lua: add helper function for printing a table

Although this is something really basic, Lua's standard library doesn't
provide anything like this. Probably because there are too many ways to
do it right or wrong.

This code tries to be really careful when dealing with mixed
arrays/maps, e.g. when a table has integer keys starting from 1, making
it look like an array, but then also has other keys.

player/lua/defaults.lua

@@ -373,4 +373,53 @@ function mp.osd_message(text, duration)
     mp.commandv("show_text", text, duration)
 end
 
+function mp.format_table(t, set)
+    if not set then
+        set = { [t] = true }
+    end
+    local res = "{"
+    -- pretty expensive but simple way to distinguish array and map parts of t
+    local keys = {}
+    local vals = {}
+    local arr = 0
+    for i = 1, #t do
+        if t[i] == nil then
+            break
+        end
+        keys[i] = i
+        vals[i] = t[i]
+        arr = i
+    end
+    for k, v in pairs(t) do
+        if not (type(k) == "number" and k >= 1 and k <= arr and keys[k]) then
+            keys[#keys + 1] = k
+            vals[#keys] = v
+        end
+    end
+    local function fmtval(v)
+        if type(v) == "string" then
+            return "\"" .. v .. "\""
+        elseif type(v) == "table" then
+            if set[v] then
+                return "[cycle]"
+            end
+            set[v] = true
+            return mp.format_table(v, set)
+        else
+            return tostring(v)
+        end
+    end
+    for i = 1, #keys do
+        if #res > 1 then
+            res = res .. ", "
+        end
+        if i > arr then
+            res = res .. fmtval(keys[i]) .. " = "
+        end
+        res = res .. fmtval(vals[i])
+    end
+    res = res .. "}"
+    return res
+end
+
 return {}