From d837a09726f5ae63350c9e04d14fc71cd713da6d Mon Sep 17 00:00:00 2001
From: Celtic Minstrel <celtic.minstrel.ca@some.place>
Date: Fri, 19 Feb 2021 01:07:45 -0500
Subject: [PATCH] Add some utility functions to help clarify the merge mode
 being used when assigning terrains

---
 data/lua/core/map.lua | 50 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)
 create mode 100644 data/lua/core/map.lua

diff --git a/data/lua/core/map.lua b/data/lua/core/map.lua
new file mode 100644
index 000000000000..20c952245486
--- /dev/null
+++ b/data/lua/core/map.lua
@@ -0,0 +1,50 @@
+--[========[Map module]========]
+print("Loading map module...")
+
+if wesnoth.kernel_type() == "Game Lua Kernel" then
+	function wesnoth.map.split_terrain_code(code)
+		return table.unpack(code:split('^', {remove_empty = false}))
+	end
+	
+	-- possible terrain string inputs:
+	-- A        A^       A^B      ^        ^B
+	-- implied mode:
+	-- both     base     both     overlay  overlay
+	function wesnoth.map.replace_base(code)
+		local base, overlay = wesnoth.map.split_terrain_code(code)
+		if base == nil then -- ^ or ^B
+			-- There's no base to replace with, so do nothing
+			return ''
+		else
+			-- Use the specified base but ignore the overlay
+			return base .. '^'
+		end
+	end
+	
+	function wesnoth.map.replace_overlay(code)
+		local base, overlay = wesnoth.map.split_terrain_code(code)
+		if overlay == nil or overlay == '' then -- A or A^
+			-- No overlay was specified, so we want to clear the overlay without touching the base
+			return '^'
+		else
+			-- An overlay was specified, so use that and ignore the base
+			return '^' .. overlay
+		end
+	end
+	
+	function wesnoth.map.replace_both(code)
+		local base, overlay = wesnoth.map.split_terrain_code(code)
+		if base == '' then -- ^ or ^B
+			-- There's no way to find a base to replace with in this case.
+			-- Could use the existing base, but that's not really replacing both, is it?
+			error('replace_both: no base terrain specified')
+		elseif overlay == '' then -- A^
+			-- This would normally mean replace base while preserving overlay,
+			-- but we actually want to replace base and clear overlay.
+			return base
+		else
+			-- It's already going to replace both, so return unchanged
+			return code
+		end
+	end
+end
\ No newline at end of file