lua: reimplement number increment <C-a> and decrement <C-x>

Based on a patch by Denis Warsow.

Stuff which could probably be improved:

 - in vim the cursor does not need to be on top of the number,
   it suffices to be on the same line.

 - decrementing beyond zero does not work for hexadecimal and
   octal numbers, vim seems to wrap around in this case.

     0x00000000 <C-x> becomes 0xffffffff

Close martanne#506

lua/plugins/number-inc-dec.lua

@@ -0,0 +1,54 @@
+-- increment/decrement number in dec/hex/oct format
+local lexer = vis.lexers
+if not lexer then return end
+
+local change = function(delta)
+
+	local win = vis.win
+	local file = win.file
+	local count = vis.count
+	if not count then count = 1 end
+	vis.count = nil -- reset count
+
+	for cursor in win:cursors_iterator() do
+		local pos = cursor.pos
+		local word = file:text_object_word(pos);
+		local s, e = word.start, word.finish;
+		if s then
+			local data = file:content(s, e - s)
+			if data and #data > 0 then
+				local base, format, padding = 10, 'd', 0
+				if lexer.oct_num:match(data) then
+					base = 8
+					format = 'o'
+					padding = #data
+				elseif lexer.hex_num:match(data) then
+					base = 16
+					format = 'x'
+					padding = #data - #"0x"
+				end
+				local number = tonumber(data, base == 8 and 8 or nil)
+				if number then
+					number = number + delta * count
+					if base ~= 10 and number < 0 then
+						-- string.format does not handle negative hex/oct values
+						-- should we wrap around?
+						number = 0
+					end
+					number = string.format((base == 16 and "0x" or "") .. "%0"..padding..format, number)
+					if base == 8 and string.sub(number, 0, 1) ~= "0" then
+						number = '0' .. number
+					end
+					file:delete(s, e - s)
+					file:insert(s, number)
+					cursor.pos = s
+				else
+					vis:info("Not a number")
+				end
+			end
+		end
+	end
+end
+
+vis:map(vis.modes.NORMAL, "<C-a>", function() change( 1) end, "Increment number")
+vis:map(vis.modes.NORMAL, "<C-x>", function() change(-1) end, "Decrement number")

lua/vis-std.lua

@@ -128,3 +128,4 @@ end)
 require('plugins/filetype')
 require('plugins/textobject-lexer')
 require('plugins/digraph')
+require('plugins/number-inc-dec')