scylladb/compound_compat.hh

/*
 * Copyright 2015 Cloudius Systems
 */

#pragma once

#include "compound.hh"

//
// This header provides adaptors between the representation used by our compound_type<>
// and representation used by Origin.
//
// For single-component keys the legacy representation is equivalent
// to the only component's serialized form. For composite keys it the following
// (See org.apache.cassandra.db.marshal.CompositeType):
//
//   <representation> ::= ( <component> )+
//   <component>      ::= <length> <value> <EOC>
//   <length>         ::= <uint16_t>
//   <EOC>            ::= <uint8_t>
//
//  <value> is component's value in serialized form. <EOC> is always 0 for partition key.
//

// Given a representation serialized using @CompoundType, provides a view on the
// representation of the same components as they would be serialized by Origin.
//
// The view is exposed in a form of a byte range. For example of use see to_legacy() function.
template <typename CompoundType>
class legacy_compound_view {
    static_assert(!CompoundType::is_prefixable, "Legacy view not defined for prefixes");
    CompoundType& _type;
    bytes_view _packed;
public:
    legacy_compound_view(CompoundType& c, bytes_view packed)
        : _type(c)
        , _packed(packed)
    { }

    class iterator : public std::iterator<std::input_iterator_tag, bytes::value_type> {
        bool _singular;
        // Offset within virtual output space of a component.
        //
        // Offset: -2             -1             0  ...  LEN-1 LEN
        // Field:  [ length MSB ] [ length LSB ] [   VALUE   ] [ EOC ]
        //
        int32_t _offset;
        typename CompoundType::iterator _i;
    public:
        struct end_tag {};

        iterator(const legacy_compound_view& v)
            : _singular(v._type.is_singular())
            , _offset(_singular ? 0 : -2)
            , _i(v._type.begin(v._packed))
        { }

        iterator(const legacy_compound_view& v, end_tag)
            : _offset(-2)
            , _i(v._type.end(v._packed))
        { }

        value_type operator*() const {
            int32_t component_size = _i->size();
            if (_offset == -2) {
                return (component_size >> 8) & 0xff;
            } else if (_offset == -1) {
                return component_size & 0xff;
            } else if (_offset < component_size) {
                return (*_i)[_offset];
            } else { // _offset == component_size
                return 0; // EOC field
            }
        }

        iterator& operator++() {
            auto component_size = (int32_t) _i->size();
            if (_offset < component_size
                // When _singular, we skip the EOC byte.
                && (!_singular || _offset != (component_size - 1)))
            {
                ++_offset;
            } else {
                ++_i;
                _offset = -2;
            }
            return *this;
        }

        bool operator==(const iterator& other) const {
            return _offset == other._offset && other._i == _i;
        }

        bool operator!=(const iterator& other) const {
            return !(*this == other);
        }
    };

    // A trichotomic comparator defined on @CompoundType representations which
    // orders them according to lexicographical ordering of their corresponding
    // legacy representations.
    //
    //   tri_comparator(t)(k1, k2)
    //
    // ...is equivalent to:
    //
    //   compare_unsigned(to_legacy(t, k1), to_legacy(t, k2))
    //
    // ...but more efficient.
    //
    struct tri_comparator {
        const CompoundType& _type;

        tri_comparator(const CompoundType& type)
            : _type(type)
        { }

        // @k1 and @k2 must be serialized using @type, which was passed to the constructor.
        int operator()(bytes_view k1, bytes_view k2) const {
            if (_type.is_singular()) {
                return compare_unsigned(*_type.begin(k1), *_type.begin(k2));
            }
            return lexicographical_tri_compare(
                _type.begin(k1), _type.end(k1),
                _type.begin(k2), _type.end(k2),
                [] (const bytes_view& c1, const bytes_view& c2) -> int {
                    if (c1.size() != c2.size()) {
                        return c1.size() < c2.size() ? -1 : 1;
                    }
                    return memcmp(c1.begin(), c2.begin(), c1.size());
                });
        }
    };

    // Equivalent to std::distance(begin(), end()), but computes faster
    size_t size() const {
        if (_type.is_singular()) {
            return _type.begin(_packed)->size();
        }
        size_t s = 0;
        for (auto&& component : _type.components(_packed)) {
            s += 2 /* length field */ + component.size() + 1 /* EOC */;
        }
        return s;
    }

    iterator begin() const {
        return iterator(*this);
    }

    iterator end() const {
        return iterator(*this, typename iterator::end_tag());
    }
};

// Converts compound_type<> representation to legacy representation
// @packed is assumed to be serialized using supplied @type.
template <typename CompoundType>
static inline
bytes to_legacy(CompoundType& type, bytes_view packed) {
    legacy_compound_view<CompoundType> lv(type, packed);
    bytes legacy_form(bytes::initialized_later(), lv.size());
    std::copy(lv.begin(), lv.end(), legacy_form.begin());
    return legacy_form;
}