Enable backward compatible map reads

[parsnips]

Implements a deprecated.MapGroup that applies to maps written by older parquet clients.

For example, AWS Kinesis Firehose's ParquetSerDe will write maps with the following schema fragment:

	optional group new {
		repeated group map {
			required binary key (STRING);
			optional group value {
				optional binary b (STRING);
				optional binary n (STRING);
			}
		}
	}

type Example struct {
    New  deprecated.MapGroup[string]string `parquet:"new,optional"`
}

Resolves #415

[achille-roussel]

Thanks for submitting this change @parsnips !

I would like to make a suggestion, instead of using a new struct tag, would it be possible to handle this special case with a type?

We have a deprecated package intended for this purpose, how about adding a type such as:

package deprecated

type Map[K comparable, V any] map[K]V

What do you think?

[parsnips]

An excellent suggestion, commit incoming :)

[achille-roussel]

What made you choose MapGroup rather than just Map? Is this term used in the parquet specification or in other client libraries?

[parsnips]

There's a constant in the package already named Map

Open to a good suggestion, I thought about DeprecatedMap and was too long, and then named so because of the syntax of the parquet being repeated map group ...so I just called this the map group :)

[achille-roussel]

Ah good call :) Thanks for pointing it out 👍

[achille-roussel]

This is looking great!

One thing to take into account is merging this change will move the minimum Go version required by the package to 1.18 since we are creating a dependency on a generics type.

We gave #428 opened to get this work done.

I'll leave it up to you to decide whether to make this code compatible with Go 1.17, or we can put the change on a short hold until we are ready to drop backwards compatibility (I would expect we get to this by end of year).

Thanks again for your contribution!

[parsnips]

@achille-roussel I think we're ok with waiting for a go 1.18 only support. I was planning on supporting a fork for the period of time until this change gets a release anyway... .and I'm not looking back ;)