Add a built-in function to reverse arrays and strings

[tsandall]

OPA does not currently include a built-in function for reversing strings and arrays. While it's possible to write a comprehension that does this, it's a bit cumbersome. We can add two simple built-in functions for reversing strings and arrays:

strings.reverse(str) => str
array.reverse(arr) => arr

[nityanandagohain]

Hey, can I pick this up ?

[srenatus]

Just a thought, the strings implementation should work on runes, not individual characters, to avoid unicode troubles we've had in other built-ins.

[srenatus]

@nityanandagohain of course! Thank you! Let us know if you need some sort of assistance.

[nityanandagohain]

I tried to make the changes locally but I am not able to build and test on M1 machine, can I get any help

➜  opa git:(main) ✗ make build
Docker not installed or not running. Skipping OPA-WASM library build.
CGO_ENABLED=1 GOFLAGS="-buildmode=exe" go generate
CGO_ENABLED=1 GOFLAGS="-buildmode=exe" go build -tags=opa_wasm -o opa_darwin_arm64 -ldflags " -X github.com/open-policy-agent/opa/version.Version=0.32.0-dev -X github.com/open-policy-agent/opa/version.Vcs=6e4377af-dirty -X github.com/open-policy-agent/opa/version.Timestamp=2021-08-22T14:01:07Z -X github.com/open-policy-agent/opa/version.Hostname=Nityanandas-MacBook-Pro.local"
# github.com/bytecodealliance/wasmtime-go
ld: library not found for -lwasmtime
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [go-build] Error 2

I did try to make it work bytecodealliance/wasmtime-go#53 (comment) , but wasn't successful

With docker the image isn't available for arm64

➜  opa git:(main) ✗ make build
WARNING: The requested image's platform (linux/amd64) does not match the detected host platform (linux/arm64/v8) and no specific platform was requested
clang++-12  -std=c++17 -MD -MP -nodefaultlibs --target=wasm32-unknown-unknown-wasm -fno-exceptions -fno-rtti -I src/lib -I src/libc++ -I /usr/lib/llvm-12/include/c++/v1 -I /usr/lib/llvm-12/lib/clang/12.0.0/include -I src/re2 -D_LIBCPP_HAS_NO_THREADS -D_LIBCPP_HAS_NO_LIBRARY_ALIGNED_ALLOCATION -O3   src/libc++/mutex.cc   -o src/libc++/mutex
clang: error: unable to execute command: Executable "wasm-ld" doesn't exist!
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [<builtin>: src/libc++/mutex] Error 1
make[1]: *** [build] Error 2
make: *** [wasm-lib-build] Error 2

[srenatus]

Can you try make build WASM_ENABLED=0? It's not relevant for this I suppose. At least not at first, we can figure this out in case you also want to work on the wasm implementation.

[srenatus]

🏓 Did you get unstuck? Let me know if you need more assistance here! 😃

[olamiko]

@srenatus, is this available?

[srenatus]

@olamiko we haven't heard back, so I suppose it is available again. Go for it! Let me know if you could use any kind of assistance.

[kristiansvalland]

Hi! Is this available, @srenatus? I can continue where @olamiko left off if possible.

[anderseknert]

Go right ahead @kristiansvalland 👍 We'll find another issue for @olamiko to work on when he returns.