Initial implementation of spam check

It checks during the sends if there are spam warnings then give to the message an attribute spam=true (only for receivers)

api.lua

@@ -83,6 +83,7 @@ function mail.send(m)
 	local entry = mail.get_storage_entry(m.from)
 	table.insert(entry.outbox, 1, msg)
 	mail.set_storage_entry(m.from, entry)
+	msg.spam = #mail.check_spam(msg) >= 1
 
 	-- add in every receivers inbox
 	for recipient in pairs(recipients) do

ui/inbox.lua

@@ -97,6 +97,9 @@ function mail.show_inbox(name, sortfieldindex, sortdirection, filter)
             if not mail.player_in_list(name, message.to) and cc_color_enable then
                 table.insert(displayed_color, "additional")
             end
+            if message.spam then
+                table.insert(displayed_color, "warning")
+            end
             formspec[#formspec + 1] = "," .. mail.get_color(displayed_color)
             formspec[#formspec + 1] = ","
             formspec[#formspec + 1] = minetest.formspec_escape(message.from)

util/init.lua

@@ -4,5 +4,6 @@ dofile(MP .. "/util/normalize.lua")
 dofile(MP .. "/util/colors.lua")
 dofile(MP .. "/util/contact.lua")
 dofile(MP .. "/util/settings.lua")
+dofile(MP .. "/util/spam.lua")
 dofile(MP .. "/util/time_ago.lua")
 dofile(MP .. "/util/uuid.lua")

util/spam.lua

@@ -0,0 +1,39 @@
+local function caps_ratio(str)
+    local total_caps = 0
+    for i = 1, #str do -- iteration through each character
+        local c = str:sub(i,i)
+        if string.lower(c) ~= c then -- do not count digits as spam
+            total_caps = total_caps + 1
+        end
+    end
+    return total_caps/(#str or 1) -- avoid division by zero
+end
+
+local function words_ratio(str, ratio)
+    local words = {}
+    local split_str = str:split(" ")
+    for _, w in ipairs(split_str) do
+        if not words[w] then
+            words[w] = 0
+        else
+            words[w] = (words[w] or 0) + 1
+        end
+    end
+    for _, n in pairs(words) do
+        if n/#split_str >= ratio then
+            return true
+        end
+    end
+    return false
+end
+
+function mail.check_spam(message)
+    spam_checks = {}
+    if caps_ratio(message.subject) == 1 or caps_ratio(message.body) > 0.4 then
+        table.insert(spam_checks, "caps")
+    end
+    if words_ratio(message.subject, 0.6) or words_ratio(message.body, 0.2) then
+        table.insert(spam_checks, "words")
+    end
+    return spam_checks
+end