package pagination

import (
	"fmt"
	"net/http"
	"net/url"
	"strconv"
	"strings"
)

func header(u *url.URL, rel string, limit, offset int) string {
	q := u.Query()
	q.Set("limit", strconv.Itoa(limit))
	q.Set("offset", strconv.Itoa(offset))
	u.RawQuery = q.Encode()
	return fmt.Sprintf("<%s>; rel=\"%s\"", u.String(), rel)
}

// Header adds an http header for pagination using a responsewriter where backwards compatibility is required.
// The header will contain links any combination of the first, last, next, or previous (prev) pages in a paginated list (given a limit and an offset, and optionally a total).
// If total is not set, then no "last" page will be calculated.
// If no limit is provided, then it will default to 1.
func Header(w http.ResponseWriter, u *url.URL, total int, limit, offset int) {
	if offset < 0 {
		offset = 0
	}

	if limit <= 0 {
		limit = 1
	}

	w.Header().Set("X-Total-Count", strconv.Itoa(total))

	// lastOffset will either equal the offset required to contain the remainder,
	// or the limit.
	var lastOffset int
	if total%limit == 0 {
		lastOffset = total - limit
	} else {
		lastOffset = (total / limit) * limit
	}

	// Check for last page
	if offset >= lastOffset {
		if total == 0 {
			w.Header().Set("Link", strings.Join([]string{
				header(u, "first", limit, 0),
				header(u, "next", limit, ((offset/limit)+1)*limit),
				header(u, "prev", limit, ((offset/limit)-1)*limit),
			}, ","))
			return
		}

		if total < limit {
			w.Header().Set("link", header(u, "first", total, 0))
			return
		}

		w.Header().Set("Link", strings.Join([]string{
			header(u, "first", limit, 0),
			header(u, "prev", limit, lastOffset-limit),
		}, ","))
		return
	}

	if offset < limit {
		w.Header().Set("Link", strings.Join([]string{
			header(u, "next", limit, limit),
			header(u, "last", limit, lastOffset),
		}, ","))
		return
	}

	w.Header().Set("Link", strings.Join([]string{
		header(u, "first", limit, 0),
		header(u, "next", limit, ((offset/limit)+1)*limit),
		header(u, "prev", limit, ((offset/limit)-1)*limit),
		header(u, "last", limit, lastOffset),
	}, ","))
	return
}