[chore] Make paging logic more generic (#901)

* make paging logic more generic
not just for timelines!

* linty linterson

internal/util/paging.go

@@ -0,0 +1,119 @@
+/*
+   GoToSocial
+   Copyright (C) 2021-2022 GoToSocial Authors admin@gotosocial.org
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Affero General Public License as published by
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Affero General Public License for more details.
+
+   You should have received a copy of the GNU Affero General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+package util
+
+import (
+	"fmt"
+	"net/url"
+
+	apimodel "github.com/superseriousbusiness/gotosocial/internal/api/model"
+	"github.com/superseriousbusiness/gotosocial/internal/config"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+)
+
+// PageableResponseParams models the parameters to pass to PackagePageableResponse.
+//
+// The given items will be provided in the paged response.
+//
+// The other values are all used to create the Link header so that callers know
+// which endpoint to query next and previously in order to do paging.
+type PageableResponseParams struct {
+	Items            []interface{} // Sorted slice of items (statuses, notifications, etc)
+	Path             string        // path to use for next/prev queries in the link header
+	NextMaxIDKey     string        // key to use for the next max id query param in the link header, defaults to 'max_id'
+	NextMaxIDValue   string        // value to use for next max id
+	PrevMinIDKey     string        // key to use for the prev min id query param in the link header, defaults to 'min_id'
+	PrevMinIDValue   string        // value to use for prev min id
+	Limit            int           // limit number of entries to return
+	ExtraQueryParams []string      // any extra query parameters to provide in the link header, should be in the format 'example=value'
+}
+
+// PackagePageableResponse is a convenience function for returning
+// a bunch of pageable items (notifications, statuses, etc), as well
+// as a Link header to inform callers of where to find next/prev items.
+func PackagePageableResponse(params PageableResponseParams) (*apimodel.PageableResponse, gtserror.WithCode) {
+	if params.NextMaxIDKey == "" {
+		params.NextMaxIDKey = "max_id"
+	}
+
+	if params.PrevMinIDKey == "" {
+		params.PrevMinIDKey = "min_id"
+	}
+
+	pageableResponse := EmptyPageableResponse()
+
+	if len(params.Items) == 0 {
+		return pageableResponse, nil
+	}
+
+	// items
+	pageableResponse.Items = params.Items
+
+	protocol := config.GetProtocol()
+	host := config.GetHost()
+
+	// next
+	nextRaw := params.NextMaxIDKey + "=" + params.NextMaxIDValue
+	if params.Limit != 0 {
+		nextRaw = fmt.Sprintf("limit=%d&", params.Limit) + nextRaw
+	}
+	for _, p := range params.ExtraQueryParams {
+		nextRaw = nextRaw + "&" + p
+	}
+	nextLink := &url.URL{
+		Scheme:   protocol,
+		Host:     host,
+		Path:     params.Path,
+		RawQuery: nextRaw,
+	}
+	nextLinkString := nextLink.String()
+	pageableResponse.NextLink = nextLinkString
+
+	// prev
+	prevRaw := params.PrevMinIDKey + "=" + params.PrevMinIDValue
+	if params.Limit != 0 {
+		prevRaw = fmt.Sprintf("limit=%d&", params.Limit) + prevRaw
+	}
+	for _, p := range params.ExtraQueryParams {
+		prevRaw = prevRaw + "&" + p
+	}
+	prevLink := &url.URL{
+		Scheme:   protocol,
+		Host:     host,
+		Path:     params.Path,
+		RawQuery: prevRaw,
+	}
+	prevLinkString := prevLink.String()
+	pageableResponse.PrevLink = prevLinkString
+
+	// link header
+	next := fmt.Sprintf("<%s>; rel=\"next\"", nextLinkString)
+	prev := fmt.Sprintf("<%s>; rel=\"prev\"", prevLinkString)
+	pageableResponse.LinkHeader = next + ", " + prev
+
+	return pageableResponse, nil
+}
+
+// EmptyPageableResponse just returns an empty
+// PageableResponse with no link header or items.
+func EmptyPageableResponse() *apimodel.PageableResponse {
+	return &apimodel.PageableResponse{
+		Items: []interface{}{},
+	}
+}