tail: Support skip from start mode

This commit adds the possibility to skip the first NUM lines of a
file by calling `tail -n +NUM filename`.

Userland/Utilities/tail.cpp

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2024, Fabian Dellwing <fabian@dellwing.net>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -20,27 +21,39 @@ static ErrorOr<void> tail_from_pos(Core::File& file, off_t startline)
     return {};
 }
 
-static ErrorOr<off_t> find_seek_pos(Core::File& file, int wanted_lines)
+static ErrorOr<off_t> find_seek_pos(Core::File& file, int wanted_lines, bool start_from_end)
 {
-    // Rather than reading the whole file, start at the end and work backwards,
-    // stopping when we've found the number of lines we want.
-    off_t pos = TRY(file.seek(0, SeekMode::FromEndPosition));
-
-    off_t end = pos;
     int lines = 0;
 
-    for (; pos >= 1; pos--) {
-        TRY(file.seek(pos - 1, SeekMode::SetPosition));
+    if (start_from_end) {
+        off_t pos = TRY(file.seek(0, SeekMode::FromEndPosition));
+        off_t end = pos;
+        for (; pos >= 1; pos--) {
+            TRY(file.seek(pos - 1, SeekMode::SetPosition));
 
+            auto ch = TRY(file.read_value<u8>());
+            if (ch == '\n' && (end - pos) > 0) {
+                lines++;
+                if (lines == wanted_lines)
+                    break;
+            }
+        }
+        return pos;
+    }
+
+    off_t file_size = TRY(file.size());
+    off_t pos = 0;
+
+    for (; pos < file_size; pos++) {
         auto ch = TRY(file.read_value<u8>());
-        if (ch == '\n' && (end - pos) > 0) {
+        if (ch == '\n') {
             lines++;
             if (lines == wanted_lines)
                 break;
         }
     }
 
-    return pos;
+    return pos + 1;
 }
 
 ErrorOr<int> serenity_main(Main::Arguments arguments)
@@ -49,12 +62,35 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
 
     bool follow = false;
     size_t wanted_line_count = DEFAULT_LINE_COUNT;
+    bool start_from_end = true;
     StringView file;
 
     Core::ArgsParser args_parser;
     args_parser.set_general_help("Print the end ('tail') of a file.");
     args_parser.add_option(follow, "Output data as it is written to the file", "follow", 'f');
-    args_parser.add_option(wanted_line_count, "Fetch the specified number of lines", "lines", 'n', "number");
+    args_parser.add_option(Core::ArgsParser::Option {
+        .argument_mode = Core::ArgsParser::OptionArgumentMode::Required,
+        .help_string = "output the last NUM lines, instead of the last 10;"
+                       " or use -n +NUM to output starting with line NUM",
+        .long_name = "lines",
+        .short_name = 'n',
+        .value_name = "[+]NUM",
+        .accept_value = [&](StringView lines) -> ErrorOr<bool> {
+            Optional<size_t> value;
+            if (lines.starts_with('+')) {
+                value = lines.substring_view(1, lines.length() - 1).to_number<size_t>();
+                start_from_end = false;
+            } else {
+                value = lines.to_number<size_t>();
+            }
+            if (!value.has_value()) {
+                warnln("Invalid number: {}", lines);
+                return false;
+            }
+            wanted_line_count = value.value();
+            return true;
+        },
+    });
     args_parser.add_positional_argument(file, "File path", "file", Core::ArgsParser::Required::No);
     args_parser.parse(arguments);
 
@@ -81,6 +117,26 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
                 continue;
             }
 
+            if (!start_from_end) {
+                if (wanted_line_count > line_count) {
+                    continue;
+                }
+                if (wanted_line_count == 0) {
+                    out("{}", StringView { bytes });
+                    continue;
+                }
+                for (size_t i = 0; i < bytes.size(); i++) {
+                    auto ch = bytes.at(i);
+                    if (ch == '\n') {
+                        line_index++;
+                    }
+                    if (line_index >= wanted_line_count)
+                        line.append(ch);
+                }
+                out("{}", line.to_byte_string().substring_view(1, line.length() - 1));
+                continue;
+            }
+
             for (size_t i = 0; i < bytes.size(); i++) {
                 auto ch = bytes.at(i);
                 line.append(ch);
@@ -98,7 +154,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
         return 0;
     }
 
-    auto pos = TRY(find_seek_pos(*f, wanted_line_count));
+    auto pos = TRY(find_seek_pos(*f, wanted_line_count, start_from_end));
     TRY(tail_from_pos(*f, pos));
 
     if (follow) {