// Package ids makes the request and approval identifiers from docs/hub-api.md.
// The shape is a ULID: 26 Crockford base32 characters, time-ordered. It needs
// no dependency, so the Hub stays a small static binary.
package ids

import (
	"crypto/rand"
	"sync"
	"time"
)

// Crockford base32 alphabet. ULID uses these 32 symbols.
const alphabet = "0123456789ABCDEFGHJKMNPQRSTVWXYZ"

var (
	mu      sync.Mutex
	lastMS  uint64
	lastRnd [10]byte
)

// New returns a 26-character ULID string. The first 10 characters hold the
// time. The last 16 hold randomness. Two calls in the same millisecond do not
// repeat.
func New() string {
	mu.Lock()
	defer mu.Unlock()

	ms := uint64(time.Now().UnixMilli())
	if ms == lastMS {
		// Same millisecond: increment the random part.
		for i := len(lastRnd) - 1; i >= 0; i-- {
			lastRnd[i]++
			if lastRnd[i] != 0 {
				break
			}
		}
	} else {
		lastMS = ms
		if _, err := rand.Read(lastRnd[:]); err != nil {
			// crypto/rand does not fail in practice. Fall back to the clock.
			for i := range lastRnd {
				lastRnd[i] = byte(ms >> (8 * (i % 8)))
			}
		}
	}

	return encodeTime(ms) + encodeRandom(lastRnd)
}

// Approval returns an approval id, for example "ap_01J8Z...".
func Approval() string {
	return "ap_" + New()
}

// encodeTime writes 10 base32 characters from the low 50 bits of ms.
func encodeTime(ms uint64) string {
	var out [10]byte
	for i := 9; i >= 0; i-- {
		out[i] = alphabet[ms&0x1f]
		ms >>= 5
	}
	return string(out[:])
}

// encodeRandom writes 16 base32 characters from 10 random bytes (80 bits).
func encodeRandom(rnd [10]byte) string {
	var out [16]byte
	var hi, lo uint64
	hi = uint64(rnd[0])<<8 | uint64(rnd[1])
	hi = hi<<8 | uint64(rnd[2])
	hi = hi<<8 | uint64(rnd[3])
	hi = hi<<8 | uint64(rnd[4])
	lo = uint64(rnd[5])<<8 | uint64(rnd[6])
	lo = lo<<8 | uint64(rnd[7])
	lo = lo<<8 | uint64(rnd[8])
	lo = lo<<8 | uint64(rnd[9])

	for i := 15; i >= 8; i-- {
		out[i] = alphabet[lo&0x1f]
		lo >>= 5
	}
	for i := 7; i >= 0; i-- {
		out[i] = alphabet[hi&0x1f]
		hi >>= 5
	}
	return string(out[:])
}