create note class

src/keypair.ts

@@ -25,7 +25,7 @@ export class KeyPair {
     return keypair;
   }
 
-  static async randomAsync(): Promise<KeyPair> {
+  static async random(): Promise<KeyPair> {
     return await KeyPair.newAsync(Field.random());
   }
 

src/note.ts

@@ -0,0 +1,53 @@
+import { BigNumberish } from 'ethers';
+
+import { Field } from './field';
+import { hash } from './hash';
+import { KeyPair } from './keypair';
+
+export class Note {
+  public amount: Field;
+
+  constructor(
+    amount: Field | BigNumberish,
+    public keypair: KeyPair,
+    public blinding: Field = Field.random()
+  ) {
+    this.amount = Field.from(amount);
+  }
+
+  static async random(amount?: Field, keypair?: KeyPair, blinding?: Field) {
+    return new Note(
+      amount ?? Field.random(),
+      keypair ?? (await KeyPair.random()),
+      blinding ?? Field.random()
+    );
+  }
+
+  static zero() {
+    return new Note(Field.zero(), KeyPair.zero(), Field.zero());
+  }
+
+  async commitment(): Promise<Field> {
+    return hash([this.amount, this.keypair.publicKey!, this.blinding]);
+  }
+
+  async commitmentHex(): Promise<string> {
+    return (await this.commitment()).hex();
+  }
+
+  async nullifier(merkle_index: Field): Promise<Field> {
+    if (merkle_index === undefined) {
+      throw new Error('merkle_index is required');
+    }
+    const commitment = await this.commitment();
+    return hash([
+      commitment,
+      merkle_index,
+      await this.keypair.sign([commitment, merkle_index]),
+    ]);
+  }
+
+  async nullifierHex(merkle_index: Field): Promise<string> {
+    return (await this.nullifier(merkle_index)).hex();
+  }
+}