From 1d04d89e320ada9d3f2f73aa3d5054e1fcbadcfe Mon Sep 17 00:00:00 2001
From: bmazzarol <bmazzarol@gmail.com>
Date: Sun, 29 Oct 2023 23:32:00 +0800
Subject: [PATCH] feat: index based Schedule.cs access

---
 Eon.Tests/Examples/PollyExamples.cs | 15 ++------
 Eon.Tests/ScheduleTests.cs          | 14 +++++++
 Eon/Schedule.cs                     | 60 +++++++++++++++++++++++++++++
 3 files changed, 78 insertions(+), 11 deletions(-)

diff --git a/Eon.Tests/Examples/PollyExamples.cs b/Eon.Tests/Examples/PollyExamples.cs
index c8d3d92..cc40ab6 100644
--- a/Eon.Tests/Examples/PollyExamples.cs
+++ b/Eon.Tests/Examples/PollyExamples.cs
@@ -10,17 +10,15 @@ public static async Task Case1()
     {
         #region Example1
 
-        // build any schedule using Eon
+        // build a schedule using Eon
         Schedule schedule = Schedule.Linear(1).Take(5);
-        // get the durations
-        var durations = schedule.ToArray();
         // now create the options
         RetryStrategyOptions options = new RetryStrategyOptions
         {
             MaxRetryAttempts = schedule.Count ?? int.MaxValue,
             Delay = TimeSpan.Zero,
             DelayGenerator = x =>
-                ValueTask.FromResult<TimeSpan?>((TimeSpan)durations[x.AttemptNumber])
+                ValueTask.FromResult<TimeSpan?>((TimeSpan)schedule[x.AttemptNumber])
         };
         ResiliencePipeline pipeline = new ResiliencePipelineBuilder().AddRetry(options).Build();
         int attempts = 0;
@@ -43,7 +41,7 @@ public static async Task Case2()
     {
         #region Example2
 
-        // build any schedule using Eon
+        // build a schedule using Eon
         Schedule schedule = Schedule.Linear(1);
         // now create the options
         RetryStrategyOptions options = new RetryStrategyOptions
@@ -51,12 +49,7 @@ public static async Task Case2()
             MaxRetryAttempts = int.MaxValue,
             Delay = TimeSpan.Zero,
             DelayGenerator = x =>
-            {
-                var enumerator = x.AttemptNumber == 0 ? schedule.GetEnumerator() : null;
-                return ValueTask.FromResult<TimeSpan?>(
-                    enumerator?.MoveNext() == true ? (TimeSpan)enumerator.Current : null
-                );
-            }
+                ValueTask.FromResult<TimeSpan?>((TimeSpan)schedule[x.AttemptNumber])
         };
         ResiliencePipeline pipeline = new ResiliencePipelineBuilder().AddRetry(options).Build();
         int attempts = 0;
diff --git a/Eon.Tests/ScheduleTests.cs b/Eon.Tests/ScheduleTests.cs
index ad07586..7e4c811 100644
--- a/Eon.Tests/ScheduleTests.cs
+++ b/Eon.Tests/ScheduleTests.cs
@@ -17,4 +17,18 @@ public static void Case2()
         var schedule = Schedule.Forever;
         ((IEnumerable)schedule).GetEnumerator().MoveNext().Should().BeTrue();
     }
+
+    [Fact(DisplayName = "Schedule can be accessed by index")]
+    public static void Case3()
+    {
+        var schedule = Schedule.Linear(100);
+        schedule[0].Should().Be(100);
+        schedule[2].Should().Be(300);
+        schedule[3].Should().Be(400);
+        schedule[4].Should().Be(500);
+        schedule[5].Should().Be(600);
+        schedule[5].Should().Be(600);
+        schedule[100].Should().Be(10100);
+        schedule[1].Should().Be(200);
+    }
 }
diff --git a/Eon/Schedule.cs b/Eon/Schedule.cs
index 6aa4c73..8c0526a 100644
--- a/Eon/Schedule.cs
+++ b/Eon/Schedule.cs
@@ -34,4 +34,64 @@ IEnumerator IEnumerable.GetEnumerator()
     {
         return GetEnumerator();
     }
+
+    private readonly object _lock = new();
+    private int _position;
+    private Duration[] _buffer = Array.Empty<Duration>();
+    private IEnumerator<Duration>? _enumerator;
+
+    /// <summary>
+    /// Provides `index` based access to a shared enumerated version of `this` <see cref="Schedule"/>.
+    /// This will grow the internal buffer in a synchronized way by doubling the current
+    /// length of the buffer when it is filled.
+    /// </summary>
+    /// <remarks>This can only ever be as large as <see cref="int.MaxValue"/></remarks>
+    /// <remarks>This can be used to integrate <see cref="Schedule"/> into places that
+    /// take delegate based back-off algorithms</remarks>
+    /// <param name="index">index</param>
+    /// <exception cref="IndexOutOfRangeException">when the <see cref="Schedule"/> has
+    /// less emissions than the provided `index`</exception>
+    public Duration this[int index]
+    {
+        get
+        {
+            lock (_lock)
+            {
+                TryFillBuffer(index);
+                return _buffer[index];
+            }
+        }
+    }
+
+    private void TryFillBuffer(int index)
+    {
+        _enumerator ??= GetEnumerator();
+
+        var currentLength = _position;
+        if (currentLength != 0 && currentLength > index)
+        {
+            return;
+        }
+
+        var size = currentLength == 0 ? 4 : currentLength * 2;
+
+        if (size <= index)
+        {
+            size = index + 1;
+        }
+
+        var newBuffer = new Duration[size];
+
+        if (_buffer.Length != 0)
+        {
+            Array.Copy(_buffer, 0, newBuffer, 0, _buffer.Length);
+        }
+
+        for (; _position < size && _enumerator.MoveNext(); _position++)
+        {
+            newBuffer[_position] = _enumerator.Current;
+        }
+
+        _buffer = newBuffer;
+    }
 }