LineEditor Re-architecture

[tompng]

Description

This pull request is a refactor of line_editor.
It also fixes many rendering, cursor position, internal state inconsistency bugs including #581 #575 #520 #491 and more, especially when input lines > screen height.

Problem of LineEditor

• Too many duplicated state, causing inconsistency between several state
• Key action modifies cursor position
• Key action controls where to rerender
• Key action sets actions to be handled in rendering methods
• Rendering is not idempotent. It also modifies state.
• Does not have differential rendering mechanism, making rendering complex.

What we should do is something like "jQuery to React"

Changes

Single source of truth - Remove duplicated state.

Important state are

• Input: @buffer_of_lines
• Cursor position: @line_index @byte_pointer
• Scroll offset: @scroll_partial_screen
• Screen size: @screen_size
• Reline-controlled line starts from: @cursor_base_y
• Current cursor y in screen: @cursor_base_y + @cursor_y

Removed these because it can be calculated
@line @highest_in_all @highest_in_this @screen_height
@cursor @cursor_max @first_line_started_from @started_from
@rest_height

Removed these because it is not a state but something like an action dispatch and a flag for rendering
@previous_line_index @just_cursor_moving @last_key
@add_newline_to_end_of_buffer @prev_mode_string @rerender_all
@move_up @check_new_auto_indent

Removed these because rendering is improved.
@cached_prompt_list @prompt_cache_time: no need to cache with time anymore.
@previous_rendered_dialog_y: not needed anymore because dialog rendering strategy is changed.

Render only from state

Don't render, don't move cursor except from single rendering method render_differential.

Lifecycle is simplified

loop do
  # Handles action caused by key input, update dialog content. does not render or move cursor
  input_keys.each { |c| @line_editor.update(c) }
  # Renders from state. Does not change state. Idempotent operation.
  @line_editor.rerender
end 

Differential rendering

render_differential calls render_line_differential(old_items, new_items) for each line.
Input line and overlay dialogs are abstracted as set of [x, width, content].

prompt> code[DIAL[dialog]DIALOG] is represented as
old_items = [[0, 16, "prompt> codecode"], [12, 20, "[DIALOGDIALOGDIALOG]"], [17, 8, "[dialog]"]]

prompt> codecode [dialog] is represented as
new_items = [[0, 16, "prompt> codecode"], nil, [17, 8, "[dialog]"]]

Calculate z-index for each column

prompt> code[DIAL[dialog]DIALOG]
00000000000011111222222221111111

prompt> codecode [dialog]
0000000000000000 22222222

Compare z-indexes and extract where to update
00000000000011111222222221111111
0000000000000000 22222222
------------0000B-------- # ('-' represents unchanged, 'B' represents blank)

Need to render "code " at column 12 and clear after column 25

[tompng]

Released this change as reline-0.5.0.pre.1

https://rubygems.org/gems/reline/versions/0.5.0.pre.1
https://github.com/ruby/reline/releases/tag/v0.5.0.pre.1

Changes and fix to 0.5.0.pre.x will be pushed to #614