From 78344d176100b519b1d7de59cca8f6c26e8e2a70 Mon Sep 17 00:00:00 2001
From: Johan Walles <johan.walles@gmail.com>
Date: Fri, 4 Nov 2022 08:57:15 +0100
Subject: [PATCH] Implement our string builder

---
 m/reusableStringBuilder.go      | 28 ++++++++++++++++++++++------
 m/reusableStringBuilder_test.go | 19 +++++++++++++++++++
 2 files changed, 41 insertions(+), 6 deletions(-)

diff --git a/m/reusableStringBuilder.go b/m/reusableStringBuilder.go
index a69fe98..43d4767 100644
--- a/m/reusableStringBuilder.go
+++ b/m/reusableStringBuilder.go
@@ -1,16 +1,32 @@
 package m
 
+import "unicode/utf8"
+
 type reusableStringBuilder struct {
+	buf    []byte
+	length int // Number of used bytes in buf
 }
 
-func (rsb reusableStringBuilder) Reset() {
-	panic("Unimplemented")
+func (rsb *reusableStringBuilder) Reset() {
+	rsb.length = 0
 }
 
-func (rsb reusableStringBuilder) WriteRune(char rune) {
-	panic("Unimplemented")
+// Inspired by the source code of strings.Builder
+func (rsb *reusableStringBuilder) WriteRune(r rune) {
+	availableBytes := cap(rsb.buf) - rsb.length
+	if availableBytes < utf8.UTFMax {
+		newLength := 2*cap(rsb.buf) + utf8.UTFMax
+		newBuf := make([]byte, newLength)
+		for i := 0; i < rsb.length; i++ {
+			newBuf[i] = rsb.buf[i]
+		}
+		rsb.buf = newBuf
+	}
+
+	encodedBytesCount := utf8.EncodeRune(rsb.buf[rsb.length:rsb.length+utf8.UTFMax], r)
+	rsb.length += encodedBytesCount
 }
 
-func (rsb reusableStringBuilder) String() string {
-	panic("Unimplemented")
+func (rsb *reusableStringBuilder) String() string {
+	return string(rsb.buf[0:rsb.length])
 }
diff --git a/m/reusableStringBuilder_test.go b/m/reusableStringBuilder_test.go
index bb4f9ad..5c08d8f 100644
--- a/m/reusableStringBuilder_test.go
+++ b/m/reusableStringBuilder_test.go
@@ -15,6 +15,10 @@ func TestReusableStringBuilder_Basics(t *testing.T) {
 
 	testMe.Reset()
 	assert.Equal(t, "", testMe.String())
+
+	// Reset again to make sure it works when empty
+	testMe.Reset()
+	assert.Equal(t, "", testMe.String())
 }
 
 // Ensure the strings returned by our builder are real, rather than just being
@@ -32,3 +36,18 @@ func TestReusableStringBuilder_Copies(t *testing.T) {
 	assert.Equal(t, s1, "a")
 	assert.Equal(t, s2, "b")
 }
+
+func TestReusableStringBuilder_AddMultiple(t *testing.T) {
+	testMe := reusableStringBuilder{}
+
+	// Should be more than utf8.UTFMax = 4 to force one expansion when we
+	// already have data
+	testMe.WriteRune('a')
+	testMe.WriteRune('b')
+	testMe.WriteRune('c')
+	testMe.WriteRune('d')
+	testMe.WriteRune('e')
+	testMe.WriteRune('f')
+
+	assert.Equal(t, testMe.String(), "abcdef")
+}