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using System;
using System.ComponentModel;
using System.Linq.Expressions;
using System.Threading.Tasks;
namespace Xamarin.PropertyEditing
{
internal static class Numeric<T>
{
public static Func<T, T> Increment
{
get
{
SetupMath();
return IncrementValue;
}
}
public static Func<T, T> Decrement
{
get
{
SetupMath();
return DecrementValue;
}
}
private static Func<T, T> IncrementValue, DecrementValue;
private static NullableConverter NullConverter;
private static Func<T, T> GetNullableVersion (Expression shiftExpression, Type realType)
{
var v = Expression.Parameter (typeof(T));
var convert = Expression.Call (Expression.Constant (NullConverter), nameof(NullableConverter.ConvertFrom), null, Expression.Convert (v, typeof(object)));
var shift = Expression.Invoke (shiftExpression, Expression.Convert (convert, realType));
var conditional = Expression.Condition (Expression.Equal (v, Expression.Constant (null)), Expression.Default (realType), shift);
// (T v) => (T)((v == null) ? default(realType) : shiftExpression ((realType)NullableConverter.ConvertFrom ((object)v));
return Expression.Lambda<Func<T, T>> (Expression.Convert (conditional, typeof(T)), v).Compile();
}
private static void SetupMath ()
{
if (IncrementValue != null)
return;
Type t = typeof(T);
bool isNullable = (t.Name == "Nullable`1");
if (isNullable) {
NullConverter = new NullableConverter (typeof(T));
t = Nullable.GetUnderlyingType (t);
}
Task<Func<T,T>> createIncrement = Task.Run (() => {
var value = Expression.Parameter (t);
Expression add;
if (t == typeof(byte) || t == typeof(sbyte)) {
// (t value) => (t)((int)value + 1);
add = Expression.Convert (Expression.Add (Expression.Convert (value, typeof(int)), Expression.Constant (1)), t);
} else {
var shiftby = Expression.Convert (Expression.Constant (1), t);
add = Expression.Add (value, shiftby);
}
var increment = Expression.Lambda (add, value);
if (isNullable)
return GetNullableVersion (increment, t);
return (Func<T, T>)increment.Compile();
});
Task<Func<T,T>> createDecrement = Task.Run (() => {
var shiftby = Expression.Convert (Expression.Constant (1), t);
var value = Expression.Parameter (t);
Expression sub;
if (t == typeof(byte) || t == typeof(sbyte)) {
sub = Expression.Convert (Expression.Subtract (Expression.Convert (value, typeof(int)), Expression.Constant (1)), t);
} else {
sub = Expression.Subtract (value, shiftby);
}
var decrement = Expression.Lambda (sub, value);
if (isNullable)
return GetNullableVersion (decrement, t);
return (Func<T, T>)decrement.Compile();
});
IncrementValue = createIncrement.Result;
DecrementValue = createDecrement.Result;
}
}
}
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