internal/lsp: introduce snippet builder object

*snippet.Builder helps you construct lsp completion snippet strings.
It handles escaping for you, and it gives a convenient interface for
writing nested placeholders.

Note that the builder does not support snippet "variables" or
associated features. They add a lot of complexity and don't seem very
useful at this point (plus they aren't supported in many editors).

Change-Id: I492ab2f6f0e08ed952154cbc0a17c86f32abf40a
GitHub-Last-Rev: 35a3f5d1cd0b6fda81b2c942a02aa1bd25c90acd
GitHub-Pull-Request: golang/tools#90
Reviewed-on: https://go-review.googlesource.com/c/tools/+/173661
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

internal/lsp/snippet/snippet_builder.go

@@ -0,0 +1,87 @@
+// 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/specification#textDocument_completion.
+//
+// A typical snippet looks like "foo(${1:i int}, ${2:s string})".
+package snippet
+
+import (
+	"fmt"
+	"strings"
+)
+
+// A Builder is used to build an LSP snippet piecemeal.
+// The zero value is ready to use. Do not copy a non-zero Builder.
+type Builder struct {
+	// currentTabStop is the index of the previous tab stop. The
+	// next tab stop will be currentTabStop+1.
+	currentTabStop int
+	sb             strings.Builder
+}
+
+// Escape characters defined in https://microsoft.github.io/language-server-protocol/specification#textDocument_completion under "Grammar".
+var replacer = strings.NewReplacer(
+	`\`, `\\`,
+	`}`, `\}`,
+	`$`, `\$`,
+)
+
+func (b *Builder) WriteText(s string) {
+	replacer.WriteString(&b.sb, s)
+}
+
+// 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, "${%d", b.nextTabStop())
+	if fn != nil {
+		b.sb.WriteByte(':')
+		fn(b)
+	}
+	b.sb.WriteByte('}')
+}
+
+// In addition to '\', '}', and '$', 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()
+}
+
+// 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
+}

internal/lsp/snippet/snippet_builder_test.go

@@ -0,0 +1,49 @@
+// 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
+
+import (
+	"testing"
+)
+
+func TestSnippetBuilder(t *testing.T) {
+	expect := func(expected string, fn func(*Builder)) {
+		var b Builder
+		fn(&b)
+		if got := b.String(); got != expected {
+			t.Errorf("got %q, expected %q", got, expected)
+		}
+	}
+
+	expect("", func(b *Builder) {})
+
+	expect(`hi { \} \$ | " , / \\`, func(b *Builder) {
+		b.WriteText(`hi { } $ | " , / \`)
+	})
+
+	expect("${1}", func(b *Builder) {
+		b.WritePlaceholder(nil)
+	})
+
+	expect("hi ${1:there}", func(b *Builder) {
+		b.WriteText("hi ")
+		b.WritePlaceholder(func(b *Builder) {
+			b.WriteText("there")
+		})
+	})
+
+	expect(`${1:id=${2:{your id\}}}`, func(b *Builder) {
+		b.WritePlaceholder(func(b *Builder) {
+			b.WriteText("id=")
+			b.WritePlaceholder(func(b *Builder) {
+				b.WriteText("{your id}")
+			})
+		})
+	})
+
+	expect(`${1|one,{ \} \$ \| " \, / \\,three|}`, func(b *Builder) {
+		b.WriteChoice([]string{"one", `{ } $ | " , / \`, "three"})
+	})
+}