package handlers

import (
	"errors"
	"net/http"
	"strconv"
	"strings"
)

// maxTargetRetries bounds a target's max_retries.
//
// Both target forms already declare max="20" on the input, so this
// enforces server-side what the UI has always advertised rather than
// introducing a new limit.
//
// The number is not cosmetic. Every attempt writes a delivery_results
// row that the event log then loads and renders, and the engine backs
// off by 2^(n-1) seconds, so attempt 20 is already about six days
// after the first. A value beyond this buys no additional durability
// and only costs rows.
const maxTargetRetries = 20

// Errors returned when a max_retries form value cannot be turned into
// a retry count.
var (
	// errRetriesInvalid signals a max_retries form value that is not
	// a non-negative whole number.
	errRetriesInvalid = errors.New(
		"retries must be a whole number of attempts",
	)

	// errRetriesTooLarge signals a max_retries form value that is a
	// whole number but above maxTargetRetries. It is distinguished
	// from errRetriesInvalid so the message can name the ceiling
	// instead of implying the input was not a number.
	errRetriesTooLarge = errors.New("retries out of range")
)

// parseMaxRetries interprets a max_retries form value.
//
// An ABSENT value — the field empty or not submitted — yields
// fallback, which lets the create path apply its default and the edit
// path leave the stored value alone. A value that is SET BUT INVALID
// is an error: unparseable, negative, or above maxTargetRetries.
//
// The distinction is the whole point of this function. max_retries=0
// means fire-and-forget, so returning 0 for input the operator typed
// but that did not parse silently disables retries on a
// store-and-forward proxy — and on the edit path it destroys a
// working retry configuration over a typo. A default answers a
// question that was not asked; it never answers one that was asked
// badly.
//
// A target stored with a count above the ceiling before this
// validation existed keeps rendering and keeps delivering — nothing
// clamps the row. Re-saving it from the edit form does have to bring
// it into range, because the form submits the pre-filled value back
// and accepting it would be the ceiling not applying to the edit
// path. The 400 names the ceiling, so the fix is one field.
func parseMaxRetries(raw string, fallback int) (int, error) {
	raw = strings.TrimSpace(raw)
	if raw == "" {
		return fallback, nil
	}

	v, err := strconv.Atoi(raw)
	if err != nil || v < 0 {
		return 0, errRetriesInvalid
	}

	if v > maxTargetRetries {
		return 0, errRetriesTooLarge
	}

	return v, nil
}

// retriesErrorMessage returns the message the create and edit forms
// show for a rejected max_retries value. Any error other than
// errRetriesTooLarge falls back to the generic wording, so an
// unrecognised parse failure still produces a sensible 400.
func retriesErrorMessage(err error) string {
	if errors.Is(err, errRetriesTooLarge) {
		return errRetriesTooLarge.Error() +
			": at most " + strconv.Itoa(maxTargetRetries) +
			" retries"
	}

	return errRetriesInvalid.Error() +
		", or 0 for fire-and-forget"
}

// targetMaxRetries reads and validates max_retries from a target form
// submission, answering the request with a 400 and reporting false
// when the value is set but invalid.
//
// Both the create and the edit path go through here, so the two
// cannot come to disagree about what a valid retry count is. The
// wording matches the timeout control on the same submission.
func targetMaxRetries(
	w http.ResponseWriter,
	r *http.Request,
	fallback int,
) (int, bool) {
	retries, err := parseMaxRetries(
		r.PostFormValue("max_retries"), fallback,
	)
	if err != nil {
		http.Error(
			w,
			"Invalid max retries: "+retriesErrorMessage(err),
			http.StatusBadRequest,
		)

		return 0, false
	}

	return retries, true
}