[Implement] Naive Buffer.from

Author: jtenner

assembly/buffer/index.ts

@@ -1,6 +1,7 @@
 import { BLOCK_MAXSIZE } from "rt/common";
 import { E_INVALIDLENGTH, E_INDEXOUTOFRANGE } from "util/error";
 import { Uint8Array } from "typedarray";
+import { ArrayBufferView } from "arraybuffer";
 
 export class Buffer extends Uint8Array {
   constructor(size: i32) {
@@ -22,4 +23,40 @@ export class Buffer extends Uint8Array {
     result.dataLength = size;
     return result;
   }
+
+  // @ts-ignore: Buffer returns on all valid branches
+  public static from<T>(value: T): Buffer {
+    if (value instanceof ArrayBuffer) {
+      let result = changetype<Buffer>(__alloc(offsetof<Buffer>(), idof<Buffer>()));
+      result.data = value;
+      result.dataStart = changetype<usize>(value);
+      result.dataLength = value.byteLength;
+      return result;
+    } else if (value instanceof String) {
+      // @ts-ignore value not instance of `string` does changetype<string>(value) work here?
+      let buffer = String.UTF8.encode(value);
+      let result = changetype<Buffer>(__alloc(offsetof<Buffer>(), idof<Buffer>()));
+      result.data = buffer;
+      result.dataStart = changetype<usize>(buffer);
+      result.dataLength = buffer.byteLength;
+      return result;
+    } else if (value instanceof Buffer) {
+      let result = changetype<Buffer>(__alloc(offsetof<Buffer>(), idof<Buffer>()));
+      result.data = value.buffer;
+      result.dataStart = value.dataStart;
+      result.dataLength = value.dataLength;
+      return result;
+    } else if (value instanceof ArrayBufferView) {
+      let length = value.length;
+      let buffer = __alloc(length, idof<ArrayBuffer>());
+      let result = changetype<Buffer>(__alloc(offsetof<Buffer>(), idof<Buffer>()));
+      // @ts-ignore: value[i] is implied to work
+      for (let i = 0; i < length; i++) store<u8>(buffer + usize(i), u8(unchecked(value[i])));
+      result.data = changetype<ArrayBuffer>(buffer);
+      result.dataStart = buffer;
+      result.dataLength = u32(length);
+      return result;
+    }
+    ERROR("Cannot call Buffer.from<T>() where T is not a string, Buffer, ArrayBuffer, Array, or Array-like Object.");
+  }
 }

assembly/node.d.ts

@@ -3,4 +3,6 @@ declare class Buffer extends Uint8Array {
   static alloc(size: i32): Buffer;
   /** This method allocates a new Buffer of indicated size. This is unsafe because the data is not zeroed. */
   static allocUnsafe(size: i32): Buffer;
+  /** This method creates a Buffer from the given reference. This method is naive and defaults to utf8 encoding for strings. */
+  static from<T>(value: T): Buffer;
 }

tests/buffer.spec.ts

@@ -1,3 +1,10 @@
+function bufferFrom<T>(values: valueof<T>[]): T {
+  let buffer = instantiate<T>(values.length);
+  // @ts-ignore
+  for (let i = 0; i < values.length; i++) buffer[i] = values[i];
+  return buffer;
+}
+
 /**
  * This is the buffer test suite. For each prototype function, put a single test
  * function call here.
@@ -42,4 +49,42 @@ describe("buffer", () => {
     // TODO: expectFn(() => { Buffer.allocUnsafe(-1); }).toThrow();
     // TODO: expectFn(() => { Buffer.allocUnsafe(BLOCK_MAXSIZE + 1); }).toThrow();
   });
+
+
+  /**
+   * This specification is a tradeoff, because Buffer.from() takes _many_ parameters.
+   * Instead, the only common parameter is the first one, which results in Buffer.from
+   * acting in a very naive fashion. Perhaps an optional encoding parameter might be
+   * possible for strings, at least. However, this makes things more complicated.
+   * There are no good solutions. Only tradeoffs. Function overloading is the only
+   * way to fix this problem.
+   */
+  test("#from", () => {
+    // TODO: Switch to expect<Buffer>() when 2.2.1 releases
+
+    // Buffer.from uses the array buffer reference
+    let buff = new ArrayBuffer(100);
+    for (let i = 0; i < 100; i++) store<u8>(changetype<usize>(buff), u8(i));
+    let abBuffer = Buffer.from<ArrayBuffer>(buff);
+    expect<ArrayBuffer>(abBuffer.buffer).toStrictEqual(buff);
+    expect<ArrayBuffer>(abBuffer.buffer).toBe(buff);
+
+    // strings are utf8 encoded by default
+    let strBuffer = Buffer.from<string>("Hello world!");
+    let strBufferExpected = bufferFrom<Buffer>([0x48, 0x65, 0x6c, 0x6c, 0x6f, 0x20, 0x77, 0x6f, 0x72, 0x6c, 0x64, 0x21]);
+    expect<ArrayBuffer>(strBuffer.buffer).toStrictEqual(strBufferExpected.buffer);
+
+    // buffer returns a new reference view to the same ArrayBuffer
+    let buff2 = Buffer.from<Buffer>(abBuffer);
+    expect<Buffer>(buff2).not.toBe(abBuffer);
+    expect<ArrayBuffer>(buff2.buffer).toBe(abBuffer.buffer);
+    expect<usize>(buff2.dataStart).toBe(abBuffer.dataStart);
+    expect<u32>(buff2.dataLength).toBe(abBuffer.dataLength);
+
+    // else if it extends ArrayBufferView simply converts all the values
+    let floats = bufferFrom<Float32Array>([1.1, 2.2, 3.3]);
+    let floatBuff = Buffer.from<Float32Array>(floats);
+    let floatBuffExpected = bufferFrom<Buffer>([1, 2, 3]);
+    expect<ArrayBuffer>(floatBuff.buffer).toStrictEqual(floatBuffExpected.buffer);
+  });
 });