Zstandard decompressor

build.zig

@@ -113,8 +113,11 @@ pub fn build(b: *std.Build) !void {
                 ".gz",
                 ".z.0",
                 ".z.9",
+                ".zstd.3",
+                ".zstd.19",
                 "rfc1951.txt",
                 "rfc1952.txt",
+                "rfc8478.txt",
                 // exclude files from lib/std/compress/deflate/testdata
                 ".expect",
                 ".expect-noinput",

lib/std/RingBuffer.zig

@@ -0,0 +1,136 @@
+//! This ring buffer stores read and write indices while being able to utilise
+//! the full backing slice by incrementing the indices modulo twice the slice's
+//! length and reducing indices modulo the slice's length on slice access. This
+//! means that whether the ring buffer if full or empty can be distinguished by
+//! looking at the difference between the read and write indices without adding
+//! an extra boolean flag or having to reserve a slot in the buffer.
+//!
+//! This ring buffer has not been implemented with thread safety in mind, and
+//! therefore should not be assumed to be suitable for use cases involving
+//! separate reader and writer threads.
+
+const Allocator = @import("std").mem.Allocator;
+const assert = @import("std").debug.assert;
+
+const RingBuffer = @This();
+
+data: []u8,
+read_index: usize,
+write_index: usize,
+
+pub const Error = error{Full};
+
+/// Allocate a new `RingBuffer`; `deinit()` should be called to free the buffer.
+pub fn init(allocator: Allocator, capacity: usize) Allocator.Error!RingBuffer {
+    const bytes = try allocator.alloc(u8, capacity);
+    return RingBuffer{
+        .data = bytes,
+        .write_index = 0,
+        .read_index = 0,
+    };
+}
+
+/// Free the data backing a `RingBuffer`; must be passed the same `Allocator` as
+/// `init()`.
+pub fn deinit(self: *RingBuffer, allocator: Allocator) void {
+    allocator.free(self.data);
+    self.* = undefined;
+}
+
+/// Returns `index` modulo the length of the backing slice.
+pub fn mask(self: RingBuffer, index: usize) usize {
+    return index % self.data.len;
+}
+
+/// Returns `index` modulo twice the length of the backing slice.
+pub fn mask2(self: RingBuffer, index: usize) usize {
+    return index % (2 * self.data.len);
+}
+
+/// Write `byte` into the ring buffer. Returns `error.Full` if the ring
+/// buffer is full.
+pub fn write(self: *RingBuffer, byte: u8) Error!void {
+    if (self.isFull()) return error.Full;
+    self.writeAssumeCapacity(byte);
+}
+
+/// Write `byte` into the ring buffer. If the ring buffer is full, the
+/// oldest byte is overwritten.
+pub fn writeAssumeCapacity(self: *RingBuffer, byte: u8) void {
+    self.data[self.mask(self.write_index)] = byte;
+    self.write_index = self.mask2(self.write_index + 1);
+}
+
+/// Write `bytes` into the ring buffer. Returns `error.Full` if the ring
+/// buffer does not have enough space, without writing any data.
+pub fn writeSlice(self: *RingBuffer, bytes: []const u8) Error!void {
+    if (self.len() + bytes.len > self.data.len) return error.Full;
+    self.writeSliceAssumeCapacity(bytes);
+}
+
+/// Write `bytes` into the ring buffer. If there is not enough space, older
+/// bytes will be overwritten.
+pub fn writeSliceAssumeCapacity(self: *RingBuffer, bytes: []const u8) void {
+    for (bytes) |b| self.writeAssumeCapacity(b);
+}
+
+/// Consume a byte from the ring buffer and return it. Returns `null` if the
+/// ring buffer is empty.
+pub fn read(self: *RingBuffer) ?u8 {
+    if (self.isEmpty()) return null;
+    return self.readAssumeLength();
+}
+
+/// Consume a byte from the ring buffer and return it; asserts that the buffer
+/// is not empty.
+pub fn readAssumeLength(self: *RingBuffer) u8 {
+    assert(!self.isEmpty());
+    const byte = self.data[self.mask(self.read_index)];
+    self.read_index = self.mask2(self.read_index + 1);
+    return byte;
+}
+
+/// Returns `true` if the ring buffer is empty and `false` otherwise.
+pub fn isEmpty(self: RingBuffer) bool {
+    return self.write_index == self.read_index;
+}
+
+/// Returns `true` if the ring buffer is full and `false` otherwise.
+pub fn isFull(self: RingBuffer) bool {
+    return self.mask2(self.write_index + self.data.len) == self.read_index;
+}
+
+/// Returns the length
+pub fn len(self: RingBuffer) usize {
+    const wrap_offset = 2 * self.data.len * @boolToInt(self.write_index < self.read_index);
+    const adjusted_write_index = self.write_index + wrap_offset;
+    return adjusted_write_index - self.read_index;
+}
+
+/// A `Slice` represents a region of a ring buffer. The region is split into two
+/// sections as the ring buffer data will not be contiguous if the desired
+/// region wraps to the start of the backing slice.
+pub const Slice = struct {
+    first: []u8,
+    second: []u8,
+};
+
+/// Returns a `Slice` for the region of the ring buffer starting at
+/// `self.mask(start_unmasked)` with the specified length.
+pub fn sliceAt(self: RingBuffer, start_unmasked: usize, length: usize) Slice {
+    assert(length <= self.data.len);
+    const slice1_start = self.mask(start_unmasked);
+    const slice1_end = @min(self.data.len, slice1_start + length);
+    const slice1 = self.data[slice1_start..slice1_end];
+    const slice2 = self.data[0 .. length - slice1.len];
+    return Slice{
+        .first = slice1,
+        .second = slice2,
+    };
+}
+
+/// Returns a `Slice` for the last `length` bytes written to the ring buffer.
+/// Does not check that any bytes have been written into the region.
+pub fn sliceLast(self: RingBuffer, length: usize) Slice {
+    return self.sliceAt(self.write_index + self.data.len - length, length);
+}

lib/std/compress.zig

@@ -6,6 +6,7 @@ pub const lzma = @import("compress/lzma.zig");
 pub const lzma2 = @import("compress/lzma2.zig");
 pub const xz = @import("compress/xz.zig");
 pub const zlib = @import("compress/zlib.zig");
+pub const zstd = @import("compress/zstandard.zig");
 
 pub fn HashedReader(
     comptime ReaderType: anytype,
@@ -44,4 +45,5 @@ test {
     _ = lzma2;
     _ = xz;
     _ = zlib;
+    _ = zstd;
 }