Capstone: Build a Bookmarks API

package main

import (
	"encoding/base64"
	"fmt"
	"strconv"
)

type Bookmark struct {
	ID    int
	Title string
}

// All bookmarks for one user, newest first (higher ID = newer).
var store = []Bookmark{{9, "Go blog"}, {8, "Chi docs"}, {7, "Gin docs"}, {6, "Echo docs"}, {5, "Fiber docs"}, {4, "slog"}, {3, "sqlc"}}

func encode(id int) string { return base64.RawURLEncoding.EncodeToString([]byte(strconv.Itoa(id))) }
func decode(c string) (int, error) {
	b, err := base64.RawURLEncoding.DecodeString(c)
	if err != nil {
		return 0, err
	}
	return strconv.Atoi(string(b))
}

// page returns up to limit bookmarks older than the cursor, plus the next cursor ("" when done).
func page(cursor string, limit int) ([]Bookmark, string) {
	before := int(^uint(0) >> 1) // no cursor: start from the newest
	if cursor != "" {
		id, err := decode(cursor)
		if err != nil {
			return nil, ""
		}
		before = id
	}
	var out []Bookmark
	for _, b := range store { // in SQL: WHERE user_id = ? AND id < ? ORDER BY id DESC LIMIT ?
		if b.ID < before && len(out) < limit {
			out = append(out, b)
		}
	}
	if len(out) < limit {
		return out, ""
	}
	return out, encode(out[len(out)-1].ID)
}

func main() {
	cursor := ""
	for n := 1; ; n++ {
		items, next := page(cursor, 3)
		fmt.Printf("page %d: %v next=%q\n", n, items, next)
		if next == "" {
			break
		}
		if n == 1 {
			store = append([]Bookmark{{10, "NEW: added while paging"}}, store...) // doesn't disturb later pages
		}
		cursor = next
	}
}