From 6c17eaa62f2e6c18bdb323a008bb5c61dd62a2b5 Mon Sep 17 00:00:00 2001
From: Brian Picciano <mediocregopher@gmail.com>
Date: Wed, 18 Jul 2018 23:01:28 +0000
Subject: [PATCH] mlog: implement KVerFunc and Prefix

---
 mlog/mlog.go      | 21 +++++++++++++++++++++
 mlog/mlog_test.go | 12 ++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/mlog/mlog.go b/mlog/mlog.go
index 5d6250b..da0ab79 100644
--- a/mlog/mlog.go
+++ b/mlog/mlog.go
@@ -104,6 +104,14 @@ type KVer interface {
 	KV() KV
 }
 
+// KVerFunc is a function which implements the KVer interface by calling itself.
+type KVerFunc func() KV
+
+// KV implements the KVer interface by calling the KVerFunc itself.
+func (kvf KVerFunc) KV() KV {
+	return kvf()
+}
+
 // KV is a set of key/value pairs which provides context for a log entry by a
 // KVer. KV is itself also a KVer.
 type KV map[string]interface{}
@@ -170,6 +178,19 @@ func MergeInto(kv KVer, kvs ...KVer) KVer {
 	return mergeInto(kv, kvs...)
 }
 
+// Prefix prefixes the all keys returned from the given KVer with the given
+// prefix string.
+func Prefix(kv KVer, prefix string) KVer {
+	return KVerFunc(func() KV {
+		kvv := kv.KV()
+		newKVV := make(KV, len(kvv))
+		for k, v := range kvv {
+			newKVV[prefix+k] = v
+		}
+		return newKVV
+	})
+}
+
 // Message describes a message to be logged, after having already resolved the
 // KVer
 type Message struct {
diff --git a/mlog/mlog_test.go b/mlog/mlog_test.go
index 152a010..b4072e8 100644
--- a/mlog/mlog_test.go
+++ b/mlog/mlog_test.go
@@ -9,6 +9,7 @@ import (
 	. "testing"
 	"time"
 
+	"github.com/mediocregopher/mediocre-go-lib/mtest/massert"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -142,3 +143,14 @@ func TestMerge(t *T) {
 		KV{"a": "a"}, KV{"a": "b"},
 	)
 }
+
+func TestPrefix(t *T) {
+	kv := KV{"foo": "bar"}
+	prefixKV := Prefix(kv, "aa")
+
+	massert.Fatal(t, massert.All(
+		massert.Equal(kv.KV(), KV{"foo": "bar"}),
+		massert.Equal(prefixKV.KV(), KV{"aafoo": "bar"}),
+		massert.Equal(kv.KV(), KV{"foo": "bar"}),
+	))
+}