Jungfraujoch/reader/HDF5ImageSource.h

// SPDX-FileCopyrightText: 2026 Filip Leonarski, Paul Scherrer Institute <filip.leonarski@psi.ch>
// SPDX-License-Identifier: GPL-3.0-only

#pragma once

#include <cstdint>
#include <map>
#include <memory>
#include <optional>
#include <vector>

#include "HDF5ImageLocator.h"
#include "../common/CompressedImage.h"

// Raw-pixel side of the reader. Turns a global image number into a CompressedImage, using
// HDF5ImageLocator to find the file (with its open-file cache). This is the part whose "links
// to files stay" constant: switching which master the per-image metadata is read from never
// touches it. Caller must hold the global hdf5_mutex (HDF5 is not thread-safe).
class HDF5ImageSource {
public:
    // Plain positional-read handle on a data file, opened alongside the HDF5 one. Owns the handle.
    class RawFile {
    public:
        explicit RawFile(const std::string &path);
        ~RawFile();
        RawFile(const RawFile &) = delete;
        RawFile &operator=(const RawFile &) = delete;

        bool IsOpen() const { return handle_ != -1; }
        // Read `size` bytes from byte `address`. Positional and stateless, so any number of threads
        // may call it on the same handle at once. Throws on a short read.
        void ReadAt(void *dst, size_t size, uint64_t address) const;

    private:
        intptr_t handle_ = -1;   // a file descriptor on POSIX, a HANDLE on Windows
    };

    // Where the bytes of one image are, and what they decode to. Everything needed to read an image
    // without calling HDF5 again.
    struct DirectChunk {
        const RawFile *file = nullptr;
        uint64_t address = 0;
        uint32_t size = 0;
        hsize_t width = 0;
        hsize_t height = 0;
        CompressedImageMode mode{};
        CompressionAlgorithm algorithm = CompressionAlgorithm::NO_COMPRESSION;
    };

    void Configure(HDF5ImageLocator::Layout layout);
    void Clear();

    // Where image `global` physically lives. Also used by the metadata source to find the data
    // file that holds a legacy/VDS image's per-image metadata.
    HDF5ImageLocator::Location Resolve(int64_t global) const;

    // Read the pixels at a resolved location into a CompressedImage backed by `buffer`.
    CompressedImage ReadImageAt(std::vector<uint8_t> &buffer, const HDF5ImageLocator::Location &loc) const;

    // Ask HDF5 where image `loc` is in the file rather than asking it for the image. This is a
    // lookup in the chunk index and nothing else - no read - so the mutex is held for a fraction of
    // what an actual read costs, and the read itself then happens on any number of threads at once
    // through ReadDirect(). Caller must hold hdf5_mutex.
    //
    // Empty when this file cannot be served that way: one chunk per image is what makes an image a
    // single contiguous run of bytes, and a chunk that has never been written has no address at all.
    // The caller falls back to ReadImageAt() then.
    std::optional<DirectChunk> PrepareDirectRead(const HDF5ImageLocator::Location &loc) const;

    // Read what PrepareDirectRead() found. Touches no HDF5 and no shared state, so it needs no
    // mutex; this is the whole point of the two-step split.
    static CompressedImage ReadDirect(std::vector<uint8_t> &buffer, const DirectChunk &chunk);

    std::vector<HDF5DataSourceMessage> GetSourceMapping(uint64_t first_image,
                                                        std::optional<uint64_t> image_count,
                                                        uint64_t total_images,
                                                        uint64_t stride = 1) const;

private:
    HDF5ImageLocator locator_;

    // /entry/data/data and everything asked of it here - its rank and dimensions, its element type,
    // its chunking, its compression - are properties of the file, identical for every image in it.
    // They used to be looked up again for each image: four HDF5 object opens per frame, inside the
    // global hdf5_mutex that every worker thread queues on. Resolve them once per file instead.
    //
    // The entry keeps the file alive, so the pointer it is keyed by cannot be recycled underneath it
    // and the dataset handle cannot outlive the file it belongs to.
    struct OpenDataset {
        std::shared_ptr<HDF5ReadOnlyFile> file;
        std::unique_ptr<HDF5DataSet> dataset;
        std::unique_ptr<RawFile> raw;
        // HDF5 addresses count from the end of the user block, so they are file offsets only once
        // its size is added. Zero for everything this project writes, but not for every file.
        uint64_t user_block = 0;
        hsize_t width = 0;
        hsize_t height = 0;
        CompressedImageMode mode{};
        CompressionAlgorithm algorithm = CompressionAlgorithm::NO_COMPRESSION;
        bool direct_chunk = false;
    };
    mutable std::map<const HDF5ReadOnlyFile *, OpenDataset> dataset_cache_;
    const OpenDataset &GetDataset(const HDF5ImageLocator::Location &loc) const;
};