// Copyright 2022 The Gitea Authors. All rights reserved.
// SPDX-License-Identifier: MIT

package vars

import (
	"fmt"
	"regexp"
	"strings"
	"sync"
	"unicode"
	"unicode/utf8"
)

// ExpandCurlyBrace replaces all variables like {var} by `vars` map,
// it always returns the expanded string regardless of errors.
// if error occurs (wrong syntax, missing variable), the error part doesn't change and is returned as it is.
func ExpandCurlyBrace(template string, vars map[string]string) (string, error) {
	// in the future, if necessary, we can introduce some escape-char,
	// for example: it will use `#' as a reversed char, templates will use `{#{}` to do escape and output char '{'.
	var buf strings.Builder
	var err error

	posBegin := 0
	strLen := len(template)
	for posBegin < strLen {
		// find the next `{`
		pos := strings.IndexByte(template[posBegin:], '{')
		if pos == -1 {
			buf.WriteString(template[posBegin:])
			break
		}

		// copy texts between vars
		buf.WriteString(template[posBegin : posBegin+pos])

		// find the var between `{` and `}`/end
		posBegin += pos
		posEnd := posBegin + 1
		for posEnd < strLen {
			if template[posEnd] == '}' {
				posEnd++
				break
			} // in the future, if we need to support escape chars, we can do: if (isEscapeChar) { posEnd+=2 }
			posEnd++
		}

		// the var part, it can be "{", "{}", "{..." or or "{...}"
		part := template[posBegin:posEnd]
		posBegin = posEnd
		if part == "{}" || part[len(part)-1] != '}' {
			// treat "{}" or "{..." as error
			err = fmt.Errorf("wrong syntax found in %s", template)
			buf.WriteString(part)
		} else {
			// now we get a valid key "{...}"
			key := part[1 : len(part)-1]
			keyFirst, _ := utf8.DecodeRuneInString(key)
			if unicode.IsSpace(keyFirst) || unicode.IsPunct(keyFirst) || unicode.IsControl(keyFirst) {
				// if the key doesn't start with a letter, then we do not treat it as a var now
				buf.WriteString(part)
			} else {
				// look up in the map
				if val, ok := vars[key]; ok {
					buf.WriteString(val)
				} else {
					// write the non-existing var as it is
					buf.WriteString(part)
					err = fmt.Errorf("the variable %s is missing for %s", key, template)
				}
			}
		}
	}

	return buf.String(), err
}

var globalVars = sync.OnceValue(func() (ret struct {
	regexpShellLike *regexp.Regexp
},
) {
	ret.regexpShellLike = regexp.MustCompile(`(\$\{[a-zA-Z_]\w*\}|\$[a-zA-Z_]\w*)`)
	return ret
})

// ExpandShellLike works like os.Expand, the difference is that this function keeps the non-existing keys
func ExpandShellLike(template string, vars map[string]string) string {
	re := globalVars().regexpShellLike
	return re.ReplaceAllStringFunc(template, func(s string) string {
		key := s[1:]
		if strings.HasPrefix(key, "{") && strings.HasSuffix(key, "}") {
			key = key[1 : len(key)-1]
		}
		if val, ok := vars[key]; ok {
			return val
		}
		return s
	})
}