/// <summary>
/// Applies a key-generating function to each element of a sequence and returns a sequence of
/// unique keys and their number of occurrences in the original sequence.
/// An additional argument specifies a comparer to use for testing equivalence of keys.
/// </summary>
/// <typeparam name="TSource">Type of the elements of the source sequence.</typeparam>
/// <typeparam name="TKey">Type of the projected element.</typeparam>
/// <param name="source">Source sequence.</param>
/// <param name="keySelector">Function that transforms each item of source sequence into a key to be compared against the others.</param>
/// <param name="comparer">The equality comparer to use to determine whether or not keys are equal.
/// If null, the default equality comparer for <typeparamref name="TSource"/> is used.</param>
/// <returns>A sequence of unique keys and their number of occurrences in the original sequence.</returns>
public static IEnumerable <KeyValuePair <TKey, int> > CountBy <TSource, TKey>(this IEnumerable <TSource> source, Func <TSource, TKey> keySelector, IEqualityComparer <TKey>?comparer)
{
if (source == null)
{
throw new ArgumentNullException(nameof(source));
}
if (keySelector == null)
{
throw new ArgumentNullException(nameof(keySelector));
}
return(_()); IEnumerable <KeyValuePair <TKey, int> > _()
{
List <TKey> keys;
List <int> counts;
// Avoid the temptation to inline the Loop method, which
// exists solely to separate the scope & lifetimes of the
// locals needed for the actual looping of the source &
// production of the results (that happens once at the start
// of iteration) from those needed to simply yield the
// results. It is harder to reason about the lifetimes (if the
// code is inlined) with respect to how the compiler will
// rewrite the iterator code as a state machine. For
// background, see:
// http://blog.stephencleary.com/2010/02/q-should-i-set-variables-to-null-to.html
Loop(comparer ?? EqualityComparer <TKey> .Default);
for (var i = 0; i < keys.Count; i++)
{
yield return(new KeyValuePair <TKey, int>(keys[i], counts[i]));
}
void Loop(IEqualityComparer <TKey> cmp)
{
var dic = new Collections.Dictionary <TKey, int>(cmp);
keys = new List <TKey>();
counts = new List <int>();
(bool, TKey)prevKey = default;
var index = 0;
foreach (var item in source)
{
var key = keySelector(item);
if (// key same as the previous? then re-use the index
prevKey is (true, {} pk) &&
cmp.GetHashCode(pk) == cmp.GetHashCode(key) &&
cmp.Equals(pk, key)
// otherwise try & find index of the key
|| dic.TryGetValue(key, out index))
{
counts[index]++;
}
/// <summary>
/// Applies an accumulator function over sequence element keys,
/// returning the keys along with intermediate accumulator states. An
/// additional parameter specifies the comparer to use to compare keys.
/// </summary>
/// <typeparam name="TSource">Type of the elements of the source sequence.</typeparam>
/// <typeparam name="TKey">The type of the key.</typeparam>
/// <typeparam name="TState">Type of the state.</typeparam>
/// <param name="source">The source sequence.</param>
/// <param name="keySelector">
/// A function that returns the key given an element.</param>
/// <param name="seedSelector">
/// A function to determine the initial value for the accumulator that is
/// invoked once per key encountered.</param>
/// <param name="accumulator">
/// An accumulator function invoked for each element.</param>
/// <param name="comparer">The equality comparer to use to determine
/// whether or not keys are equal. If <c>null</c>, the default equality
/// comparer for <typeparamref name="TSource"/> is used.</param>
/// <returns>
/// A sequence of keys paired with intermediate accumulator states.
/// </returns>
public static IEnumerable <KeyValuePair <TKey, TState> > ScanBy <TSource, TKey, TState>(
this IEnumerable <TSource> source,
Func <TSource, TKey> keySelector,
Func <TKey, TState> seedSelector,
Func <TState, TKey, TSource, TState> accumulator,
IEqualityComparer <TKey>?comparer)
{
if (source == null)
{
throw new ArgumentNullException(nameof(source));
}
if (keySelector == null)
{
throw new ArgumentNullException(nameof(keySelector));
}
if (seedSelector == null)
{
throw new ArgumentNullException(nameof(seedSelector));
}
if (accumulator == null)
{
throw new ArgumentNullException(nameof(accumulator));
}
return(_(comparer ?? EqualityComparer <TKey> .Default));
IEnumerable <KeyValuePair <TKey, TState> > _(IEqualityComparer <TKey> comparer)
{
var stateByKey = new Collections.Dictionary <TKey, TState>(comparer);
(bool, TKey, TState)prev = default;
foreach (var item in source)
{
var key = keySelector(item);
var state = // key same as the previous? then re-use the state
prev is (true, {} pk, {} ps) &&
comparer.GetHashCode(pk) == comparer.GetHashCode(key) &&
comparer.Equals(pk, key) ? ps
: // otherwise try & find state of the key
stateByKey.TryGetValue(key, out var ns) ? ns
: seedSelector(key);
state = accumulator(state, key, item);
stateByKey[key] = state;
yield return(new KeyValuePair <TKey, TState>(key, state));
prev = (true, key, state);
}
}
}
