initial support for playlist modifiers

supported modifiers are --playlist-start, --playlist-end, --playlist-reverse, --max-downloads

server/playlist/modifiers.go

@@ -0,0 +1,86 @@
+package playlist
+
+import (
+	"slices"
+	"strconv"
+
+	"github.com/marcopiovanello/yt-dlp-web-ui/v3/server/common"
+)
+
+/*
+	Applicable modifiers
+
+				full									|		   short		  |					description
+	---------------------------------------------------------------------------------
+	--playlist-start NUMBER     |    -I NUMBER:	  |	  discard first N entries
+	--playlist-end NUMBER       |    -I :NUMBER   |   discard last N entries
+	--playlist-reverse          |    -I ::-1			|   self explanatory
+	--max-downloads NUMBER      |                 |   stops after N completed downloads
+*/
+
+func ApplyModifiers(entries *[]common.DownloadInfo, args []string) error {
+	for i, modifier := range args {
+		switch modifier {
+		case "--playlist-start":
+			return playlistStart(i, modifier, args, entries)
+
+		case "--playlist-end":
+			return playlistEnd(i, modifier, args, entries)
+
+		case "--max-downloads":
+			return maxDownloads(i, modifier, args, entries)
+
+		case "--playlist-reverse":
+			slices.Reverse(*entries)
+			return nil
+		}
+	}
+	return nil
+}
+
+func playlistStart(i int, modifier string, args []string, entries *[]common.DownloadInfo) error {
+	if !guard(i, len(modifier)) {
+		return nil
+	}
+
+	n, err := strconv.Atoi(args[i+1])
+	if err != nil {
+		return err
+	}
+
+	*entries = (*entries)[n:]
+
+	return nil
+}
+
+func playlistEnd(i int, modifier string, args []string, entries *[]common.DownloadInfo) error {
+	if !guard(i, len(modifier)) {
+		return nil
+	}
+
+	n, err := strconv.Atoi(args[i+1])
+	if err != nil {
+		return err
+	}
+
+	*entries = (*entries)[:n]
+
+	return nil
+}
+
+func maxDownloads(i int, modifier string, args []string, entries *[]common.DownloadInfo) error {
+	if !guard(i, len(modifier)) {
+		return nil
+	}
+
+	n, err := strconv.Atoi(args[i+1])
+	if err != nil {
+		return err
+	}
+
+	*entries = (*entries)[0:n]
+
+	return nil
+}
+
+func guard(i, len int) bool { return i+1 < len-1 }