1 files changed, 111 insertions, 0 deletions
diff --git a/gopls/internal/lsp/snippet/snippet_builder.go b/gopls/internal/lsp/snippet/snippet_builder.go
new file mode 100644
index 000000000..fa63e8d83
--- /dev/null
+++ b/gopls/internal/lsp/snippet/snippet_builder.go
@@ -0,0 +1,111 @@
+// Copyright 2019 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package snippet implements the specification for the LSP snippet format.
+//
+// Snippets are "tab stop" templates returned as an optional attribute of LSP
+// completion candidates. As the user presses tab, they cycle through a series of
+// tab stops defined in the snippet. Each tab stop can optionally have placeholder
+// text, which can be pre-selected by editors. For a full description of syntax
+// and features, see "Snippet Syntax" at
+// https://microsoft.github.io/language-server-protocol/specifications/specification-3-14/#textDocument_completion.
+//
+// A typical snippet looks like "foo(${1:i int}, ${2:s string})".
+package snippet
+
+import (
+	"fmt"
+	"strings"
+)
+
+// A Builder is used to build an LSP snippet piecemeal.
+// The zero value is ready to use. Do not copy a non-zero Builder.
+type Builder struct {
+	// currentTabStop is the index of the previous tab stop. The
+	// next tab stop will be currentTabStop+1.
+	currentTabStop int
+	sb             strings.Builder
+}
+
+// Escape characters defined in https://microsoft.github.io/language-server-protocol/specifications/specification-3-14/#textDocument_completion under "Grammar".
+var replacer = strings.NewReplacer(
+	`\`, `\\`,
+	`}`, `\}`,
+	`$`, `\$`,
+)
+
+func (b *Builder) WriteText(s string) {
+	replacer.WriteString(&b.sb, s)
+}
+
+func (b *Builder) PrependText(s string) {
+	rawSnip := b.String()
+	b.sb.Reset()
+	b.WriteText(s)
+	b.sb.WriteString(rawSnip)
+}
+
+func (b *Builder) Write(data []byte) (int, error) {
+	return b.sb.Write(data)
+}
+
+// WritePlaceholder writes a tab stop and placeholder value to the Builder.
+// The callback style allows for creating nested placeholders. To write an
+// empty tab stop, provide a nil callback.
+func (b *Builder) WritePlaceholder(fn func(*Builder)) {
+	fmt.Fprintf(&b.sb, "${%d:", b.nextTabStop())
+	if fn != nil {
+		fn(b)
+	}
+	b.sb.WriteByte('}')
+}
+
+// WriteFinalTabstop marks where cursor ends up after the user has
+// cycled through all the normal tab stops. It defaults to the
+// character after the snippet.
+func (b *Builder) WriteFinalTabstop() {
+	fmt.Fprint(&b.sb, "$0")
+}
+
+// In addition to '\', '}', and '$', snippet choices also use '|' and ',' as
+// meta characters, so they must be escaped within the choices.
+var choiceReplacer = strings.NewReplacer(
+	`\`, `\\`,
+	`}`, `\}`,
+	`$`, `\$`,
+	`|`, `\|`,
+	`,`, `\,`,
+)
+
+// WriteChoice writes a tab stop and list of text choices to the Builder.
+// The user's editor will prompt the user to choose one of the choices.
+func (b *Builder) WriteChoice(choices []string) {
+	fmt.Fprintf(&b.sb, "${%d|", b.nextTabStop())
+	for i, c := range choices {
+		if i != 0 {
+			b.sb.WriteByte(',')
+		}
+		choiceReplacer.WriteString(&b.sb, c)
+	}
+	b.sb.WriteString("|}")
+}
+
+// String returns the built snippet string.
+func (b *Builder) String() string {
+	return b.sb.String()
+}
+
+// Clone returns a copy of b.
+func (b *Builder) Clone() *Builder {
+	var clone Builder
+	clone.sb.WriteString(b.String())
+	return &clone
+}
+
+// nextTabStop returns the next tab stop index for a new placeholder.
+func (b *Builder) nextTabStop() int {
+	// Tab stops start from 1, so increment before returning.
+	b.currentTabStop++
+	return b.currentTabStop
+}