Correctly handle sparse arrays in cc.pretty

This also swaps the order we display mixed array/maps in, so that the
array part comes first. I think this is more sensible.

Closes #777

src/main/resources/data/computercraft/lua/rom/modules/main/cc/pretty.lua

@@ -409,18 +409,24 @@ local function pretty_impl(obj, options, tracking)
         local doc = setmetatable({ tag = "concat", n = 1, space_line }, Doc)
 
         local length, keys, keysn = #obj, {}, 1
-        for k in pairs(obj) do keys[keysn], keysn = k, keysn + 1 end
+        for k in pairs(obj) do
+            if type(k) ~= "number" or k % 1 ~= 0 or k < 1 or k > length then
+                keys[keysn], keysn = k, keysn + 1
+            end
+        end
         table.sort(keys, key_compare)
 
-        for i = 1, keysn - 1 do
+        for i = 1, length do
             if i > 1 then append(doc, comma) append(doc, space_line) end
+            append(doc, pretty_impl(obj[i], options, tracking))
+        end
+
+        for i = 1, keysn - 1 do
+            if i > 1 or length >= 1 then append(doc, comma) append(doc, space_line) end
 
             local k = keys[i]
             local v = obj[k]
-            local ty = type(k)
+            if type(k) == "string" and not keywords[k] and k:match("^[%a_][%a%d_]*$") then
-            if ty == "number" and k % 1 == 0 and k >= 1 and k <= length then
-                append(doc, pretty_impl(v, options, tracking))
-            elseif ty == "string" and not keywords[k] and k:match("^[%a_][%a%d_]*$") then
                 append(doc, text(k .. " = "))
                 append(doc, pretty_impl(v, options, tracking))
             else

src/test/resources/test-rom/spec/modules/cc/pretty_spec.lua

@@ -177,7 +177,7 @@ describe("cc.pretty", function()
             end)
 
             it("displays mixed tables", function()
-                expect(pretty({ n = 3, 1, 2, 3 })):eq("{ n = 3, 1, 2, 3 }")
+                expect(pretty({ n = 3, 1, 2, 3 })):eq("{ 1, 2, 3, n = 3 }")
             end)
 
             it("escapes keys", function()
@@ -191,6 +191,13 @@ describe("cc.pretty", function()
             it("groups tables", function()
                 expect(pretty({ 1, 2, 3 }, 4)):eq("{\n  1,\n  2,\n  3\n}")
             end)
+
+            it("handles sparse tables", function()
+                local tbl = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }
+                tbl[4] = nil
+
+                expect(tostring(pp.pretty(tbl))):eq("{ 1, 2, 3, nil, 5, 6, 7, 8, 9, 10 }")
+            end)
         end)
 
         it("shows numbers", function()