pub const ElfModule = struct {
    base_address: usize,
    dwarf: Dwarf,
    mapped_memory: []align(std.heap.page_size_min) const u8,
    external_mapped_memory: ?[]align(std.heap.page_size_min) const u8,

    pub fn deinit(self: *@This(), allocator: Allocator) void {
        self.dwarf.deinit(allocator);
        std.posix.munmap(self.mapped_memory);
        if (self.external_mapped_memory) |m| std.posix.munmap(m);
    }

    pub fn getSymbolAtAddress(self: *@This(), allocator: Allocator, address: usize) !std.debug.Symbol {
        // Translate the VA into an address into this object
        const relocated_address = address - self.base_address;
        return self.dwarf.getSymbol(allocator, relocated_address);
    }

    pub fn getDwarfInfoForAddress(self: *@This(), allocator: Allocator, address: usize) !?*Dwarf {
        _ = allocator;
        _ = address;
        return &self.dwarf;
    }

    pub const LoadError = error{
        InvalidDebugInfo,
        MissingDebugInfo,
        InvalidElfMagic,
        InvalidElfVersion,
        InvalidElfEndian,
        /// TODO: implement this and then remove this error code
        UnimplementedDwarfForeignEndian,
        /// The debug info may be valid but this implementation uses memory
        /// mapping which limits things to usize. If the target debug info is
        /// 64-bit and host is 32-bit, there may be debug info that is not
        /// supportable using this method.
        Overflow,

        PermissionDenied,
        LockedMemoryLimitExceeded,
        MemoryMappingNotSupported,
    } || Allocator.Error || std.fs.File.OpenError || OpenError;

    /// Reads debug info from an already mapped ELF file.
    ///
    /// If the required sections aren't present but a reference to external debug
    /// info is, then this this function will recurse to attempt to load the debug
    /// sections from an external file.
    pub fn load(
        gpa: Allocator,
        mapped_mem: []align(std.heap.page_size_min) const u8,
        build_id: ?[]const u8,
        expected_crc: ?u32,
        parent_sections: *Dwarf.SectionArray,
        parent_mapped_mem: ?[]align(std.heap.page_size_min) const u8,
        elf_filename: ?[]const u8,
    ) LoadError!Dwarf.ElfModule {
        if (expected_crc) |crc| if (crc != std.hash.crc.Crc32.hash(mapped_mem)) return error.InvalidDebugInfo;

        const hdr: *const elf.Ehdr = @ptrCast(&mapped_mem[0]);
        if (!mem.eql(u8, hdr.e_ident[0..4], elf.MAGIC)) return error.InvalidElfMagic;
        if (hdr.e_ident[elf.EI_VERSION] != 1) return error.InvalidElfVersion;

        const endian: std.builtin.Endian = switch (hdr.e_ident[elf.EI_DATA]) {
            elf.ELFDATA2LSB => .little,
            elf.ELFDATA2MSB => .big,
            else => return error.InvalidElfEndian,
        };
        if (endian != native_endian) return error.UnimplementedDwarfForeignEndian;

        const shoff = hdr.e_shoff;
        const str_section_off = shoff + @as(u64, hdr.e_shentsize) * @as(u64, hdr.e_shstrndx);
        const str_shdr: *const elf.Shdr = @ptrCast(@alignCast(&mapped_mem[cast(usize, str_section_off) orelse return error.Overflow]));
        const header_strings = mapped_mem[str_shdr.sh_offset..][0..str_shdr.sh_size];
        const shdrs = @as(
            [*]const elf.Shdr,
            @ptrCast(@alignCast(&mapped_mem[shoff])),
        )[0..hdr.e_shnum];

        var sections: Dwarf.SectionArray = Dwarf.null_section_array;

        // Combine section list. This takes ownership over any owned sections from the parent scope.
        for (parent_sections, &sections) |*parent, *section_elem| {
            if (parent.*) |*p| {
                section_elem.* = p.*;
                p.owned = false;
            }
        }
        errdefer for (sections) |opt_section| if (opt_section) |s| if (s.owned) gpa.free(s.data);

        var separate_debug_filename: ?[]const u8 = null;
        var separate_debug_crc: ?u32 = null;

        for (shdrs) |*shdr| {
            if (shdr.sh_type == elf.SHT_NULL or shdr.sh_type == elf.SHT_NOBITS) continue;
            const name = mem.sliceTo(header_strings[shdr.sh_name..], 0);

            if (mem.eql(u8, name, ".gnu_debuglink")) {
                const gnu_debuglink = try chopSlice(mapped_mem, shdr.sh_offset, shdr.sh_size);
                const debug_filename = mem.sliceTo(@as([*:0]const u8, @ptrCast(gnu_debuglink.ptr)), 0);
                const crc_offset = mem.alignForward(usize, debug_filename.len + 1, 4);
                const crc_bytes = gnu_debuglink[crc_offset..][0..4];
                separate_debug_crc = mem.readInt(u32, crc_bytes, native_endian);
                separate_debug_filename = debug_filename;
                continue;
            }

            var section_index: ?usize = null;
            inline for (@typeInfo(Dwarf.Section.Id).@"enum".fields, 0..) |sect, i| {
                if (mem.eql(u8, "." ++ sect.name, name)) section_index = i;
            }
            if (section_index == null) continue;
            if (sections[section_index.?] != null) continue;

            const section_bytes = try chopSlice(mapped_mem, shdr.sh_offset, shdr.sh_size);
            sections[section_index.?] = if ((shdr.sh_flags & elf.SHF_COMPRESSED) > 0) blk: {
                var section_stream = std.io.fixedBufferStream(section_bytes);
                const section_reader = section_stream.reader();
                const chdr = section_reader.readStruct(elf.Chdr) catch continue;
                if (chdr.ch_type != .ZLIB) continue;

                var zlib_stream = std.compress.zlib.decompressor(section_reader);

                const decompressed_section = try gpa.alloc(u8, chdr.ch_size);
                errdefer gpa.free(decompressed_section);

                const read = zlib_stream.reader().readAll(decompressed_section) catch continue;
                assert(read == decompressed_section.len);

                break :blk .{
                    .data = decompressed_section,
                    .virtual_address = shdr.sh_addr,
                    .owned = true,
                };
            } else .{
                .data = section_bytes,
                .virtual_address = shdr.sh_addr,
                .owned = false,
            };
        }

        const missing_debug_info =
            sections[@intFromEnum(Dwarf.Section.Id.debug_info)] == null or
            sections[@intFromEnum(Dwarf.Section.Id.debug_abbrev)] == null or
            sections[@intFromEnum(Dwarf.Section.Id.debug_str)] == null or
            sections[@intFromEnum(Dwarf.Section.Id.debug_line)] == null;

        // Attempt to load debug info from an external file
        // See: https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html
        if (missing_debug_info) {

            // Only allow one level of debug info nesting
            if (parent_mapped_mem) |_| {
                return error.MissingDebugInfo;
            }

            // $XDG_CACHE_HOME/debuginfod_client/<buildid>/debuginfo
            // This only opportunisticly tries to load from the debuginfod cache, but doesn't try to populate it.
            // One can manually run `debuginfod-find debuginfo PATH` to download the symbols
            if (build_id) |id| blk: {
                var debuginfod_dir: std.fs.Dir = switch (builtin.os.tag) {
                    .wasi, .windows => break :blk,
                    else => dir: {
                        if (std.posix.getenv("DEBUGINFOD_CACHE_PATH")) |path| {
                            break :dir std.fs.openDirAbsolute(path, .{}) catch break :blk;
                        }
                        if (std.posix.getenv("XDG_CACHE_HOME")) |cache_path| {
                            if (cache_path.len > 0) {
                                const path = std.fs.path.join(gpa, &[_][]const u8{ cache_path, "debuginfod_client" }) catch break :blk;
                                defer gpa.free(path);
                                break :dir std.fs.openDirAbsolute(path, .{}) catch break :blk;
                            }
                        }
                        if (std.posix.getenv("HOME")) |home_path| {
                            const path = std.fs.path.join(gpa, &[_][]const u8{ home_path, ".cache", "debuginfod_client" }) catch break :blk;
                            defer gpa.free(path);
                            break :dir std.fs.openDirAbsolute(path, .{}) catch break :blk;
                        }
                        break :blk;
                    },
                };
                defer debuginfod_dir.close();

                const filename = std.fmt.allocPrint(
                    gpa,
                    "{s}/debuginfo",
                    .{std.fmt.fmtSliceHexLower(id)},
                ) catch break :blk;
                defer gpa.free(filename);

                const path: Path = .{
                    .root_dir = .{ .path = null, .handle = debuginfod_dir },
                    .sub_path = filename,
                };

                return loadPath(gpa, path, null, separate_debug_crc, &sections, mapped_mem) catch break :blk;
            }

            const global_debug_directories = [_][]const u8{
                "/usr/lib/debug",
            };

            // <global debug directory>/.build-id/<2-character id prefix>/<id remainder>.debug
            if (build_id) |id| blk: {
                if (id.len < 3) break :blk;

                // Either md5 (16 bytes) or sha1 (20 bytes) are used here in practice
                const extension = ".debug";
                var id_prefix_buf: [2]u8 = undefined;
                var filename_buf: [38 + extension.len]u8 = undefined;

                _ = std.fmt.bufPrint(&id_prefix_buf, "{s}", .{std.fmt.fmtSliceHexLower(id[0..1])}) catch unreachable;
                const filename = std.fmt.bufPrint(
                    &filename_buf,
                    "{s}" ++ extension,
                    .{std.fmt.fmtSliceHexLower(id[1..])},
                ) catch break :blk;

                for (global_debug_directories) |global_directory| {
                    const path: Path = .{
                        .root_dir = std.Build.Cache.Directory.cwd(),
                        .sub_path = try std.fs.path.join(gpa, &.{
                            global_directory, ".build-id", &id_prefix_buf, filename,
                        }),
                    };
                    defer gpa.free(path.sub_path);

                    return loadPath(gpa, path, null, separate_debug_crc, &sections, mapped_mem) catch continue;
                }
            }

            // use the path from .gnu_debuglink, in the same search order as gdb
            if (separate_debug_filename) |separate_filename| blk: {
                if (elf_filename != null and mem.eql(u8, elf_filename.?, separate_filename))
                    return error.MissingDebugInfo;

                exe_dir: {
                    var exe_dir_buf: [std.fs.max_path_bytes]u8 = undefined;
                    const exe_dir_path = std.fs.selfExeDirPath(&exe_dir_buf) catch break :exe_dir;
                    var exe_dir = std.fs.openDirAbsolute(exe_dir_path, .{}) catch break :exe_dir;
                    defer exe_dir.close();

                    // <exe_dir>/<gnu_debuglink>
                    if (loadPath(
                        gpa,
                        .{
                            .root_dir = .{ .path = null, .handle = exe_dir },
                            .sub_path = separate_filename,
                        },
                        null,
                        separate_debug_crc,
                        &sections,
                        mapped_mem,
                    )) |debug_info| {
                        return debug_info;
                    } else |_| {}

                    // <exe_dir>/.debug/<gnu_debuglink>
                    const path: Path = .{
                        .root_dir = .{ .path = null, .handle = exe_dir },
                        .sub_path = try std.fs.path.join(gpa, &.{ ".debug", separate_filename }),
                    };
                    defer gpa.free(path.sub_path);

                    if (loadPath(gpa, path, null, separate_debug_crc, &sections, mapped_mem)) |debug_info| return debug_info else |_| {}
                }

                var cwd_buf: [std.fs.max_path_bytes]u8 = undefined;
                const cwd_path = std.posix.realpath(".", &cwd_buf) catch break :blk;

                // <global debug directory>/<absolute folder of current binary>/<gnu_debuglink>
                for (global_debug_directories) |global_directory| {
                    const path: Path = .{
                        .root_dir = std.Build.Cache.Directory.cwd(),
                        .sub_path = try std.fs.path.join(gpa, &.{ global_directory, cwd_path, separate_filename }),
                    };
                    defer gpa.free(path.sub_path);
                    if (loadPath(gpa, path, null, separate_debug_crc, &sections, mapped_mem)) |debug_info| return debug_info else |_| {}
                }
            }

            return error.MissingDebugInfo;
        }

        var di: Dwarf = .{
            .endian = endian,
            .sections = sections,
            .is_macho = false,
        };

        try Dwarf.open(&di, gpa);

        return .{
            .base_address = 0,
            .dwarf = di,
            .mapped_memory = parent_mapped_mem orelse mapped_mem,
            .external_mapped_memory = if (parent_mapped_mem != null) mapped_mem else null,
        };
    }

    pub fn loadPath(
        gpa: Allocator,
        elf_file_path: Path,
        build_id: ?[]const u8,
        expected_crc: ?u32,
        parent_sections: *Dwarf.SectionArray,
        parent_mapped_mem: ?[]align(std.heap.page_size_min) const u8,
    ) LoadError!Dwarf.ElfModule {
        const elf_file = elf_file_path.root_dir.handle.openFile(elf_file_path.sub_path, .{}) catch |err| switch (err) {
            error.FileNotFound => return missing(),
            else => return err,
        };
        defer elf_file.close();

        const end_pos = elf_file.getEndPos() catch return bad();
        const file_len = cast(usize, end_pos) orelse return error.Overflow;

        const mapped_mem = std.posix.mmap(
            null,
            file_len,
            std.posix.PROT.READ,
            .{ .TYPE = .SHARED },
            elf_file.handle,
            0,
        ) catch |err| switch (err) {
            error.MappingAlreadyExists => unreachable,
            else => |e| return e,
        };
        errdefer std.posix.munmap(mapped_mem);

        return load(
            gpa,
            mapped_mem,
            build_id,
            expected_crc,
            parent_sections,
            parent_mapped_mem,
            elf_file_path.sub_path,
        );
    }
}