switch Nodes from array to table

We need keys to be fixed as we add/delete nodes because we're going to
start recording them next inside Nodes to encode edges.

Since there isn't a clear name for nodes in this app, I came up with a
way to autogenerate keys.

0028-A

@@ -2,8 +2,8 @@ A = function(preserve_screen_top_of_cursor_node)
 	print('A')
 	-- translate Nodes to Surface
 	while #Surface > 0 do table.remove(Surface) end
-	for i,node in ipairs(Nodes) do
-		node.id = i
+	for key,node in pairs(Nodes) do
+		node.id = key
 		compute_layout(node, node.x,node.y, Surface, preserve_screen_top_of_cursor_node)
 	end
 	-- continue the pipeline

0031-Nodes

@@ -1,3 +1,3 @@
 Nodes = {
-	{type='text', x=0,y=0, margin=0, width=120},
+	a={type='text', x=0,y=0, margin=0, width=120},
 }

0036-on_move_bar

@@ -1,9 +1,9 @@
 on_move_bar = function(sx,sy)
-	for _,node in ipairs(Nodes) do
+	for _,node in pairs(Nodes) do
 		if sx >= node.x-10 and sx < node.x-10+node.w/2 then
 			if sy >= node.y-15-10 and sy < node.y-15 then
 				return node
 			end
 		end
 	end
-end
+end

0037-on_resize

@@ -1,5 +1,5 @@
 on_resize = function(sx,sy)
-	for _,node in ipairs(Nodes) do
+	for _,node in pairs(Nodes) do
 		if sx >= node.x+node.w+20 and sx < node.x+node.w+24 then
 			local buffer_height = math.max(node.h, 3*node.editor.line_height)
 			if sy >= node.y and sy < node.y+buffer_height then
@@ -7,4 +7,4 @@ on_resize = function(sx,sy)
 			end
 		end
 	end
-end
+end

0044-First_available_id

@@ -0,0 +1 @@
+First_available_id = 1

0045-next_key

@@ -0,0 +1,5 @@
+next_key = function()
+	local result = to_key(First_available_id)
+	First_available_id = First_available_id+1
+	return result
+end

0046-to_key

@@ -0,0 +1,15 @@
+to_key = function(n)
+	-- represent an integer n in base-26 using a-z
+	local result = {}
+	n = math.floor(n)
+	local a = string.byte('a')
+	while n > 0 do
+		local digit = n % 26
+		table.insert(result, 1, string.char(digit+a))
+		n = math.floor(n/26)
+	end
+	if #result == 0 then
+		return 'a'
+	end
+	return table.concat(result)
+end

0047-test_to_key

@@ -0,0 +1,11 @@
+test_to_key = function()
+	check_eq(to_key(0), 'a', 0)
+	check_eq(to_key(1), 'b', 1)
+	-- ...
+	check_eq(to_key(24), 'y', 24)
+	check_eq(to_key(25), 'z', 25)
+	check_eq(to_key(26), 'ba', 26)
+	check_eq(to_key(26*26), 'baa', '3 digit')
+	check_eq(to_key(26*26+1), 'bab', '3 digit/2')
+	check_eq(to_key(26*26*26), 'baaa', '4 digit')
+end