perf: skip metric name normalization for symbol inputs

src/Humanizer/MetricNumeralExtensions.cs

@@ -255,7 +255,11 @@ static string CleanRepresentation(string input)
         ArgumentNullException.ThrowIfNull(input);
 
         input = input.Trim();
-        input = ReplaceNameBySymbol(input);
+        if (input.Length > 0 && char.IsLetter(input[^1]) && !HasSymbolSuffix(input))
+        {
+            input = ReplaceNameBySymbol(input);
+        }
+
         if (input.Length == 0 || input.IsInvalidMetricNumeral())
         {
             throw new ArgumentException("Empty or invalid Metric string.", nameof(input));
@@ -264,6 +268,10 @@ static string CleanRepresentation(string input)
         return input.Replace(" ", string.Empty);
     }
 
+    static bool HasSymbolSuffix(string input) =>
+        UnitPrefixes.ContainsKey(input[^1]) &&
+        (input.Length == 1 || !char.IsLetter(input[^2]));
+
     /// <summary>
     /// Build a number from a metric representation or from a number
     /// </summary>
@@ -303,9 +311,15 @@ static double BuildMetricNumber(string input, char last)
     /// </summary>
     /// <param name="input">Metric representation with a name or a symbol</param>
     /// <returns>A metric representation with a symbol</returns>
-    static string ReplaceNameBySymbol(string input) =>
-        UnitPrefixes.Aggregate(input, (current, unitPrefix) =>
-            current.Replace(unitPrefix.Value.Name, unitPrefix.Key.ToString()));
+    static string ReplaceNameBySymbol(string input)
+    {
+        foreach (var unitPrefix in UnitPrefixes)
+        {
+            input = input.Replace(unitPrefix.Value.Name, GetSymbolText(unitPrefix.Key));
+        }
+
+        return input;
+    }
 
     /// <summary>
     /// Build a Metric representation of the number.
@@ -496,9 +510,31 @@ static string GetUnitText(char symbol, MetricNumeralFormats? formats)
             }
         }
 
-        return symbol.ToString();
+        return GetSymbolText(symbol);
     }
 
+    static string GetSymbolText(char symbol) =>
+        symbol switch
+        {
+            'k' => "k",
+            'M' => "M",
+            'G' => "G",
+            'T' => "T",
+            'P' => "P",
+            'E' => "E",
+            'Z' => "Z",
+            'Y' => "Y",
+            'm' => "m",
+            'μ' => "μ",
+            'n' => "n",
+            'p' => "p",
+            'f' => "f",
+            'a' => "a",
+            'z' => "z",
+            'y' => "y",
+            _ => symbol.ToString()
+        };
+
     /// <summary>
     /// Check if a Metric representation is out of the valid range.
     /// </summary>

tests/Humanizer.Tests/MetricNumeralTests.cs

@@ -20,6 +20,13 @@ public class MetricNumeralTests
     public void FromMetric(double expected, string input) =>
         Assert.Equal(expected, input.FromMetric());
 
+    [Theory]
+    [InlineData(1E6, "1 mega")]
+    [InlineData(1E-18, "1a")]
+    [InlineData(1E-18, "1 a")]
+    public void FromMetricDistinguishesNamesFromSymbolSuffixes(double expected, string input) =>
+        Assert.Equal(expected, input.FromMetric());
+
     [Theory]
     [InlineData("")]
     [InlineData(" ")]