2 files changed, 164 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
+}
diff --git a/modules/templates/vars/vars_test.go b/modules/templates/vars/vars_test.go
new file mode 100644
index 0000000..c543422
--- /dev/null
+++ b/modules/templates/vars/vars_test.go
@@ -0,0 +1,72 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package vars
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestExpandVars(t *testing.T) {
+	kases := []struct {
+		tmpl  string
+		data  map[string]string
+		out   string
+		error bool
+	}{
+		{
+			tmpl: "{a}",
+			data: map[string]string{
+				"a": "1",
+			},
+			out: "1",
+		},
+		{
+			tmpl: "expand {a}, {b} and {c}, with non-var { } {#}",
+			data: map[string]string{
+				"a": "1",
+				"b": "2",
+				"c": "3",
+			},
+			out: "expand 1, 2 and 3, with non-var { } {#}",
+		},
+		{
+			tmpl: "中文内容 {一}, {二} 和 {三} 中文结尾",
+			data: map[string]string{
+				"一": "11",
+				"二": "22",
+				"三": "33",
+			},
+			out: "中文内容 11, 22 和 33 中文结尾",
+		},
+		{
+			tmpl: "expand {{a}, {b} and {c}",
+			data: map[string]string{
+				"a": "foo",
+				"b": "bar",
+			},
+			out:   "expand {{a}, bar and {c}",
+			error: true,
+		},
+		{
+			tmpl:  "expand } {} and {",
+			out:   "expand } {} and {",
+			error: true,
+		},
+	}
+
+	for _, kase := range kases {
+		t.Run(kase.tmpl, func(t *testing.T) {
+			res, err := Expand(kase.tmpl, kase.data)
+			assert.EqualValues(t, kase.out, res)
+			if kase.error {
+				require.Error(t, err)
+			} else {
+				require.NoError(t, err)
+			}
+		})
+	}
+}