LibLine: Complete only common prefixes, and tweak suggestion logic

On the first tab, only complete the token without suggesting extra
stuff, and on further tabs, show suggestions.
Except in one case where the completion is all in the buffer, then
we show the suggestions, but do no completion.

1 files changed, 84 insertions, 13 deletions

diff --git a/Libraries/LibLine/Editor.cpp b/Libraries/LibLine/Editor.cpp
index 71e5f0a79d..3958bec31d 100644
--- a/Libraries/LibLine/Editor.cpp
+++ b/Libraries/LibLine/Editor.cpp
@@ -290,12 +290,38 @@ String Editor::get_line(const String& prompt)
                 String token = is_empty_token ? String() : String(&m_buffer[token_start], m_cursor - token_start);
 
                 // ask for completions only on the first tab
+                // and scan for the largest common prefix to display
                 // further tabs simply show the cached completions
                 if (m_times_tab_pressed == 1) {
                     if (is_first_token)
                         m_suggestions = on_tab_complete_first_token(token);
                     else
                         m_suggestions = on_tab_complete_other_token(token);
+                    size_t common_suggestion_prefix { 0 };
+                    if (m_suggestions.size() == 1) {
+                        m_largest_common_suggestion_prefix_length = m_suggestions[0].length();
+                    } else if (m_suggestions.size()) {
+                        char last_valid_suggestion_char;
+                        for (;; ++common_suggestion_prefix) {
+                            if (m_suggestions[0].length() <= common_suggestion_prefix)
+                                goto no_more_commons;
+
+                            last_valid_suggestion_char = m_suggestions[0][common_suggestion_prefix];
+
+                            for (const auto& suggestion : m_suggestions) {
+                                if (suggestion.length() < common_suggestion_prefix || suggestion[common_suggestion_prefix] != last_valid_suggestion_char) {
+                                    goto no_more_commons;
+                                }
+                            }
+                        }
+                    no_more_commons:;
+                        m_largest_common_suggestion_prefix_length = common_suggestion_prefix;
+                    } else {
+                        m_largest_common_suggestion_prefix_length = 0;
+                        // there are no suggestions, beep~
+                        putchar('\a');
+                        fflush(stdout);
+                    }
                     m_prompt_lines_at_suggestion_initiation = num_lines();
                 }
 
@@ -314,19 +340,57 @@ String Editor::get_line(const String& prompt)
 
                 auto current_suggestion_index = m_next_suggestion_index;
                 if (m_next_suggestion_index < m_suggestions.size()) {
+                    auto can_complete = m_next_suggestion_invariant_offset < m_largest_common_suggestion_prefix_length;
                     if (!m_last_shown_suggestion.is_null()) {
-                        auto actual_offset = m_times_tab_pressed == 1 ? m_cursor : m_cursor - m_last_shown_suggestion.length() + m_next_suggestion_invariant_offset;
-                        for (size_t i = m_next_suggestion_invariant_offset; i < m_last_shown_suggestion.length(); ++i)
+                        size_t actual_offset;
+                        size_t shown_length = m_last_shown_suggestion_display_length;
+                        switch (m_times_tab_pressed) {
+                        case 1:
+                            actual_offset = m_cursor;
+                            break;
+                        case 2:
+                            actual_offset = m_cursor - m_largest_common_suggestion_prefix_length + m_next_suggestion_invariant_offset;
+                            if (can_complete)
+                                shown_length = m_largest_common_suggestion_prefix_length;
+                            break;
+                        default:
+                            if (m_last_shown_suggestion_display_length == 0)
+                                actual_offset = m_cursor;
+                            else
+                                actual_offset = m_cursor - m_last_shown_suggestion_display_length + m_next_suggestion_invariant_offset;
+                            break;
+                        }
+
+                        for (size_t i = m_next_suggestion_invariant_offset; i < shown_length; ++i)
                             m_buffer.remove(actual_offset);
                         m_cursor = actual_offset;
                         m_refresh_needed = true;
                     }
                     m_last_shown_suggestion = m_suggestions[m_next_suggestion_index];
-                    insert(m_last_shown_suggestion.substring_view(m_next_suggestion_invariant_offset, m_last_shown_suggestion.length() - m_next_suggestion_invariant_offset));
-                    if (m_tab_direction == TabDirection::Forward)
-                        increment_suggestion_index();
-                    else
-                        decrement_suggestion_index();
+                    m_last_shown_suggestion_display_length = m_last_shown_suggestion.length();
+                    m_last_shown_suggestion_was_complete = true;
+                    if (m_times_tab_pressed == 1) {
+                        // This is the first time, so only auto-complete *if possible*
+                        if (can_complete) {
+                            insert(m_last_shown_suggestion.substring_view(m_next_suggestion_invariant_offset, m_largest_common_suggestion_prefix_length - m_next_suggestion_invariant_offset));
+                            m_last_shown_suggestion_display_length = m_largest_common_suggestion_prefix_length;
+                            // do not increment the suggestion index, as the first tab should only be a *peek*
+                            if (m_suggestions.size() == 1) {
+                                // if there's one suggestion, commit and forget
+                                m_times_tab_pressed = 0;
+                            }
+                        } else {
+                            m_last_shown_suggestion_display_length = 0;
+                        }
+                        ++m_times_tab_pressed;
+                        m_last_shown_suggestion_was_complete = false;
+                    } else {
+                        insert(m_last_shown_suggestion.substring_view(m_next_suggestion_invariant_offset, m_last_shown_suggestion.length() - m_next_suggestion_invariant_offset));
+                        if (m_tab_direction == TabDirection::Forward)
+                            increment_suggestion_index();
+                        else
+                            decrement_suggestion_index();
+                    }
                 } else {
                     m_next_suggestion_index = 0;
                 }
@@ -373,7 +437,8 @@ String Editor::get_line(const String& prompt)
                         if (lines_used + m_prompt_lines_at_suggestion_initiation >= m_num_lines)
                             break;
 
-                        if (index == current_suggestion_index) {
+                        // only apply colour to the selection if something is *actually* added to the buffer
+                        if (m_last_shown_suggestion_was_complete && index == current_suggestion_index) {
                             vt_apply_style({ Style::Foreground(Style::Color::Blue) });
                             fflush(stdout);
                         }
@@ -385,7 +450,7 @@ String Editor::get_line(const String& prompt)
                             num_printed += fprintf(stderr, "%-*s", static_cast<int>(longest_suggestion_length) + 2, suggestion.characters());
                         }
 
-                        if (index == current_suggestion_index) {
+                        if (m_last_shown_suggestion_was_complete && index == current_suggestion_index) {
                             vt_apply_style({});
                             fflush(stdout);
                         }
@@ -405,6 +470,7 @@ String Editor::get_line(const String& prompt)
                     // after it, as if it were auto-completed
                     suggest(0, 0);
                     m_last_shown_suggestion = String::empty();
+                    m_last_shown_suggestion_display_length = 0;
                     m_suggestions.clear();
                     m_times_tab_pressed = 0;
                 }
@@ -420,6 +486,7 @@ String Editor::get_line(const String& prompt)
                     m_refresh_needed = true;
                     m_lines_used_for_last_suggestions = 0;
                 }
+                m_last_shown_suggestion_display_length = 0;
                 m_last_shown_suggestion = String::empty();
                 m_suggestions.clear();
                 suggest(0, 0);
@@ -462,7 +529,8 @@ String Editor::get_line(const String& prompt)
                 m_refresh_needed = true;
                 continue;
             }
-            if (ch == 0xc) {                    // ^L
+            // ^L
+            if (ch == 0xc) {
                 printf("\033[3J\033[H\033[2J"); // Clear screen.
                 vt_move_absolute(1, 1);
                 m_origin_x = 1;
@@ -470,18 +538,21 @@ String Editor::get_line(const String& prompt)
                 m_refresh_needed = true;
                 continue;
             }
-            if (ch == 0x01) { // ^A
+            // ^A
+            if (ch == 0x01) {
                 m_cursor = 0;
                 continue;
             }
-            if (ch == m_termios.c_cc[VEOF]) { // Normally ^D
+            // Normally ^D
+            if (ch == m_termios.c_cc[VEOF]) {
                 if (m_buffer.is_empty()) {
                     printf("<EOF>\n");
                     exit(0);
                 }
                 continue;
             }
-            if (ch == 0x05) { // ^E
+            // ^E
+            if (ch == 0x05) {
 
                 m_cursor = m_buffer.size();
                 continue;