(0)
	case 1:
		randInt = max.Int // the underlying big int with all precision bits.
	default: // NOTE: there are 10 total cases.
		randInt = big.NewInt(0).Rand(r, max.Int)
	}
	return sdk.NewDecFromBigIntWithPrec(randInt, sdk.Precision)
}

// RandTimestamp generates a random timestamp
func RandTimestamp(r *rand.Rand) time.Time {
	// json.Marshal breaks for timestamps greater with year greater than 9999
	unixTime := r.Int63n(253373529600)
	return time.Unix(unixTime, 0)
}

// RandIntBetween returns a random int between two numbers inclusively.
func RandIntBetween(r *rand.Rand, min, max int) int {
	return r.Intn(max-min) + min
}

// Derive a new rand deterministically from a rand.
// Unlike rand.New(rand.NewSource(seed)), the result is "more random"
// depending on the source and state of r.
// NOTE: not crypto safe.
func DeriveRand(r *rand.Rand) *rand.Rand {
	const num = 8 // TODO what's a good number?  Too large is too slow.
	ms := multiSource(make([]rand.Source, num))
	for i := 0; i < num; i++ {
		ms[i] = rand.NewSource(r.Int63())
	}
	return rand.New(ms)
}

type multiSource []rand.Source

func (ms multiSource) Int63() (r int64) {
	for _, source := range ms {
		r ^= source.Int63()
	}
	return r
}

func (ms multiSource) Seed(seed int64) {
	panic("multiSource Seed should not be called")
}