fix zig-zag decoding #51
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To make bitwise operations platform independent the [docs](https://dart.dev/guides/language/numbers#what-should-you-do) recommend coercing to signed 32 bit integers. Fixes greensopinion#7
| @@ -24,7 +24,7 @@ _Command _decodeCommand(int command) { | |||
| int _decodeCommandLength(int command) => command >> 3; | |||
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| @pragma('vm:prefer-inline') | |||
| int _decodeZigZag(int value) => (value >> 1) ^ -(value & 1); | |||
| int _decodeZigZag(int value) => ((value >> 1) ^ -(value & 1)).toSigned(32); | |||
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Thanks for the PR, this looks fantastic.
The vector tile spec 2.1 indicates:
A geometry is encoded as a sequence of 32 bit unsigned integers
Any concern with using signed 32-bit integers? Wondering if it might affect the results.
Also curious as to why toSigned(32) is applied after the bitwise operations, rather than before. Should we be doing something like this instead?
int _decodeZigZag(int value) {
final value32 = value.toSigned(32)
return (value32 >> 1) ^ -(value32 & 1);
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The decoded parameter values fit into signed 32-bit integers:
Parameter values greater than pow(2,31) - 1 or less than -1 * (pow(2,31) - 1) are not supported.
The results of bitwise operations are different between native and web because of the different underlying representations. I'm not sure how this works exactly, but for 32-bit chunks we can get consistent results everywhere by interpreting the results of bit-wise operations as signed 32-bit numbers.
For bit manipulation, consider explicitly operating on 32-bit chunks, which are consistent on all platforms. To force a signed interpretation of a 32-bit chunk, use int.toSigned(32).
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Screenshot with this and #52 in a web browser 🎉
Still appears to work fine with this and [vector_map_tiles](https://github.com/greensopinion/flutter-vector-map-tiles) (mac application tested)
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To make bitwise operations platform independent the
docs
recommend coercing to signed 32 bit integers.
Fixes #7