long_double

pub const long_double = struct {
    const U80 = @Int(.unsigned, 80);

    inline fn bitWidth(x: anytype) u16 {
        const T = @TypeOf(x);
        return switch (T) {
            f80, f128, c_longdouble => @typeInfo(T).float.bits,
            else => @compileError("Unsupported type: " ++ @typeName(T) ++ "\nPass a `f80`, `f128`, or `c_longdouble`."),
        };
    }

    /// Returns the sign + exponent bits of a `long double`.
    pub fn signExponent(x: anytype) u16 {
        const bit_width = bitWidth(x);
        switch (bit_width) {
            80 => {
                const bits: U80 = @bitCast(x);
                return @intCast(bits >> 64);
            },
            128 => {
                const bits: u128 = @bitCast(x);
                return @intCast(bits >> 112);
            },
            // `c_longdouble` can have <80 bits on some targets, we want to error on that
            else => @compileError(std.fmt.comptimePrint("`signExponent` supports floats of only `80` and `128` bit width, got bit width: {d}", .{bit_width})),
        }
    }

    test "signExponent" {
        try expectEqual(signExponent(@as(f80, -0.0)), 0x8000);
        try expectEqual(signExponent(@as(f128, 0.0)), 0x0000);
        try expectEqual(signExponent(@as(f128, 42.0)), 0x4004);
        try expectEqual(signExponent(nan(c_longdouble)), 0x7FFF);
    }

    /// Takes the top 16 bits of a `long double`'s mantissa.
    pub fn mantissaTop(x: anytype) u16 {
        const bit_width = bitWidth(x);
        switch (bit_width) {
            80 => {
                const bits: U80 = @bitCast(x);
                return @intCast((bits >> 48) & 0xFFFF);
            },
            128 => {
                const bits: u128 = @bitCast(x);
                return @intCast((bits >> 96) & 0xFFFF);
            },
            // `c_longdouble` can have <80 bits on some targets, we want to error on that
            else => @compileError(std.fmt.comptimePrint("`mantissaTop` supports floats of only `80` and `128` bit width, got bit width: {d}", .{bit_width})),
        }
    }

    test "mantissaTop" {
        try expectEqual(mantissaTop(@as(f80, -0.0)), 0x0000);
        try expectEqual(mantissaTop(nan(f128)), 0x8000);
        try expectEqual(mantissaTop(@as(f128, 42.0)), 0x5000);
    }
}