std.compress.zstandard: add doc comments for RingBuffer

lib/std/compress/zstandard/RingBuffer.zig

@@ -1,8 +1,8 @@
 //! 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 the bit of information
-//! distinguishing whether the buffer is full or empty in an implementation utilising
-//! and extra flag is stored in difference of the indices.
+//! indices modulo the slice's length on slice access. This means that whether the ring buffer
+//! if full or empty can be distinguised by looking at the different between the read and write
+//! indices without adding an extra boolean flag or having to reserve a slot in the buffer.
 
 const assert = @import("std").debug.assert;
 
@@ -12,33 +12,45 @@ data: []u8,
 read_index: usize,
 write_index: usize,
 
+/// Returns `index` modulo the length of the backing slice.
 pub fn mask(self: RingBuffer, index: usize) usize {
     return index % self.data.len;
 }
 
+/// Returns `index` module 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) !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 bufffer. Returns `error.Full` if the ring
+/// buffer does not have enough space, without writing any data.
 pub fn writeSlice(self: *RingBuffer, bytes: []const u8) !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;
     const byte = self.data[self.mask(self.read_index)];
@@ -46,24 +58,32 @@ pub fn read(self: *RingBuffer) ?u8 {
     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 adjusted_write_index = self.write_index + @boolToInt(self.write_index < self.read_index) * 2 * self.data.len;
     return adjusted_write_index - self.read_index;
 }
 
-const Slice = struct {
+/// 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 staring 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);
@@ -76,6 +96,7 @@ pub fn sliceAt(self: RingBuffer, start_unmasked: usize, length: usize) Slice {
     };
 }
 
+/// Returns a `Slice` for the last `length` bytes written to the ring buffer.
 pub fn sliceLast(self: RingBuffer, length: usize) Slice {
     return self.sliceAt(self.write_index + self.data.len - length, length);
 }