Fixes and optimizations

Added FragManager so each frag block only gets decompressed once
Returned to C for decompression (only zstd stateless ATM)

src/frag.zig

@@ -1,7 +1,84 @@
+const std = @import("std");
+const Io = std.Io;
+
 const BlockSize = @import("inode_data/file.zig").BlockSize;
+const LookupTable = @import("lookup_table.zig");
+const Decompressor = @import("util/decompressor.zig");
+const MetadataReader = @import("util/metadata.zig");
+const OffsetFile = @import("util/offset_file.zig");
+
+const FragManager = @This();
 
 pub const FragEntry = extern struct {
     start: u64,
     size: BlockSize,
     _: u32,
 };
+
+alloc: std.mem.Allocator,
+fil: OffsetFile,
+decomp: *const Decompressor,
+block_size: u32,
+
+entries: []FragEntry,
+
+frag_cache: std.array_hash_map.Auto(u32, []u8),
+cache_mut: std.Io.Mutex = .init,
+
+pub fn init(alloc: std.mem.Allocator, io: Io, fil: OffsetFile, decomp: *const Decompressor, frag_start: u64, frag_num: u32, block_size: u32) !FragManager {
+    var buf: [8 * 1024]u8 = undefined;
+    var rdr = try fil.readerAt(io, frag_start, &buf);
+    var first_offset: u64 = undefined;
+    try rdr.interface.readSliceEndian(u64, @ptrCast(&first_offset), .little);
+
+    rdr = try fil.readerAt(io, first_offset, &buf);
+    var meta: MetadataReader = .init(alloc, &rdr.interface, decomp);
+
+    const entries = try alloc.alloc(FragEntry, frag_num);
+    errdefer alloc.free(entries);
+
+    try meta.interface.readSliceEndian(FragEntry, entries, .little);
+
+    return .{
+        .alloc = alloc,
+        .fil = fil,
+        .decomp = decomp,
+        .block_size = block_size,
+
+        .entries = entries,
+
+        .frag_cache = .empty,
+    };
+}
+pub fn deinit(self: *FragManager, io: Io) void {
+    self.cache_mut.lockUncancelable(io);
+    self.alloc.free(self.entries);
+    for (self.frag_cache.values()) |v|
+        self.alloc.free(v);
+    self.frag_cache.deinit(self.alloc);
+}
+
+pub fn get(self: *FragManager, io: Io, idx: u32) ![]u8 {
+    if (self.frag_cache.contains(idx))
+        return self.frag_cache.get(idx).?;
+
+    try self.cache_mut.lock(io);
+    defer self.cache_mut.unlock(io);
+
+    const entry = self.entries[idx];
+
+    const out = try self.alloc.alloc(u8, if (entry.size.uncompressed) entry.size.size else self.block_size);
+
+    var buf: [1024 * 1024]u8 = undefined;
+    var rdr = try self.fil.readerAt(io, entry.start, &buf);
+    if (entry.size.uncompressed) {
+        try rdr.interface.readSliceAll(out);
+    } else {
+        @branchHint(.likely);
+        try rdr.interface.fill(entry.size.size);
+        _ = try self.decomp.Decompress(self.alloc, buf[0..entry.size.size], out);
+    }
+
+    try self.frag_cache.put(self.alloc, idx, out);
+    return out;
+}