3 files changed, 219 insertions, 0 deletions
diff --git a/Util/Extensions.cs b/Util/Extensions.cs
new file mode 100644
index 0000000..bbf9a12
--- /dev/null
+++ b/Util/Extensions.cs
@@ -0,0 +1,33 @@
+namespace HyperBooru.Util;
+
+public static class Extensions {
+    public static readonly string[] MagnitudeOrders = new[] {
+        "K", "M", "G", "T", "P", "E", "Z", "Y", "R", "Q"
+    };
+
+    public static DateTime? TryParseDateTimeUtc(this string s) {
+        bool success = DateTime.TryParse(s, out var dateTime);
+        return success ? DateTime.SpecifyKind(dateTime, DateTimeKind.Utc) : null;
+    }
+
+    public static string ToBytesSI(this long x) {
+        var exp = (int) Math.Log10(x);
+        var suffix = MagnitudeOrders.ElementAtOrDefault(exp / 3 - 1);
+        if(suffix is null)
+            return x.ToString();
+        double n = x / Math.Pow(10, exp / 3 * 3);
+        return $"{Math.Round(n, 2 - (exp % 3))} {suffix}B";
+    }
+
+    public static string ToStringHumanReadable(this TimeSpan t) {
+        if(t.TotalMilliseconds < 1000)
+            return string.Format("{0:0}ms", t.TotalMilliseconds);
+        if(t.TotalSeconds < 60)
+            return string.Format("{0:0.00}s", t.TotalSeconds);
+        if(t.TotalMinutes < 60)
+            return string.Format("{0:0}m{0:0}s", t.TotalMinutes, t.Seconds);
+        if(t.TotalHours < 24)
+            return string.Format("{0:0}h{0:0}m", t.TotalHours, t.Minutes);
+        return string.Format("{0:0.00}d", t.TotalDays);
+    }
+}
diff --git a/Util/LimitedConcurrencyTaskScheduler.cs b/Util/LimitedConcurrencyTaskScheduler.cs
new file mode 100644
index 0000000..8d5adf2
--- /dev/null
+++ b/Util/LimitedConcurrencyTaskScheduler.cs
@@ -0,0 +1,88 @@
+namespace HyperBooru.Util;
+
+public class LimitedConcurrencyTaskScheduler : TaskScheduler {
+    public sealed override int MaximumConcurrencyLevel =>
+        maxConcurrency;
+
+    private int maxConcurrency;
+
+    [ThreadStatic]
+    private static bool threadIsProcessingItems;
+
+    private readonly LinkedList<Task> tasks = new();
+
+    private int delegatesQueuedOrRunning = 0;
+
+    public LimitedConcurrencyTaskScheduler() {
+        maxConcurrency = Environment.ProcessorCount;
+    }
+
+    public LimitedConcurrencyTaskScheduler(int maxConcurrency) {
+        if(maxConcurrency < 1)
+            throw new ArgumentOutOfRangeException("maxConcurrency must be greater than 0");
+        this.maxConcurrency = (int) maxConcurrency;
+    }
+
+    protected sealed override void QueueTask(Task task) {
+        lock(tasks) {
+            tasks.AddLast(task);
+            if(delegatesQueuedOrRunning < maxConcurrency) {
+                delegatesQueuedOrRunning++;
+                NotifyThreadPoolOfPendingWork();
+            }
+        }
+    }
+
+    private void NotifyThreadPoolOfPendingWork() {
+        ThreadPool.UnsafeQueueUserWorkItem(_ => {
+            threadIsProcessingItems = true;
+            try {
+                while(true) {
+                    Task item;
+                    lock(tasks) {
+                        if(tasks.Count == 0) {
+                            delegatesQueuedOrRunning--;
+                            break;
+                        } else {
+                            item = tasks.First.Value;
+                            tasks.RemoveFirst();
+                        }
+                    }
+                    TryExecuteTask(item);
+                }
+            } finally {
+                threadIsProcessingItems = false;
+            }
+        }, null);
+    }
+
+    protected sealed override bool TryExecuteTaskInline(Task task, bool taskWasPreviouslyQueued) {
+        if(!threadIsProcessingItems)
+            return false;
+
+        if(taskWasPreviouslyQueued)
+            return TryDequeue(task) ? TryExecuteTask(task) : false;
+        else
+            return TryExecuteTask(task);
+    }
+
+    protected sealed override bool TryDequeue(Task task) {
+        lock(tasks) {
+            return tasks.Remove(task);
+        }
+    }
+
+    protected sealed override IEnumerable<Task> GetScheduledTasks() {
+        bool lockTaken = false;
+        try {
+            Monitor.TryEnter(tasks, ref lockTaken);
+            if(lockTaken)
+                return tasks;
+            else
+                throw new NotSupportedException();
+        } finally {
+            if(lockTaken)
+                Monitor.Exit(tasks);
+        }
+    }
+}
diff --git a/Util/MemoryCache.cs b/Util/MemoryCache.cs
new file mode 100644
index 0000000..ba314cf
--- /dev/null
+++ b/Util/MemoryCache.cs
@@ -0,0 +1,98 @@
+using System.Collections;
+
+namespace HyperBooru.Util;
+
+public class MemoryCache<TKey, TValue> : IEnumerable<KeyValuePair<TKey, TValue>> where TKey : struct {
+    /// <summary>Maximum number of items this cache may hold (unlimited if null)</summary>
+    public int?                 MaxItems   { get; init; }
+    /// <summary>Maximum amount of time an item may stay in cache before it is removed (unlimited if null)</summary>
+    public TimeSpan?            MaxAge     { get; init; }
+    /// <summary>Function that will be called to populate the cache in the event of a cache-miss</summary>
+    public Func<TKey, TValue?>? DataSource { get; init; }
+
+    private Dictionary<TKey, (DateTime createTime, TValue value)> cache = new();
+
+    public TValue this[TKey key] {
+        get => GetValue(key);
+        set {
+            Prune();
+            cache[key] = (DateTime.Now, value);
+        }
+    }
+
+    public TValue GetValue(TKey key) {
+        bool success = cache.TryGetValue(key, out var result);
+        if(success) {
+            if(MaxAge is null)
+                return result.value;
+            if(result.createTime > DateTime.Now - MaxAge)
+                return result.value;
+        }
+
+        if(DataSource is null)
+            throw new KeyNotFoundException();
+
+        TValue? value = DataSource(key);
+
+        if(value is null)
+            throw new KeyNotFoundException();
+
+        Prune();
+        cache[key] = (DateTime.Now, value);
+        return value;
+    }
+
+    public bool TryGetValue(TKey key, out TValue? value) {
+        try {
+            value = GetValue(key);
+            return true;
+        } catch(KeyNotFoundException) {
+            value = default;
+            return false;
+        }
+    }
+
+    public bool Remove(TKey key) => cache.Remove(key);
+
+    public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator() {
+        DateTime? expiry = MaxAge is null ? null : DateTime.Now - MaxAge;
+
+        foreach(var kv in cache) {
+            // Don't return expired cache items
+            if(expiry is not null)
+                if(kv.Value.createTime < expiry)
+                    continue;
+
+            yield return new KeyValuePair<TKey, TValue>(kv.Key, kv.Value.value);
+        }
+    }
+
+    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+
+    private void Prune() {
+        DateTime? expiry = MaxAge is null ? null : DateTime.Now - MaxAge;
+
+        // If an expiry time for cache items was
+        // specified, remove expired cache items.
+        if(expiry is not null) {
+            foreach(var kv in cache) {
+                if(kv.Value.createTime < expiry)
+                    cache.Remove(kv.Key);
+            }
+        }
+
+        // If this cache was created with a maximum size,
+        // remove elements until that size is reached.
+        if(MaxItems is null || cache.Count() < MaxItems)
+            return;
+
+        var toRemove = cache
+            .OrderBy(kv => kv.Value.createTime)
+            .Take(cache.Count() - (int) MaxItems + 1)
+            .Select(kv => kv.Key)
+            .ToArray();
+
+        foreach(var key in toRemove)
+            cache.Remove(key);
+    }
+}