spec: bytevectors (make-bytevector/u8-ref/u8-set!/utf8->string/etc)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hosts/javascript/platform.py

@@ -1817,6 +1817,73 @@ PRIMITIVES_JS_MODULES: dict[str, str] = {
     });
     return out;
   };
+''',
+    "stdlib.bytevectors": '''
+  // stdlib.bytevectors — R7RS bytevector type backed by Uint8Array
+  function SxBytevector(size_or_buf) {
+    if (size_or_buf instanceof Uint8Array) {
+      this.data = size_or_buf;
+    } else {
+      this.data = new Uint8Array(typeof size_or_buf === "number" ? size_or_buf : 0);
+    }
+    this._bytevector = true;
+  }
+  SxBytevector.prototype._type = "bytevector";
+  PRIMITIVES["make-bytevector"] = function(n, fill) {
+    var bv = new SxBytevector(n);
+    if (fill !== undefined) bv.data.fill(fill & 0xff);
+    return bv;
+  };
+  PRIMITIVES["bytevector?"] = function(v) { return v instanceof SxBytevector; };
+  PRIMITIVES["bytevector-length"] = function(bv) { return bv.data.length; };
+  PRIMITIVES["bytevector-u8-ref"] = function(bv, i) { return bv.data[i]; };
+  PRIMITIVES["bytevector-u8-set!"] = function(bv, i, byte) { bv.data[i] = byte & 0xff; return NIL; };
+  PRIMITIVES["bytevector-copy"] = function(bv, start, end_) {
+    var s = start === undefined ? 0 : start;
+    var e = end_ === undefined ? bv.data.length : end_;
+    return new SxBytevector(bv.data.slice(s, e));
+  };
+  PRIMITIVES["bytevector-copy!"] = function(dst, at, src, start, end_) {
+    var s = start === undefined ? 0 : start;
+    var e = end_ === undefined ? src.data.length : end_;
+    dst.data.set(src.data.subarray(s, e), at);
+    return NIL;
+  };
+  PRIMITIVES["bytevector-append"] = function() {
+    var total = 0;
+    for (var i = 0; i < arguments.length; i++) total += arguments[i].data.length;
+    var result = new Uint8Array(total);
+    var pos = 0;
+    for (var i = 0; i < arguments.length; i++) {
+      result.set(arguments[i].data, pos);
+      pos += arguments[i].data.length;
+    }
+    return new SxBytevector(result);
+  };
+  PRIMITIVES["utf8->string"] = function(bv, start, end_) {
+    var s = start === undefined ? 0 : start;
+    var e = end_ === undefined ? bv.data.length : end_;
+    var dec = new TextDecoder("utf-8");
+    return dec.decode(bv.data.subarray(s, e));
+  };
+  PRIMITIVES["string->utf8"] = function(str, start, end_) {
+    var enc = new TextEncoder();
+    var full = enc.encode(str);
+    var s = start === undefined ? 0 : start;
+    var e = end_ === undefined ? full.length : end_;
+    return new SxBytevector(full.slice(s, e));
+  };
+  PRIMITIVES["bytevector->list"] = function(bv) {
+    var out = [];
+    for (var i = 0; i < bv.data.length; i++) out.push(bv.data[i]);
+    return out;
+  };
+  PRIMITIVES["list->bytevector"] = function(lst) {
+    if (!Array.isArray(lst)) lst = [];
+    var b = new Uint8Array(lst.length);
+    for (var i = 0; i < lst.length; i++) b[i] = lst[i] & 0xff;
+    return new SxBytevector(b);
+  };
 ''',
 }
 # Modules to include by default (all)
@@ -1864,6 +1931,7 @@ PLATFORM_JS_PRE = '''
     if (x._hash_table) return "hash-table";
     if (x._sxset) return "set";
     if (x._regexp) return "regexp";
+    if (x._bytevector) return "bytevector";
     if (x._rational) return "rational";
     if (typeof Node !== "undefined" && x instanceof Node) return "dom-node";
     if (Array.isArray(x)) return "list";