Merge pull request #120 from HicServices/feature/singlepassdate

Feature/singlepassdate

CHANGELOG.md

@@ -6,6 +6,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+- Single pass non-copying date format guessing
+
 ## [1.2.6] - 2024-07-16
 
 - Throw exceptions on invalid conversion attempts

TypeGuesser/Deciders/DateTimeTypeDecider.cs

@@ -180,21 +180,32 @@ protected override object ParseImpl(ReadOnlySpan<char> value)
     /// </summary>
     public void GuessDateFormat(IEnumerable<string> samples)
     {
-        if (!AllowCultureGuessing)
-            return;
-
-        samples = samples.Where(static s => !string.IsNullOrWhiteSpace(s)).ToList();
+        var total = 0;
+        var simple = 0;
+        var m = 0;
+        var d = 0;
 
-        //if they are all valid anyway
-        if (samples.All(s => DateTime.TryParse(s, Culture, DateTimeStyles.None, out _)))
+        if (!AllowCultureGuessing)
             return;
 
-        _dateFormatToUse = DateFormatsDM;
-        var countDm = samples.Count(s => TryBruteParse(s, out _));
-        _dateFormatToUse = DateFormatsMD;
-        var countMd = samples.Count(s => TryBruteParse(s, out _));
+        foreach (var sSample in samples.Where(static sSample => !string.IsNullOrWhiteSpace(sSample)))
+        {
+            var sample = sSample.AsSpan();
+            total++;
+            if (DateTime.TryParse(sample, Culture, DateTimeStyles.None, out _))
+                simple++;
+            else
+            {
+                _dateFormatToUse = DateFormatsDM;
+                if (TryBruteParse(sample, out _))
+                    d++;
+                _dateFormatToUse = DateFormatsMD;
+                if (TryBruteParse(sample, out _))
+                    m++;
+            }
+        }
 
-        if (countDm >= countMd)
+        if (simple < total && d > m)
             _dateFormatToUse = DateFormatsDM;
     }