refactor(grid): change schar_T representation to be more compact

Previously, a screen cell would occupy 28+4=32 bytes per cell
as we always made space for up to MAX_MCO+1 codepoints in a cell.

As an example, even a pretty modest 50*80 screen would consume

50*80*2*32 = 256000, i e a quarter megabyte

With the factor of two due to the TUI side buffer, and even more when
using msg_grid and/or ext_multigrid.

This instead stores a 4-byte union of either:
- a valid UTF-8 sequence up to 4 bytes
- an escape char which is invalid UTF-8 (0xFF) plus a 24-bit index to a
  glyph cache

This avoids allocating space for huge composed glyphs _upfront_, while
still keeping rendering such glyphs reasonably fast (1 hash table lookup
+ one plain index lookup). If the same large glyphs are using repeatedly
on the screen, this is still a net reduction of memory/cache
consumption. The only case which really gets worse is if you blast
the screen full with crazy emojis and zalgo text and even this case
only leads to 4 extra bytes per char.

When only <= 4-byte glyphs are used, plus the 4-byte attribute code,
i e 8 bytes in total there is a factor of four reduction of memory use.
Memory which will be quite hot in cache as the screen buffer is scanned
over in win_line() buffer text drawing

A slight complication is that the representation depends on host byte
order. I've tested this manually by compling and running this
in qemu-s390x and it works fine. We might add a qemu based solution
to CI at some point.

1 files changed, 16 insertions, 0 deletions

diff --git a/test/functional/ui/multibyte_spec.lua b/test/functional/ui/multibyte_spec.lua
index 5cecd423d7..417c7b797c 100644
--- a/test/functional/ui/multibyte_spec.lua
+++ b/test/functional/ui/multibyte_spec.lua
@@ -160,6 +160,22 @@ describe("multibyte rendering", function()
       {1:~                                                                }|
                                                                        |
     ]]}
+
+    -- nvim will reset the zalgo text^W^W glyph cache if it gets too full.
+    -- this should be exceedingly rare, but fake it to make sure it works
+    meths._invalidate_glyph_cache()
+    screen:expect{grid=[[
+      ^L̓̉̑̒̌̚ơ̗̌̒̄̀ŕ̈̈̎̐̕è̇̅̄̄̐m̖̟̟̅̄̚ ̛̓̑̆̇̍i̗̟̞̜̅̐p̗̞̜̉̆̕s̟̜̘̍̑̏ū̟̞̎̃̉ḿ̘̙́́̐ ̖̍̌̇̉̚d̞̄̃̒̉̎ò́̌̌̂̐l̞̀̄̆̌̚ȯ̖̞̋̀̐r̓̇̌̃̃̚ ̗̘̀̏̍́s̜̀̎̎̑̕i̟̗̐̄̄̚t̝̎̆̓̐̒ ̘̇̔̓̊̚ȃ̛̟̗̏̅m̜̟̙̞̈̓é̘̞̟̔̆t̝̂̂̈̑̔,̜̜̖̅̄̍ ̛̗̊̓̆̚c̟̍̆̍̈̔ȯ̖̖̝̑̀n̜̟̎̊̃̚s̟̏̇̎̒̚e̙̐̈̓̌̚c̙̍̈̏̅̕ť̇̄̇̆̓e̛̓̌̈̓̈t̟̍̀̉̆̅u̝̞̎̂̄̚r̘̀̅̈̅̐ ̝̞̓́̇̉ã̏̀̆̅̕d̛̆̐̉̆̋ȉ̞̟̍̃̚p̛̜̊̍̂̓ȋ̏̅̃̋̚ṥ̛̏̃̕č̛̞̝̀̂í̗̘̌́̎n̔̎́̒̂̕ǧ̗̜̋̇̂ ̛̜̔̄̎̃ê̛̔̆̇̕l̘̝̏̐̊̏ĩ̛̍̏̏̄t̟̐́̀̐̎,̙̘̍̆̉̐ ̋̂̏̄̌̅s̙̓̌̈́̇e̛̗̋̒̎̏d̜̗̊̍̊̚     |
+      ď̘̋̌̌̕ǒ̝̗̔̇̕ ̙̍́̄̄̉è̛̛̞̌̌i̜̖̐̈̆̚ȕ̇̈̓̃̓ŝ̛̞̙̉̋m̜̐̂̄̋̂ȯ̈̎̎̅̕d̜̙̓̔̋̑ ̞̗̄̂̂̚t̝̊́̃́̄e̛̘̜̞̓̑m̊̅̏̉̌̕p̛̈̂̇̀̐ỏ̙̘̈̉̔r̘̞̋̍̃̚ ̝̄̀̇̅̇ỉ̛̖̍̓̈n̛̛̝̎̕̕c̛̛̊̅́̐ĭ̗̓̀̍̐d̞̜̋̐̅̚i̟̙̇̄̊̄d̞̊̂̀̇̚ủ̝̉̑̃̕n̜̏̇̄̐̋ť̗̜̞̋̉ ̝̒̓̌̓̚ȕ̖̙̀̚̕t̖̘̎̉̂̌ ̛̝̄̍̌̂l̛̟̝̃̑̋á̛̝̝̔̅b̝̙̜̗̅̒ơ̖̌̒̄̆r̒̇̓̎̈̄e̛̛̖̅̏̇ ̖̗̜̝̃́e̛̛̘̅̔̌ẗ̛̙̗̐̕ ̖̟̇̋̌̈d̞̙̀̉̑̕ŏ̝̂́̐̑l̞̟̗̓̓̀ơ̘̎̃̄̂r̗̗̖̔̆̍ẻ̖̝̞̋̅ ̜̌̇̍̈̊m̈̉̇̄̒̀a̜̞̘̔̅̆g̗̖̈̃̈̉n̙̖̄̈̉̄â̛̝̜̄̃ ̛́̎̕̕̚ā̊́́̆̌l̟̙̞̃̒́i̖̇̎̃̀̋q̟̇̒̆́̊ủ́̌̇̑̚ã̛̘̉̐̚.̛́̏̐̍̊   |
+      U̝̙̎̈̐̆t̜̍̌̀̔̏ ̞̉̍̇̈̃e̟̟̊̄̕̕n̝̜̒̓̆̕i̖̒̌̅̇̚m̞̊̃̔̊̂ ̛̜̊̎̄̂a̘̜̋̒̚̚d̟̊̎̇̂̍ ̜̖̏̑̉̕m̜̒̎̅̄̚i̝̖̓̂̍̕n̙̉̒̑̀̔ỉ̖̝̌̒́m̛̖̘̅̆̎ ̖̉̎̒̌̕v̖̞̀̔́̎e̖̙̗̒̎̉n̛̗̝̎̀̂ȉ̞̗̒̕̚ȧ̟̜̝̅̚m̆̉̐̐̇̈,̏̐̎́̍́ ̜̞̙̘̏̆q̙̖̙̅̓̂ủ̇́̀̔̚í̙̟̟̏̐s̖̝̍̏̂̇ ̛̘̋̈̕̕ń̛̞̜̜̎o̗̜̔̔̈̆s̞̘̘̄̒̋t̛̅̋́̔̈ȓ̓̒́̇̅ủ̜̄̃̒̍d̙̝̘̊̏̚ ̛̟̞̄́̔e̛̗̝̍̃̀x̞̖̃̄̂̅e̖̅̇̐̔̃r̗̞̖̔̎̚c̘̜̖̆̊̏ï̙̝̙̂̕t̖̏́̓̋̂ă̖̄̆̑̒t̜̟̍̉̑̏i̛̞̞̘̒̑ǒ̜̆̅̃̉ṅ̖̜̒̎̚               |
+      u̗̞̓̔̈̏ĺ̟̝́̎̚l̛̜̅̌̎̆a̒̑̆̔̇̃m̜̗̈̊̎̚ċ̘̋̇̂̚ơ̟̖̊́̕ ̖̟̍̉̏̚l̙̔̓̀̅̏ä̞̗̘̙̅ḃ̟̎̄̃̕o̞̎̓̓̓̚r̗̜̊̓̈̒ï̗̜̃̃̅s̀̒̌̂̎̂ ̖̗̗̋̎̐n̝̟̝̘̄̚i̜̒̀̒̐̕s̘̘̄̊̃̀ī̘̜̏̌̕ ̗̖̞̐̈̒ư̙̞̄́̌t̟̘̖̙̊̚ ̌̅̋̆̚̚ä̇̊̇̕̕l̝̞̘̋̔̅i̍̋́̆̑̈q̛̆̐̈̐̚ư̏̆̊́̚î̜̝̑́̊p̗̓̅̑̆̏ ̆́̓̔̋̋e̟̊̋̏̓̚x̗̍̑̊̎̈ ̟̞̆̄̂̍ë̄̎̄̃̅a̛̜̅́̃̈ ̔̋̀̎̐̀c̖̖̍̀̒̂ơ̛̙̖̄̒m̘̔̍̏̆̕ḿ̖̙̝̏̂ȍ̓̋̈̀̕d̆̂̊̅̓̚o̖̔̌̑̚̕ ̙̆́̔̊̒c̖̘̖̀̄̍o̓̄̑̐̓̒ñ̞̒̎̈̚s̞̜̘̈̄̄e̙̊̀̇̌̋q̐̒̓́̔̃ư̗̟̔̔̚å̖̙̞̄̏t̛̙̟̒̇̏.̙̗̓̃̓̎         |
+      D̜̖̆̏̌̌ư̑̃̌̍̕i̝̊̊̊̊̄s̛̙̒́̌̇ ̛̃̔̄̆̌ă̘̔̅̅̀ú̟̟̟̃̃t̟̂̄̈̈̃e̘̅̌̒̂̆ ̖̟̐̉̉̌î̟̟̙̜̇r̛̙̞̗̄̌ú̗̗̃̌̎r̛̙̘̉̊̕e̒̐̔̃̓̋ ̊̊̍̋̑̉d̛̝̙̉̀̓o̘̜̐̐̓̐l̞̋̌̆̍́o̊̊̐̃̃̚ṙ̛̖̘̃̕ ̞̊̀̍̒̕ȉ́̑̐̇̅ǹ̜̗̜̞̏ ̛̜̐̄̄̚r̜̖̈̇̅̋ĕ̗̉̃̔̚p̟̝̀̓̔̆r̜̈̆̇̃̃e̘̔̔̏̎̓h̗̒̉̑̆̚ė̛̘̘̈̐n̘̂̀̒̕̕d̗̅̂̋̅́ê̗̜̜̜̕r̟̋̄̐̅̂i̛̔̌̒̂̕t̛̗̓̎̀̎ ̙̗̀̉̂̚ȉ̟̗̐̓̚n̙̂̍̏̓̉ ̙̘̊̋̍̕v̜̖̀̎̆̐ő̜̆̉̃̎l̑̋̒̉̔̆ư̙̓̓́̚p̝̘̖̎̏̒t̛̘̝̞̂̓ȁ̘̆̔́̊t̖̝̉̒̐̎e̞̟̋̀̅̄ ̆̌̃̀̑̔v̝̘̝̍̀̇ȅ̝̊̄̓̕l̞̝̑̔̂̋ĭ̝̄̅̆̍t̝̜̉̂̈̇        |
+      ē̟̊̇̕̚s̖̘̘̒̄̑s̛̘̀̊̆̇e̛̝̘̒̏̚ ̉̅̑̂̐̎c̛̟̙̎̋̓i̜̇̒̏̆̆l̟̄́̆̊̌l̍̊̋̃̆̌ủ̗̙̒̔̚m̛̘̘̖̅̍ ̖̙̈̎̂̕d̞̟̏̋̈̔ơ̟̝̌̃̄l̗̙̝̂̉̒õ̒̃̄̄̚ŕ̗̏̏̊̍ê̞̝̞̋̈ ̜̔̒̎̃̚e̞̟̞̒̃̄ư̖̏̄̑̃ ̛̗̜̄̓̎f̛̖̞̅̓̃ü̞̏̆̋̕g̜̝̞̑̑̆i̛̘̐̐̅̚à̜̖̌̆̎t̙̙̎̉̂̍ ̋̔̈̎̎̉n̞̓́̔̊̕ư̘̅̋̔̚l̗̍̒̄̀̚l̞̗̘̙̓̍â̘̔̒̎̚ ̖̓̋̉̃̆p̛̛̘̋̌̀ä̙̔́̒̕r̟̟̖̋̐̋ì̗̙̎̓̓ȃ̔̋̑̚̕t̄́̎̓̂̋ư̏̈̂̑̃r̖̓̋̊̚̚.̒̆̑̆̊̎ ̘̜̍̐̂̚E̞̅̐̇́̂x̄́̈̌̉̕ć̘̃̉̃̕è̘̂̑̏̑p̝̘̑̂̌̆t̔̐̅̍̌̂ȇ̞̈̐̚̕ű̝̞̜́̚ŕ̗̝̉̆́           |
+      š̟́̔̏̀ȉ̝̟̝̏̅n̑̆̇̒̆̚t̝̒́̅̋̏ ̗̑̌̋̇̚ơ̙̗̟̆̅c̙̞̙̎̊̎c̘̟̍̔̊̊a̛̒̓̉́̐e̜̘̙̒̅̇ć̝̝̂̇̕ả̓̍̎̂̚t̗̗̗̟̒̃ ̘̒̓̐̇́c̟̞̉̐̓̄ȕ̙̗̅́̏p̛̍̋̈́̅i̖̓̒̍̈̄d̞̃̈̌̆̐a̛̗̝̎̋̉t̞̙̀̊̆̇a̛̙̒̆̉̚t̜̟̘̉̓̚ ̝̘̗̐̇̕n̛̘̑̏̂́ō̑̋̉̏́ň̞̊̆̄̃ ̙̙̙̜̄̏p̒̆̋̋̓̏r̖̖̅̉́̚ơ̜̆̑̈̚i̟̒̀̃̂̌d̛̏̃̍̋̚ë̖̞̙̗̓n̛̘̓̒̅̎t̟̗̙̊̆̚,̘̙̔̊̚̕ ̟̗̘̜̑̔s̜̝̍̀̓̌û̞̙̅̇́n̘̗̝̒̃̎t̗̅̀̅̊̈ ̗̖̅̅̀̄i̛̖̍̅̋̂n̙̝̓̓̎̚ ̞̋̅̋̃̚c̗̒̀̆̌̎ū̞̂̑̌̓ĺ̛̐̍̑́p̝̆̌̎̈̚a̖̙̒̅̈̌ ̝̝̜̂̈̀q̝̖̔̍̒̚ư̔̐̂̎̊ǐ̛̟̖̘̕          |
+      o̖̜̔̋̅̚f̛̊̀̉́̕f̏̉̀̔̃̃i̘̍̎̐̔̎c̙̅̑̂̐̅ȋ̛̜̀̒̚a̋̍̇̏̀̋ ̖̘̒̅̃̒d̗̘̓̈̇̋é̝́̎̒̄š̙̒̊̉̋e̖̓̐̀̍̕r̗̞̂̅̇̄ù̘̇̐̉̀n̐̑̀̄̍̐t̟̀̂̊̄̚ ̟̝̂̍̏́m̜̗̈̂̏̚ő̞̊̑̇̒l̘̑̏́̔̄l̛̛̇̃̋̊i̓̋̒̃̉̌t̛̗̜̏̀̋ ̙̟̒̂̌̐a̙̝̔̆̏̅n̝̙̙̗̆̅i̍̔́̊̃̕m̖̝̟̒̍̚ ̛̃̃̑̌́ǐ̘̉̔̅̚d̝̗̀̌̏̒ ̖̝̓̑̊̚ȇ̞̟̖̌̕š̙̙̈̔̀t̂̉̒̍̄̄ ̝̗̊̋̌̄l̛̞̜̙̘̔å̝̍̂̍̅b̜̆̇̈̉̌ǒ̜̙̎̃̆r̝̀̄̍́̕ư̋̊́̊̕m̜̗̒̐̕̚.̟̘̀̒̌̚                     |
+      {1:~                                                                }|
+                                                                       |
+    ]], reset=true}
   end)
 end)