1 files changed, 92 insertions, 0 deletions
diff --git a/modules/templates/vars/vars.go b/modules/templates/vars/vars.go
new file mode 100644
index 0000000..cc9d0e9
--- /dev/null
+++ b/modules/templates/vars/vars.go
@@ -0,0 +1,92 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package vars
+
+import (
+	"fmt"
+	"strings"
+	"unicode"
+	"unicode/utf8"
+)
+
+// ErrWrongSyntax represents a wrong syntax with a template
+type ErrWrongSyntax struct {
+	Template string
+}
+
+func (err ErrWrongSyntax) Error() string {
+	return fmt.Sprintf("wrong syntax found in %s", err.Template)
+}
+
+// ErrVarMissing represents an error that no matched variable
+type ErrVarMissing struct {
+	Template string
+	Var      string
+}
+
+func (err ErrVarMissing) Error() string {
+	return fmt.Sprintf("the variable %s is missing for %s", err.Var, err.Template)
+}
+
+// Expand replaces all variables like {var} by `vars` map, it always returns the expanded string regardless of errors
+// if error occurs, the error part doesn't change and is returned as it is.
+func Expand(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 = ErrWrongSyntax{Template: 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) {
+				// the if 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 = ErrVarMissing{Template: template, Var: key}
+				}
+			}
+		}
+	}
+
+	return buf.String(), err
+}