package push

import (
	"fmt"
	"strconv"
	"sync/atomic"
)

// cursorWidth is the zero-padded decimal width applied to every cursor.
// 20 digits accommodate the full uint64 range so lexicographic order
// matches numeric order across the entire process lifetime.
const cursorWidth = 20

// cursorGenerator hands out monotonically increasing uint64 sequence
// numbers. Cursors restart from 0 on process boot; the ring buffer's
// freshness-window TTL bounds how long a cursor remains valid, so a
// fresh process intentionally invalidates every previously-issued
// cursor.
type cursorGenerator struct {
	seq atomic.Uint64
}

// next returns the next sequence number. The first call returns 1.
func (g *cursorGenerator) next() uint64 {
	return g.seq.Add(1)
}

// formatCursor renders n in the canonical zero-padded form so cursor
// strings sort identically to their numeric counterparts.
func formatCursor(n uint64) string {
	return fmt.Sprintf("%0*d", cursorWidth, n)
}

// parseCursor decodes a cursor string back to its numeric value. An
// empty string maps to 0 ("subscribe from now"); malformed input also
// maps to 0 with ok=false so callers can log without rejecting the
// subscription — gateway is trusted but reconnects can race against a
// process restart that scrambled the in-memory sequence.
func parseCursor(s string) (uint64, bool) {
	if s == "" {
		return 0, true
	}
	n, err := strconv.ParseUint(s, 10, 64)
	if err != nil {
		return 0, false
	}
	return n, true
}