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@@ -201,6 +201,39 @@ if errors.As(err, &cerr) && errors.Is(cerr, cepis.ErrRateLimited) { answering instead of it — a captive Wi-Fi portal, a corporate proxy, an HTML error page. Useful as a diagnosis: the problem is not in your code. +### Which city the missing CEP was from + +A CEP that fails is rarely a random number — it is usually someone who knows +the city and guessed the rest. The classic guess is the `-000`: a small town +often has a single city-wide CEP ending that way, but 957 municipalities are +coded street by street and never got one. Areal/RJ answers from `25845-001` up, +never at `25845-000`. + +When the API can name the municipality the number would fall in, it comes back +on the error: + +```go +_, err := cepis.Lookup(ctx, "25845-000") + +var cerr *cepis.Error +if errors.As(err, &cerr) && cerr.Faixa != nil { + fmt.Printf("%s/%s has %d CEPs, from %s to %s\n", + cerr.Faixa.Localidade, cerr.Faixa.UF, cerr.Faixa.CEPs, + cerr.Faixa.CEPIni, cerr.Faixa.CEPFim) + // Areal/RJ has 118 CEPs, from 25845001 to 25849899 +} +``` + +`CEPIni` and `CEPFim` are the smallest and largest CEP that **exist** there, not +the bounds the Correios publish — the published start is the `-000` that just +failed. The range is sparse, which is what `CEPs` is for: those 118 are +scattered across nearly five thousand numbers, so do not read the pair as a list +of valid CEPs. + +`Faixa` is `nil` whenever the API did not send one, including for a CEP in a +band that belongs to no municipality at all. It is an extra on top of an error +that is complete without it. + ## Development Everything runs through `make`: |