/********************************************************************************
 *                                                                              *
 * This file is part of IfcOpenShell.                                           *
 *                                                                              *
 * IfcOpenShell is free software: you can redistribute it and/or modify         *
 * it under the terms of the Lesser GNU General Public License as published by  *
 * the Free Software Foundation, either version 3.0 of the License, or          *
 * (at your option) any later version.                                          *
 *                                                                              *
 * IfcOpenShell is distributed in the hope that it will be useful,              *
 * but WITHOUT ANY WARRANTY; without even the implied warranty of               *
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the                 *
 * Lesser GNU General Public License for more details.                          *
 *                                                                              *
 * You should have received a copy of the Lesser GNU General Public License     *
 * along with this program. If not, see <http://www.gnu.org/licenses/>.         *
 *                                                                              *
 ********************************************************************************/

// v13 sidecar layout (matched against SidecarCache.cpp):
//
//   SidecarHeader (12 bytes)
//   uint32 num_vertex_bytes
//   uint8[num_vertex_bytes] vertex data            <-- streaming skips
//   uint32 num_indices
//   uint32[num_indices] index data                  <-- streaming skips
//   uint32 num_meshes + MeshInfo[]                  <-- streaming reads
//   uint32 num_instances + InstanceCpu[]            <-- streaming reads
//   uint32 has_coord_op + double[16] + 2× double    <-- streaming reads
//   uint32 num_elements + PackedElementInfo[]       <-- streaming reads
//   uint32 string_table_bytes + char[]              <-- streaming reads
//
// Streaming reader returns offsets to the two skipped sections so chunks
// can be range-read on demand. File handle is closed before return.

#include "WgpuStreamingLoader.h"

#include <algorithm>
#include <cstdio>
#include <cstring>

namespace {

struct SidecarHeaderRaw {
    uint32_t magic;
    uint32_t version;
    uint32_t endian;
};

template<typename T>
bool readVec(FILE* f, std::vector<T>& v) {
    uint32_t n;
    if (std::fread(&n, 4, 1, f) != 1) return false;
    v.resize(n);
    if (n > 0 && std::fread(v.data(), sizeof(T), n, f) != n) return false;
    return true;
}

std::string sidecarPath(const std::string& ifc_path) {
    std::string p = ifc_path;
    while (!p.empty() && (p.back() == '/' || p.back() == '\\')) p.pop_back();
    auto slash = p.find_last_of("/\\");
    auto dot   = p.find_last_of('.');
    std::string stem = (dot != std::string::npos &&
                        (slash == std::string::npos || dot > slash))
                           ? p.substr(0, dot)
                           : p;
    return stem + ".ifcview";
}

}  // namespace

std::optional<StreamingSidecar> readSidecarMetadataOnly(const std::string& ifc_path) {
    const std::string path = sidecarPath(ifc_path);
    FILE* f = std::fopen(path.c_str(), "rb");
    if (!f) return std::nullopt;

    auto fail = [&]() -> std::optional<StreamingSidecar> {
        std::fclose(f);
        return std::nullopt;
    };

    SidecarHeaderRaw hdr;
    if (std::fread(&hdr, sizeof(hdr), 1, f) != 1)      return fail();
    if (hdr.magic   != SIDECAR_MAGIC)                  return fail();
    if (hdr.version != SIDECAR_VERSION)                return fail();
    if (hdr.endian  != SIDECAR_ENDIAN)                 return fail();

    StreamingSidecar out;
    out.file_path = path;

    // Vertex section: read count, record offset of data, seek past.
    uint32_t num_vertex_bytes = 0;
    if (std::fread(&num_vertex_bytes, 4, 1, f) != 1)   return fail();
    out.vertex_section_offset = uint64_t(std::ftell(f));
    out.vertex_total_bytes    = num_vertex_bytes;
    if (std::fseek(f, long(num_vertex_bytes), SEEK_CUR) != 0) return fail();

    // Index section: same dance, in u32 units.
    uint32_t num_indices = 0;
    if (std::fread(&num_indices, 4, 1, f) != 1)        return fail();
    out.index_section_offset = uint64_t(std::ftell(f));
    out.index_total_count    = num_indices;
    if (std::fseek(f, long(num_indices) * 4, SEEK_CUR) != 0) return fail();

    // Mesh dict + instance dict — small, load into meta.
    if (!readVec(f, out.meta.meshes))    return fail();
    if (!readVec(f, out.meta.instances)) return fail();

    // v11 georef block (148 bytes total).
    if (std::fread(&out.meta.has_coordinate_operation, 4, 1, f) != 1) return fail();
    if (std::fread(out.meta.coordinate_operation_meters,
                   sizeof(double), 16, f) != 16)                      return fail();
    if (std::fread(&out.meta.project_length_to_meters,
                   sizeof(double), 1, f) != 1)                        return fail();
    if (std::fread(&out.meta.map_unit_to_meters,
                   sizeof(double), 1, f) != 1)                        return fail();

    // Element table + string table.
    if (!readVec(f, out.meta.elements))                return fail();
    uint32_t stbl_len = 0;
    if (std::fread(&stbl_len, 4, 1, f) != 1)           return fail();
    out.meta.string_table.resize(stbl_len);
    if (stbl_len > 0 &&
        std::fread(out.meta.string_table.data(), 1, stbl_len, f) != stbl_len)
        return fail();

    std::fclose(f);
    return out;
}

bool readSidecarVertexChunk(const std::string& ifc_path,
                            uint64_t vertex_section_offset,
                            uint64_t chunk_byte_offset,
                            uint64_t chunk_byte_size,
                            std::vector<uint8_t>& out_bytes) {
    if (chunk_byte_size == 0) { out_bytes.clear(); return true; }

    const std::string path = sidecarPath(ifc_path);
    FILE* f = std::fopen(path.c_str(), "rb");
    if (!f) return false;
    if (std::fseek(f, long(vertex_section_offset + chunk_byte_offset), SEEK_SET) != 0) {
        std::fclose(f);
        return false;
    }
    out_bytes.resize(size_t(chunk_byte_size));
    const size_t got = std::fread(out_bytes.data(), 1, size_t(chunk_byte_size), f);
    std::fclose(f);
    return got == size_t(chunk_byte_size);
}

bool readSidecarIndexChunk(const std::string& ifc_path,
                           uint64_t index_section_offset,
                           uint64_t chunk_first_index,
                           uint64_t chunk_index_count,
                           std::vector<uint32_t>& out_indices) {
    if (chunk_index_count == 0) { out_indices.clear(); return true; }

    const std::string path = sidecarPath(ifc_path);
    FILE* f = std::fopen(path.c_str(), "rb");
    if (!f) return false;
    const uint64_t byte_offset = index_section_offset + chunk_first_index * 4u;
    if (std::fseek(f, long(byte_offset), SEEK_SET) != 0) {
        std::fclose(f);
        return false;
    }
    out_indices.resize(size_t(chunk_index_count));
    const size_t got = std::fread(out_indices.data(), sizeof(uint32_t),
                                  size_t(chunk_index_count), f);
    std::fclose(f);
    return got == size_t(chunk_index_count);
}

// Coalesce ranges that are close in file order into single reads. The
// input order is preserved in the destination buffer; we just merge
// reads on the file side. A `max_gap_bytes` tolerance lets us swallow
// small file gaps when reading would be cheaper than seeking.
//
// SIDE EFFECT: callers must give the dst buffer in INPUT order; the
// reader scatters bytes via per-input-range dst offsets after a single
// coalesced fread. Returns false on any I/O failure.
namespace {

struct ReadPlan {
    uint64_t file_offset;   // absolute file offset
    uint64_t read_size;     // total bytes to read
    // Per input range: where its bytes land in this read, and where to
    // copy them into the destination buffer.
    struct Slice {
        uint64_t src_offset;   // offset within the read buffer
        uint64_t dst_offset;   // offset within the destination buffer
        uint64_t bytes;
    };
    std::vector<Slice> slices;
};

// Build a plan that merges adjacent file ranges into single reads.
// `ranges` are (section-relative offset, size). `max_gap_bytes` is the
// largest "wasted bytes" we'll read to bridge two ranges into one read.
std::vector<ReadPlan> buildReadPlan(
        uint64_t section_offset,
        const std::vector<std::pair<uint64_t, uint64_t>>& ranges,
        uint64_t max_gap_bytes) {
    // Sort by file offset, remembering original order so we can scatter
    // to the destination correctly.
    struct Indexed { uint64_t off, size, dst; };
    std::vector<Indexed> sorted;
    sorted.reserve(ranges.size());
    uint64_t dst_cursor = 0;
    for (const auto& [off, sz] : ranges) {
        sorted.push_back({off, sz, dst_cursor});
        dst_cursor += sz;
    }
    std::sort(sorted.begin(), sorted.end(),
              [](const Indexed& a, const Indexed& b) { return a.off < b.off; });

    std::vector<ReadPlan> plans;
    for (const auto& r : sorted) {
        if (r.size == 0) continue;
        if (!plans.empty()) {
            ReadPlan& back = plans.back();
            const uint64_t end_of_back = back.file_offset + back.read_size;
            const uint64_t r_file = section_offset + r.off;
            if (r_file >= end_of_back && r_file - end_of_back <= max_gap_bytes) {
                // Merge: extend the read to include r (plus any gap).
                const uint64_t new_size = (r_file + r.size) - back.file_offset;
                back.slices.push_back({
                    r_file - back.file_offset,  // src within read
                    r.dst,
                    r.size,
                });
                back.read_size = new_size;
                continue;
            }
        }
        ReadPlan np;
        np.file_offset = section_offset + r.off;
        np.read_size   = r.size;
        np.slices.push_back({0, r.dst, r.size});
        plans.push_back(std::move(np));
    }
    return plans;
}

}  // namespace

bool readSidecarVertexRanges(const std::string& ifc_path,
                             uint64_t vertex_section_offset,
                             const std::vector<std::pair<uint64_t, uint64_t>>& ranges,
                             std::vector<uint8_t>& out_bytes) {
    uint64_t total = 0;
    for (const auto& r : ranges) total += r.second;
    out_bytes.resize(size_t(total));
    if (total == 0) return true;

    // 64 KB max gap: on SSDs a small contiguous read is much cheaper
    // than a seek + fresh read, even if some bytes are discarded.
    auto plans = buildReadPlan(vertex_section_offset, ranges, 64 * 1024);

    const std::string path = sidecarPath(ifc_path);
    FILE* f = std::fopen(path.c_str(), "rb");
    if (!f) return false;

    std::vector<uint8_t> scratch;
    for (const auto& p : plans) {
        scratch.resize(size_t(p.read_size));
        if (std::fseek(f, long(p.file_offset), SEEK_SET) != 0) { std::fclose(f); return false; }
        if (std::fread(scratch.data(), 1, scratch.size(), f) != scratch.size()) {
            std::fclose(f); return false;
        }
        for (const auto& s : p.slices) {
            std::memcpy(out_bytes.data() + s.dst_offset,
                        scratch.data() + s.src_offset, size_t(s.bytes));
        }
    }
    std::fclose(f);
    return true;
}

bool readSidecarIndexRanges(const std::string& ifc_path,
                            uint64_t index_section_offset,
                            const std::vector<std::pair<uint64_t, uint64_t>>& ranges,
                            std::vector<uint32_t>& out_indices) {
    uint64_t total = 0;
    for (const auto& r : ranges) total += r.second;
    out_indices.resize(size_t(total));
    if (total == 0) return true;

    // Convert u32-range (first_u32, count_u32) to byte-range
    // (file_offset, byte_size). Then coalesce + read.
    std::vector<std::pair<uint64_t, uint64_t>> byte_ranges;
    byte_ranges.reserve(ranges.size());
    uint64_t out_byte_cursor = 0;
    for (const auto& [first_u32, count] : ranges) {
        // Store byte offsets relative to the index section.
        byte_ranges.emplace_back(first_u32 * 4u, count * 4u);
        out_byte_cursor += count * 4u;
    }
    auto plans = buildReadPlan(index_section_offset, byte_ranges, 64 * 1024);

    const std::string path = sidecarPath(ifc_path);
    FILE* f = std::fopen(path.c_str(), "rb");
    if (!f) return false;

    std::vector<uint8_t> scratch;
    uint8_t* out_bytes = reinterpret_cast<uint8_t*>(out_indices.data());
    for (const auto& p : plans) {
        scratch.resize(size_t(p.read_size));
        if (std::fseek(f, long(p.file_offset), SEEK_SET) != 0) { std::fclose(f); return false; }
        if (std::fread(scratch.data(), 1, scratch.size(), f) != scratch.size()) {
            std::fclose(f); return false;
        }
        for (const auto& s : p.slices) {
            std::memcpy(out_bytes + s.dst_offset,
                        scratch.data() + s.src_offset, size_t(s.bytes));
        }
    }
    std::fclose(f);
    return true;
}