move repetitive logic to ExpiryToken type (potentially also useful from other use-cases)

Author: mgravell

src/StackExchange.Redis/RedisDatabase.ExpiryToken.cs

@@ -0,0 +1,76 @@
+using System;
+
+namespace StackExchange.Redis;
+
+internal partial class RedisDatabase
+{
+    /// <summary>
+    /// Parses, validates and represents, for example: "EX 10", "KEEPTTL" or "".
+    /// </summary>
+    internal readonly struct ExpiryToken
+    {
+        private static readonly ExpiryToken s_Persist = new(RedisLiterals.PERSIST), s_KeepTtl = new(RedisLiterals.KEEPTTL), s_Null = new(RedisValue.Null);
+
+        public RedisValue Operand { get; }
+        public long Value { get; }
+        public int Tokens => Value == long.MinValue ? (Operand.IsNull ? 0 : 1) : 2;
+        public bool HasValue => Value != long.MinValue;
+        public bool HasOperand => !Operand.IsNull;
+
+        public static ExpiryToken Persist(TimeSpan? expiry, bool persist)
+        {
+            if (expiry.HasValue)
+            {
+                if (persist) throw new ArgumentException("Cannot specify both expiry and persist", nameof(persist));
+                return new(expiry.GetValueOrDefault()); // EX 10
+            }
+
+            return persist ? s_Persist : s_Null; // PERSIST (or nothing)
+        }
+
+        public static ExpiryToken KeepTtl(TimeSpan? expiry, bool keepTtl)
+        {
+            if (expiry.HasValue)
+            {
+                if (keepTtl) throw new ArgumentException("Cannot specify both expiry and keepTtl", nameof(keepTtl));
+                return new(expiry.GetValueOrDefault()); // EX 10
+            }
+
+            return keepTtl ? s_KeepTtl : s_Null; // KEEPTTL (or nothing)
+        }
+
+        private ExpiryToken(RedisValue operand, long value = long.MinValue)
+        {
+            Operand = operand;
+            Value = value;
+        }
+
+        public ExpiryToken(TimeSpan expiry)
+        {
+            long milliseconds = expiry.Ticks / TimeSpan.TicksPerMillisecond;
+            var useSeconds = milliseconds % 1000 == 0;
+
+            Operand = useSeconds ? RedisLiterals.EX : RedisLiterals.PX;
+            Value = useSeconds ? (milliseconds / 1000) : milliseconds;
+        }
+
+        public ExpiryToken(DateTime expiry)
+        {
+            long milliseconds = GetUnixTimeMilliseconds(expiry);
+            var useSeconds = milliseconds % 1000 == 0;
+
+            Operand = useSeconds ? RedisLiterals.EXAT : RedisLiterals.PXAT;
+            Value = useSeconds ? (milliseconds / 1000) : milliseconds;
+        }
+
+        public override string ToString() => Tokens switch
+        {
+            2 => $"{Operand} {Value}",
+            1 => Operand.ToString(),
+            _ => "",
+        };
+
+        public override int GetHashCode() => throw new NotSupportedException();
+        public override bool Equals(object? obj) => throw new NotSupportedException();
+    }
+}