// Package unicode provides an alternative to the SQLite ICU extension.
//
// Like the [ICU extension], it provides Unicode aware:
//   - upper() and lower() functions,
//   - LIKE and REGEXP operators,
//   - collation sequences.
//
// The implementation is not 100% compatible with the [ICU extension]:
//   - upper() and lower() use [strings.ToUpper], [strings.ToLower] and [cases];
//   - the LIKE operator follows [strings.EqualFold] rules;
//   - the REGEXP operator uses Go [regexp/syntax];
//   - collation sequences use [collate].
//
// Expect subtle differences (e.g.) in the handling of Turkish case folding.
//
// [ICU extension]: https://sqlite.org/src/dir/ext/icu
package unicode

import (
	"bytes"
	"errors"
	"regexp"
	"strings"
	"unicode/utf8"

	"github.com/ncruces/go-sqlite3"
	"github.com/ncruces/go-sqlite3/internal/util"
	"golang.org/x/text/cases"
	"golang.org/x/text/collate"
	"golang.org/x/text/language"
)

// Register registers Unicode aware functions for a database connection.
func Register(db *sqlite3.Conn) error {
	flags := sqlite3.DETERMINISTIC | sqlite3.INNOCUOUS
	return errors.Join(
		db.CreateFunction("like", 2, flags, like),
		db.CreateFunction("like", 3, flags, like),
		db.CreateFunction("upper", 1, flags, upper),
		db.CreateFunction("upper", 2, flags, upper),
		db.CreateFunction("lower", 1, flags, lower),
		db.CreateFunction("lower", 2, flags, lower),
		db.CreateFunction("regexp", 2, flags, regex),
		db.CreateFunction("icu_load_collation", 2, sqlite3.DIRECTONLY,
			func(ctx sqlite3.Context, arg ...sqlite3.Value) {
				name := arg[1].Text()
				if name == "" {
					return
				}

				err := RegisterCollation(db, arg[0].Text(), name)
				if err != nil {
					ctx.ResultError(err)
					return // notest
				}
			}))
}

// RegisterCollation registers a Unicode collation sequence for a database connection.
func RegisterCollation(db *sqlite3.Conn, locale, name string) error {
	tag, err := language.Parse(locale)
	if err != nil {
		return err
	}
	return db.CreateCollation(name, collate.New(tag).Compare)
}

func upper(ctx sqlite3.Context, arg ...sqlite3.Value) {
	if len(arg) == 1 {
		ctx.ResultRawText(bytes.ToUpper(arg[0].RawText()))
		return
	}
	cs, ok := ctx.GetAuxData(1).(cases.Caser)
	if !ok {
		t, err := language.Parse(arg[1].Text())
		if err != nil {
			ctx.ResultError(err)
			return // notest
		}
		c := cases.Upper(t)
		ctx.SetAuxData(1, c)
		cs = c
	}
	ctx.ResultRawText(cs.Bytes(arg[0].RawText()))
}

func lower(ctx sqlite3.Context, arg ...sqlite3.Value) {
	if len(arg) == 1 {
		ctx.ResultRawText(bytes.ToLower(arg[0].RawText()))
		return
	}
	cs, ok := ctx.GetAuxData(1).(cases.Caser)
	if !ok {
		t, err := language.Parse(arg[1].Text())
		if err != nil {
			ctx.ResultError(err)
			return // notest
		}
		c := cases.Lower(t)
		ctx.SetAuxData(1, c)
		cs = c
	}
	ctx.ResultRawText(cs.Bytes(arg[0].RawText()))
}

func regex(ctx sqlite3.Context, arg ...sqlite3.Value) {
	re, ok := ctx.GetAuxData(0).(*regexp.Regexp)
	if !ok {
		r, err := regexp.Compile(arg[0].Text())
		if err != nil {
			ctx.ResultError(err)
			return // notest
		}
		re = r
		ctx.SetAuxData(0, r)
	}
	ctx.ResultBool(re.Match(arg[1].RawText()))
}

func like(ctx sqlite3.Context, arg ...sqlite3.Value) {
	escape := rune(-1)
	if len(arg) == 3 {
		var size int
		b := arg[2].RawText()
		escape, size = utf8.DecodeRune(b)
		if size != len(b) {
			ctx.ResultError(util.ErrorString("ESCAPE expression must be a single character"))
			return
		}
	}

	type likeData struct {
		*regexp.Regexp
		escape rune
	}

	re, ok := ctx.GetAuxData(0).(likeData)
	if !ok || re.escape != escape {
		re = likeData{
			regexp.MustCompile(like2regex(arg[0].Text(), escape)),
			escape,
		}
		ctx.SetAuxData(0, re)
	}
	ctx.ResultBool(re.Match(arg[1].RawText()))
}

func like2regex(pattern string, escape rune) string {
	var re strings.Builder
	start := 0
	literal := false
	re.Grow(len(pattern) + 10)
	re.WriteString(`(?is)\A`) // case insensitive, . matches any character
	for i, r := range pattern {
		if start < 0 {
			start = i
		}
		if literal {
			literal = false
			continue
		}
		var symbol string
		switch r {
		case '_':
			symbol = `.`
		case '%':
			symbol = `.*`
		case escape:
			literal = true
		default:
			continue
		}
		re.WriteString(regexp.QuoteMeta(pattern[start:i]))
		re.WriteString(symbol)
		start = -1
	}
	if start >= 0 {
		re.WriteString(regexp.QuoteMeta(pattern[start:]))
	}
	re.WriteString(`\z`)
	return re.String()
}