4 files changed, 120 insertions, 4 deletions
diff --git a/src/main/kotlin/org/cepis/Cepis.kt b/src/main/kotlin/org/cepis/Cepis.kt
index b9e0e34..2cd3516 100644
--- a/src/main/kotlin/org/cepis/Cepis.kt
+++ b/src/main/kotlin/org/cepis/Cepis.kt
@@ -11,7 +11,7 @@ const val DEFAULT_BASE_URL: String = "https://cep.is"
 const val MAX_BATCH: Int = 500
 
 /** This library's version, as it appears in the default `User-Agent`. */
-const val VERSION: String = "0.1.0"
+const val VERSION: String = "0.2.0"
 
 private val DIGITS_ONLY = Regex("""^\d{8}$""")
 
diff --git a/src/main/kotlin/org/cepis/Client.kt b/src/main/kotlin/org/cepis/Client.kt
index e315b74..6a054ae 100644
--- a/src/main/kotlin/org/cepis/Client.kt
+++ b/src/main/kotlin/org/cepis/Client.kt
@@ -95,6 +95,10 @@ class Client(
      * API's own message: the format already passed local validation, so the
      * server knows something this client does not.
      *
+     * A 404 may carry a [Faixa] in [CepNotFoundException.faixa] naming the
+     * municipality the CEP would have belonged to. It is still a 404 — the
+     * block is context for the failure, not an address.
+     *
      * @throws InvalidCepException the CEP is not 8 digits (no request is made).
      * @throws CepNotFoundException well-formed, but not in the database.
      * @throws RateLimitedException 429 after every attempt.
@@ -107,7 +111,12 @@ class Client(
         val reply = request("/$number")
 
         if (reply.status == 404) {
-            throw CepNotFoundException("CEP $number does not exist in the DNE database")
+            val faixa = faixaFrom(reply.body)
+            val where = faixa?.let { " — that number falls in ${it.localidade}/${it.uf}" } ?: ""
+            throw CepNotFoundException(
+                "CEP $number does not exist in the DNE database$where",
+                faixa,
+            )
         }
         if (reply.status == 400) {
             throw InvalidCepException(errorMessage(reply.body, "CEP $number rejected"))
@@ -210,6 +219,24 @@ class Client(
         return parsed as Map<String, Any?>
     }
 
+    /**
+     * The `faixa` block of a 404 body, or `null`.
+     *
+     * Swallows everything: a body that is not JSON, is not an object, or has
+     * no usable `faixa`. The lookup has already failed and the caller is going
+     * to get [CepNotFoundException] either way — losing that exception to a
+     * parse error over an optional field would be a bad trade.
+     */
+    private fun faixaFrom(body: ByteArray): Faixa? {
+        val parsed =
+            try {
+                Json.parse(body.toString(StandardCharsets.UTF_8))
+            } catch (e: JsonSyntaxException) {
+                return null
+            }
+        return Faixa.fromJson((parsed as? Map<*, *>)?.get("faixa"))
+    }
+
     private fun errorMessage(
         body: ByteArray,
         fallback: String,
diff --git a/src/main/kotlin/org/cepis/Errors.kt b/src/main/kotlin/org/cepis/Errors.kt
index 8d3f1ff..d80df57 100644
--- a/src/main/kotlin/org/cepis/Errors.kt
+++ b/src/main/kotlin/org/cepis/Errors.kt
@@ -40,8 +40,19 @@ sealed class CepisException(
  */
 class InvalidCepException(message: String) : CepisException(message)
 
-/** The CEP is well-formed but does not exist in the DNE database. */
-class CepNotFoundException(message: String) : CepisException(message)
+/**
+ * The CEP is well-formed but does not exist in the DNE database.
+ *
+ * @property faixa the municipality the CEP would have belonged to, when the
+ *   API can name one, and `null` when the number falls in a band no
+ *   municipality owns. It is **context for the failure, not an address**: this
+ *   is still a 404 and there is still no CEP. Read [Faixa] before showing its
+ *   range to anyone — the two endpoints are not a list of valid CEPs.
+ */
+class CepNotFoundException(
+    message: String,
+    val faixa: Faixa? = null,
+) : CepisException(message)
 
 /**
  * The request never completed: DNS, refused connection, timeout, TLS.
diff --git a/src/main/kotlin/org/cepis/Models.kt b/src/main/kotlin/org/cepis/Models.kt
index 058244e..7841082 100644
--- a/src/main/kotlin/org/cepis/Models.kt
+++ b/src/main/kotlin/org/cepis/Models.kt
@@ -39,6 +39,84 @@ data class Geo(
 }
 
 /**
+ * The municipality a **nonexistent** CEP would have belonged to.
+ *
+ * This is context for an error, never an address. It arrives on the 404 of
+ * [Client.lookup], in [CepNotFoundException.faixa], and it does not make the
+ * CEP exist: the lookup still failed.
+ *
+ * It is there because a CEP that fails is rarely a random number — it is
+ * usually someone who knows the city and guessed the digits. The classic guess
+ * is the `-000`, which 957 municipalities never received.
+ *
+ * Two things about the range that are easy to get wrong:
+ *
+ * - [cepIni] and [cepFim] are the smallest and the largest CEP that **actually
+ *   exist** in that municipality, not the bounds the Correios publish. For
+ *   Areal the DNE's own start is `25845000` — which is exactly the CEP that
+ *   just failed. Echoing that back would be useless, so the API does not.
+ * - The range is **sparse**. Areal's 118 CEPs are spread across some 4,900
+ *   numbers. [cepIni]/[cepFim] are two endpoints, not a list of valid CEPs,
+ *   and [ceps] is there to keep that from being forgotten.
+ *
+ * @property localidade the municipality's name.
+ * @property uf its two-letter state code.
+ * @property estado the state's full name.
+ * @property regiao the region of Brazil it sits in.
+ * @property ibge the municipality's IBGE code — the field to use to look the
+ *   place up somewhere else.
+ * @property cepIni the smallest CEP that exists there. The API calls this
+ *   field `cep_ini`.
+ * @property cepFim the largest CEP that exists there. The API calls this field
+ *   `cep_fim`.
+ * @property ceps how many CEPs exist between those two endpoints. Almost
+ *   always far fewer than the numeric distance between them.
+ */
+data class Faixa(
+    val localidade: String = "",
+    val uf: String = "",
+    val estado: String = "",
+    val regiao: String = "",
+    val ibge: String = "",
+    val cepIni: String = "",
+    val cepFim: String = "",
+    val ceps: Int = 0,
+) {
+    companion object {
+        /**
+         * Build from the `faixa` block of a 404 body, or `null`.
+         *
+         * Takes `Any?` rather than a map, and returns `null` for anything that
+         * is not a usable block — absent, not an object, empty, or without a
+         * municipality name. The API **omits** `faixa` (it never sends null)
+         * when the CEP falls in a numeric band no municipality owns, and that
+         * is the common case this has to survive.
+         *
+         * Nothing here throws, deliberately. The caller is already handling a
+         * failed lookup; a surprise in this block must not turn a clean
+         * [CepNotFoundException] into a parse error.
+         */
+        fun fromJson(block: Any?): Faixa? {
+            if (block !is Map<*, *>) return null
+            @Suppress("UNCHECKED_CAST")
+            val fields = block as Map<String, Any?>
+            val localidade = fields.text("localidade")
+            if (localidade.isEmpty()) return null
+            return Faixa(
+                localidade = localidade,
+                uf = fields.text("uf"),
+                estado = fields.text("estado"),
+                regiao = fields.text("regiao"),
+                ibge = fields.text("ibge"),
+                cepIni = fields.text("cep_ini"),
+                cepFim = fields.text("cep_fim"),
+                ceps = fields.number("ceps")?.toInt() ?: 0,
+            )
+        }
+    }
+}
+
+/**
  * A CEP and everything the database knows about it.
  *
  * **Field names are the API's own, in Portuguese** — `logradouro`, `bairro`,