optimize path_get_index_by_offset

Instead of calling path_get_adjacent twice (which calls path_get
twice, i.e. 4 times overall), we call path_get directly only 2 times
with min and max indices to generate all the path we need for
calculations.

advtrains/path.lua

@@ -286,16 +286,26 @@ end
 -- This function determines the index resulting from moving along the path by 'offset' meters
 -- starting from 'index'. See also the comment on the top of the file.
 function advtrains.path_get_index_by_offset(train, index, offset)
+	local advtrains_path_get = advtrains.path_get
+
 	-- Step 1: determine my current absolute pos on the path
-	local start_index_f = math.floor(index)
-	local _, _, frac = advtrains.path_get_adjacent(train, index)
+	local start_index_f = atfloor(index)
+	local end_index_f = start_index_f + 1
+	local c_idx = atfloor(index + offset)
+	local c_idx_f = c_idx + 1
+
+	local frac = index - start_index_f
+
+	advtrains_path_get(train, math.min(start_index_f, end_index_f, c_idx, c_idx_f))
+	advtrains_path_get(train, math.max(start_index_f, end_index_f, c_idx, c_idx_f))
+
 	local dist1, dist2 = train.path_dist[start_index_f], train.path_dist[start_index_f+1]
-	local start_dist = n_interpolate(dist1, dist2, frac)
+	local start_dist = dist1 + (dist2-dist1)*frac
 	
 	-- Step 2: determine the total end distance and estimate the index we'd come out
 	local end_dist = start_dist + offset
 	
-	local c_idx = math.floor(index + offset)
+	local c_idx = atfloor(index + offset)
 	
 	-- Step 3: move forward/backward to find real index
 	-- We assume here that the distance between 2 path items is never smaller than 1.
@@ -306,9 +316,7 @@ function advtrains.path_get_index_by_offset(train, index, offset)
 	--  Desired position:  -------#------
 	--  Path items      :  --|--|--|--|--
 	--  c_idx           :       ^
-	
-	advtrains.path_get_adjacent(train, c_idx)
-	
+
 	while train.path_dist[c_idx] < end_dist do
 		c_idx = c_idx + 1
 	end