Function timingSafeCompare [src]

Alias for std.crypto.timing_safe.compare

Compare two integers serialized as arrays of the same size, in constant time. Returns .lt if ab and .eq if a=b

Prototype

pub fn compare(comptime T: type, a: []const T, b: []const T, endian: Endian) Order

Parameters

T: typea: []const Tb: []const Tendian: Endian

Example

 test compare {
    const expectEqual = std.testing.expectEqual;
    var a = [_]u8{10} ** 32;
    var b = [_]u8{10} ** 32;
    try expectEqual(compare(u8, &a, &b, .big), .eq);
    try expectEqual(compare(u8, &a, &b, .little), .eq);
    a[31] = 1;
    try expectEqual(compare(u8, &a, &b, .big), .lt);
    try expectEqual(compare(u8, &a, &b, .little), .lt);
    a[0] = 20;
    try expectEqual(compare(u8, &a, &b, .big), .gt);
    try expectEqual(compare(u8, &a, &b, .little), .lt);
}

Source

  pub fn compare(comptime T: type, a: []const T, b: []const T, endian: Endian) Order {
    assert(a.len == b.len);
    const bits = switch (@typeInfo(T)) {
        .int => |cinfo| if (cinfo.signedness != .unsigned) @compileError("Elements to be compared must be unsigned") else cinfo.bits,
        else => @compileError("Elements to be compared must be integers"),
    };
    const Cext = std.meta.Int(.unsigned, bits + 1);
    var gt: T = 0;
    var eq: T = 1;
    if (endian == .little) {
        var i = a.len;
        while (i != 0) {
            i -= 1;
            const x1 = a[i];
            const x2 = b[i];
            gt |= @as(T, @truncate((@as(Cext, x2) -% @as(Cext, x1)) >> bits)) & eq;
            eq &= @as(T, @truncate((@as(Cext, (x2 ^ x1)) -% 1) >> bits));
        }
    } else {
        for (a, 0..) |x1, i| {
            const x2 = b[i];
            gt |= @as(T, @truncate((@as(Cext, x2) -% @as(Cext, x1)) >> bits)) & eq;
            eq &= @as(T, @truncate((@as(Cext, (x2 ^ x1)) -% 1) >> bits));
        }
    }
    if (gt != 0) {
        return Order.gt;
    } else if (eq != 0) {
        return Order.eq;
    }
    return Order.lt;
}  