From db9fbd98cc2c89dcb34673becc32cdd930a41295 Mon Sep 17 00:00:00 2001
From: Jesse Tane <jesse.tane@gmail.com>
Date: Mon, 15 Dec 2014 19:17:57 -0500
Subject: [PATCH] do not encode partial or invalid code points to utf8

---
 index.js | 106 +++++++++++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 92 insertions(+), 14 deletions(-)

diff --git a/index.js b/index.js
index 1a7144a..f01d6ac 100644
--- a/index.js
+++ b/index.js
@@ -341,7 +341,7 @@ function hexWrite (buf, string, offset, length) {
 }
 
 function utf8Write (buf, string, offset, length) {
-  var charsWritten = blitBuffer(utf8ToBytes(string), buf, offset, length)
+  var charsWritten = blitBuffer(utf8ToBytes(string, buf.length - offset), buf, offset, length)
   return charsWritten
 }
 
@@ -1171,22 +1171,100 @@ function toHex (n) {
   return n.toString(16)
 }
 
-function utf8ToBytes (str) {
-  var byteArray = []
-  for (var i = 0; i < str.length; i++) {
-    var b = str.charCodeAt(i)
-    if (b <= 0x7F) {
-      byteArray.push(b)
-    } else {
-      var start = i
-      if (b >= 0xD800 && b <= 0xDFFF) i++
-      var h = encodeURIComponent(str.slice(start, i+1)).substr(1).split('%')
-      for (var j = 0; j < h.length; j++) {
-        byteArray.push(parseInt(h[j], 16))
+function utf8ToBytes(string, units) {
+  var codePoint, length = string.length
+  var leadSurrogate = null
+  var units = units || Infinity
+  var bytes = []
+  var i = 0
+
+  for (; i<length; i++) {
+    codePoint = string.charCodeAt(i)
+
+    // is surrogate component
+    if (codePoint > 0xD7FF && codePoint < 0xE000) {
+
+      // last char was a lead
+      if (leadSurrogate) {
+
+        // 2 leads in a row
+        if (codePoint < 0xDC00) {
+          if ((units -= 3) > -1) bytes.push(0xEF, 0xBF, 0xBD)
+          leadSurrogate = codePoint
+          continue
+        }
+
+        // valid surrogate pair
+        else {
+          codePoint = leadSurrogate - 0xD800 << 10 | codePoint - 0xDC00 | 0x10000
+          leadSurrogate = null
+        }
+      }
+
+      // no lead yet
+      else {
+
+        // unexpected trail
+        if (codePoint > 0xDBFF) {
+          if ((units -= 3) > -1) bytes.push(0xEF, 0xBF, 0xBD)
+          continue
+        }
+
+        // unpaired lead
+        else if (i + 1 === length) {
+          if ((units -= 3) > -1) bytes.push(0xEF, 0xBF, 0xBD)
+          continue
+        }
+
+        // valid lead
+        else {
+          leadSurrogate = codePoint
+          continue
+        }
       }
     }
+
+    // valid bmp char, but last char was a lead
+    else if (leadSurrogate) {
+      if ((units -= 3) > -1) bytes.push(0xEF, 0xBF, 0xBD)
+      leadSurrogate = null
+    }
+
+    // encode utf8
+    if (codePoint < 0x80) {
+      if ((units -= 1) < 0) break
+      bytes.push(codePoint)
+    }
+    else if (codePoint < 0x800) {
+      if ((units -= 2) < 0) break
+      bytes.push(
+        codePoint >> 0x6 | 0xC0,
+        codePoint & 0x3F | 0x80
+      );
+    }
+    else if (codePoint < 0x10000) {
+      if ((units -= 3) < 0) break
+      bytes.push(
+        codePoint >> 0xC | 0xE0,
+        codePoint >> 0x6 & 0x3F | 0x80,
+        codePoint & 0x3F | 0x80
+      );
+    }
+    else if (codePoint < 0x200000) {
+      if ((units -= 4) < 0) break
+      bytes.push(
+        codePoint >> 0x12 | 0xF0,
+        codePoint >> 0xC & 0x3F | 0x80,
+        codePoint >> 0x6 & 0x3F | 0x80,
+        codePoint & 0x3F | 0x80
+      );
+    }
+    else {
+      throw new Error('Invalid code point')
+    }
   }
-  return byteArray
+
+  return bytes
 }
 
 function asciiToBytes (str) {
-- 
2.34.1