From 4f02fb27da9a7e7a655eb7e42557743e06d8eeb5 Mon Sep 17 00:00:00 2001
From: Sijmen Schoon <me@sijmenschoon.nl>
Date: Sun, 4 Jul 2021 14:36:21 +0200
Subject: [PATCH] Parse DURATION property from iCal

---
 BlazorApp/Data/Calendar.cs     | 58 +++++++++++++++++++++++++++++++++-
 BlazorApp/Pages/Calendar.razor |  4 +--
 BlazorApp/Pages/Event.razor    |  4 +--
 3 files changed, 61 insertions(+), 5 deletions(-)

diff --git a/BlazorApp/Data/Calendar.cs b/BlazorApp/Data/Calendar.cs
index bcc68ab..abafcb4 100644
--- a/BlazorApp/Data/Calendar.cs
+++ b/BlazorApp/Data/Calendar.cs
@@ -16,6 +16,17 @@ namespace BlazorApp.Data
         public DateTime? DtStart { get; set; }
         public DateTime? DtEnd { get; set; }
         public string Description { get; set; }
+        public TimeSpan? Duration { get; set; }
+
+        public DateTime? CalculatedEnd
+        {
+            get
+            {
+                if (Duration != null)
+                    return DtStart + Duration;
+                return DtEnd;
+            }
+        }
     }
 
     public class Calendar
@@ -40,6 +51,48 @@ namespace BlazorApp.Data
             return DateTime.ParseExact(s, format, CultureInfo.InvariantCulture);
         }
 
+        /// <summary>
+        /// Very non-strict parser of durations as specified in
+        /// https://datatracker.ietf.org/doc/html/rfc5545#section-3.3.6
+        /// </summary>
+        private static TimeSpan? _parseDuration(string s)
+        {
+            var timeSpan = new TimeSpan();
+            var positive = s[0] != '-';
+            var value = 0;
+            foreach (var c in s)
+            {
+                if (char.IsDigit(c))
+                {
+                    value = value * 10 + (c - '0');
+                    continue;
+                }
+
+                switch (c)
+                {
+                    case 'W':
+                        timeSpan += TimeSpan.FromDays(value * 7);
+                        break;
+                    case 'D':
+                        timeSpan += TimeSpan.FromDays(value);
+                        break;
+                    case 'H':
+                        timeSpan += TimeSpan.FromHours(value);
+                        break;
+                    case 'M':
+                        timeSpan += TimeSpan.FromMinutes(value);
+                        break;
+                    case 'S':
+                        timeSpan += TimeSpan.FromSeconds(value);
+                        break;
+                }
+
+                value = 0;
+            }
+
+            return positive ? timeSpan : -timeSpan;
+        }
+
         private static List<Event> ParseICal(string iCal)
         {
             Event @event = null;
@@ -108,6 +161,9 @@ namespace BlazorApp.Data
                     case "DTEND":
                         @event.DtEnd = _parseDateTime(value);
                         break;
+                    case "DURATION":
+                        @event.Duration = _parseDuration(value);
+                        break;
                 }
             }
 
@@ -122,7 +178,7 @@ namespace BlazorApp.Data
             var client = new HttpClient();
             if (AuthorizationHeader != null)
                 client.DefaultRequestHeaders.Add("Authorization", AuthorizationHeader);
-            
+
             var body = await client.GetStringAsync(Url);
             var events = ParseICal(body);
 
diff --git a/BlazorApp/Pages/Calendar.razor b/BlazorApp/Pages/Calendar.razor
index ff79110..88c6a26 100644
--- a/BlazorApp/Pages/Calendar.razor
+++ b/BlazorApp/Pages/Calendar.razor
@@ -28,10 +28,10 @@ else
                 </div>
             }
             
-            @if (@event.DtEnd != null)
+            @if (@event.CalculatedEnd != null)
             {
                 <div>
-                    <strong>End:</strong> @(@event.DtEnd)
+                    <strong>End:</strong> @(@event.CalculatedEnd)
                 </div>
             }
             
diff --git a/BlazorApp/Pages/Event.razor b/BlazorApp/Pages/Event.razor
index 3782c1e..577a728 100644
--- a/BlazorApp/Pages/Event.razor
+++ b/BlazorApp/Pages/Event.razor
@@ -22,10 +22,10 @@ else
         </div>
     }
 
-    @if (_event.DtEnd != null)
+    @if (_event.CalculatedEnd != null)
     {
         <div>
-            <strong>End:</strong> @_event.DtEnd
+            <strong>End:</strong> @_event.CalculatedEnd
         </div>
     }