package applog

import (
	"encoding/base64"
	"encoding/json"
	"time"
)

// Cursor names a position in the merged timeline — the last record a reader
// has seen, by exactly the (TS, Box, Seq) triple MergeRecords orders by.
// Opaque base64 on the wire so its shape is the server's to evolve.
type Cursor struct {
	TS  time.Time `json:"ts"`
	Box string    `json:"box"`
	Seq uint64    `json:"seq"`
}

// Before reports whether c sits strictly before r in merge order — the
// follow test: records with c.Before(r) are new since the cursor.
func (c Cursor) Before(r Record) bool {
	if !c.TS.Equal(r.TS) {
		return c.TS.Before(r.TS)
	}
	if c.Box != r.Box {
		return c.Box < r.Box
	}
	return c.Seq < r.Seq
}

// EncodeCursor renders the position just after r.
func EncodeCursor(r Record) string {
	b, _ := json.Marshal(Cursor{TS: r.TS, Box: r.Box, Seq: r.Seq}) // cannot fail on this shape
	return base64.RawURLEncoding.EncodeToString(b)
}

// DecodeCursor parses what EncodeCursor produced; ok=false for anything
// else (including future shapes this build cannot read — the caller treats
// that as "no cursor", which re-tails rather than erroring a follow).
func DecodeCursor(s string) (Cursor, bool) {
	b, err := base64.RawURLEncoding.DecodeString(s)
	if err != nil {
		return Cursor{}, false
	}
	var c Cursor
	if json.Unmarshal(b, &c) != nil || c.TS.IsZero() {
		return Cursor{}, false
	}
	return c, true
}