feat(fuzzy)!: treat empty word as matching any candidate

Details:
- This behavior is usually more useful in practice.

Author: echasnovski

CHANGELOG.md

@@ -52,6 +52,7 @@
 ## mini.fuzzy
 
 - BREAKING: update `process_lsp_items()` to only use `filterText` and `label` item fields during fuzzy matching (instead of `textEdit.newText`, `insertText`, and `label` as before). This is more aligned with LSP specification.
+- BREAKING: treat empty `word` as matching any candidate (matched positions is empty array and score is -1). This behavior is usually more useful in practice.
 
 ## mini.hues
 

doc/mini-fuzzy.txt

@@ -43,7 +43,7 @@ See |mini.nvim-buffer-local-config| for more details.
 General design uses only width of found match and index of first letter
 match. No special characters or positions (like in fzy and fzf) are used.
 
-Given input `word` and target `candidate`:
+Given non-empty input `word` and target `candidate`:
 - The goal is to find matching between `word`'s letters and letters in
   `candidate` which minimizes certain score. It is assumed that order of
   letters in `word` and those matched in `candidate` should be the same.
@@ -108,9 +108,10 @@ Parameters ~
 Return ~
 `(table)` Matching information:
   - <positions> `(table|nil)` - array with letter indexes inside `candidate`
-    which matched to corresponding letters in `word`. It is `nil` if no match.
+    which matched to corresponding letters in `word`. It is empty array if
+    `word` is empty string and `nil` if no match.
   - <score> `number` - positive number representing how good the match is
-    (lower is better). It is `-1` if no match.
+    (lower is better). It is `-1` if no match or word is empty string.
 
 ------------------------------------------------------------------------------
                                                         *MiniFuzzy.filtersort()*

lua/mini/fuzzy.lua

@@ -41,7 +41,7 @@
 --- General design uses only width of found match and index of first letter
 --- match. No special characters or positions (like in fzy and fzf) are used.
 ---
---- Given input `word` and target `candidate`:
+--- Given non-empty input `word` and target `candidate`:
 --- - The goal is to find matching between `word`'s letters and letters in
 ---   `candidate` which minimizes certain score. It is assumed that order of
 ---   letters in `word` and those matched in `candidate` should be the same.
@@ -111,9 +111,10 @@ MiniFuzzy.config = {
 ---
 ---@return table Matching information:
 ---   - <positions> `(table|nil)` - array with letter indexes inside `candidate`
----     which matched to corresponding letters in `word`. It is `nil` if no match.
+---     which matched to corresponding letters in `word`. It is empty array if
+---     `word` is empty string and `nil` if no match.
 ---   - <score> `number` - positive number representing how good the match is
----     (lower is better). It is `-1` if no match.
+---     (lower is better). It is `-1` if no match or word is empty string.
 MiniFuzzy.match = function(word, candidate)
   -- Use 'smart case'
   candidate = word == word:lower() and candidate:lower() or candidate
@@ -236,7 +237,8 @@ end
 ---@private
 H.find_best_positions = function(letters, candidate)
   local n_candidate, n_letters = #candidate, #letters
-  if n_letters == 0 or n_candidate < n_letters then return nil end
+  if n_letters == 0 then return {} end
+  if n_candidate < n_letters then return nil end
 
   -- Search forward to find matching positions with left-most last letter match
   local pos_last = 0

tests/test_fuzzy.lua

@@ -72,7 +72,7 @@ T['match()']['handles cases when match is impossible'] = function()
   eq(child.lua_get([[MiniFuzzy.match('ab', 'a')]]), { score = -1 })
 
   -- Empty word
-  eq(child.lua_get([[MiniFuzzy.match('', 'abc')]]), { score = -1 })
+  eq(child.lua_get([[MiniFuzzy.match('', 'abc')]]), { positions = {}, score = -1 })
 end
 
 local validate_match = function(word, candidate, positions)
@@ -189,7 +189,7 @@ T['filtersort()']['preserves original order with equal matching score'] = functi
 end
 
 T['filtersort()']['works with empty arguments'] = function()
-  validate_filtersort('', { 'a', 'b', '_a' }, {})
+  validate_filtersort('', { 'a', 'b', '_a' }, { 'a', 'b', '_a' })
   validate_filtersort('a', {}, {})
 end
 
@@ -205,6 +205,10 @@ T['process_lsp_items()']['works'] = function()
   local validate = function(items) eq(process_lsp_items(items, 'a'), { items[3], items[2], items[1] }) end
   validate({ new_item('___a'), new_item('__a'), new_item('_a') })
   validate({ new_item(nil, '___a'), new_item(nil, '__a'), new_item(nil, '_a') })
+
+  -- Should match all as is for empty string `base`
+  local items = { new_item(nil, '___a'), new_item(nil, '__a'), new_item(nil, '_a') }
+  eq(process_lsp_items(items, ''), items)
 end
 
 T['process_lsp_items()']['correctly extracts candidate from fields'] = function()