105 lines
3.7 KiB
C
105 lines
3.7 KiB
C
#include <facts.h>
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#include <stdio.h>
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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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int main(int argc, char *argv[]) {
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ecs_world_t *ecs = ecs_init_w_args(argc, argv);
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ECS_TAG(ecs, Likes); // Likes relationship
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ecs_entity_t bob = ecs_new_entity(ecs, "Bob");
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ecs_entity_t alice = ecs_new_entity(ecs, "Alice");
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ecs_entity_t john = ecs_new_entity(ecs, "John");
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ecs_entity_t jane = ecs_new_entity(ecs, "Jane");
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ecs_add_pair(ecs, bob, Likes, alice);
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ecs_add_pair(ecs, alice, Likes, bob);
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ecs_add_pair(ecs, john, Likes, jane);
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ecs_add_pair(ecs, jane, Likes, john);
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ecs_add_pair(ecs, bob, Likes, jane); // inserting a 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 of this 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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//
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// See the setting_variables example for more details
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ecs_rule_t *friends = ecs_rule(ecs, {
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.terms = {
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{
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.first.id = Likes,
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.src = { .name = "X", .flags = EcsIsVariable },
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.second = { .name = "Y", .flags = EcsIsVariable }
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},
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{
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.first.id = Likes,
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.src = { .name = "Y", .flags = EcsIsVariable },
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.second = { .name = "X", .flags = EcsIsVariable }
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}
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}
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});
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// Lookup the index of the variables.
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int x_var = ecs_rule_find_var(friends, "X");
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int y_var = ecs_rule_find_var(friends, "Y");
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ecs_iter_t it = ecs_rule_iter(ecs, friends);
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ecs_iter_set_var(&it, x_var, bob);
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ecs_iter_set_var(&it, y_var, alice);
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printf("Are Bob and Alice friends? %s\n",
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ecs_iter_is_true(&it) ? "Yes" : "No");
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it = ecs_rule_iter(ecs, friends);
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ecs_iter_set_var(&it, x_var, bob);
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ecs_iter_set_var(&it, y_var, john);
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printf("Are Bob and John friends? %s\n",
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ecs_iter_is_true(&it) ? "Yes" : "No");
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it = ecs_rule_iter(ecs, friends);
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ecs_iter_set_var(&it, x_var, jane);
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ecs_iter_set_var(&it, y_var, john);
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printf("Are Jane and John friends? %s\n",
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ecs_iter_is_true(&it) ? "Yes" : "No");
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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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it = ecs_rule_iter(ecs, friends);
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ecs_iter_set_var(&it, x_var, john);
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ecs_iter_set_var(&it, y_var, jane);
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printf("Are John and Jane friends? %s\n",
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ecs_iter_is_true(&it) ? "Yes" : "No");
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ecs_rule_fini(friends);
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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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return ecs_fini(ecs);
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}
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