Report the municipality behind a not-found CEPHEADv0.2.0main

A CEP that fails is rarely a random number. It is usually someone who
knows the city and guessed the last digits, and the classic guess is the
-000 that 957 municipalities never received. The API now answers those
404s with a faixa block naming the municipality the CEP would have
belonged to, and telling a caller "that CEP does not exist, did you mean
somewhere in Areal/RJ?" is worth far more than telling them "no".

It rides on cepis_error_t rather than on cepis_address_t because it is
context for a failure, not an address: the lookup still returns
CEPIS_ERR_NOT_FOUND and still hands back no address. Embedded by value
with a has_faixa flag, so the error keeps owning nothing and keeps
living on the stack with nothing to free; cepis_error_faixa gives the
absent case the same NULL-or-not shape as cepis_address_t::geo, which is
what the API's omit-never-null encoding calls for.

Parsing is total on purpose. A body that is not JSON, a faixa that is
null, a string, an array, or an object with no localidade all mean "the
API named no municipality". A 404 is already the answer, and failing to
read the garnish must never turn it into a different one.

The bounds need reading with care, hence the length of the comments:
cep_ini and cep_fim are the smallest and largest CEP that actually
exist, not the range the Correios publish -- the DNE's own start for
Areal is 25845000, exactly the CEP that just failed -- and the range is
sparse, 118 CEPs scattered over the ~4900 numbers up to 25849899. That
is what ceps is for.

1 files changed, 15 insertions, 0 deletions

diff --git a/src/internal.h b/src/internal.h
index 57487c2..700b1ee 100644
--- a/src/internal.h
+++ b/src/internal.h
@@ -71,4 +71,19 @@ cepis_address_t *cepis_address_from_csv(const csv_row_t *header,
 /** @brief Build a health report from a parsed JSON object. */
 cepis_health_t *cepis_health_from_json(const json_value_t *object);
 
+/**
+ * @brief Read the `faixa` block of a 404 body into @p out.
+ * @param object The decoded body, which may be `NULL`.
+ * @param out    Filled and returning `true`, or untouched and returning
+ *               `false`.
+ * @return `true` only when the block is present, is an object and names a
+ *         municipality.
+ *
+ * Deliberately total: a body that is not JSON, a `faixa` that is null, a
+ * string, an array or an object without `localidade` all mean "the API named
+ * no municipality". A 404 is already the answer, and failing to read the
+ * garnish must never turn it into a different one.
+ */
+bool cepis_faixa_from_json(const json_value_t *object, cepis_faixa_t *out);
+
 #endif