Add workaround for inconsistent statusline eval padding

It seems that neovim applies inconsistent padding when evaluating the
statusline:

- When padding a normal component, e.g. "%5l", then the global fillchars
  option is used
- When padding a grouped component with minwid, e.g. "%2(a%)", then the
  fillchars parameter for nvim_eval_statusline() is used

It is this inconsistent padding that causes our statusline split
heuristic to mistakenly think that a grouped component is the actual
split of the statusline (like "%="), while it is actually not.
This caused some statuslines (in particular heirline, luckily not many
other popular statuslines use grouped components like this) to have very
weird and early right alignment.
This is obviously not the fault of those statuslines, but it turns out
that this bug is not very easy to fix in upstream neovim.
Ideally we would have API in nvim_eval_statusline() that would tell us
the split point directly, then we could avoid these nasty heuristics
altogether.

So instead I came up with a workaround in the meantime. Keep in mind,
this workaround is absolutely horrid, but at least it works. Hopefully
we can resolve this problem properly, but for now I have added a unit
test to make sure that we don't regress this further.

Also fuck whoever masochist came up with 1-based indexes in Lua. It's
causing off-by-one errors literally everywhere, and unfortunately that's
not even the worst part about this horrible language.

Fixes #58

Author: vimpostor

lua/tpipeline/main.lua

@@ -35,6 +35,19 @@ function M.color(grp)
 	return string.format('#[fg=%s,bg=%s%s]', fg, bg, st)
 end
 
+function M.eval_stl(stl, width)
+	local evl = vim.api.nvim_eval_statusline(stl, {fillchar = vim.g.tpipeline_fillchar, highlights = 1, use_tabline = vim.g.tpipeline_tabline, maxwidth = width})
+	local res = evl.str
+	local i = 0
+	for k, hl in pairs(evl.highlights) do
+		local grp = M.color(hl.group)
+		-- unfortunately we need to use vimscript strpart() as Lua's UTF 8 support is absolutely horrendous
+		res = vim.fn.strpart(res, 0, hl.start + i) .. grp .. vim.fn.strpart(res, hl.start + i)
+		i = i + string.len(grp)
+	end
+	return res
+end
+
 function M.update()
 	was_bold = false
 	was_italic = false
@@ -43,16 +56,26 @@ function M.update()
 	if stl == '' then
 		stl = vim.o.stl
 	end
-	local evl = vim.api.nvim_eval_statusline(stl, {fillchar = vim.g.tpipeline_fillchar, highlights = 1, use_tabline = vim.g.tpipeline_tabline, maxwidth = vim.g.tpipeline_size})
-	local res = evl.str
-	local i = 0
-	for k, hl in pairs(evl.highlights) do
-		local grp = M.color(hl.group)
-		-- unfortunately we need to use vimscript strpart() as Lua's UTF 8 support is absolutely horrendous
-		res = vim.fn.strpart(res, 0, hl.start + i) .. grp .. vim.fn.strpart(res, hl.start + i)
-		i = i + string.len(grp)
+
+	local res = M.eval_stl(stl, vim.g.tpipeline_size)
+
+	if #vim.split(res, split_pattern) <= 2 then
+		res = res:gsub(split_pattern, '%%=')
+	else
+		-- sometimes the split point is not unique, in which case we have to find it out manually
+		local retry = M.eval_stl(stl, vim.g.tpipeline_size + 2)
+		local i = 1
+		local start = 1
+		while i <= res:len() and res:sub(i, i) == retry:sub(i, i) do
+			if res:sub(i, i) ~= vim.g.tpipeline_fillchar then
+				start = i
+			end
+			i = i + 1
+		end
+		res = string.gsub(res:sub(1, start), vim.g.tpipeline_fillchar, ' ') .. string.gsub(res:sub(start + 1, i - 1), split_pattern, '%%=') .. string.gsub(res:sub(i), vim.g.tpipeline_fillchar, ' ')
 	end
-	return res:gsub(split_pattern, '%%=')
+
+	return res
 end
 
 return M

tests/test_general.vim

@@ -30,6 +30,11 @@ func Test_loaded()
 	call assert_equal(1, g:loaded_tpipeline)
 endfunc
 
+func Test_can_debug()
+	let info = tpipeline#debug#info()
+	call assert_true(len(info))
+endfunc
+
 func Test_job_runs()
 	let job = tpipeline#debug#info()
 	" background job is still running
@@ -208,7 +213,10 @@ func Test_quoted_strings()
 	call assert_equal(string(g:ReturnNumber()), Strip_hl(s:left))
 endfunc
 
-func Test_can_debug()
-	let info = tpipeline#debug#info()
-	call assert_true(len(info))
+func Test_minwid_padded()
+	" padding stl groups with minwid should not confuse the statusline splitter to cause right alignment where there is none
+	let g:tpipeline_statusline = '%2(a%)%='
+	call Read_socket()
+	call assert_match("a$", s:left)
+	call assert_true(empty(s:right))
 endfunc