91 lines
2.9 KiB
C++
91 lines
2.9 KiB
C++
#include <facts.h>
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#include <iostream>
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// This example shows how to use rules for testing facts. A fact is a query that
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// has no variable elements. Consider a regular ECS query like this:
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// Position, Velocity
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//
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// When written out in full, this query looks like:
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// Position($This), Velocity($This)
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//
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// "This" is a (builtin) query variable that is unknown before we evaluate the
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// query. Therefore this query does not test a fact, we can't know which values
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// This will assume.
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//
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// An example of a fact-checking query is:
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// IsA(Cat, Animal)
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//
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// This is a fact: the query has no elements that are unknown before evaluating
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// the query. A rule that checks a fact does not return entities, but will
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// instead return the reasons why a fact is true (if it is true).
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struct Likes { };
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int main(int, char *[]) {
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flecs::world ecs;
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flecs::entity bob = ecs.entity("Bob");
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flecs::entity alice = ecs.entity("Alice");
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flecs::entity jane = ecs.entity("Jane");
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flecs::entity john = ecs.entity("John");
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bob.add<Likes>(alice);
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alice.add<Likes>(bob);
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jane.add<Likes>(john);
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john.add<Likes>(jane);
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bob.add<Likes>(john); // bit of drama
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// Create a rule that checks if two entities like each other. By itself this
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// rule is not a fact, but we can use it to check facts by populating both
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// of its variables.
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//
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// The equivalent query in the DSL is:
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// Likes($X, $Y), Likes($Y, $X)
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//
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// Instead of using variables we could have created a rule that referred the
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// entities directly, but then we would have to create a rule for each
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// fact, vs reusing a single rule for multiple facts.
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flecs::rule<> friends = ecs.rule_builder()
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.with<Likes>("$Y").src("$X")
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.with<Likes>("$X").src("$Y")
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.build();
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int x_var = friends.find_var("X");
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int y_var = friends.find_var("Y");
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// Check a few facts
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std::cout << "Are Bob and Alice friends? " <<
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(friends.iter()
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.set_var(x_var, bob)
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.set_var(y_var, alice)
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.is_true() ? "Yes\n" : "No\n");
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std::cout << "Are Bob and John friends? " <<
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(friends.iter()
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.set_var(x_var, bob)
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.set_var(y_var, john)
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.is_true() ? "Yes\n" : "No\n");
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std::cout << "Are Jane and John friends? " <<
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(friends.iter()
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.set_var(x_var, jane)
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.set_var(y_var, john)
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.is_true() ? "Yes\n" : "No\n");
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// It doesn't matter who we assign to X or Y. After the variables are
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// substituted, either yields a fact that is true.
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std::cout << "Are John and Jane friends? " <<
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(friends.iter()
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.set_var(x_var, john)
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.set_var(y_var, jane)
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.is_true() ? "Yes\n" : "No\n");
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// Output
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// Are Bob and Alice friends? Yes
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// Are Bob and John friends? No
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// Are Jane and John friends? Yes
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// Are John and Jane friends? Yes
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}
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