Name the municipality behind a CEP that does not existHEADv0.2.0main

The API now hangs an optional faixa block off the 404 of GET /{cep}, and
this exposes it as CepNotFoundException.Faixa. A CEP that fails is rarely
random: it is usually someone who knows the city and guessed the number,
most often the -000 that 957 municipalities never received. Knowing where
the guess landed turns a dead-end "CEP not found" into a question worth
asking the user.

It stays an error. The block names the municipality the number would
belong to, never an address, and the lookup still throws. CepIni and
CepFim are the smallest and largest CEP that actually exist there, not the
range the Correios publish — the published start for Areal is 25845000,
exactly the CEP that fails — and the interval between them is sparse,
which is what Ceps is for.

Parsing is deliberately toothless: absent, misshapen or nameless, the
block becomes null and the caller gets the same clean not-found it got
before the field existed. A courtesy on top of a 404 must never be able to
turn it into a parse failure.

1 files changed, 98 insertions, 0 deletions

diff --git a/src/Cepis/FaixaReader.cs b/src/Cepis/FaixaReader.cs
new file mode 100644
index 0000000..da3c6a2
--- /dev/null
+++ b/src/Cepis/FaixaReader.cs
@@ -0,0 +1,98 @@
+using System.Globalization;
+using System.Text.Json;
+
+namespace Cepis;
+
+/// <summary>
+/// Reads the <c>faixa</c> block of a 404 body, yielding <see langword="null"/>
+/// for anything it cannot make sense of.
+/// </summary>
+/// <remarks>
+/// <para>
+/// A reader rather than a <see cref="System.Text.Json.Serialization.JsonConverter{T}"/>
+/// because <c>faixa</c> is not a field of any model this library deserializes:
+/// it hangs off an error body that is otherwise only read for its message.
+/// </para>
+/// <para>
+/// Nothing in here throws. The block is a courtesy on top of a 404 — a missing,
+/// misshapen or half-empty one must leave the caller with the same clean
+/// <see cref="CepNotFoundException"/> it would have got before the API grew the
+/// field, never with a parse failure on top of a not-found.
+/// </para>
+/// </remarks>
+internal static class FaixaReader
+{
+    /// <summary>Turn the <c>faixa</c> element into a range, or null.</summary>
+    /// <param name="element">the block, as it came off the wire</param>
+    /// <returns>the range, or null when there is nothing usable in it</returns>
+    internal static Faixa? Read(JsonElement element)
+    {
+        if (element.ValueKind != JsonValueKind.Object)
+        {
+            return null;
+        }
+
+        var localidade = ReadString(element, "localidade");
+        if (string.IsNullOrWhiteSpace(localidade))
+        {
+            return null;
+        }
+
+        return new Faixa
+        {
+            Localidade = localidade,
+            UF = ReadString(element, "uf"),
+            Estado = ReadString(element, "estado"),
+            Regiao = ReadString(element, "regiao"),
+            Ibge = ReadString(element, "ibge"),
+            CepIni = ReadString(element, "cep_ini"),
+            CepFim = ReadString(element, "cep_fim"),
+            Ceps = ReadInt(element, "ceps"),
+        };
+    }
+
+    /// <summary>
+    /// The property as text, or the empty string when it is absent or of a
+    /// shape that is not text.
+    /// </summary>
+    /// <remarks>
+    /// A number is accepted and taken verbatim, the way
+    /// <see cref="LenientStringConverter"/> does it for the address fields.
+    /// </remarks>
+    private static string ReadString(JsonElement owner, string name)
+    {
+        if (!owner.TryGetProperty(name, out var element))
+        {
+            return "";
+        }
+
+        return element.ValueKind switch
+        {
+            JsonValueKind.String => element.GetString() ?? "",
+            JsonValueKind.Number => element.GetRawText(),
+            _ => "",
+        };
+    }
+
+    /// <summary>The property as a count, or zero when it cannot be read.</summary>
+    private static int ReadInt(JsonElement owner, string name)
+    {
+        if (!owner.TryGetProperty(name, out var element))
+        {
+            return 0;
+        }
+
+        return element.ValueKind switch
+        {
+            JsonValueKind.Number => element.TryGetInt32(out var number) ? number : 0,
+            JsonValueKind.String => int.TryParse(
+                element.GetString(),
+                NumberStyles.Integer,
+                CultureInfo.InvariantCulture,
+                out var parsed)
+                ? parsed
+                : 0,
+            _ => 0,
+        };
+    }
+}