From 8ef8c8187fcea23761175f4e4d15485127e32803 Mon Sep 17 00:00:00 2001
From: Kegan Dougal <kegan@matrix.org>
Date: Tue, 6 Dec 2016 14:24:27 +0000
Subject: [PATCH] Add user ID handling methods

---
 userids.go      | 15 +++++++++++++++
 userids_test.go | 22 ++++++++++++++++++++++
 2 files changed, 37 insertions(+)

diff --git a/userids.go b/userids.go
index cf2f8c2..817a006 100644
--- a/userids.go
+++ b/userids.go
@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"encoding/hex"
 	"fmt"
+	"strings"
 )
 
 const lowerhex = "0123456789abcdef"
@@ -44,6 +45,7 @@ func isValidEscapedChar(b byte) bool {
 }
 
 // EncodeUserLocalpart encodes the given string into Matrix-compliant user ID localpart form.
+// See http://matrix.org/docs/spec/intro.html#mapping-from-other-character-sets
 //
 // This returns a string with only the characters "a-z0-9._=-". The uppercase range A-Z
 // are encoded using leading underscores ("_"). Characters outside the aforementioned ranges
@@ -67,6 +69,7 @@ func EncodeUserLocalpart(str string) string {
 
 // DecodeUserLocalpart decodes the given string back into the original input string.
 // Returns an error if the given string is not a valid user ID localpart encoding.
+// See http://matrix.org/docs/spec/intro.html#mapping-from-other-character-sets
 //
 // This decodes quoted-printable bytes back into UTF8, and unescapes casing. For
 // example:
@@ -113,3 +116,15 @@ func DecodeUserLocalpart(str string) (string, error) {
 	}
 	return outputBuffer.String(), nil
 }
+
+// ExtractUserLocalpart extracts the localpart portion of a user ID.
+// See http://matrix.org/docs/spec/intro.html#user-identifiers
+func ExtractUserLocalpart(userID string) (string, error) {
+	if len(userID) == 0 || userID[0] != '@' {
+		return "", fmt.Errorf("%s is not a valid user id")
+	}
+	return strings.TrimPrefix(
+		strings.SplitN(userID, ":", 2)[0], // @foo:bar:8448 => [ "@foo", "bar:8448" ]
+		"@", // remove "@" prefix
+	), nil
+}
diff --git a/userids_test.go b/userids_test.go
index 26d183e..b896740 100644
--- a/userids_test.go
+++ b/userids_test.go
@@ -62,3 +62,25 @@ func TestDecodeUserLocalpartErrors(t *testing.T) {
 		}
 	}
 }
+
+var localparttests = []struct {
+	Input        string
+	ExpectOutput string
+}{
+	{"@foo:bar", "foo"},
+	{"@foo:bar:8448", "foo"},
+	{"@foo.bar:baz.quuz", "foo.bar"},
+}
+
+func TestExtractUserLocalpart(t *testing.T) {
+	for _, u := range localparttests {
+		out, err := ExtractUserLocalpart(u.Input)
+		if err != nil {
+			t.Errorf("TestExtractUserLocalpart(%s) => Error: %s", u.Input, err)
+			continue
+		}
+		if out != u.ExpectOutput {
+			t.Errorf("TestExtractUserLocalpart(%s) => Got: %s, Want %s", u.Input, out, u.ExpectOutput)
+		}
+	}
+}