nix-super/src/libexpr/pos-table.hh

#pragma once

#include <cinttypes>
#include <numeric>
#include <vector>

#include "chunked-vector.hh"
#include "pos-idx.hh"
#include "position.hh"

namespace nix {

class PosTable
{
public:
    class Origin
    {
        friend PosTable;
    private:
        // must always be invalid by default, add() replaces this with the actual value.
        // subsequent add() calls use this index as a token to quickly check whether the
        // current origins.back() can be reused or not.
        mutable uint32_t idx = std::numeric_limits<uint32_t>::max();

        // Used for searching in PosTable::[].
        explicit Origin(uint32_t idx)
            : idx(idx)
            , origin{std::monostate()}
        {
        }

    public:
        const Pos::Origin origin;

        Origin(Pos::Origin origin)
            : origin(origin)
        {
        }
    };

    struct Offset
    {
        uint32_t line, column;
    };

private:
    std::vector<Origin> origins;
    ChunkedVector<Offset, 8192> offsets;

public:
    PosTable()
        : offsets(1024)
    {
        origins.reserve(1024);
    }

    PosIdx add(const Origin & origin, uint32_t line, uint32_t column)
    {
        const auto idx = offsets.add({line, column}).second;
        if (origins.empty() || origins.back().idx != origin.idx) {
            origin.idx = idx;
            origins.push_back(origin);
        }
        return PosIdx(idx + 1);
    }

    Pos operator[](PosIdx p) const
    {
        if (p.id == 0 || p.id > offsets.size())
            return {};
        const auto idx = p.id - 1;
        /* we want the last key <= idx, so we'll take prev(first key > idx).
           this is guaranteed to never rewind origin.begin because the first
           key is always 0. */
        const auto pastOrigin = std::upper_bound(
            origins.begin(), origins.end(), Origin(idx), [](const auto & a, const auto & b) { return a.idx < b.idx; });
        const auto origin = *std::prev(pastOrigin);
        const auto offset = offsets[idx];
        return {offset.line, offset.column, origin.origin};
    }
};

}