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Splitting the data into two static ReadOnlySpans (one for keys, one for values) would probably be the most efficient, space-wise, and codegen-wise. Then just do a for loop, indexing into both spans and adding into dictionary.

Splitting the data into two static ReadOnlySpans (one for keys, one for values) would probably be the most efficient, space-wise, and codegen-wise. Then just do a for loop, indexing into both spans and adding into dictionary.

Nice idea, wonder if it's possible to hide those two spans under the hood of the source gen then and let the user to specify any type of such immutable dictionaries

Ah, I just realized that the static ReadOnlySpan trick won't work because these areints (the static ReadOnlySpan currently only works for bytes due to endianness issues with anything bigger than that). Might as well just make them arrays. The storage for these will be similarly optimal but we'll have a useless allocation and a memcpy operation into the array.

Yeah, we need#24961, if anyone wants to tackle that in the runtime and compiler. :-)

You can have just one Span of bytes with the data and do the endianness conversion yourself.

         ReadOnlySpan<byte> data = ...         var d = new Dictionary<long, char>(data.Length / (sizeof(long) + sizeof(char)));         while (!data.IsEmpty)         {              d[BinaryPrimitives.ReadInt64LittleEndian(data)] = (char)BinaryPrimitives.ReadInt16LittleEndian(data.Slice(sizeof(long)));              data = data.Slice(sizeof(long) + sizeof(char));         }

Or if you wanted to optimize the heck out of this, you can create a perfect or nearly perfect hash function for this set and use theReadOnlySpan<byte> directly without copying it into Dictionary. The lookup function would be then just:

         ReadOnlySpan<byte> data = ...         long key = ToUInt64Key(entity);         int index = (sizeof(long) + sizeof(char)) * PerfectHash(key);         if (BinaryPrimitives.ReadInt64LittleEndian(data.Slice(index)) == key) return BinaryPrimitives.ReadInt64LittleEndian(data.Slice(index + sizeof(long)));