Floats from bytes in Motoko

Developers Language Support
1

I am currently trying to convert a byte array to a Float (16, 32 and 64 bit) without success. I understand the binary structure of a float but not sure how to translate it in motoko.
I don’t see any methods in the Float base module that would help and I’m not even sure where to look. Doesn’t seem like bitshifting out the components will help either but I could be missing something.
Any help would be great

2

I think I figured it out using bitshifting, needed to approach it from a different angle


    public func decodeFloat(bytes: [Nat8]) : ?Float {
        var bits: Nat64 = Binary.BigEndian.toNat64(bytes);
        let (exponentBitLength: Nat64, mantissaBitLength: Nat64) = switch(bytes.size()) {
            case (2) {
                (5: Nat64, 10: Nat64);
            };
            case (4) {
                (8: Nat64, 23: Nat64);
            };
            case (8) {
                (11: Nat64, 52: Nat64);
            };
            case (a) return null; 
        };
        // Bitshift to get mantissa, exponent and sign bits
        let mantissaBits: Nat64 = bits & (Nat64.pow(2, mantissaBitLength) - 1);
        let exponentBits: Nat64 = (bits >> mantissaBitLength) & (Nat64.pow(2, exponentBitLength) - 1);
        let signBits: Nat64 = (bits >> (mantissaBitLength + exponentBitLength)) & 0x01;

        // Convert bits into numbers
        let e: Int64 = Int64.pow(2, Int64.fromNat64(exponentBits) - ((Int64.fromNat64(Nat64.pow(2, exponentBitLength) / 2)) - 1));
        let maxOffsetInverse: Float = Float.pow(2, Float.fromInt64(Int64.fromNat64(mantissaBitLength)) * -1);
        let m: Float = 1.0 + (Float.fromInt64(Int64.fromNat64(mantissaBits)) * maxOffsetInverse);

        var floatValue: Float = Float.fromInt64(e) * m;

        // Make negative if sign bit is 1
        if (signBits == 1) {
            floatValue := Float.mul(floatValue, -1.0);
        };
        
        ?floatValue;
    }

BUT im still having issues with things like INFINITY and NaN, which dont seem to exist in Motoko.
Anyone have thoughts on those? not sure if it matters

3

Curious why you cannot pass Float directly as argument?

4

He’s building CBOR and Candid Interpreters for Motoko so we can use http_outgoing_request from a canister to call services on the IC use t-ECDSA signatures. :grimacing:

1 Like
5

Okay, then you can use from_candid(blob) : ?Float. Append the blob with prefix DIDL\00\01\72 to indicate this is a float64 value.

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6

Is there a list of those prefixes somewhere?

7

Yes, it’s in the Candid spec: candid/Candid.md at master · dfinity/candid · GitHub

8

I wonder how hard it would be to expose that float parsing functionality to Motoko itself

9

Generally the RTS can do a lot of stuff (it’s written in Rust). But it is a delicate game to only include things that are relevant for the broad community, as bloat will weigh heavy on every installed canister.

10

Because I was implementing byte encoding libraries, I went ahead and made a library for number encoding/extensions for Nat, Int and Float

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