ScalarDouble

const ScalarDouble = struct

File

Code

const ScalarDouble = struct {
    x1: Fe,
    x2: Fe,
    x3: Fe,

    fn fromBytes(comptime bits: usize, s_: [bits / 8]u8, endian: std.builtin.Endian) ScalarDouble {
        debug.assert(bits > 0 and bits <= 512 and bits >= Fe.saturated_bits and bits <= Fe.saturated_bits * 3);

        var s = s_;
        if (endian == .big) {
            for (s_, 0..) |x, i| s[s.len - 1 - i] = x;
        }
        var t = ScalarDouble{ .x1 = undefined, .x2 = Fe.zero, .x3 = Fe.zero };
        {
            var b: [encoded_length]u8 = @splat(0);
            const len = @min(s.len, 24);
            b[0..len].* = s[0..len].*;
            t.x1 = Fe.fromBytes(b, .little) catch unreachable;
        }
        if (s_.len >= 24) {
            var b: [encoded_length]u8 = @splat(0);
            const len = @min(s.len - 24, 24);
            b[0..len].* = s[24..][0..len].*;
            t.x2 = Fe.fromBytes(b, .little) catch unreachable;
        }
        if (s_.len >= 48) {
            var b: [encoded_length]u8 = @splat(0);
            const len = s.len - 48;
            b[0..len].* = s[48..][0..len].*;
            t.x3 = Fe.fromBytes(b, .little) catch unreachable;
        }
        return t;
    }

    fn reduce(expanded: ScalarDouble, comptime bits: usize) Scalar {
        debug.assert(bits > 0 and bits <= Fe.saturated_bits * 3 and bits <= 512);
        var fe = expanded.x1;
        if (bits >= 192) {
            const st1 = Fe.fromInt(1 << 192) catch unreachable;
            fe = fe.add(expanded.x2.mul(st1));
            if (bits >= 384) {
                const st2 = st1.sq();
                fe = fe.add(expanded.x3.mul(st2));
            }
        }
        return Scalar{ .fe = fe };
    }
}