FixedBufferStream

pub fn FixedBufferStream(comptime Buffer: type) type {
    return struct {
        /// `Buffer` is either a `[]u8` or `[]const u8`.
        buffer: Buffer,
        pos: usize,

        pub const ReadError = error{};
        pub const WriteError = error{NoSpaceLeft};
        pub const SeekError = error{};
        pub const GetSeekPosError = error{};

        pub const Reader = io.GenericReader(*Self, ReadError, read);
        pub const Writer = io.GenericWriter(*Self, WriteError, write);

        const Self = @This();

        pub fn reader(self: *Self) Reader {
            return .{ .context = self };
        }

        pub fn writer(self: *Self) Writer {
            return .{ .context = self };
        }

        pub fn read(self: *Self, dest: []u8) ReadError!usize {
            const size = @min(dest.len, self.buffer.len - self.pos);
            const end = self.pos + size;

            @memcpy(dest[0..size], self.buffer[self.pos..end]);
            self.pos = end;

            return size;
        }

        /// If the returned number of bytes written is less than requested, the
        /// buffer is full. Returns `error.NoSpaceLeft` when no bytes would be written.
        /// Note: `error.NoSpaceLeft` matches the corresponding error from
        /// `std.fs.File.WriteError`.
        pub fn write(self: *Self, bytes: []const u8) WriteError!usize {
            if (bytes.len == 0) return 0;
            if (self.pos >= self.buffer.len) return error.NoSpaceLeft;

            const n = @min(self.buffer.len - self.pos, bytes.len);
            @memcpy(self.buffer[self.pos..][0..n], bytes[0..n]);
            self.pos += n;

            if (n == 0) return error.NoSpaceLeft;

            return n;
        }

        pub fn seekTo(self: *Self, pos: u64) SeekError!void {
            self.pos = @min(std.math.lossyCast(usize, pos), self.buffer.len);
        }

        pub fn seekBy(self: *Self, amt: i64) SeekError!void {
            if (amt < 0) {
                const abs_amt = @abs(amt);
                const abs_amt_usize = std.math.cast(usize, abs_amt) orelse std.math.maxInt(usize);
                if (abs_amt_usize > self.pos) {
                    self.pos = 0;
                } else {
                    self.pos -= abs_amt_usize;
                }
            } else {
                const amt_usize = std.math.cast(usize, amt) orelse std.math.maxInt(usize);
                const new_pos = std.math.add(usize, self.pos, amt_usize) catch std.math.maxInt(usize);
                self.pos = @min(self.buffer.len, new_pos);
            }
        }

        pub fn getEndPos(self: *Self) GetSeekPosError!u64 {
            return self.buffer.len;
        }

        pub fn getPos(self: *Self) GetSeekPosError!u64 {
            return self.pos;
        }

        pub fn getWritten(self: Self) Buffer {
            return self.buffer[0..self.pos];
        }

        pub fn reset(self: *Self) void {
            self.pos = 0;
        }
    };
}