1const std = @import("std");
 2const File = std.fs.File;
 3const Writer = std.fs.File.Writer;
 4
 5pub fn main() !void {
 6    const in = std.io.getStdIn();
 7    const out = std.io.getStdOut();
 8    Uf.init().streamCopy(in, out) catch |err| {
 9        try std.io.getStdErr().writer().print("Error {}", .{err});
10    };
11}
12
13const buf_size = 1_000_000;
14
15const Uf = struct {
16    const Self = @This();
17
18    var out_buf: [buf_size]u8 = undefined;
19    var out_index: u32 = 0;
20
21    fn init() *const Self {
22        return &Self{};
23    }
24
25    fn flush(_: *const Self, out: Writer) !void {
26        _ = try out.write(out_buf[0..out_index]);
27        out_index = 0;
28    }
29
30    fn writeChar(self: *const Self, out: Writer, c: u8) !void {
31        if (out_index >= buf_size) {
32            try self.flush(out);
33        }
34        out_buf[out_index] = c;
35        out_index += 1;
36    }
37
38    fn streamCopy(self: *const Self, in: File, out: File) !void {
39        const reader = in.reader();
40        const writer = out.writer();
41
42        var hit = false;
43        var second = false;
44        var in_buf: [buf_size]u8 = undefined;
45        while (true) {
46            const size = try reader.read(&in_buf);
47
48            for (in_buf[0..size]) |c| {
49                if (hit and c == '\n') {
50                    if (second == false) {
51                        second = true;
52                    } else {
53                        continue;
54                    }
55                } else if (hit and c != '\n') {
56                    if (second) {
57                        try self.writeChar(writer, '\n');
58                    }
59                    hit = false;
60                    second = false;
61                } else if (c == '\n') {
62                    hit = true;
63                    continue;
64                }
65
66                try self.writeChar(writer, c);
67            }
68
69            if (size != in_buf.len) {
70                try self.flush(writer);
71                return;
72            }
73        }
74    }
75};