propose adding BufferedTee to the std.io

lib/std/io.zig

@@ -411,6 +411,9 @@ pub const BufferedAtomicFile = @import("io/buffered_atomic_file.zig").BufferedAt
 
 pub const StreamSource = @import("io/stream_source.zig").StreamSource;
 
+pub const BufferedTee = @import("io/buffered_tee.zig").BufferedTee;
+pub const bufferedTee = @import("io/buffered_tee.zig").bufferedTee;
+
 pub const tty = @import("io/tty.zig");
 
 /// A Writer that doesn't write to anything.
@@ -692,4 +695,5 @@ test {
     _ = @import("io/seekable_stream.zig");
     _ = @import("io/stream_source.zig");
     _ = @import("io/test.zig");
+    _ = @import("io/buffered_tee.zig");
 }

lib/std/io/buffered_tee.zig

@@ -0,0 +1,379 @@
+const std = @import("std");
+const io = std.io;
+const assert = std.debug.assert;
+const testing = std.testing;
+
+/// BufferedTee provides reader interface to the consumer. Data read by consumer
+/// is also written to the output. Output is hold lookahead_size bytes behind
+/// consumer. Allowing consumer to put back some bytes to be read again. On flush
+/// all consumed bytes are flushed to the output.
+///
+///       input   ->   tee   ->   consumer
+///                     |
+///                  output
+///
+/// input - underlying unbuffered reader
+/// output - writer, receives data read by consumer
+/// consumer - uses provided reader interface
+///
+/// If lookahead_size is zero output always has same bytes as consumer.
+///
+pub fn BufferedTee(
+    comptime buffer_size: usize, // internal buffer size in bytes
+    comptime lookahead_size: usize, // lookahead, number of bytes to hold output behind consumer
+    comptime InputReaderType: type,
+    comptime OutputWriterType: type,
+) type {
+    comptime assert(buffer_size > lookahead_size);
+
+    return struct {
+        input: InputReaderType,
+        output: OutputWriterType,
+
+        buf: [buffer_size]u8 = undefined, // internal buffer
+        tail: usize = 0, // buffer is filled up to this position with bytes from input
+        rp: usize = 0, // reader pointer; consumer has read up to this position
+        wp: usize = 0, // writer pointer; data is sent to the output up to this position
+
+        pub const Error = InputReaderType.Error || OutputWriterType.Error;
+        pub const Reader = io.Reader(*Self, Error, read);
+
+        const Self = @This();
+
+        pub fn read(self: *Self, dest: []u8) Error!usize {
+            var dest_index: usize = 0;
+
+            while (dest_index < dest.len) {
+                const written = @min(dest.len - dest_index, self.tail - self.rp);
+                if (written == 0) {
+                    try self.preserveLookahead();
+                    // fill upper part of the buf
+                    const n = try self.input.read(self.buf[self.tail..]);
+                    if (n == 0) {
+                        // reading from the unbuffered stream returned nothing
+                        // so we have nothing left to read.
+                        return dest_index;
+                    }
+                    self.tail += n;
+                } else {
+                    @memcpy(dest[dest_index..][0..written], self.buf[self.rp..][0..written]);
+                    self.rp += written;
+                    dest_index += written;
+                    try self.flush_(lookahead_size);
+                }
+            }
+            return dest.len;
+        }
+
+        /// Move lookahead_size bytes to the buffer start.
+        fn preserveLookahead(self: *Self) !void {
+            assert(self.tail == self.rp);
+            if (lookahead_size == 0) {
+                // Flush is called on each read so wp must follow rp when lookahead_size == 0.
+                assert(self.wp == self.rp);
+                // Nothing to preserve rewind pointer to the buffer start
+                self.rp = 0;
+                self.wp = 0;
+                self.tail = 0;
+                return;
+            }
+            if (self.tail <= lookahead_size) {
+                // There is still palce in the buffer, append to buffer from tail position.
+                return;
+            }
+            try self.flush_(lookahead_size);
+            const head = self.tail - lookahead_size;
+            // Preserve head..tail at the start of the buffer.
+            std.mem.copyForwards(u8, self.buf[0..lookahead_size], self.buf[head..self.tail]);
+            self.wp -= head;
+            assert(self.wp <= lookahead_size);
+            self.rp = lookahead_size;
+            self.tail = lookahead_size;
+        }
+
+        /// Flush to the output all but lookahead size bytes.
+        fn flush_(self: *Self, lookahead: usize) !void {
+            if (self.rp <= self.wp + lookahead) return;
+            const new_wp = self.rp - lookahead;
+            try self.output.writeAll(self.buf[self.wp..new_wp]);
+            self.wp = new_wp;
+        }
+
+        /// Flush to the output all consumed bytes.
+        pub fn flush(self: *Self) !void {
+            try self.flush_(0);
+        }
+
+        /// Put back some bytes to be consumed again. Usefull when we overshoot
+        /// reading and want to return that overshoot bytes. Can return maximum
+        /// of lookahead_size number of bytes.
+        pub fn putBack(self: *Self, n: usize) void {
+            assert(n <= lookahead_size and n <= self.rp);
+            self.rp -= n;
+        }
+
+        pub fn reader(self: *Self) Reader {
+            return .{ .context = self };
+        }
+    };
+}
+
+pub fn bufferedTee(
+    comptime buffer_size: usize,
+    comptime lookahead_size: usize,
+    input: anytype,
+    output: anytype,
+) BufferedTee(
+    buffer_size,
+    lookahead_size,
+    @TypeOf(input),
+    @TypeOf(output),
+) {
+    return .{ .input = input, .output = output };
+}
+
+// Running test from std.io.BufferedReader on BufferedTee
+// It should act as BufferedReader for consumer.
+
+fn BufferedReader(comptime buffer_size: usize, comptime ReaderType: type) type {
+    return BufferedTee(buffer_size, 0, ReaderType, @TypeOf(io.null_writer));
+}
+
+fn bufferedReader(reader: anytype) BufferedReader(4096, @TypeOf(reader)) {
+    return .{
+        .input = reader,
+        .output = io.null_writer,
+    };
+}
+
+test "io.BufferedTee io.BufferedReader OneByte" {
+    const OneByteReadReader = struct {
+        str: []const u8,
+        curr: usize,
+
+        const Error = error{NoError};
+        const Self = @This();
+        const Reader = io.Reader(*Self, Error, read);
+
+        fn init(str: []const u8) Self {
+            return Self{
+                .str = str,
+                .curr = 0,
+            };
+        }
+
+        fn read(self: *Self, dest: []u8) Error!usize {
+            if (self.str.len <= self.curr or dest.len == 0)
+                return 0;
+
+            dest[0] = self.str[self.curr];
+            self.curr += 1;
+            return 1;
+        }
+
+        fn reader(self: *Self) Reader {
+            return .{ .context = self };
+        }
+    };
+
+    const str = "This is a test";
+    var one_byte_stream = OneByteReadReader.init(str);
+    var buf_reader = bufferedReader(one_byte_stream.reader());
+    const stream = buf_reader.reader();
+
+    const res = try stream.readAllAlloc(testing.allocator, str.len + 1);
+    defer testing.allocator.free(res);
+    try testing.expectEqualSlices(u8, str, res);
+}
+
+test "io.BufferedTee io.BufferedReader Block" {
+    const BlockReader = struct {
+        block: []const u8,
+        reads_allowed: usize,
+        curr_read: usize,
+
+        const Error = error{NoError};
+        const Self = @This();
+        const Reader = io.Reader(*Self, Error, read);
+
+        fn init(block: []const u8, reads_allowed: usize) Self {
+            return Self{
+                .block = block,
+                .reads_allowed = reads_allowed,
+                .curr_read = 0,
+            };
+        }
+
+        fn read(self: *Self, dest: []u8) Error!usize {
+            if (self.curr_read >= self.reads_allowed) return 0;
+            @memcpy(dest[0..self.block.len], self.block);
+
+            self.curr_read += 1;
+            return self.block.len;
+        }
+
+        fn reader(self: *Self) Reader {
+            return .{ .context = self };
+        }
+    };
+
+    const block = "0123";
+
+    // len out == block
+    {
+        var test_buf_reader: BufferedReader(4, BlockReader) = .{
+            .input = BlockReader.init(block, 2),
+            .output = io.null_writer,
+        };
+        var out_buf: [4]u8 = undefined;
+        _ = try test_buf_reader.read(&out_buf);
+        try testing.expectEqualSlices(u8, &out_buf, block);
+        _ = try test_buf_reader.read(&out_buf);
+        try testing.expectEqualSlices(u8, &out_buf, block);
+        try testing.expectEqual(try test_buf_reader.read(&out_buf), 0);
+    }
+
+    // len out < block
+    {
+        var test_buf_reader: BufferedReader(4, BlockReader) = .{
+            .input = BlockReader.init(block, 2),
+            .output = io.null_writer,
+        };
+        var out_buf: [3]u8 = undefined;
+        _ = try test_buf_reader.read(&out_buf);
+        try testing.expectEqualSlices(u8, &out_buf, "012");
+        _ = try test_buf_reader.read(&out_buf);
+        try testing.expectEqualSlices(u8, &out_buf, "301");
+        const n = try test_buf_reader.read(&out_buf);
+        try testing.expectEqualSlices(u8, out_buf[0..n], "23");
+        try testing.expectEqual(try test_buf_reader.read(&out_buf), 0);
+    }
+
+    // len out > block
+    {
+        var test_buf_reader: BufferedReader(4, BlockReader) = .{
+            .input = BlockReader.init(block, 2),
+            .output = io.null_writer,
+        };
+        var out_buf: [5]u8 = undefined;
+        _ = try test_buf_reader.read(&out_buf);
+        try testing.expectEqualSlices(u8, &out_buf, "01230");
+        const n = try test_buf_reader.read(&out_buf);
+        try testing.expectEqualSlices(u8, out_buf[0..n], "123");
+        try testing.expectEqual(try test_buf_reader.read(&out_buf), 0);
+    }
+
+    // len out == 0
+    {
+        var test_buf_reader: BufferedReader(4, BlockReader) = .{
+            .input = BlockReader.init(block, 2),
+            .output = io.null_writer,
+        };
+        var out_buf: [0]u8 = undefined;
+        _ = try test_buf_reader.read(&out_buf);
+        try testing.expectEqualSlices(u8, &out_buf, "");
+    }
+
+    // len bufreader buf > block
+    {
+        var test_buf_reader: BufferedReader(5, BlockReader) = .{
+            .input = BlockReader.init(block, 2),
+            .output = io.null_writer,
+        };
+        var out_buf: [4]u8 = undefined;
+        _ = try test_buf_reader.read(&out_buf);
+        try testing.expectEqualSlices(u8, &out_buf, block);
+        _ = try test_buf_reader.read(&out_buf);
+        try testing.expectEqualSlices(u8, &out_buf, block);
+        try testing.expectEqual(try test_buf_reader.read(&out_buf), 0);
+    }
+}
+
+test "io.BufferedTee with zero lookahead" {
+    // output has same bytes as consumer
+    const data = [_]u8{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 } ** 12;
+    var in = io.fixedBufferStream(&data);
+    var out = std.ArrayList(u8).init(testing.allocator);
+    defer out.deinit();
+
+    var bt = bufferedTee(8, 0, in.reader(), out.writer());
+
+    var buf: [16]u8 = undefined;
+    var read_len: usize = 0;
+    for (0..buf.len) |i| {
+        const n = try bt.read(buf[0..i]);
+        try testing.expectEqual(i, n);
+        read_len += i;
+        try testing.expectEqual(read_len, out.items.len);
+    }
+}
+
+test "io.BufferedTee with lookahead" {
+    // output is lookahead bytes behind consumer
+    inline for (1..8) |lookahead| {
+        const data = [_]u8{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 } ** 12;
+        var in = io.fixedBufferStream(&data);
+        var out = std.ArrayList(u8).init(testing.allocator);
+        defer out.deinit();
+
+        var bt = bufferedTee(8, lookahead, in.reader(), out.writer());
+        var buf: [16]u8 = undefined;
+
+        var read_len: usize = 0;
+        for (1..buf.len) |i| {
+            const n = try bt.read(buf[0..i]);
+            try testing.expectEqual(i, n);
+            read_len += i;
+            const out_len = if (read_len < lookahead) 0 else read_len - lookahead;
+            try testing.expectEqual(out_len, out.items.len);
+        }
+        try testing.expectEqual(read_len, out.items.len + lookahead);
+        try bt.flush();
+        try testing.expectEqual(read_len, out.items.len);
+    }
+}
+
+test "io.BufferedTee internal state" {
+    const data = [_]u8{ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 } ** 2;
+    var in = io.fixedBufferStream(&data);
+    var out = std.ArrayList(u8).init(testing.allocator);
+    defer out.deinit();
+
+    var bt = bufferedTee(8, 4, in.reader(), out.writer());
+
+    var buf: [16]u8 = undefined;
+    var n = try bt.read(buf[0..3]);
+    try testing.expectEqual(3, n);
+    try testing.expectEqualSlices(u8, data[0..3], buf[0..n]);
+    try testing.expectEqual(8, bt.tail);
+    try testing.expectEqual(3, bt.rp);
+    try testing.expectEqual(0, out.items.len);
+
+    n = try bt.read(buf[0..6]);
+    try testing.expectEqual(6, n);
+    try testing.expectEqualSlices(u8, data[3..9], buf[0..n]);
+    try testing.expectEqual(8, bt.tail);
+    try testing.expectEqual(5, bt.rp);
+    try testing.expectEqualSlices(u8, data[4..12], &bt.buf);
+    try testing.expectEqual(5, out.items.len);
+
+    n = try bt.read(buf[0..9]);
+    try testing.expectEqual(9, n);
+    try testing.expectEqualSlices(u8, data[9..18], buf[0..n]);
+    try testing.expectEqual(8, bt.tail);
+    try testing.expectEqual(6, bt.rp);
+    try testing.expectEqualSlices(u8, data[12..20], &bt.buf);
+    try testing.expectEqual(14, out.items.len);
+
+    try bt.flush();
+    try testing.expectEqual(18, out.items.len);
+
+    bt.putBack(4);
+    n = try bt.read(buf[0..4]);
+    try testing.expectEqual(4, n);
+    try testing.expectEqualSlices(u8, data[14..18], buf[0..n]);
+
+    try testing.expectEqual(18, out.items.len);
+    try bt.flush();
+    try testing.expectEqual(18, out.items.len);
+}