perf(diagnostic): avoid table copies to filter by severity (#28491)

Instead of adding all diagnostics matching lnum filters to a table, and
then copying that table to another table while applying the severity
filter, this changes the flow to only add diagnostics matching both
filters in the first pass.

1 files changed, 33 insertions, 26 deletions

diff --git a/runtime/lua/vim/diagnostic.lua b/runtime/lua/vim/diagnostic.lua
index b42eece4c2..3321b6ad71 100644
--- a/runtime/lua/vim/diagnostic.lua
+++ b/runtime/lua/vim/diagnostic.lua
@@ -372,43 +372,46 @@ local function to_severity(severity)
 end
 
 --- @param severity vim.diagnostic.SeverityFilter
---- @param diagnostics vim.Diagnostic[]
---- @return vim.Diagnostic[]
-local function filter_by_severity(severity, diagnostics)
-  if not severity then
-    return diagnostics
-  end
-
+--- @return fun(vim.Diagnostic):boolean
+local function severity_predicate(severity)
   if type(severity) ~= 'table' then
     severity = assert(to_severity(severity))
-    --- @param t vim.Diagnostic
-    return vim.tbl_filter(function(t)
-      return t.severity == severity
-    end, diagnostics)
+    ---@param d vim.Diagnostic
+    return function(d)
+      return d.severity == severity
+    end
   end
-
   if severity.min or severity.max then
     --- @cast severity {min:vim.diagnostic.Severity,max:vim.diagnostic.Severity}
     local min_severity = to_severity(severity.min) or M.severity.HINT
     local max_severity = to_severity(severity.max) or M.severity.ERROR
 
-    --- @param t vim.Diagnostic
-    return vim.tbl_filter(function(t)
-      return t.severity <= min_severity and t.severity >= max_severity
-    end, diagnostics)
+    --- @param d vim.Diagnostic
+    return function(d)
+      return d.severity <= min_severity and d.severity >= max_severity
+    end
   end
 
   --- @cast severity vim.diagnostic.Severity[]
-
   local severities = {} --- @type table<vim.diagnostic.Severity,true>
   for _, s in ipairs(severity) do
     severities[assert(to_severity(s))] = true
   end
 
-  --- @param t vim.Diagnostic
-  return vim.tbl_filter(function(t)
-    return severities[t.severity]
-  end, diagnostics)
+  --- @param d vim.Diagnostic
+  return function(d)
+    return severities[d.severity]
+  end
+end
+
+--- @param severity vim.diagnostic.SeverityFilter
+--- @param diagnostics vim.Diagnostic[]
+--- @return vim.Diagnostic[]
+local function filter_by_severity(severity, diagnostics)
+  if not severity then
+    return diagnostics
+  end
+  return vim.tbl_filter(severity_predicate(severity), diagnostics)
 end
 
 --- @param bufnr integer
@@ -714,10 +717,18 @@ local function get_diagnostics(bufnr, opts, clamp)
     end,
   })
 
+  local match_severity = opts.severity and severity_predicate(opts.severity)
+    or function(_)
+      return true
+    end
+
   ---@param b integer
   ---@param d vim.Diagnostic
   local function add(b, d)
-    if not opts.lnum or (opts.lnum >= d.lnum and opts.lnum <= (d.end_lnum or d.lnum)) then
+    if
+      match_severity(d)
+      and (not opts.lnum or (opts.lnum >= d.lnum and opts.lnum <= (d.end_lnum or d.lnum)))
+    then
       if clamp and api.nvim_buf_is_loaded(b) then
         local line_count = buf_line_count[b] - 1
         if
@@ -771,10 +782,6 @@ local function get_diagnostics(bufnr, opts, clamp)
     end
   end
 
-  if opts.severity then
-    diagnostics = filter_by_severity(opts.severity, diagnostics)
-  end
-
   return diagnostics
 end