From dc59df0b648b74794b5ca70478cb8cdeb02c7aea Mon Sep 17 00:00:00 2001
From: Skye <63878374+MierenManz@users.noreply.github.com>
Date: Fri, 21 Apr 2023 10:42:50 +0200
Subject: [PATCH] feat: add `I32LEB128` (#11)
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Co-authored-by: Elias Sjögreen <eliassjogreen1@gmail.com>
---
 types/varint/leb128.ts      | 40 ++++++++++++++++
 types/varint/leb128_test.ts | 94 +++++++++++++++++++++++++++++++++++++
 types/varint/mod.ts         |  1 +
 3 files changed, 135 insertions(+)
 create mode 100644 types/varint/leb128.ts
 create mode 100644 types/varint/leb128_test.ts
 create mode 100644 types/varint/mod.ts

diff --git a/types/varint/leb128.ts b/types/varint/leb128.ts
new file mode 100644
index 0000000..3a93921
--- /dev/null
+++ b/types/varint/leb128.ts
@@ -0,0 +1,40 @@
+import { Type } from "../types.ts";
+
+const SEGMENT_BITS = 0x7F;
+const CONTINUE_BIT = 0x80;
+
+export class I32LEB128 implements Type<number> {
+  read(dataView: DataView, byteOffset = 0): number {
+    let value = 0, position = 0;
+    while (true) {
+      const currentByte = dataView.getInt8(byteOffset);
+      value |= (currentByte & SEGMENT_BITS) << position;
+
+      if ((currentByte & CONTINUE_BIT) === 0) break;
+
+      position += 7;
+      byteOffset++;
+
+      if (position >= 32) {
+        throw new TypeError("I32LEB128 cannot exceed 32 bits in length");
+      }
+    }
+
+    return value;
+  }
+
+  write(value: number, dataView: DataView, byteOffset = 0): void {
+    while (true) {
+      if ((value & ~SEGMENT_BITS) === 0) {
+        dataView.setInt8(byteOffset, value);
+        return;
+      }
+
+      dataView.setInt8(byteOffset, value & SEGMENT_BITS | CONTINUE_BIT);
+      byteOffset++;
+      value >>>= 7;
+    }
+  }
+}
+
+export const i32leb128 = new I32LEB128();
diff --git a/types/varint/leb128_test.ts b/types/varint/leb128_test.ts
new file mode 100644
index 0000000..35b455a
--- /dev/null
+++ b/types/varint/leb128_test.ts
@@ -0,0 +1,94 @@
+import { I32LEB128 } from "./mod.ts";
+import {
+  assertEquals,
+  assertThrows,
+} from "https://deno.land/std@0.183.0/testing/asserts.ts";
+
+Deno.test({
+  name: "Read Positive varint",
+  fn: () => {
+    let data = Uint8Array.of(127);
+    let result = new I32LEB128().read(new DataView(data.buffer));
+    assertEquals(result, 127);
+
+    data = Uint8Array.of(128, 1);
+    result = new I32LEB128().read(new DataView(data.buffer));
+    assertEquals(result, 128);
+
+    data = Uint8Array.of(221, 199, 1);
+    result = new I32LEB128().read(new DataView(data.buffer));
+    assertEquals(result, 25565);
+
+    data = Uint8Array.of(255, 255, 255, 255, 7);
+    result = new I32LEB128().read(new DataView(data.buffer));
+    assertEquals(result, 2147483647);
+  },
+});
+
+Deno.test({
+  name: "Read Negative varint",
+  fn: () => {
+    let data = Uint8Array.of(255, 255, 255, 255, 15);
+    let result = new I32LEB128().read(new DataView(data.buffer));
+    assertEquals(result, -1);
+
+    data = Uint8Array.of(128, 128, 128, 128, 8);
+    result = new I32LEB128().read(new DataView(data.buffer));
+    assertEquals(result, -2147483648);
+  },
+});
+
+Deno.test({
+  name: "Read Bad varint",
+  fn: () => {
+    const data = Uint8Array.of(255, 255, 255, 255, 255, 15);
+    assertThrows(() => new I32LEB128().read(new DataView(data.buffer)));
+  },
+});
+
+Deno.test({
+  name: "Write Positive varint",
+  fn: () => {
+    let data = new Uint8Array(1);
+    new I32LEB128().write(127, new DataView(data.buffer));
+    assertEquals(data, Uint8Array.of(127));
+
+    data = new Uint8Array(2);
+    new I32LEB128().write(128, new DataView(data.buffer));
+    assertEquals(data, Uint8Array.of(128, 1));
+
+    data = new Uint8Array(3);
+    new I32LEB128().write(25565, new DataView(data.buffer));
+    assertEquals(data, Uint8Array.of(221, 199, 1));
+
+    data = new Uint8Array(5);
+    new I32LEB128().write(2147483647, new DataView(data.buffer));
+    assertEquals(data, Uint8Array.of(255, 255, 255, 255, 7));
+  },
+});
+
+Deno.test({
+  name: "Write Negative varint",
+  fn: () => {
+    let data = new Uint8Array(5);
+    new I32LEB128().write(-1, new DataView(data.buffer));
+    assertEquals(data, Uint8Array.of(255, 255, 255, 255, 15));
+
+    data = new Uint8Array(5);
+    new I32LEB128().write(-2147483648, new DataView(data.buffer));
+    assertEquals(data, Uint8Array.of(128, 128, 128, 128, 8));
+  },
+});
+
+Deno.test({
+  name: "Write & read i32 MAX",
+  fn: () => {
+    const value = 2_147_483_647;
+    const decoder = new I32LEB128();
+    const bytes = new Uint8Array(5);
+    const dt = new DataView(bytes.buffer);
+    decoder.write(value, dt, 0);
+    const decodedValue = decoder.read(dt);
+    assertEquals(decodedValue, value);
+  },
+});
diff --git a/types/varint/mod.ts b/types/varint/mod.ts
new file mode 100644
index 0000000..53fea10
--- /dev/null
+++ b/types/varint/mod.ts
@@ -0,0 +1 @@
+export * from "./leb128.ts";