adding reference impementation for reading files from fat

src/reference_implementations/fat.zig

@@ -0,0 +1,117 @@
+const std = @import("std");
+
+const Errors = error{ InvalidSyntax, ReadSectors, FileNotFound };
+
+var buffer: [1024]u8 = undefined;
+
+var g_Fat: []u8 = undefined;
+
+const BootSector = extern struct {
+    BootJumpInstruction: [3]u8 align(1),
+    OemIdentifier: [8]u8 align(1),
+    BytesPerSector: u16 align(1),
+    SectorsPerCluster: u8 align(1),
+    ReservedSectors: u16 align(1),
+    FatCount: u8 align(1),
+    DirEntryCount: u16 align(1),
+    TotalSectors: u16 align(1),
+    MediaDescriptorType: u8 align(1),
+    SectorsPerFat: u16 align(1),
+    SectorsPerTrack: u16 align(1),
+    Heads: u16 align(1),
+    HiddenSectors: u32 align(1),
+    LargeSectorCount: u32 align(1),
+
+    // ebr
+    DriveNumber: u8 align(1),
+    _Reserved: u8 align(1),
+    Signature: u8 align(1),
+    VolumeId: u32 align(1),
+    VolumeLaber: [11]u8 align(1),
+    SystemId: [8]u8 align(1),
+};
+
+var g_BootSector: BootSector = undefined;
+
+const DirectoryEntry = extern struct {
+    Name: [11]u8 align(1),
+    Attributes: u8 align(1),
+    _Reserved: u8 align(1),
+    CreatedTimeTenths: u8 align(1),
+    CreatedTime: u16 align(1),
+    CreatedDate: u16 align(1),
+    AccessedDate: u16 align(1),
+    FirstClusterHigh: u16 align(1),
+    ModifiedTime: u16 align(1),
+    ModifiedDate: u16 align(1),
+    FirstClusterLow: u16 align(1),
+    Size: u32 align(1),
+};
+
+var g_RootDirectory: []DirectoryEntry = undefined;
+
+fn readStruct(reader: *std.fs.File.Reader, T: type) !T {
+    var ret: [@sizeOf(T)]u8 = undefined;
+    _ = try reader.*.read(&ret);
+    return @bitCast(ret);
+}
+
+fn readBootSector(disk: *std.fs.File) !void {
+    var reader = disk.*.reader(&buffer);
+    g_BootSector = try readStruct(&reader, BootSector);
+}
+
+fn readSectors(disk: *std.fs.File, lba: u32, count: u32, bufferOut: *[]anyopaque) !void {
+    const dest: []u8 = try std.heap.smp_allocator.alloc(u8, g_BootSector.BytesPerSector * count);
+    defer std.heap.smp_allocator.free(dest);
+    var stream = disk.readerStreaming(&buffer);
+    try stream.seekTo(lba * g_BootSector.BytesPerSector);
+    if (try stream.readStreaming(dest) != g_BootSector.BytesPerSector * count) return Errors.ReadSectors;
+    @memcpy(@as(*[]u8, @ptrCast(bufferOut)).*, dest);
+}
+
+fn readFat(disk: *std.fs.File) !void {
+    g_Fat = try std.heap.smp_allocator.alloc(u8, g_BootSector.SectorsPerFat * g_BootSector.BytesPerSector);
+    errdefer std.heap.smp_allocator.free(g_Fat);
+    try readSectors(disk, g_BootSector.ReservedSectors, g_BootSector.SectorsPerFat, &g_Fat);
+}
+
+fn readRootDirectory(disk: std.fs.File) !void {
+    const lba: u32 = g_BootSector.ReservedSectors + g_BootSector.SectorsPerFat + g_BootSector.FatCount;
+    const size: u32 = @sizeOf(DirectoryEntry) * g_BootSector.DirEntryCount;
+    const sectors: u32 = (size / g_BootSector.BytesPerSector);
+    if (@rem(size, g_BootSector.BytesPerSector > 0))
+        sectors += 1;
+
+    g_RootDirectory = try std.heap.smp_allocator.alloc(DirectoryEntry, sectors * g_BootSector.BytesPerSector);
+    errdefer std.heap.smp_allocator.free(g_RootDirectory);
+    try readSectors(disk, lba, sectors, &g_RootDirectory);
+}
+
+fn findFile(name: []const u8) !*DirectoryEntry {
+    for (g_RootDirectory, 0..) |Entry, i|
+        if (std.mem.eql(u8, Entry.Name, name))
+            return &g_RootDirectory[i];
+    return Errors.FileNotFound;
+}
+
+pub fn main() !void {
+    if (std.os.argv.len < 3) {
+        std.log.err("syntax: {s} <disk image> <file name>", .{std.os.argv[0]});
+        return Errors.InvalidSyntax;
+    }
+
+    var disk = try std.fs.cwd().openFileZ(std.os.argv[1], .{ .mode = .read_only });
+
+    _ = &disk;
+    try readBootSector(&disk);
+
+    try readFat(&disk);
+    defer std.heap.smp_allocator.free(g_Fat);
+
+    try readRootDirectory(disk);
+    defer std.heap.smp_allocator.free(g_RootDirectory);
+
+    const fileEntry = try findFile(std.os.argv[2]);
+    _ = fileEntry;
+}