From dccab569d2f97f694af3b2f486c64df094df97a1 Mon Sep 17 00:00:00 2001
From: willmcpherson2 <willmcpherson2@gmail.com>
Date: Sun, 15 Sep 2019 23:12:43 +1000
Subject: [PATCH] Shell: Tab completion for programs in PATH

This patch adds a function to LineEditor that takes the current shell
buffer, searches PATH for the first program that starts with that
buffer and then compares that to any other programs starting with the
buffer to remove any mismatching characters off the end. The result is
appended to the buffer.

This may be faster with a data structure but that seems overkill.
---
 Shell/LineEditor.cpp | 70 +++++++++++++++++++++++++++++++++++++++++++-
 Shell/LineEditor.h   |  4 +++
 2 files changed, 73 insertions(+), 1 deletion(-)

diff --git a/Shell/LineEditor.cpp b/Shell/LineEditor.cpp
index 5e36ca16c55..ebfaf10150c 100644
--- a/Shell/LineEditor.cpp
+++ b/Shell/LineEditor.cpp
@@ -37,6 +37,60 @@ void LineEditor::append(const String& string)
     m_cursor = m_buffer.size();
 }
 
+void LineEditor::tab_complete_first_token()
+{
+    auto input = String::copy(m_buffer);
+
+    String path = getenv("PATH");
+    if (path.is_empty())
+        return;
+    auto directories = path.split(':');
+
+    String match;
+
+    // Go through the files in PATH.
+    for (const auto& directory : directories) {
+        CDirIterator programs(directory.characters(), CDirIterator::SkipDots);
+        while (programs.has_next()) {
+            String program = programs.next_path();
+            if (!program.starts_with(input))
+                continue;
+
+            // Check that the file is an executable program.
+            struct stat program_status;
+            StringBuilder program_path;
+            program_path.append(directory.characters());
+            program_path.append('/');
+            program_path.append(program.characters());
+            int stat_error = stat(program_path.to_string().characters(), &program_status);
+            if (stat_error || !(program_status.st_mode & S_IXUSR))
+                continue;
+
+            // Set `match` to the first one that starts with `input`.
+            if (match.is_empty()) {
+                match = program;
+            } else {
+                // Remove characters from the end of `match` if they're
+                // different from another `program` starting with `input`.
+                int i = input.length();
+                while (i < match.length() && i < program.length() && match[i] == program[i])
+                    ++i;
+                match = match.substring(0, i);
+            }
+
+            if (match.length() == input.length())
+                return;
+        }
+    }
+
+    if (match.is_empty())
+        return;
+
+    // Then append `match` to the buffer, excluding the `input` part which is
+    // already in the buffer.
+    append(match.substring(input.length(), match.length() - input.length()).characters());
+}
+
 String LineEditor::get_line(const String& prompt)
 {
     fputs(prompt.characters(), stdout);
@@ -174,7 +228,21 @@ String LineEditor::get_line(const String& prompt)
             }
 
             if (ch == '\t') {
-                // FIXME: Implement tab-completion.
+                if (m_buffer.is_empty())
+                    continue;
+
+                bool is_first_token = true;
+                for (const auto& character : m_buffer) {
+                    if (isspace(character)) {
+                        is_first_token = false;
+                        break;
+                    }
+                }
+
+                // FIXME: Implement tab-completion for other tokens (paths).
+                if (is_first_token)
+                    tab_complete_first_token();
+
                 continue;
             }
 
diff --git a/Shell/LineEditor.h b/Shell/LineEditor.h
index 5f5a4344d7a..b2a5b1c6dc2 100644
--- a/Shell/LineEditor.h
+++ b/Shell/LineEditor.h
@@ -1,7 +1,10 @@
 #pragma once
 
 #include <AK/String.h>
+#include <AK/StringBuilder.h>
 #include <AK/Vector.h>
+#include <LibCore/CDirIterator.h>
+#include <sys/stat.h>
 
 class LineEditor {
 public:
@@ -16,6 +19,7 @@ public:
 private:
     void clear_line();
     void append(const String&);
+    void tab_complete_first_token();
     void vt_save_cursor();
     void vt_restore_cursor();
     void vt_clear_to_end_of_line();