// Copyright (c) 2026 Zeni Kim <zenik@smarteching.com>
// Use of this source code is governed by MIT-style
// license that can be found in the LICENSE file.

package core

import (
	"embed"
	"fmt"
	"html/template"
	"io/fs"
	"math"
	"reflect"
	"strings"
	"time"
	"unicode"
	"golang.org/x/text/language"
	"golang.org/x/text/message"
)

var tmpl *template.Template = nil

// ─────────────────────────────────────────────
// Struct helpers
// ─────────────────────────────────────────────

// hasField checks if a struct has a given field name.
// It can be used inside Go html/templates like:
//
//	{{if hasField . "FieldA"}}<a>{{.FieldA}}</a>{{end}}
func hasField(v interface{}, name string) bool {
	rv := reflect.ValueOf(v)
	if rv.Kind() == reflect.Ptr {
		rv = rv.Elem()
	}
	if rv.Kind() != reflect.Struct {
		return false
	}
	return rv.FieldByName(name).IsValid()
}

// ─────────────────────────────────────────────
// String helpers
// ─────────────────────────────────────────────

// capitalize uppercases the first letter of a string and lowercases the rest.
//
//	{{capitalize "hello world"}} → "Hello world"
func capitalize(s string) string {
	if s == "" {
		return ""
	}
	runes := []rune(s)
	runes[0] = unicode.ToUpper(runes[0])
	for i := 1; i < len(runes); i++ {
		runes[i] = unicode.ToLower(runes[i])
	}
	return string(runes)
}

// prepend adds a prefix string to the beginning of s.
//
//	{{prepend "World" "Hello, "}} → "Hello, World"
func prepend(s, prefix string) string {
	return prefix + s
}

// strAppend adds a suffix string to the end of s.
// Named strAppend to avoid collision with the builtin append keyword.
//
//	{{strAppend "Hello" "!"}} → "Hello!"
func strAppend(s, suffix string) string {
	return s + suffix
}

// split divides s into a slice of substrings separated by sep.
// Useful when combined with range in templates.
//
//	{{range split .Tags ","}}...{{end}}
func split(s, sep string) []string {
	return strings.Split(s, sep)
}

// truncate cuts s to at most n characters. Designed for pipeline use:
//   {{.Title | truncate 30}}
func truncate(n int, s string) string {
    runes := []rune(s)
    if len(runes) <= n {
        return s
    }
    return string(runes[:n]) + "…"
}

// ─────────────────────────────────────────────
// Number helpers
// ─────────────────────────────────────────────

// fmtNumber formats an integer or float with thousands separators
// using the English locale (e.g. 1000000 → "1,000,000").
//
//	{{fmtNumber .Price}}
func fmtNumber(v interface{}) string {
	p := message.NewPrinter(language.English)
	switch n := v.(type) {
	case int:
		return p.Sprintf("%d", n)
	case int64:
		return p.Sprintf("%d", n)
	case float64:
		// trim unnecessary trailing zeros
		formatted := p.Sprintf("%.2f", n)
		return formatted
	case float32:
		return p.Sprintf("%.2f", n)
	default:
		return fmt.Sprintf("%v", v)
	}
}

// ─────────────────────────────────────────────
// Date & time helpers
// ─────────────────────────────────────────────

// fmtDate formats a time.Time value using a named layout or a custom Go layout string.
// Named layouts: "short" → "02 Jan 2006", "long" → "02 January 2006",
// "iso" → "2006-01-02", "datetime" → "02 Jan 2006 15:04".
// Any other string is used directly as a Go time layout.
//
//	{{fmtDate .PublishedAt "short"}}
//	{{fmtDate .CreatedAt "02/01/2006"}}
func fmtDate(t time.Time, layout string) string {
	switch layout {
	case "short":
		return t.Format("02 Jan 2006")
	case "long":
		return t.Format("02 January 2006")
	case "iso":
		return t.Format("2006-01-02")
	case "datetime":
		return t.Format("02 Jan 2006 15:04")
	default:
		return t.Format(layout)
	}
}

// timeAgo returns a human-readable relative time string from now.
//
//	{{timeAgo .PublishedAt}} → "3 hours ago", "2 days ago", "just now"
func timeAgo(t time.Time) string {
	diff := time.Since(t)

	switch {
	case diff < time.Minute:
		return "just now"
	case diff < time.Hour:
		m := int(math.Round(diff.Minutes()))
		return plural(m, "minute") + " ago"
	case diff < 24*time.Hour:
		h := int(math.Round(diff.Hours()))
		return plural(h, "hour") + " ago"
	case diff < 7*24*time.Hour:
		d := int(math.Round(diff.Hours() / 24))
		return plural(d, "day") + " ago"
	case diff < 30*24*time.Hour:
		w := int(math.Round(diff.Hours() / 24 / 7))
		return plural(w, "week") + " ago"
	case diff < 365*24*time.Hour:
		mo := int(math.Round(diff.Hours() / 24 / 30))
		return plural(mo, "month") + " ago"
	default:
		y := int(math.Round(diff.Hours() / 24 / 365))
		return plural(y, "year") + " ago"
	}
}

// plural is an internal helper that returns "1 item" or "N items".
func plural(n int, unit string) string {
	if n == 1 {
		return fmt.Sprintf("%d %s", n, unit)
	}
	return fmt.Sprintf("%d %ss", n, unit)
}

// ─────────────────────────────────────────────
// Collection helpers
// ─────────────────────────────────────────────

// first returns the first element of a slice, or nil if empty.
//
//	{{with first .Items}}{{.Name}}{{end}}
func first(slice interface{}) interface{} {
	rv := reflect.ValueOf(slice)
	if rv.Kind() != reflect.Slice || rv.Len() == 0 {
		return nil
	}
	return rv.Index(0).Interface()
}

// last returns the last element of a slice, or nil if empty.
//
//	{{with last .Items}}{{.Name}}{{end}}
func last(slice interface{}) interface{} {
	rv := reflect.ValueOf(slice)
	if rv.Kind() != reflect.Slice || rv.Len() == 0 {
		return nil
	}
	return rv.Index(rv.Len() - 1).Interface()
}

// sliceOf returns a sub-range of a slice from index start (inclusive) to end (exclusive).
// Named sliceOf to avoid collision with the builtin slice expression.
//
//	{{range sliceOf .Items 0 3}}...{{end}}
func sliceOf(slice interface{}, start, end int) interface{} {
	rv := reflect.ValueOf(slice)
	if rv.Kind() != reflect.Slice {
		return nil
	}
	if start < 0 {
		start = 0
	}
	if end > rv.Len() {
		end = rv.Len()
	}
	return rv.Slice(start, end).Interface()
}

// contains reports whether item is present in slice (or substr in a string).
//
//	{{if contains .Tags "go"}}...{{end}}
//	{{if contains .Bio "engineer"}}...{{end}}
func contains(collection, item interface{}) bool {
	// string substring check
	if s, ok := collection.(string); ok {
		if sub, ok := item.(string); ok {
			return strings.Contains(s, sub)
		}
		return false
	}
	rv := reflect.ValueOf(collection)
	if rv.Kind() != reflect.Slice {
		return false
	}
	for i := 0; i < rv.Len(); i++ {
		if reflect.DeepEqual(rv.Index(i).Interface(), item) {
			return true
		}
	}
	return false
}

// join concatenates the elements of a string slice with sep as separator.
//
//	{{join .Tags ", "}}
func join(slice []string, sep string) string {
	return strings.Join(slice, sep)
}

// ─────────────────────────────────────────────
// Logic helpers
// ─────────────────────────────────────────────

// defaultVal returns fallback if value is the zero value or an empty string.
//
//	{{defaultVal "N/A" .OptionalField}}
func defaultVal(fallback, value interface{}) interface{} {
	if value == nil {
		return fallback
	}
	rv := reflect.ValueOf(value)
	if rv.IsZero() {
		return fallback
	}
	return value
}

// ternary returns trueVal if condition is true, falseVal otherwise.
//
//	{{ternary "Active" "Inactive" .IsActive}}
func ternary(trueVal, falseVal interface{}, condition bool) interface{} {
	if condition {
		return trueVal
	}
	return falseVal
}

// coalesce returns the first non-empty, non-nil value from the provided list.
//
//	{{coalesce .Nickname .FullName "Anonymous"}}
func coalesce(values ...interface{}) interface{} {
	for _, v := range values {
		if v == nil {
			continue
		}
		rv := reflect.ValueOf(v)
		if !rv.IsZero() {
			return v
		}
	}
	return nil
}

// ─────────────────────────────────────────────
// FuncMap registry
// ─────────────────────────────────────────────

func funcMap() template.FuncMap {
	return template.FuncMap{
		// Struct
		"hasField": hasField,
		// String
		"capitalize": capitalize,
		"prepend":    prepend,
		"strAppend":  strAppend,
		"split":      split,
		"truncate":   truncate,
		// Number
		"fmtNumber": fmtNumber,
		// Date & time
		"fmtDate": fmtDate,
		"timeAgo": timeAgo,
		// Collections
		"first":   first,
		"last":    last,
		"sliceOf": sliceOf,
		"contains": contains,
		"join":    join,
		// Logic
		"defaultVal": defaultVal,
		"ternary":    ternary,
		"coalesce":   coalesce,
	}
}

// ─────────────────────────────────────────────
// Template initializer
// ─────────────────────────────────────────────

func NewTemplates(components embed.FS, templates embed.FS) {
	var paths []string
	fs.WalkDir(components, ".", func(path string, d fs.DirEntry, err error) error {
		if strings.Contains(d.Name(), ".html") {
			paths = append(paths, path)
		}
		return nil
	})

	var pathst []string
	fs.WalkDir(templates, ".", func(patht string, d fs.DirEntry, err error) error {
		if strings.Contains(d.Name(), ".html") {
			pathst = append(pathst, patht)
		}
		return nil
	})

	tmpl = template.Must(
		template.New("").
			Funcs(funcMap()).
			ParseFS(components, paths...),
	)
	tmpl = template.Must(tmpl.ParseFS(templates, pathst...))
}