From 3063608df6ff1668f683d9d22c57fea29d728d0d Mon Sep 17 00:00:00 2001
From: Marcos Magueta <maguetamarcos@gmail.com>
Date: Tue, 11 Nov 2025 00:00:43 -0300
Subject: migrating to erlang to attach to mnesia

---
 src/main.app.src   | 15 +++++++++++++++
 src/main.erl       |  7 +++++++
 src/operations.erl | 15 +++++++++++++++
 src/relation.erl   | 42 ++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 79 insertions(+)
 create mode 100644 src/main.app.src
 create mode 100644 src/main.erl
 create mode 100644 src/operations.erl
 create mode 100644 src/relation.erl

(limited to 'src')

diff --git a/src/main.app.src b/src/main.app.src
new file mode 100644
index 0000000..0b7700a
--- /dev/null
+++ b/src/main.app.src
@@ -0,0 +1,15 @@
+{application, main, [
+    {description, "A relational algebra engine"},
+    {vsn, "0.1.0"},
+    {registered, []},
+    {mod, {main_app, []}},
+    {applications, [
+        kernel,
+        stdlib,
+        mnesia
+    ]},
+    {env, []},
+    {modules, []},
+    {licenses, ["Apache-2.0"]},
+    {links, []}
+ ]}.
diff --git a/src/main.erl b/src/main.erl
new file mode 100644
index 0000000..3c34a31
--- /dev/null
+++ b/src/main.erl
@@ -0,0 +1,7 @@
+-module(main).
+-export([fibonacci/1]).
+
+fibonacci(0) -> 0;
+fibonacci(1) -> 1;
+fibonacci(N) ->
+    fibonacci(N-2) + fibonacci(N-1).
diff --git a/src/operations.erl b/src/operations.erl
new file mode 100644
index 0000000..7846caf
--- /dev/null
+++ b/src/operations.erl
@@ -0,0 +1,15 @@
+-module(operations).
+-export([select/2, project/2, cartesian_product/2]).
+
+%% @doc Selection - filter tuples based on a predicate
+%% `Predicate` is a function that takes a tuple and returns true/false
+select(Relation, Predicate) ->
+    sets:filter(Predicate, Relation).
+
+%% @doc Projection - select specific attributes from a relation
+%% Note that `AttributeNames` can have duplicate keys, which are ignored
+project(Relation, AttributeNames) when is_list(AttributeNames) ->
+    sets:map(fun (Tuple) -> maps:with(AttributeNames, Tuple) end, Relation).
+
+cartesian_product(Relation1, Relation2) ->
+    sets:from_list([maps:merge(Tuple1, Tuple2) || Tuple1 <- sets:to_list(Relation1), Tuple2 <- sets:to_list(Relation2)], [{version, 2}]).
diff --git a/src/relation.erl b/src/relation.erl
new file mode 100644
index 0000000..66930d2
--- /dev/null
+++ b/src/relation.erl
@@ -0,0 +1,42 @@
+-module(relation).
+-export([new_attribute/2, new_tuple/1, new_relation/1]).
+
+%% Attribute: A pair of (Name, Value), eg. ("Name", "Augusto")
+%% Tuple: A set of attributes, eg. (("Name", "Augusto"), ("Age", 20), ("Surname", "Rengel"))
+%% Relation: A set of tuples organized through some selector
+%% Example Relation: f(x: Nat) = x + 1
+%% |{(0, 1), (1, 2), (2, 3), ...}| = |N|
+%% RelationCustomer = [(Username, Text), (Password, Text)]
+%% RelationProduct = [(Name, Text), (Price, Real)]
+%% RelationOrder = [(OrderId, Natural), ((Username, Text), 
+%%                                       (Password, Text), RelationCustomer), ((Name, Text), 
+%%                                                                             (Price, Real), RelationProduct)]
+
+new_attribute(Name, Value) ->
+    #{name => Name, value => Value}.
+
+new_tuple(Attributes) when is_list(Attributes) ->
+    Fun = fun (#{name := Name, value := Value}, Acc) ->
+		  maps:put(Name, Value, Acc)
+	  end,
+    lists:foldl(Fun, #{}, Attributes).
+
+new_relation(Tuples) when is_list(Tuples) ->
+    sets:from_list(Tuples, [{version, 2}]).
+
+%% SoapName1 = relation:new_attribute(name, "ABC").
+%% SoapPrice1 = relation:new_attribute(price, 1.0).
+%% SoapName2 = relation:new_attribute(name, "XYZ").
+%% SoapPrice2 = relation:new_attribute(price, 1.25).
+%% BazookaName = relation:new_attribute(name, "Bazzum").
+%% BazookaPrice = relation:new_attribute(price, 413.0).
+%% SwordName = relation:new_attribute(name, "Sword").
+%% SwordPrice = relation:new_attribute(price, 100.0).
+%% relation:new_relation([relation:new_tuple([SoapName, SoapPrice]), relation:new_tuple([BazookaName, BazookaPrice]), relation:new_tuple([SwordName, SwordPrice])]).
+
+%% Jules = relation:new_attribute(customer_name, "Jules").
+%% Emily = relation:new_attribute(customer_name, "Emily").
+%% Customers = relation:new_relation([relation:new_tuple([Jules]),
+%% 				   relation:new_tuple([Emily])]).
+
+%% operations:project(operations:select(operations:cartesian_product(relation:new_relation([relation:new_tuple([SoapName1, SoapPrice1]), relation:new_tuple([SoapName2, SoapPrice2]), relation:new_tuple([BazookaName, BazookaPrice]), relation:new_tuple([SwordName, SwordPrice])]), Customers), fun (Tuple) -> maps:get(price, Tuple) =< 1.0 end), [name, customer_name]).
-- 
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