Spreadsheet: Implement infinit-scroll for columns

This naturally also implements multi-char columns, and also integrates
it into the js runtime (such columns can be named in ranges too).

Applications/Spreadsheet/Spreadsheet.cpp

@@ -99,28 +99,57 @@ size_t Sheet::add_row()
     return m_rows++;
 }
 
+static String convert_to_string(size_t value, unsigned base = 26, StringView map = {})
+{
+    if (map.is_null())
+        map = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+
+    ASSERT(base >= 2 && base <= map.length());
+
+    // The '8 bits per byte' assumption may need to go?
+    Array<char, round_up_to_power_of_two(sizeof(size_t) * 8 + 1, 2)> buffer;
+    size_t i = 0;
+    do {
+        buffer[i++] = map[value % base];
+        value /= base;
+    } while (value > 0);
+
+    // NOTE: Weird as this may seem, the thing that comes after 'A' is 'AA', which as a number would be '00'
+    //       to make this work, only the most significant digit has to be in a range of (1..25) as opposed to (0..25),
+    //       but only if it's not the only digit in the string.
+    if (i > 1)
+        --buffer[i - 1];
+
+    for (size_t j = 0; j < i / 2; ++j)
+        swap(buffer[j], buffer[i - j - 1]);
+
+    return String { ReadonlyBytes(buffer.data(), i) };
+}
+
+static size_t convert_from_string(StringView str, unsigned base = 26, StringView map = {})
+{
+    if (map.is_null())
+        map = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+
+    ASSERT(base >= 2 && base <= map.length());
+
+    size_t value = 0;
+    for (size_t i = str.length(); i > 0; --i) {
+        auto digit_value = map.find_first_of(str[i - 1]).value_or(0);
+        // NOTE: Refer to the note in `convert_to_string()'.
+        if (i == str.length() && str.length() > 1)
+            ++digit_value;
+        value = value * base + digit_value;
+    }
+
+    return value;
+}
+
 String Sheet::add_column()
 {
-    if (m_current_column_name_length == 0) {
-        m_current_column_name_length = 1;
-        m_columns.append("A");
-        return "A";
-    }
-
-    if (m_current_column_name_length == 1) {
-        auto last_char = m_columns.last()[0];
-        if (last_char == 'Z') {
-            m_current_column_name_length = 2;
-            m_columns.append("AA");
-            return "AA";
-        }
-
-        last_char++;
-        m_columns.append({ &last_char, 1 });
-        return m_columns.last();
-    }
-
-    TODO();
+    auto next_column = convert_to_string(m_columns.size());
+    m_columns.append(next_column);
+    return next_column;
 }
 
 void Sheet::update()
@@ -207,6 +236,31 @@ Optional<Position> Sheet::parse_cell_name(const StringView& name)
     return Position { col, row.to_uint().value() };
 }
 
+Optional<size_t> Sheet::column_index(const StringView& column_name) const
+{
+    auto index = convert_from_string(column_name);
+    if (m_columns.size() <= index || m_columns[index] != column_name)
+        return {};
+
+    return index;
+}
+
+Optional<String> Sheet::column_arithmetic(const StringView& column_name, int offset)
+{
+    auto maybe_index = column_index(column_name);
+    if (!maybe_index.has_value())
+        return {};
+
+    auto index = maybe_index.value() + offset;
+    if (m_columns.size() > index)
+        return m_columns[index];
+
+    for (size_t i = m_columns.size(); i <= index; ++i)
+        add_column();
+
+    return m_columns.last();
+}
+
 Cell* Sheet::from_url(const URL& url)
 {
     auto maybe_position = position_from_url(url);
@@ -335,6 +389,11 @@ RefPtr<Sheet> Sheet::from_json(const JsonObject& object, Workbook& workbook)
         return IterationDecision::Continue;
     });
 
+    if (sheet->m_columns.size() < default_column_count && sheet->columns_are_standard()) {
+        for (size_t i = sheet->m_columns.size(); i < default_column_count; ++i)
+            sheet->add_column();
+    }
+
     auto cells = object.get("cells").as_object();
     auto json = sheet->interpreter().global_object().get("JSON");
     auto& parse_function = json.as_object().get("parse").as_function();
@@ -425,15 +484,28 @@ Position Sheet::written_data_bounds() const
 {
     Position bound;
     for (auto& entry : m_cells) {
-        if (entry.key.row > bound.row)
+        if (entry.key.row >= bound.row)
             bound.row = entry.key.row;
-        if (entry.key.column > bound.column)
+        if (entry.key.column >= bound.column)
             bound.column = entry.key.column;
     }
 
     return bound;
 }
 
+/// The sheet is allowed to have nonstandard column names
+/// this checks whether all existing columns are 'standard'
+/// (i.e. as generated by 'convert_to_string()'
+bool Sheet::columns_are_standard() const
+{
+    for (size_t i = 0; i < m_columns.size(); ++i) {
+        if (m_columns[i] != convert_to_string(i))
+            return false;
+    }
+
+    return true;
+}
+
 JsonObject Sheet::to_json() const
 {
     JsonObject object;
@@ -448,12 +520,13 @@ JsonObject Sheet::to_json() const
 
     auto bottom_right = written_data_bounds();
 
-    auto columns = JsonArray();
-    for (auto& column : m_columns)
-        columns.append(column);
-    object.set("columns", move(columns));
-
-    object.set("rows", bottom_right.row);
+    if (!columns_are_standard()) {
+        auto columns = JsonArray();
+        for (auto& column : m_columns)
+            columns.append(column);
+        object.set("columns", move(columns));
+    }
+    object.set("rows", bottom_right.row + 1);
 
     JsonObject cells;
     for (auto& it : m_cells) {
@@ -520,12 +593,21 @@ Vector<Vector<String>> Sheet::to_xsv() const
     auto bottom_right = written_data_bounds();
 
     // First row = headers.
-    data.append(m_columns);
+    size_t column_count = m_columns.size();
+    if (columns_are_standard()) {
+        column_count = convert_from_string(bottom_right.column) + 1;
+        Vector<String> cols;
+        for (size_t i = 0; i < column_count; ++i)
+            cols.append(m_columns[i]);
+        data.append(move(cols));
+    } else {
+        data.append(m_columns);
+    }
 
     for (size_t i = 0; i <= bottom_right.row; ++i) {
         Vector<String> row;
-        row.resize(m_columns.size());
-        for (size_t j = 0; j < m_columns.size(); ++j) {
+        row.resize(column_count);
+        for (size_t j = 0; j < column_count; ++j) {
             auto cell = at({ m_columns[j], i });
             if (cell)
                 row[j] = cell->typed_display();
@@ -546,6 +628,10 @@ RefPtr<Sheet> Sheet::from_xsv(const Reader::XSV& xsv, Workbook& workbook)
     sheet->m_columns = cols;
     for (size_t i = 0; i < max(rows, Sheet::default_row_count); ++i)
         sheet->add_row();
+    if (sheet->columns_are_standard()) {
+        for (size_t i = sheet->m_columns.size(); i < Sheet::default_column_count; ++i)
+            sheet->add_column();
+    }
 
     for (auto row : xsv) {
         for (size_t i = 0; i < cols.size(); ++i) {