From 4ebb31f26efed74f8fcb049fe27ddf7cf5388a08 Mon Sep 17 00:00:00 2001
From: Murilo Ijanc <murilo@ijanc.org>
Date: Mon, 27 Jul 2026 20:31:55 -0300
Subject: Carry the municipality of a CEP that does not exist

A CEP that fails is rarely a random number: it is someone who knows the
city and guessed the digits, and the classic guess is the -000 that 957
municipalities never received. The API now answers the 404 with the
municipality that band belongs to, which is enough to tell the user
"Areal/RJ, but not that number" instead of a bare failure.

It stays an error. Faixa hangs off CEPNotFound rather than becoming a
result, cep_ini/cep_fim are documented as the CEPs that exist there and
not as a range to iterate, and an absent or unreadable block yields
undefined so that context can never cost the caller the clean not-found
it was already getting.
---
 src/models.ts | 99 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 99 insertions(+)

(limited to 'src/models.ts')

diff --git a/src/models.ts b/src/models.ts
index 7e20b54..7125336 100644
--- a/src/models.ts
+++ b/src/models.ts
@@ -246,6 +246,105 @@ export class Address {
   }
 }
 
+/** Every field of a {@link Faixa}, as plain data. */
+export interface FaixaFields {
+  localidade: string;
+  uf: string;
+  estado: string;
+  regiao: string;
+  ibge: string;
+  cep_ini: string;
+  cep_fim: string;
+  ceps: number;
+}
+
+/**
+ * The municipality a CEP that does not exist *would* belong to.
+ *
+ * It comes attached to {@link CEPNotFound}, on a response that is still a
+ * **404**. It is context for an error, never an address: nothing here says the
+ * CEP exists, and code that treats a `faixa` as a result turns a clean
+ * not-found into a wrong delivery.
+ *
+ * It exists because a failing CEP is rarely random — it is someone who knows
+ * the city and guessed the number, and the classic guess is the `-000` that 957
+ * municipalities never received.
+ *
+ * `cep_ini` and `cep_fim` are the smallest and the largest CEP that **actually
+ * exist** in that municipality, not the range the Correios publish for it. For
+ * Areal/RJ the DNE's own start is `25845000` — precisely the CEP that just
+ * failed — while `cep_ini` is `25845001`.
+ *
+ * The range is **sparse**. Areal's 118 CEPs are scattered across the ~4,900
+ * numbers between `25845001` and `25849899`, so the pair is not a list of valid
+ * CEPs and iterating it produces mostly 404s. `ceps` is there to make that
+ * density visible: 118 out of 4,900.
+ */
+export class Faixa implements FaixaFields {
+  /** Municipality that owns the band the CEP falls in. */
+  readonly localidade: string;
+  /** Two-letter state code, such as `"RJ"`. */
+  readonly uf: string;
+  /** State name, spelled out. */
+  readonly estado: string;
+  /** Region of the country, such as `"Sudeste"`. */
+  readonly regiao: string;
+  /** IBGE municipality code. */
+  readonly ibge: string;
+  /** Smallest CEP that exists there — not the band's published start. */
+  readonly cep_ini: string;
+  /** Largest CEP that exists there — not the band's published end. */
+  readonly cep_fim: string;
+  /** How many CEPs actually exist between `cep_ini` and `cep_fim`. */
+  readonly ceps: number;
+
+  constructor(fields: FaixaFields) {
+    this.localidade = fields.localidade;
+    this.uf = fields.uf;
+    this.estado = fields.estado;
+    this.regiao = fields.regiao;
+    this.ibge = fields.ibge;
+    this.cep_ini = fields.cep_ini;
+    this.cep_fim = fields.cep_fim;
+    this.ceps = fields.ceps;
+  }
+
+  /** The municipality as `"Areal/RJ"`, for a message shown to a person. */
+  get label(): string {
+    return this.uf === "" ? this.localidade : `${this.localidade}/${this.uf}`;
+  }
+
+  /**
+   * Build from the `faixa` block of a 404 body, or `undefined` when there is
+   * none to read.
+   *
+   * `undefined`, not `null`, because the API **omits** the field rather than
+   * sending null when the CEP falls in a band no municipality owns. Absent and
+   * unreadable are the same answer here: a malformed block must never cost the
+   * caller the not-found error it was already getting.
+   */
+  static fromJson(data: unknown): Faixa | undefined {
+    if (typeof data !== "object" || data === null || Array.isArray(data)) {
+      return undefined;
+    }
+    const block = data as RawObject;
+    const localidade = toText(block.localidade).trim();
+    if (localidade === "") {
+      return undefined;
+    }
+    return new Faixa({
+      localidade,
+      uf: toText(block.uf),
+      estado: toText(block.estado),
+      regiao: toText(block.regiao),
+      ibge: toText(block.ibge),
+      cep_ini: toText(block.cep_ini),
+      cep_fim: toText(block.cep_fim),
+      ceps: toNumber(block.ceps) ?? 0,
+    });
+  }
+}
+
 /** Every field of a {@link Health} report, as plain data. */
 export interface HealthFields {
   status: string;
-- 
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