feat: support specifies the bufnrs for the buffer source

as [cmp-buffer](https://github.com/hrsh7th/cmp-buffer?tab=readme-ov-file#get_bufnrs-type-fun-number)

Author: gh-liu

README.md

@@ -598,6 +598,17 @@ MiniDeps.add({
         name = 'Buffer',
         module = 'blink.cmp.sources.buffer',
         fallback_for = { 'lsp' },
+        -- opts = {
+        --   -- Specifies the buffer numbers to complete
+        --   -- Example usage for getting all visible buffers
+        --   get_bufnrs = function()
+        --     local bufs = {}
+        --     for _, win in ipairs(vim.api.nvim_list_wins()) do
+        --       bufs[vim.api.nvim_win_get_buf(win)] = true
+        --     end
+        --     return vim.tbl_keys(bufs)
+        --   end,
+        -- }
       },
     },
   },

lua/blink/cmp/sources/buffer.lua

@@ -4,9 +4,9 @@
 
 local uv = vim.uv
 
+--- @param bufnr integer
 ---@return string
-local function get_buf_text()
-  local bufnr = vim.api.nvim_get_current_buf()
+local function get_buf_text(bufnr)
   local lines = vim.api.nvim_buf_get_lines(bufnr, 0, -1, false)
 
   -- exclude word under the cursor
@@ -62,19 +62,30 @@ local function run_async(buf_text, callback)
   worker:queue(buf_text)
 end
 
+--- @class blink.cmp.BufferOpts
+--- @field get_bufnrs fun(): integer[]
+
 --- Public API
 
---- @class blink.cmp.Source
 local buffer = {}
 
-function buffer.new() return setmetatable({}, { __index = buffer }) end
+function buffer.new(opts)
+  opts = opts or {} ---@type blink.cmp.BufferOpts
+  local self = setmetatable({}, { __index = buffer })
+  self.get_bufnrs = opts.get_bufnrs or function() return { vim.api.nvim_get_current_buf() } end
+  return self
+end
 
 function buffer:get_completions(_, callback)
   local transformed_callback = function(items)
     callback({ is_incomplete_forward = false, is_incomplete_backward = false, items = items })
   end
 
-  local buf_text = get_buf_text()
+  local buf_texts = {}
+  for _, buf in ipairs(self.get_bufnrs()) do
+    table.insert(buf_texts, get_buf_text(buf))
+  end
+  local buf_text = table.concat(buf_texts, '\n')
   -- should take less than 2ms
   if #buf_text < 20000 then
     run_sync(buf_text, transformed_callback)
@@ -91,3 +102,4 @@ function buffer:get_completions(_, callback)
 end
 
 return buffer
+