src/defs.js: Refactor to_jsstring

Now fails on non-utf8 input.
Previously it would convert to equivalent unicode codepoints as bytes, which did not round-trip

1 files changed, 28 insertions, 32 deletions

diff --git a/src/defs.js b/src/defs.js
index 5ebaad9..e55e3dc 100644
--- a/src/defs.js
+++ b/src/defs.js
@@ -138,43 +138,39 @@ const is_luastring = function(s) {
 const to_jsstring = function(value, from, to) {
     assert(is_luastring(value), "jsstring expect a array of bytes");
 
-    let u0, u1, u2, u3, u4, u5;
-    let idx = 0;
     value = value.slice(from ? from : 0, to);
 
-    var str = '';
-    while (1) {
-        // For UTF8 byte structure, see http://en.wikipedia.org/wiki/UTF-8#Description and https://www.ietf.org/rfc/rfc2279.txt and https://tools.ietf.org/html/rfc3629
-        u0 = value[idx++];
-        if (u0 === 0) { str += "\0"; continue; } // Lua string embed '\0'
-        if (!u0) return str;
-        if (!(u0 & 0x80)) { str += String.fromCharCode(u0); continue; }
-        u1 = value[idx++] & 63;
-        if ((u0 & 0xE0) == 0xC0) { str += String.fromCharCode(((u0 & 31) << 6) | u1); continue; }
-        u2 = value[idx++] & 63;
-        if ((u0 & 0xF0) == 0xE0) {
-            u0 = ((u0 & 15) << 12) | (u1 << 6) | u2;
-        } else {
-            u3 = value[idx++] & 63;
-            if ((u0 & 0xF8) == 0xF0) {
-                u0 = ((u0 & 7) << 18) | (u1 << 12) | (u2 << 6) | u3;
-            } else {
-                u4 = value[idx++] & 63;
-                if ((u0 & 0xFC) == 0xF8) {
-                    u0 = ((u0 & 3) << 24) | (u1 << 18) | (u2 << 12) | (u3 << 6) | u4;
-                } else {
-                    u5 = value[idx++] & 63;
-                    u0 = ((u0 & 1) << 30) | (u1 << 24) | (u2 << 18) | (u3 << 12) | (u4 << 6) | u5;
-                }
-            }
-        }
-        if (u0 < 0x10000) {
-            str += String.fromCharCode(u0);
+    let str = "";
+    for (let i = 0; i < value.length;) {
+        let u;
+        let u0 = value[i++];
+        if (u0 < 0x80) {
+            /* single byte sequence */
+            u = u0;
+        } else if (u0 < 0xC2 || u0 > 0xF4) {
+            throw RangeError("cannot convert invalid utf8 to javascript string");
+        } else if (u0 <= 0xDF) {
+            /* two byte sequence */
+            if (i >= value.length) throw RangeError("cannot convert invalid utf8 to javascript string");
+            let u1 = value[i++];
+            u = ((u0 & 0x1F) << 6) + (u1 & 0x3F);
+        } else if (u0 <= 0xEF) {
+            /* three byte sequence */
+            if (i+1 >= value.length) throw RangeError("cannot convert invalid utf8 to javascript string");
+            let u1 = value[i++];
+            let u2 = value[i++];
+            u = ((u0 & 0x0F) << 12) + ((u1 & 0x3F) << 6) + (u2 & 0x3F);
         } else {
-            var ch = u0 - 0x10000;
-            str += String.fromCharCode(0xD800 | (ch >> 10), 0xDC00 | (ch & 0x3FF));
+            /* four byte sequence */
+            if (i+2 >= value.length) throw RangeError("cannot convert invalid utf8 to javascript string");
+            let u1 = value[i++];
+            let u2 = value[i++];
+            let u3 = value[i++];
+            u = ((u0 & 0x07) << 18) + ((u1 & 0x3F) << 12) + ((u2 & 0x3F) << 6) + (u3 & 0x3F);
         }
+        str += String.fromCodePoint(u);
     }
+    return str;
 };
 
 const to_luastring_cache = {};