/// <summary>
/// Because the input <see cref="IDictionary{TKey, TValue}"/> is not an <see cref="IDistinctValuedDictionary{TKey, TValue}"/>, duplicate value handling must be specified.
/// </summary>
public static Dictionary <TValue, TKey> Invert <TKey, TValue>(this IDictionary <TKey, TValue> dictionary,
DuplicateValueHandling duplicateValueHandling = DuplicateValueHandling.Error)
{
var output = new Dictionary <TValue, TKey>();
foreach (var pair in dictionary)
{
var key = pair.Value;
var value = pair.Key;
if (output.ContainsKey(key))
{
var existing = output[key];
var chosen = duplicateValueHandling.Choose(existing, value);
output[key] = chosen;
}
else
{
output.Add(key, value);
}
}
return(output);
}
public static TValue Choose <TValue>(this DuplicateValueHandling duplicateValueHandling,
TValue existing,
TValue duplicate)
{
if (duplicateValueHandling.ChooseWouldFail(existing, duplicate, out TValue chosen))
{
throw new DuplicateValueException <TValue>(existing, duplicate);
}
return(chosen);
}
/// <summary>
/// Note: assumes that <paramref name="existing"/> and <paramref name="duplicate"/> have already been determined to be duplicates.
/// This method is meant for use in an if()-block, and informs as to whether choosing a duplicate would succeed, or if an exception would be thrown based on the <see cref="DuplicateValueHandling"/> value.
/// In case of false, the output <paramref name="chosen"/> value is the default value of <typeparamref name="TValue"/>.
/// In case of true, the chosen value is either the existing (<see cref="DuplicateValueHandling.KeepFirst"/>) or duplicate (<see cref="DuplicateValueHandling.UseLast"/>) value depending on the value of <paramref name="duplicateValueHandling"/>.
/// </summary>
public static bool ChooseWouldFail <TValue>(this DuplicateValueHandling duplicateValueHandling,
TValue existing,
TValue duplicate,
out TValue chosen)
{
if (duplicateValueHandling == DuplicateValueHandling.Error)
{
chosen = default;
return(true);
}
if (duplicateValueHandling == DuplicateValueHandling.KeepFirst)
{
chosen = existing;
}
else // Use last.
{
chosen = duplicate;
}
return(false);
}
