package render

import (
	"fmt"
	"html/template"
	"strconv"
	"strings"
	"time"

	"khosra/internal/content"
)

// Chrome is every word the engine puts on a page that the author did not write, in each language it
// knows (ADR-0034). Keyed by phrase then language, so both forms sit side by side and a missing one is
// visible while reading rather than at request time.
//
// A phrase may carry %s placeholders, filled in order by the caller. Templates reach these through the
// `t`, `num` and `day` functions; nothing here is content, and content never comes from here.
var chrome = map[string]map[string]string{
	"newer":    {"en": "Newer", "bn": "নতুন"},
	"older":    {"en": "Older", "bn": "পুরোনো"},
	"empty":    {"en": "Nothing here yet.", "bn": "এখনও কিছু নেই।"},
	"page-of":  {"en": "Page %s of %s", "bn": "পৃষ্ঠা %s / %s"},
	"position": {"en": "%s of %s", "bn": "%s / %s"},
}

// months are Gregorian month names per language, indexed by [time.Month]-1.
var months = map[string][]string{
	"en": {"January", "February", "March", "April", "May", "June",
		"July", "August", "September", "October", "November", "December"},
	"bn": {"জানুয়ারি", "ফেব্রুয়ারি", "মার্চ", "এপ্রিল", "মে", "জুন",
		"জুলাই", "আগস্ট", "সেপ্টেম্বর", "অক্টোবর", "নভেম্বর", "ডিসেম্বর"},
}

// digits are the decimal digits of a script, indexed 0-9. A language absent here uses ASCII.
var digits = map[string][]rune{
	"bn": []rune("০১২৩৪৫৬৭৮৯"),
}

// funcs are the chrome helpers a template may call. Registered before parsing, so a site root's override
// blocks may use them too.
var funcs = template.FuncMap{"t": text, "num": numerals, "day": day}

// text is one chrome phrase in a language, with any %s placeholders filled.
//
// It cannot fail: an unknown language falls back to the default locale and an unknown phrase returns its
// own key, because a missing translation must never blank a page or break a render (invariant 1).
func text(lang, key string, args ...string) string {
	forms := chrome[key]
	phrase, ok := forms[lang]
	if !ok {
		phrase = forms[content.DefaultLang]
	}
	if phrase == "" {
		return key
	}
	if len(args) == 0 {
		return phrase
	}
	filled := make([]any, len(args))
	for i, a := range args {
		filled[i] = a
	}
	return fmt.Sprintf(phrase, filled...)
}

// numerals is an integer in the script of a language: 12 in English, ১২ in Bengali.
func numerals(lang string, n int) string {
	return localiseDigits(lang, strconv.Itoa(n))
}

// day is a date as a reader of that language would read it — "8 March 2026", "৮ মার্চ ২০২৬".
//
// One path for every language rather than [time.Time.Format] for the default locale and a hand-built
// string for the rest, so English cannot drift from the others. The machine-readable form belongs in a
// datetime attribute and stays ASCII (ADR-0034).
func day(lang string, t time.Time) string {
	if t.IsZero() {
		return ""
	}
	names, ok := months[lang]
	if !ok {
		names = months[content.DefaultLang]
	}
	return strings.Join([]string{
		numerals(lang, t.Day()),
		names[t.Month()-1],
		numerals(lang, t.Year()),
	}, " ")
}

// localiseDigits swaps ASCII digits for a script's own, leaving everything else alone.
func localiseDigits(lang, s string) string {
	set, ok := digits[lang]
	if !ok {
		return s
	}
	out := []rune(s)
	for i, r := range out {
		if r >= '0' && r <= '9' {
			out[i] = set[r-'0']
		}
	}
	return string(out)
}