lua: add file:match_at method to match LPeg pattern around a position

lua/vis.lua

@@ -190,4 +190,35 @@ end
 
 vis.events = events
 
+---
+-- @type File
+
+--- Check whether LPeg pattern matches at a given file position.
+-- @function match_at
+-- @param pattern the LPeg pattern
+-- @tparam int the absolute position in bytes to test for a match
+-- @tparam[opt] int horizon the number of bytes around `pos` to consider (defaults to 1K)
+-- @treturn int start,end the range of the matched region or `nil`
+vis.types['vis.file'].match_at = function(file, pattern, pos, horizon)
+	horizon = horizon or 1024
+	local lpeg = vis.lpeg
+	if not lpeg then return nil end
+	local before, after = pos - horizon, pos + horizon
+	if before < 0 then before = 0 end
+	local data = file:content(before, after - before)
+	local string_pos = pos - before + 1
+
+	local I = lpeg.Cp()
+	local p = lpeg.P{ I * pattern * I + 1 * lpeg.V(1) }
+	local s, e = 1
+	while true do
+		s, e = p:match(data, s)
+		if not s then return nil end
+		if s <= string_pos and string_pos < e then
+			return before + s - 1, before + e - 1
+		end
+		s = e
+	end
+end
+
 require('vis-std')