Expand the reduce function to both fold and reduce (#6337)

.luacheckrc

@@ -14,4 +14,4 @@ global=false
 unused=false
 -- excluded files due to using lua 5.4 syntax that currently gets flagged as a syntax error
 -- clear out once a newer Ubuntu LTS base is used for our docker images, which would then also have a newer luacheck available
-exclude_files={"data/lua/core/wml.lua","data/lua/wml-flow.lua","data/lua/wml/find_path.lua","data/lua/wml/harm_unit.lua","data/lua/wml/modify_unit.lua","data/lua/wml/random_placement.lua"}
+exclude_files={"data/lua/core/wml.lua","data/lua/wml-flow.lua","data/lua/wml/find_path.lua","data/lua/wml/harm_unit.lua","data/lua/wml/modify_unit.lua","data/lua/wml/random_placement.lua","data/lua/functional.lua"}

data/lua/functional.lua

@@ -126,15 +126,28 @@ local known_operators = {
 	['or'] = function(a, b) return a or b end,
 }
 
-function functional.reduce(input, operator, identity)
-	if type(operator) == 'string' then
-		operator = known_operators[operator]
+--- Reduce the elements of array t into a single value. operator is called as
+--- 'operator(accumulator, element)' for every element in t. If a 3rd argument
+--- is provided, which may be nil, it will be used as the accumulator when
+--- calling operator on the first element. If there is no 3rd argument, the
+--- first operator call will be on the first two elements. If there is no 3rd
+--- argument and the array is empty, return nil. operator may be a function or a
+--- binary Lua operator as a string.
+function functional.reduce(t, operator, ...)
+	local f <const> = known_operators[operator] or operator
+
+	local function loop(init, i)
+		local value <const> = t[i]
+		if value == nil then
+			return init
+		end
+		return loop(f(init, value), i + 1)
 	end
-	local result = identity or 0
-	for _, v in ipairs(input) do
-		result = operator(result, v)
+
+	if select('#', ...) == 0 then
+		return loop(t[1], 2)
 	end
-	return result
+	return loop(select(1, ...), 1)
 end
 
 function functional.take_while(input, condition)

data/test/scenarios/lua_functional.cfg

@@ -0,0 +1,29 @@
+#textdomain wesnoth-test
+
+# Tests for the functional.lua library
+{GENERIC_UNIT_TEST "lua_functional" (
+    [event]
+        name = start
+        [lua]
+            code = <<
+                local fp <const> = wesnoth.require'lua/functional'
+
+                unit_test.assert_equal(
+                    fp.reduce({}, function() end),
+                    nil,
+                    "reduce with empty array and no init doesn't return nil.")
+
+                unit_test.assert(
+                    not fp.reduce({false, false}, 'or', false),
+                    "reduce with false elements and 'or' operator returns a truthy value.")
+
+                unit_test.assert_equal(
+                    fp.reduce({4, 5, 2}, '+', 0),
+                    11,
+                    "reduce does not calculate a sum correctly.")
+
+                unit_test.succeed()
+            >>
+        [/lua]
+    [/event]
+)}

wml_test_schedule

@@ -351,6 +351,7 @@
 9 unknown_scenario_1_0
 
 # Lua API tests
+0 lua_functional
 0 lua_map_find
 0 mapgen_filter_range
 0 mapgen_filter_terrain